HomeMy WebLinkAboutLIterature Review and Data Gap Analysis Task 2 draft v2
COMMUNITY-BASED HILO BAY
RESILIENCE AND WATERSHED
MANAGEMENT PLAN
Task 2: Literature Review & Data Gap Analysis
Deliverable Prepared by: Lynker Corporation, Fisheries Immersed Sciences Hawaii Inc (FISH),
Arizona State University (ASU), Roth Ecological Design International (REDI), and Engineering,
Science, and Technology, Inc., PBC (EA).
Date: 6/30/2026
EA Engineering, Science, and Technology, Inc., PBC.................................................................3
1. Site Description and Background..............................................................................................3
2. Modeling Approach....................................................................................................................3
3. Data Review................................................................................................................................6
4. Data Gaps and Conclusions....................................................................................................12
5. References............................................................................................................................... 14
Roth Ecological Design Int......................................................................................................16
1. Data Needs...............................................................................................................................16
2. Data Currently Acquired...........................................................................................................17
Arizona State University.........................................................................................................18
1. Authorization and Purpose......................................................................................................18
2. Introduction..............................................................................................................................18
3. Study Area................................................................................................................................19
4. Climate, Geology, Flow Routing, and Hydrologic Metrics......................................................20
5. Land Use and Land Cover........................................................................................................22
6. Water Quality............................................................................................................................25
7. Flooding....................................................................................................................................33
8. Sea Level Rise..........................................................................................................................33
9. Data Gaps.................................................................................................................................35
10. Proposed Sampling Points for Suspended Sediment Concentration
and Nutrient Monitoring.............................................................................................................. 38
11. Field Data Collection Progress Report, June 2026..............................................................39
References....................................................................................................................................43
EA Engineering, Science,
and Technology, Inc., PBC
EA Engineering, Science, and Technology, Inc., PBC (EA) prepared this Technical Memorandum
for Lynker Corporation on behalf of the Hawaii County Department of Research and
Development as part of the Community-Based Hilo Bay Resilience and Watershed Management
Plan. This memorandum describes the proposed hydraulic modeling approach and summarizes
findings from a desktop data review and data gap analysis. The objective is to identify data
requirements necessary for completing Task 5b: Hydraulic Modeling and Flood Hazard Maps.
1.SITE DESCRIPTION AND BACKGROUND
The Hilo Bay Watershed covers approximately 470 square miles (1,200 square kilometers). It
stretches from the summits of Mauna Kea and Mauna Loa down to Hilo Bay, and encompasses
agricultural, forest, conservation, urban, and rural lands. About 69 percent of the watershed is
state-owned, and it receives between 75 and 300 inches of rainfall annually.
For decades, Hilo Bay and its sub-watersheds have been impaired, impacting endangered
habitats and local communities. In 2005, the University of Hawaiʻi and the Hilo Bay Advisory
Group created a restoration plan identifying pollutants and data gaps (Hawaii Department of
Health 2005). In 2023, the U.S. Army Corps of Engineers (USACE) recommended comprehensive
water quality and sediment analysis, mapping sediment sources, and developing a watershed
management plan with prioritized strategies and funding pathways (USACE 2023).
The new plan will integrate these goals with EPA requirements for eligibility under Section 319,
aiming to conserve and restore biodiversity, restore water quality, protect groundwater, manage
surface water, and reflect community values. It will guide sustainable infrastructure and
resilience efforts as Hilo adapts to climate change and population growth.
2.MODELING APPROACH
Hydraulic models for Honoli’i Stream, Wailuku River, and Alenaio Stream will be developed using
the USACE Hydrologic Engineering Center – River Analysis System (HEC-RAS). These models
will simulate flood events with annual exceedance probabilities (AEP) of 10 percent, 4 percent, 2
percent, 1 percent, and 0.2 percent (corresponding to 10-, 25-, 50-, 100-, and 500-year return
periods). The primary objectives of hydraulic modeling includes:
●Generating floodplain boundaries
●Estimating flood elevations and water surface profiles
●Identifying areas susceptible to erosion and sediment delivery under various
storm scenarios
●Supporting broader watershed management strategies
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
2.1 Model Development
The hydraulic models will be HEC-RAS two-dimensional (2D) and will use unsteady-flow input.
Key components of the HEC-RAS 2D model will include:
●Boundary Conditions: Upstream boundaries will use flow rates from streamgage
analyses and/or hydrologic models; downstream boundaries will incorporate sea-level
with tidal influences. Future boundary conditions, including flow rates and sea-level, will
be derived from available literature regarding changing climate conditions.
●Terrain: Derived from available light detecting and ranging (LiDAR) and bathymetric data,
with a 2D mesh applied to the model domain. Mesh cells will be oriented perpendicular
to channel flow and sized to maintain low Courant numbers (ratio of flow velocity and
time step vs cell size) for stability.
●Roughness: Manning’s roughness values will be assigned based on land use and
vegetation cover.
●Structures: Bridges, culverts, and levees will be integrated as inline structures to
accurately represent hydraulic conveyance or flow restrictions through these features.
AEP flow rate inputs for Wailuku River, Alenaio Stream, and Honoli’i Stream watersheds will be
derived from U.S. Geological Survey (USGS) streamgage records. The Honoli’i Stream analysis
will be conducted by other project team members, and EA will obtain streamgage flow data
from online resources for the other watersheds. EA will estimate boundary condition flow rates
for the hydraulic models by applying drainage area scaling to gaged flow data. Flow rates in
streams without streamgage records will be estimated from similar adjacent streams. Historical
hydraulic analyses will be used to validate scaled estimates, and USGS StreamStats will be used
to generate supplemental estimates of AEP flow rates for calibration and validation. In addition,
the completed Wailoa River hydrologic and hydraulic model (USACE 2017) has been obtained
and will be reviewed and integrated to provide the hydraulic modeling results for that river.
The modeling extents will encompass the coastal plain and extend toward the mountains
(mauka) until all floodplains are fully contained within ravines. Upland areas where flood flows
remain confined within ravines and stormwater is conveyed through multiple smaller tributaries
will not be included in the modeling. The approximate extents of proposed modeling domains
are shown on Figure 1.
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
Figure 1. Proposed Modeling Domains
2.2 Scenarios and Calibration
Four scenarios will be evaluated:
1.Existing Conditions
2.Existing under Future Climate Conditions (2075): Increased flow rates and sea-level rise
with expected future climate conditions in 2075.
3.Proposed Conditions: Incorporating green stormwater infrastructure adaptation
strategies (relevant/updated model inputs will be provided by other team members to
support this scenario).
4.Proposed under Future Climate Conditions (2075)
Model calibration and validation will be performed using available historical records, such as
observed flood events, streamgage data, and high-water marks. Where formal data is limited,
anecdotal information from local stakeholders and community observations may be considered
to provide context and validate model performance under real-world conditions.
2.3 Deliverables
Results will be documented in a Hydraulic Modeling Technical Memorandum, including:
●Summary of model inputs and development process
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
● Results for existing, future, and proposed conditions
● Figures, tables, and supporting attachments
3. DATA REVIEW
Hydraulic modeling will utilize publicly available data, published literature, and
government-furnished information. The following data sources have been identified as valuable
for supporting these modeling efforts.
3.1 FEMA – FIS, FIRM, NFHL, Effective Models
The FEMA conducts Flood Insurance Studies (FIS) to develop Flood Insurance Rate Maps
(FIRMs). These maps illustrate flood hazard information, including flood extents for various
storm events, base flood elevations, and the locations of hydraulic structures (FEMA 2017,
2023, and 2026). FIRM data is also available in shapefile format through the National Flood
Hazard Layer (NFHL), as shown in Figure 2. FEMA develops hydrologic and hydraulic models to
establish the base flood elevations and flood extents. These effective models have been
obtained from FEMA for a fee.
Figure 2. NFHL
The FIS report, NFHL files, and effective models were acquired and reviewed. The FIS report
provides watershed descriptions, flood protection measures, and summaries of hydraulic
EA Engineering, Science, and Technology, Inc., PBC | PAGE 6
Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
analyses. It includes tables of peak discharges for flooding sources and base flood water
surface elevations at modeled cross sections. FIRMs identify special flood hazard areas subject
to inundation by the 1 percent annual chance flood for streams that have been studied. The
1 percent annual chance floodplain is shown on the FIRMs for Wailuku River, Alenaio Stream,
and Wailoa River. Honoli’i Stream has not been studied and therefore lacks flood hazard
mapping. NFHL files contain floodplain boundaries, stream centerlines, cross sections,
waterbodies, and other relevant features in shapefile format.
The effective model files were developed using multiple versions of HEC software. The files
obtained include HEC-1 printouts in PDF format, HEC-2 files, and HEC-RAS files. These models
were reviewed for existing structure data, such as bridge and culvert geometry. Most files
pertain to streams outside the current modeling scope, including numerous files for the Wailoa
watershed. Structure data was identified for Alenaio Stream, but only limited data was found for
Wailuku River, and none for Honoli’i Stream.
3.2 Google Earth
Google Earth provides both current and historical aerial imagery for the project site (Google
Earth 2026). The latest imagery can be used to verify and refine land cover delineation,
characterize surface conditions, and assign Manning’s roughness values for hydraulic modeling.
Aerial imagery can be valuable when referencing FEMA’s effective models and other data
sources such as the Hawaii Statewide geographic information system (GIS) Program data, as it
helps identify model elements (matching hydraulic structures to road crossings). Comparing
historical and current imagery can reveal changes in topography, surface roughness, or the
number of structures, ensuring that use of the effective model data account for any changes.
3.3 Hawaii Statewide GIS Program
The Hawaii Statewide GIS Program provides a centralized repository of geospatial data
maintained by the State of Hawaii Office of Planning and Sustainable Development (Hawaii
Statewide GIS Program 2026). This resource offers a wide range of datasets relevant to
watershed and hydraulic modeling, including stream centerlines, roads, bridges, parcels,
vegetation, and agricultural land use, as shown in Figure 3. These layers will be integrated into
the hydraulic modeling process to enhance model accuracy and ensure comprehensive
representation of physical and land-use conditions. Stream centerlines may be used to identify
and label channel thalwegs within the modeling domains. Bridge data includes deck width and
span length, but does not include deck thickness, deck elevation, or pier information. The limited
bridge data available in this database may be used in concert with Google Earth and the terrain
data to approximate structure geometry for hydraulic models. Additional bridge and culvert data
was found to be available from county resources, including detailed as-built design drawings for
many bridges within the project area, and will be incorporated into the modeling process.
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
Figure 3. Hawaii Statewide GIS Layers
3.4 NOAA – Land Cover, LiDAR
The National Oceanic and Atmospheric Administration (NOAA) Coastal Change Analysis
Program provides a free, publicly accessible database of land cover and land change
information for U.S. coastal regions (NOAA 2026). These datasets classify land cover types by
the following: wetlands, developed areas, forests, and cultivated lands, with an emphasis on
coastal and estuarine environments. NOAA land cover data can support the estimation of
Manning’s roughness coefficients for hydraulic modeling. High resolution land cover data from
2010 is shown in Figure 4.
LiDAR is a remote sensing method used to collect detailed topographic data. The NOAA Office
for Coastal Management curates LiDAR, including both topographic and bathymetric data
datasets, for coastal areas. These high-resolution elevation data can be represented as a digital
elevation model, which serves as the terrain input for HEC-RAS hydraulic models. LiDAR surveys
conducted in 2013, 2018, and 2023 across different portions of the watershed will be merged to
produce a comprehensive terrain model of the entire watershed for hydraulic analysis. The
merged LiDAR terrain is shown in Figure 4.
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
Figure 4. NOAA Land Cover and LiDAR
3.5 USDA – Web Soil Survey
The web Soil Survey is a publicly available, free, online database of soil data produced by the
U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (USDA 2026).
Surface and subsurface soil data, such as hydrologic soil group, are available in the Web Soil
Survey database. Soil classification can be used to support estimation of Manning’s roughness
coefficients for hydraulic models. A map of soil types from the web soil survey is shown in
Figure 5.
EA Engineering, Science, and Technology, Inc., PBC | PAGE 9
Figure 5. Web Soil Survey
Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
3.6 USGS – Streamgages
The USGS operates streamgages to monitor hydrologic conditions in rivers and streams (USGS
2026a). These instruments collect continuous data such as discharge, gage height, water level,
and velocity. Multiple gages in a region can provide contextual information, but site-specific
data are generally preferred for accurate modeling and analysis.
Several active gages are located on streams within the project area, including Honoli‘i Stream
and Wailuku River. Historical records are also available for Waiakea Stream (at Mountain View),
which was monitored from 1967 to 1995, and reestablished by our team in 2025. These
datasets enable computation of AEP flow rates. While streamgage coverage is robust for major
streams, data gaps exist for smaller tributaries and un-gaged locations. These gaps can be
addressed using regional regression equations and supplemental tools such as USGS
StreamStats (USGS 2026b).
3.7 USGS – Streamstats
The USGS StreamStats application is a web-based tool that provides hydrologic and basin
characteristics for user-defined points on streams across the U.S. (USGS 2026c). The tool
integrates geospatial datasets and regression equations to compute peak-flow estimates for
various recurrence intervals. StreamStats can be used to delineate watershed boundaries and
estimate flow rates for un-gaged locations within the modeled watersheds. These estimates
can supplement USGS streamgage data and provide input for hydraulic modeling of stream
locations where continuous flow records are unavailable.
3.8 Updated Hydraulic and Hydrologic Modeling for the Waiakea-Palai
CAP 205 Project
This document presents a detailed hydrologic and hydraulic analysis of the Wailoa River system,
focusing on Waiakea Stream and Palai Stream (USACE 2021a). The updated model incorporates
high-resolution LiDAR data collected between 2018 and 2023, NOAA topobathymetric data for
Hilo Bay, and field measurements of bridges and culverts obtained in 2024. These
enhancements provide a robust terrain foundation and accurate structural geometry for the 2D
HEC-RAS model. Hydrologic inputs were calculated using a calibrated Hydrologic Engineering
Center Hydraulic Modeling System model informed by NOAA Atlas 14 precipitation frequency
data and local rainfall records. Synthetic inflow hydrographs were developed for events ranging
from the 10-year to the 500-year recurrence interval. The resulting hydraulic model will provide
flood hazard maps, flood elevations, and water surface profiles of the Wailoa River.
3.9 Literature Review
The Hawaii County Department of Research and Development curated an extensive list of
literature that is relevant to the watershed planning and presented this literature on their website
(County of Hawai’i Research & Development 2026). This literature was reviewed to identify the
documents which include information relevant to hydraulic modeling and flood hazard mapping.
The following documents were identified:
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
●2020 Hawaiʻi Coastal Resilience Assessment (National Environmental Modeling &
Analysis Center 2020)
○Includes a discussion of flooding hazards and incorporates the FEMA flood
extents in exposure index calculations. Includes anecdotal accounts of flooding
after tropical storms, and discussion on coastal flooding from storm surge and
sea-level rise.
●U.S. Army Corps of Engineers – Hilo Bay Watershed Planning Assistance to States
Report (USACE 2023)
○Includes description of the watershed and FEMA flood extents.
●Hilo Bay Water Circulation and Water Quality Study (USACE 2009)
○Includes baseflow estimates for Wailuku and Wailoa Rivers entering the bay.
●Hilo Bay Watershed-Based Restoration Plan (Hawaii Department of Health 2005)
○Lists average flows (million gallon(s) per day) for major rivers and provides a
summary of watershed hydrology.
●Hilo Bay Muliwai National Estuarine Reserve System Site Designation Application (Hilo
Bay Muliwai 2013)
○Includes watershed information and FEMA flood extents.
●Waiakea-Palai Streams – Section 205 Flood Risk Management Final Integrated
Feasibility Report and EA (USACE 2021a)
○Includes hydrology and hydraulics modeling for Wailoa River using HMS and 1D
RAS. This modeling was updated in 2024 for the Waiakea-Calai CAP 205 Project.
●Mauna Kea Watershed Management Plan (Mauna Kea Watershed Alliance 2010)
○Provides watershed data for all Mauna Kea watersheds.
●Rapid Watershed Assessment – Hilo Watershed (USDA National Resources
Conservation Service 2009)
○Includes watershed data for the Hilo Watershed (Hydrologic Unit Code
2001000003).
●Alenaio Stream – Harbors and Rivers in Hawaii: Final Survey Report and Environmental
Impact Statement (USACE 1982)
○Contains historical hydrology flow data. This data is outdated for current use.
●Wailuku-Alenaio Watershed Project Work Plan (Soil Conservation Service Forest
Service 1976)
○Includes historic watershed and flooding data; outdated for current use but may
inform original design of flood control structures.
●Alenaio Stream LB – Levee, Floodwall C & Lined Channel (USACE 2016a)
○Levee information which may be used for hydraulic model setup.
●Alenaio Stream – Floodwall A, B – RB & Lined Channel (USACE 2016b)
○Levee information which may be used for hydraulic model setup.
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
●Wailoa Stream RB – Diversion Levee 1–4 & Channel (USACE 2017)
○Levee information which may be used for hydraulic model setup.
●Hawaiʻi Sea Level Rise Vulnerability and Adaptation Report (Hawai’i State Climate
Commission 2022)
○Sea-level rise data applicable for future downstream boundary conditions.
●Hawaiian Islands National Shoreline Management Study (USACE 2021b)
○Sea-level rise data applicable for future downstream boundary conditions.
●A Water-Budget Model and Assessment of Groundwater Recharge for the Island of
Hawai‘i (USGS 2011)
○Presents rainfall and runoff modeling calculations, including future with changing
climate condition rainfall and runoff estimates.
●Agricultural Water Use and Development Plan (State of Hawaii Department of
Agriculture 2019)
○Includes agricultural water use and water control structure data.
●2020 County of Hawaiʻi Multi-Hazard Mitigation Plan (County of Hawai’i 2020)
○Describes flood hazards, levee information, and historic flooding. Uses FEMA
floodplain mapping. Includes a table summarizing peak discharges from the
FEMA FIS.
4.DATA GAPS AND CONCLUSIONS
A comprehensive review of available resources indicates that sufficient data exists to develop
hydraulic models for the Hilo Bay watershed; however, several gaps remain that require
attention. These gaps and proposed mitigation strategies are summarized below.
4.1 Identified Data Gaps
●Bridge Geometry: Limited information is available on bridge dimensions and
structural details.
●Culverts: Uncertainty exists regarding the presence and size of culverts within the
modeling domain.
●Stream Bathymetry: Existing LiDAR and topographic datasets may not capture
underwater channel geometry.
●Flow Rates for Honoli‘i, Wailuku, and Alenaio Streams: Hydrologic models are
unavailable for these streams.
●Future Flow Rates: Limited literature addresses projected changes in runoff in future
with changing climate conditions scenarios.
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
4.2 Planned Solutions
●Bridges: Field data collection is currently outside the project scope. Bridge data,
including detailed as-built bridge drawings, was obtained from county resources and will
be incorporated into the modeling process.
●Culverts: If present, culverts are likely small and will have negligible impact on large
flood events (e.g., 100-year storms). They may be excluded from the model if no data is
available. Culvert data may be available from county resources and if this information
becomes available before modeling begins it will be incorporated into the modeling
process.
●Stream Bathymetry: In the absence of bathymetric data, riverbeds will be approximated
using water surface elevations from LiDAR. This conservative approach will be
documented.
●Flow Rates: Flow estimates will be estimated using USGS streamgage data and drainage
area scaling, with calibration using hydraulic data from historical records at
streamgages.
●Future Flow Rates: Available literature on climate-driven rainfall changes will be reviewed
to correlate projected precipitation with runoff for the modeled watersheds. Additional
sources will be sought and EA’s internal resources will be used to strengthen these
estimates.
4.3 Conclusions
Despite these gaps, adequate information is available to construct reliable hydraulic models for
the project area. Where assumptions are necessary, they will be clearly documented. These
limitations are not expected to significantly affect the overall quality or utility of the modeling
results for flood hazard mapping.
EA Engineering, Science, and Technology, Inc., PBC | PAGE 13
Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
5.REFERENCES
County of Hawai’i Research & Development. (2026). Creating a Community-based Hilo Bay
Resilience and Watershed Management Plan.
https://www.rd.hawaiicounty.gov/economic-development/hilo-bay. Accessed January 2026.
County of Hawai’i. (2020). Multi-Hazard Mitigation Plan. May.
Federal Emergency Management Agency. (2017). Flood Insurance Study Numbers 1551660902F
- 1551660904F. September. https://msc.fema.gov/portal/search.
Federal Emergency Management Agency. (2023). Flood Insurance Rate Maps 18089C0126F,
18089C0127E, 18089C0131E, 18089C0132E, 18089C0151E, 18089C0152F, 18089C0156F, and
18089C0157F.
Federal Emergency Management Agency. (2026). FEMA’s National Flood Hazard Layer (NFHL)
Viewer.
https://www.arcgis.com/apps/webappviewer/index.html?id=8b0adb51996444d4879338b55
29aa9cd.
Google Earth (n.d.). Google Earth (Imagery of Hilo Bay Watershed). Accessed January 2026.
Hawaii Department of Health. (2005). Hilo Bay Watershed-Based Restoration Plan. October.
Hawai’i State Climate Commission. (2022). Hawai’i Sea Level Rise and Vulnerability and
Adaptation Report.
Hawaii Statewide GIS Program. (2026). Geospatial Data Portal. https://geoportal.hawaii.gov/.
Accessed January 2026.
Hilo Bay Muliwai Hui: Edith Kanakaole Foundation, University of Hawai’i at Hilo. (2013). National
Estuarine Reserve System Site Designation for Hawai’i State. August.
Mauna Kea Watershed Alliance. (2010). Mauna Kea Watershed Management Plan. 15 April.
National Environmental Modeling & Analysis Center. (2020). Hawai’i Coastal Resilience
Assessment.
National Oceanic and Atmospheric Administration. (2026). Data Access Viewer,
https://coast.noaa.gov/dataviewer/#/. Accessed January 2026.
Soil Conservation Service Forest Service. (1976). Watershed Work Plan, Wailuku-Alenaio
Watershed, Hawaii County, Hawaii. March.
State of Hawaii Department of Agriculture. (2019). Agricultural Water Use and Development Plan
Update. December.
U.S. Army Corps of Engineers. (1982). Alenaio Stream, Island of Hawaii Harbors and Rivers in
Hawaii, Final Survey Report and Environmental Impact Statement. July.
U.S. Army Corps of Engineers. (2009). Hilo Bay Water Circulation and Water Quality Study.
January.
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
U.S. Army Corps of Engineers. (2016a). Alenaio Stream LB - Levee, Floodwall C & Lined Channel
(ASFC). https://levees.sec.usace.army.mil/levees/3205010103. Accessed January 2026.
U.S. Army Corps of Engineers. (2016b). Alenaio Stream - Floodwall A, B - RB & Lined Channel
(ASFA). https://levees.sec.usace.army.mil/levees/3205010101. Accessed January 2026.
U.S. Army Corps of Engineers. (2017). Wailoa Stream River Basin – Diversion Levee 1, 2, 3, 4 &
Channel (WSRB). https://levees.sec.usace.army.mil/levees/3205010202. Accessed January
2026.
U.S. Army Corps of Engineers. (2021a). Waiakea-Palai Streams.
U.S. Army Corps of Engineers. (2021b). Hawaii Regional Assessment National Shoreline
Management Study. December.
U.S. Army Corps of Engineers, Honolulu District. (2023). Hilo Bay Watershed Planning Assistance
to States. February.
U.S. Department of Agriculture. (2026). United States Department of Agriculture Web Soil Survey,
https://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx. Accessed January 2026.
USDA National Resources Conservation Service. (2009). Rapid Watershed Assessment, Hilo
Watershed, Hawai’i, Hydrologic Unit Code – 2001000003. March.
U.S. Geological Survey. (2011). A Water-Budget Model and Assessment of Groundwater Recharge
for the Island of Hawai’i. May.
U.S. Geological Survey. (2026a). USGS Current Water Data for the Nation.
https://waterdata.usgs.gov/nwis/rt. Accessed January 2026.
U.S. Geological Survey. (2026b). StreamStats. https://streamstats.usgs.gov/ss/. Accessed
January 2026.
U.S. Geological Survey. (2026c). National Water Information System: Web Interface.
https://www.usgs.gov/node/279857. Accessed January 2026.
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Roth Ecological Design Int.
Roth Ecological Design Int (REDI) will be undergoing geospatial analysis to identify priority
areas/parcels where green stormwater infrastructure could have the most benefit.
1.DATA NEEDS
This memo summarizes the data needs for Roth Ecological Design Int (REDI)’s scope or work.
This includes datasets currently available for the Hilo analysis, identifies remaining data needs,
and requests confirmation of preferred data sources where applicable. The goal is to ensure
alignment on inputs prior to final modeling and analysis.
Figure 1. Proposed study area.
●Watershed Boundaries
○Preferred watershed scale (e.g., HUC12 vs. HUC8) and applicable GIS files
●Total Maximum Daily Loads (TMDLs)
○Applicable known or future TMDLs for the study area (e.g., enterococci,
turbidity, nutrients)
●Water Quality Standards
○Applicable state or local standards to be used for assessment
and exceedance tracking
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
● Modeled Catchment Boundaries
○ To be provided following completion of the relevant contractor scope item
● Modeled Flood Extents
○ Flood extents for specified return periods; to be provided following completion of
the relevant contractor scope item
2. DATA CURRENTLY ACQUIRED
The following datasets have already been identified and/or acquired and are proposed for use
unless otherwise directed:
● Water Quality and Hydrology
○ EPA and USGS water quality data (1973–2024): Water Quality Data Portal
○ USGS water level data: USGS Station 16717000
○ NOAA water level data: NOAA Tides and Currents Station 1617760
● Infrastructure and Land Use
○ Cesspool locations: Hawai‘i GeoPortal
○ County zoning data: Hawai‘i GeoPortal
○ Agricultural land use: Hawai‘i GeoPortal
○ Parcel data for Hawai‘i County (to be supplemented with ReGrid if needed):
Hawai‘i GeoPortal
● Land Cover and Physical Characteristics
○ Land cover, impervious surface, and tree canopy: NOAA Digital Coast
○ Elevation data: NOAA Digital Coast
○ NHD waterbodies: Hawai‘i GeoPortal
○ Stream network: Hawai‘i GeoPortal
○ NRCS Soil Web Survey
● Climate and Precipitation
○ Annual rainfall: Hawai‘i GeoPortal
○ NOAA Atlas 14 precipitation frequency estimates (Hilo Airport)
● Risk and Contextual Indicators
○ 2020 Social Vulnerability Indicators: Hawai‘i GeoPortal
○ FEMA National Flood Hazard Layer
○ Base Flood Elevations: Hawai‘i GeoPortal
Roth Ecological Design Int. | PAGE 17
Arizona State University
Jasper Oshun & Kelly Hondula
Center for Global Discovery and Conservation Science, Arizona State University
Hilo, Hawai‘i
1.AUTHORIZATION AND PURPOSE
The following document details the ‘state of knowledge’ with respect to hydrologic processes
within the Hilo Bay Watershed and delivery of contaminants to Hilo Bay. Knowledge sources
include peer-reviewed publications, published books, professional reports, master’s theses,
publicly available datasets, and watershed management plans. This document focuses on the
identification and quantification of sources of contaminants within the watershed and delivery
mechanisms to Hilo Bay. We conclude with a list of ‘data gaps’ and our planned
approach/suggested approaches to fill these data gaps.
2.INTRODUCTION
The Hilo Bay Watershed encompasses one of the six major moku-o-loko, or districts of Hawai‘i
Island (Maly, 1996) and spans at least 24 ahupua‘a from ʻŌlaʻa in the south to Pāpaʻikou in the
north. The upper limits of the watershed begin in wao akua (realm of the gods) on the slopes of
Mauna Kea and Mauna Loa, and extend across the reefs of Hilo Bay. Kānaka Maoli (Native
Hawaiians) considered Hilo Bay to be a resource rich place integrating systems of fresh and sea
water. Here, they managed fishponds across the varying salinities of the ‘broad waters’ of the
Waiākea/Wailoa estuary (Kelly et al., 1981) and harvested wood to construct canoes on Hilo’s
broad black sand beaches (Desha, 2000). The abundant rainfall and fertile land allowed Hilo
Paliku (Hilo of the upright cliffs) to be ‘cultivated from the sea upward to the line of frost’
(Kalākaua,1972:284).
Nowadays, Hilo Bay serves as an economic center, linking the shipment of goods onto and away
from the island, and is used for fishing, sailing, canoeing, surfing, and swimming. The Bay also
provides scenic beauty for the benefit of residents and visitors. The eastern portion of the bay,
extending through Keaukaha, is rich in corals, fish, and other forms of aquatic life and are
popular destinations for traditional fishing and tourism.
Since at least the mid 20th century, however, the Hilo community has documented pollution in
the Bay. The department of health measured high levels of fecal coliform in the bay as early as
the 1950s (Cox & Gordon, 1970). Hilo Bay has exceeded state water quality standards (due to
high suspended load, high levels of contamination, or both) since the late 1970s, and portions of
the bay and watershed have been federally listed as impaired waters by the Environmental
Protection Agency (EPA) under section 303(d) of the Clean Water Act.
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Pollution to Hilo Bay has two primary sources: 1) chronic and episodic groundwater delivery of
untreated sewage, which leaches from on-site-sewage-disposal systems (OSDS) through young
and highly porous volcanic rock; and 2) episodic delivery of sediment and other contaminants
mobilized from agricultural or urban surfaces via high-intensity rainfall, overland flow, and
stream flooding. In Hilo, only 30-40 % of the urban population is connected to the wastewater
collection sewer system (Silvius et al., 2005), while there over 10,000 OSDS (primarily
cesspools) throughout the watershed (Mezzacapo and Shuler, 2022) leaching large volumes of
untreated sewage into the subsurface. Although the problem has been recognized for many
years, the collection of data, community knowledge, and political momentum have not yet
aligned to generate lasting positive change. Specifically, the Hilo Bay Watershed-Based
Restoration Plan (Silvius et al, 2005) noted that despite the numerous studies of contaminant
sources and concentrations in Hilo Bay, ‘none of the data sets are lengthy’ and ‘none of the data
sets pinpoint the sources of pollutants by land use type.’
Now, a new community-supported process has begun to collect measurements and samples
following EPA procedural guidelines, to address knowledge gaps, and to produce the necessary
data to inform a comprehensive watershed management plan. Our goal is to engage with the
community to learn, follow federal guidelines to collect and interpret the necessary data, and
provide specific recommendations to the County. Our hope is that the community and the
County will use the results of our study to craft a watershed management plan to prioritize
mitigation and acquire funding to restore Hilo Bay to a healthy ecosystem. The following
summarizes the state of knowledge on the problem of pollution to Hilo Bay and identifies key
knowledge gaps we aim to fill over the course of this study.
3.STUDY AREA
The Hilo Bay Watershed, as defined by the County of Hawai‘i in their Request for Proposal,
includes three primary watersheds: Honoli‘i, Wailuku, and Waiākea/Wailoa (Figure 1). Together,
the three watersheds cover 1106 km2 (273,299 acres) of rocky uplands, native forest,
agricultural and rural, suburban, urban, and industrial land uses. Honoli‘i is the northernmost
watershed and exhibits a relatively thin and narrow shape, discharging to the ocean at the
northern end of Hilo Bay. Wailuku is the largest watershed in the state of Hawai‘i, and
discharges to the western side of Hilo Bay at the northern boundary of Hilo’s urban center.
Waiākea/Wailoa forms a large watershed which discharges to Hilo Bay through the Wailoa Pond
estuary on the southern margin of Hilo Bay. On the ocean side, this study is focusing on the Hilo
Bay ocean area adjacent to these watersheds out to three miles.
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Figure 1. Map of three watersheds draining to Hilo Bay. Contours show mean annual precipitation in mm.
4. CLIMATE, GEOLOGY, FLOW ROUTING, AND HYDROLOGIC METRICS
4.1 Precipitation
Mean annual rainfall in Hilo Bay Watershed varies from 3000 mm in the lower watershed around
Hilo to greater than 7000 mm in the upper Honoli‘i watershed north of Hilo (Longman et al.,
2024) (Figure 1). Mean annual precipitation increases with elevation until approximately 2,000
meters above sea level, at which point a temperature inversion due to descending dry air (e.g.,
Schroeder, 1981) leads to decreased mean annual precipitation (Giambelluca et al, 2013) across
the cool grasslands, scrub lands, and bare earth of the wao akua, or upper slopes of Mauna Kea
and Mauna Loa. Mean monthly rainfall at Hilo International Airport is 244 mm and varies from a
high of 353 mm in December to a low of 142 mm in June. May through July is typically drier,
while October through December is typically wetter. Intense rainfall can occur any time of year
and sustained heavy rainfall produces flashy stream responses throughout the watershed,
resulting in large and episodic stormflow delivery to Hilo Bay.
4.2 Streamflow and Runoff Generation
The USGS provides current and long-term streamflow data at Honoliʻi (station #16717000) and
Wailuku at Pi‘ihonua (# 16704000), as well as historical long-term streamflow data from
Waiākea at Mountain View (#16700000) and Wailuku in Hilo (#16713000). The first three
stations are located at the downstream edge of the forest boundary and thus provide good
estimates of natural flow. Flow duration (for mean daily flow) and flood frequency (annual peak
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flows) are shown in Figure 2. The flow duration curves show the percentage of time (number of
days on record) mean daily discharge at each station is in exceedance of specific flows. Mean
daily discharge is consistently largest at Wailuku (Hilo) and smallest at Waiākea. The middle
section of the curves shows the consistency of medium flows and robust baseflow at the
Wailuku sites and Honoliʻi. Waiākea shows less ‘flashiness’ at large mean daily discharge (low
exceedance probability) and smaller baseflows (steep decline at high exceedance probabilities).
For a given recurrence interval, peak discharge is largest in Wailuku and smallest in Waiākea.
Figure 2. Flow duration curves (mean daily flows) and flood frequency curves (annual peak flows) for streams with
long-term USGS data in the Hilo Bay Watershed.
The mean age of the underlying substrate in each watershed decreases from 60 ky (thousand
years) in the northernmost watershed, Honoliʻi, to 31 ky in Wailuku, and to 2.2 ky in the
southernmost watershed, Waiākea. The routing of precipitation into surface and subsurface
(groundwater) runoff appears to be controlled by the hydrologic properties of the different aged
substrate. Whereas in the older Honoliʻi watershed, 54 % of rainfall leaves as surface flow, only
16 and 4 % of rainfall, respectively, leaves as surface runoff from Wailuku and Waiākea
watersheds (Table 1). Although all three watersheds have similar vegetation cover in the upper
reaches and similar mean annual evapotranspiration, increasing ‘leakage’ or an increasing
percentage of rainfall leaving the watershed as groundwater draining below the USGS gages
measuring surface flow is seen from Honoliʻi to Wailuku and Waiākea. This difference in the
‘routing’ of precipitation into surface and subsurface flow controls the delivery of contaminants
from these watersheds to Hilo Bay. Native Hawaiians observed this phenomenon and named
the flashy ‘Wailuku’ (waters of destruction) and ‘Wailoa’ (broad river), accordingly. Wailoa
became one of the most productive Loko I‘a as the mixing of fresh water and salt water allowed
for kalo cultivation and fish nurseries (Maly, 1996).
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Despite large mean annual precipitation in the Waiākea watershed (~ 3600 mm/year) the
geologically young and highly porous substrate results in only an intermittent stream through
Hilo. The vast Kea‘au aquifer extending below and beyond the Waiākea watershed has an
estimated sustainable yield of 395 MGD, while the estimated sustainable yield of Onomea
aquifer (which includes Honoli‘i watershed, which routes more precipitation to surface flow) is
147 MGD (DWS). Freshwater inputs (carrying nutrients and contaminants) to Hilo Bay can thus
be conceptualized as arriving to two sections: a surface water dominated western half, and a
groundwater dominated eastern half (sensu Lucas et al., 2023). The Wailuku River is the primary
input to the western half of the Bay, while the Wailoa River, and submarine groundwater
discharge dominate the eastern half. Mean daily discharge from the Wailuku River to the
western side of Hilo Bay (at station 16713000, active 1977 – 1983) was ~ 1.0 million cubic
meters per day (~ 264 MGD), while submarine groundwater discharge primarily from the
Waiākea Watershed to the eastern portion of the bay is estimated at 1.8 million cubic meters
per day (~ 476 MGD) (M & E Pacific, 1980).
5. LAND USE AND LAND COVER
Nearly three quarters of the watersheds draining to Hilo Bay are uplands covered in forest, bare
earth, pasture (NOAA C-CAP) and other land use types (Figure 3) that are primarily designated
for conservation. These lands include native forests, grasslands, and bare soil and lava rock
(Table 2), and are concentrated across the uplands of the study area. Forests span elevations
across the zone of highest mean annual precipitation and provide many ecosystem services to
the community of Hilo below. Forests reduce the risk of flood through canopies that lower
rainfall intensity and organic rich soils promoting infiltration and groundwater flow (e.g.,
Yamamoto and Duffy, 1963). Trees assimilate and store carbon above ground (e.g. Asner et al.,
2011) and below (Selmants et al., 2014, 2017), and may also stabilize steep slopes through the
cohesive strength of their roots (e.g., Ziemer, 1981; Zhu & Anderson, 1998). Native,
well-managed forests should therefore be considered an important asset to Hilo Bay, or a ‘green
infrastructure’ playing an important role in regulating fluxes of water, soil, carbon, and oxygen.
Across Hawai‘i, deforestation and degradation of native forests have led to reductions in the
infiltration capacity of surface soils, which promotes overland flow and soil loss (e.g., Izuka,
2012; Stock and Cerovski-Darriau, 2021). Increased delivery of sediments to streams impairs
water quality in freshwater and saltwater environments (DOH, 2018). The episodic delivery of
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sediment laden stormflow to the nearshore environment is also an acute stressor to coral reefs
(e.g., Field et al., 2008 Gove et al, 2023).
Hilo’s native forests are threatened by invasive species (feral ungulates, and strawberry guava;
Risch et al., unpublished; Denslow et al., 2024), and pathogens (e.g., rapid ‘ōhi‘a death, ‘ROD’,
Fortini et al., 2019; Asner et al., 2018). Feral pigs are pervasive throughout the upper forests (e.g.
Risch et al., unpublished) and their activity denudes landscapes allowing for the invasion of
non-native species (e.g., Hess et al., 2010; Katahira, 1980). Feral pig activity wounds ‘ōhi‘a, which
may provide entry points for fungal pathogens (Mortenson et al., 2016; Fortini et al., 2019). Feral
pigs are also a direct source of fecal indicator bacteria (e.g., Dunkell et al., 2011a; Strauch et al.,
2014; Strauch, 2017). Feral pig rooting and wallowing also likely plays a role in sediment delivery
to streams, although studies have shown both increases and decreases in total suspended
solids downstream of forest exclusion plots (e.g., Browning, 2008; Dunkell et al., 2011b).
Whereas the exclusion of feral pigs does allow forests, and in particular ground cover to
regenerate (over ~ 6 years), control of non-natives over much longer periods may be necessary
to allow for the recruitment and sustained growth of native species (Cole et al., 2012; Cole &
Litton, 2014). This project has an opportunity to learn from and inform ongoing ungulate control
projects that are ongoing in the upper parts of Wailuku and Honoli‘i watersheds (USDA,
NRCS, 2022).
Cropland, pasture, shrub/brush, and rangeland represent the next most prevalent land uses.
Former sugarcane plantations, which were once a primary source of sediment, were left fallow
or converted to diversified agriculture in the 1990s (Michaud & Wiegner, 2011). Nowadays,
agriculture within the Hilo Bay Watershed is primarily pasture (59 %), and macadamia nuts (22
%) (USDA, NRCS, 2022).
Developed land represents a relatively small proportion of the study area concentrated in the
lower parts of all watersheds. The highest density of developed land is found in the lower
Waiākea/Wailoa, in close proximity to Hilo Bay. The urban center of Hilo is a source of arsenic
from cane production (Hallacher et al., 1985), as well as a likely source of petrochemicals from
urban runoff. Land use and land cover provide information on contaminant sources to surface
flow (e.g., loose soil in agricultural lands, or urban runoff) and subsurface flow (e.g., high density
urban areas with cesspools, see below).
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Figure 3. Map of land use and land cover throughout the Hilo Bay Watersheds.
Table 2. Percentage of land use/land cover across the three primary watersheds of Hilo Bay.
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6. WATER QUALITY
6.1 Suspended Sediment Concentration and Turbidity in Streamflow
Suspended sediment concentration (SSC) was measured in the lower Wailuku River near the
outlet to Hilo Bay (USGS station 16713000) from 1977 to 1983. Although these data were
collected at the height of sugar cane production and do not identify specific sources of
sediment within the watershed, they do provide an important metric against which to compare
our forthcoming results. Over the monitoring period, the average total daily load (TDL, metric
Tons/day) was 64 T/day, and the maximum daily load was greater than 10,000 T. Only about 1 %
of the cumulative measured load occurred below a threshold discharge of 9.6 m3/s (30 %
exceedance probability) and the data show a weak power law relationship between discharges
greater than 9.6 m3/s and SSC (Figure 4). However, the largest mean daily flow for which SSC
was sampled was 453 m3/s and there are 6 days for which mean daily flow was between 453
and 1217 m3/s. Thus the data set is missing SSC in the highest flows. In the Waimea River
(Kaua‘i), as is common in many large rivers, it is the more frequent flows (those occurring every
1 – 5 years) that are most effective at transporting the bulk of the suspended sediment load
(Gomez, 2018; Wolman & Miller, 1960). This is likely the case in the Wailuku River, however, we
will not know until we capture SSC measurements across the highest flows. Similar to data from
the Waimea River (Gomez, 2018), the slope of the power law relationship between discharge
and SSC at Wailuku is less than 1 (0.54), which suggests sediment delivery is supply limited
within the watershed.
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Figure 4. Relationship between mean daily discharge and suspended sediment concentration (SSC) in the lower
Wailuku River. Data collected by the USGS from 1977 – 1983.
Although the USGS station in Hilo is no longer active, mean daily discharge correlates well with
the upstream gage (Pi‘ihonua) during the period of overlap: (QHilo = 1.48QPi’ihonua + 1.08, R2 = 0.98).
Our data collection will fill this gap and assess current sediment delivery through Wailuku River.
SSC was measured in Waiākea at Hoaka Road (USGS station 16700600) from 2004 to 2006
(Presley, 2008). Over the monitoring period mean daily flow ranged from 0 (no flow) to 12.46
m3/s. The maximum measured daily load was only about 6 T/d. There is a weak positive power
law relationship between mean daily discharge and SSC (R2 = 0.31). Over the monitoring period
(745 days), SSC was greater than 0 on only 65 days (< 9%), indicating the relatively small
amount of sediment transport in Waiākea stream. The report also includes downstream
monitoring on Waiākea stream within Hilo (USGS station 16701300). Here, the mean daily load
was only 0.65 T/d.
The same USGS report (Presley et al., 2008) includes data (2004 to 2006) from two locations in
Alenaio stream (16701600 and 16701650, at the Kilauea Bridge). Across 15 samples, maximum
SSC was 121 and maximum nitrate concentration was 0.141 mg/L. Maximum TDL was
estimated as 317 T/d. The slope of the trend line between discharge and SSC (0.45) and
discharge and nitrate (0.2) is less than 0, indicating a stream that dilutes at higher flows.
Alenaio stream is an infrequent, but important contributor of sediment and nutrients to Hilo Bay.
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There are no continuous records of suspended sediment concentration or turbidity at the
Honoli‘i gauge, however there are data relating estimated discharge at the mouth of Honoli‘i
with Turbidity (NTU) measurements (Strauch, 2017). Turbidity exhibited a non-linear and
accelerating response to rises in discharge, and higher turbidity carried higher loads of fecal
indicator bacteria (FIB). These FIBs were associated with pig activity in the upper watershed
(Strauch et al., 2014).
Low levels of turbidity (NTU) were measured throughout the wet season on upper and lower
Wailuku, Pukaihae, Maile, and Honoli‘i (Young and Gadszak, 2008). The authors measured a
maximum TDL of about 16 T/d at Honoli‘i when mean daily flow was 0.68 m3/s (65%
exceedance probability). In summary, the SSC data show little sediment is transported to Hilo
Bay in Waiākea, but that Wailuku, and to a lesser extent Alenaio, transport large amounts of
sediment. There are few data to assess sediment loads in Honoli‘i, but visual observations cite
Honoli‘i as a large sediment contributor. There are no spatial data identifying sediment sources
in any of the streams.
6.2 Nutrients and Contaminants
High nutrient concentrations and bacteria levels have frequently been documented in rivers and
nearshore waters of Hilo Bay watershed (Steadmon et al. 2024; Wiegner et al. 2017; Waiki et al.
2025), including antibiotic-resistant strains of Staphylococcus (Gerken et al. 2021; Saingam et al
2021), posing ecological and public health risks. Temporal and spatial patterns of nutrient and
bacteria data along the coast and in river plumes have
indicated that bacteria are transported from the
watershed in surface runoff, especially during high flow
events, from urban land uses and human sources
(Wiegner et al. 2017; Economy et al. 2019). Although
water quality along the coast has high variability in
space and time, the Wailuku river mouth is a common
hotspot of turbidity and bacteria contamination (Wiegner
et al. 2017; Saingam et al. 2021). The Wailoa River
plume is the more likely dominant source of nutrients
into the bay, evidenced by high concentrations of nitrate after storm events. The primary source
of nitrates and phosphates is leaching from on-site-sewage-disposal systems (‘OSDS’, Whittier
and El Kady, 2014). Only 30% of urban Hilo is connected to sewer (Silvius et al, 2005). There are
over 10,000 cesspools in the greater Hilo area with the greatest concentration of cesspools per
square kilometer in the south-central region of Hilo (Figure 5). Combined, the OSDS dump an
estimated 5.6 MGD of untreated effluent into Hilo Bay (HDOH, 2017). Contamination of drinking
water, as measured by DOH, led to the closure of the Kaumana spring (one of Hilo’s first drinking
water sources) in the 1960’s (Young and Godszak, 2008). Previous Water Quality Monitoring
(ibid) has identified streams with consistently elevated nutrient concentrations that isolate
areas contaminated by OSDS and includes maps of sampling locations from which to build an
effective monitoring program. The report also identified stream corridors subject to periodic
flooding and subsequent transport of contaminants to Hilo Bay. The urban nature of the lower
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Waiākea/Wailoa watershed requires assessment of existing floodwater engineering and a
modeling assessment of predicted flooding under different sea level rise scenarios.
Cesspools are predominantly found within the Waiākea/Wailoa watershed (Table 3). The
highest densities of cesspools (> 500/km2) are located west of the Kanoelehua Avenue between
Puainako and Haihai streets and along Kaumana Drive mauka of Mohouli Street. Hilo’s
municipal water serves over 13,000 connections and is sourced from groundwater at Pana‘ewa
Well Nos. 1, 2 and 3, Pi‘ihonua Well Nos. A, B, and C, Saddle Road Well “A”, and the UH Hilo Well.
The Hawai‘i County Department of Water Supply Water Quality Report for Hilo (2024) found a
maximum nitrate of 0.39 ppm. However, the UH Hilo and Pi‘ihonua A & B wells are downstream
of high OSDS density areas and may be at risk of contamination. In 2017, the Hawai‘i State
Legislature passed Act 125 to require the replacement of all cesspools by 2050 (Hawaiʻi State
Legislature, 2017). Hawai‘i County must expand sewer service and initiate a cesspool
conversion plan by 2026.
Figure 5. Map of OSDS concentration (number of OSDS per square kilometer) in Hilo Bay Watersheds.
In-stream nutrient criteria for streams (HAR 11-54) require geometric means below 30 ug/L (in
dry season) and 70 ug/L (in wet season) (0.03 and 0.07 ppm), and below a 2% exceedance
value of 0.170 and 0.300 ppm (dry/wet season). High levels of nitrate were measured at
Kaumana Waiākea and Waiolama Springs and at the Kaluiiki branch of the Waipahoehoe (Young
and Godszak, 2008). Waiolama spring (489 – 850 μg/L) and Kaumana spring (161 – 352 μg/L)
showed high levels of nitrate, and nitrate was also found at high concentrations (379 – 487
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μg/L) in Waiākea Pond. The authors pinpointed elevated spring concentrations leading to
impairment of river conditions downstream.
Continuous monitoring on Waiākea stream at Hoaka (from Presley et al., 2008) shows steady
increases in nitrate and phosphate concentrations with increasing discharge, but overall low
levels of nitrate (< 110 μg/L at all flows) and phosphate (< 125 μg/L at all flows) typical of
background flows. The approximately linear relationship between mean daily discharge and
kg/day of nitrate and phosphate shows a good fit (R2 = 0.80 and 0.76, respectively). Total
maximum daily loads were 32.3 kg/d for nitrate and 75.4 kg/d for phosphate. These data
suggest little to no nutrient loading upstream of Hoaka.
Analysis of stream nutrient concentrations and yields in relation to land cover and watershed
characteristics for 24 east Hawaii Island streams (Michaud and Wiegner 2011) found elevated
nitrate concentrations compared to a global baseline for unpolluted tropical streams and
concluded that anthropogenic activity in the Mauna Kea watershed has resulted in increased
nitrate concentrations. This study did not find strong relationships between water quality
conditions and watershed characteristics including watershed area, mean annual rainfall,
population density, or agricultural land cover. However, their sampling was limited to baseflow
conditions (based on feasibility) and did not include any spatial variability within stream
networks.
6.3 Turbidity and water quality in Hilo Bay
A power law relationship was shown between discharge in Wailuku River (Pi‘ihonua) and
turbidity (NTU) measured in Hilo Bay (Mead and Wiegner, 2010). Discharges exceeding modest
flows of approximately 3.5 m3/s at the Pi‘ihonua station (~ 34 % probability exceedance)
resulted in turbidity rising above the water quality threshold of 3 NTU. A more recent study of
point measurements across the bay (Lucas et al., 2023) showed turbidity to exceed the same
water quality thresholds throughout the western (‘surface-water dominant’) portion of the bay
following high flow (90th percentile) events. The authors found that during the same high flow
events, the entire bay exceeded water quality standards for chlorophyll. At mean flows, turbidity
in excess of water quality standards were concentrated along the western shore to beyond the
breakwater. At low flows, turbidity in excess of water quality standards was only found at the
mouth of Wailuku River; however, chlorophyll concentrations exceeded water quality standards
in many areas of the eastern (‘groundwater dominant’) portion of the bay.
Since at least 2021, the Pacific Islands Ocean Observing System (Hawaiʻi Climate Change
Mitigation and Adaptation Commission, 2017) buoy has recorded temperature, salinity, turbidity,
chlorophyll, oxygen, and saturated oxygen at 15-minute intervals less than a kilometer from the
mouth of the Wailuku River. Although the data are quite noisy (presumably due to biofouling and
deferred maintenance), there appears to be a relationship between peak event stormflow in
Wailuku (Pi‘ihonua) and the maximum turbidity recorded in Hilo Bay (Figure 6).
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Figure 6. Peak event discharge in Wailuku (Pi‘ihonua) and maximum turbidity (within a 12 h period) recorded at the
Hilo PACIOOS buoy. Data points are colored according to 7-day Antecedent Precipitation Index (API) at the ‘Quarry’
rain gauge (USGS). Magenta circle indicates event on June 2nd, 2026, which was the largest recorded flow since fall
2024. Erroneous points (turbidity > 150 NTU when peak discharge < 200 m3/s, and all events with maximum turbidity
less than 5 NTU) were excluded.
There is an opportunity to compare monitoring in Wailuku and Honoli‘i streams with turbidity in
Hilo Bay (i.e., maximum recorded turbidity response and the duration of turbidity in excess of
water quality thresholds). This comparison will help us to better understand sediment and
nutrient production and the resulting impact (magnitude and duration) in the bay.
6.4 Impaired Streams
A recent assessment from the Hawai‘i Department of Health listed all major streams in the Hilo
Bay Watershed as impaired based on nutrient or turbidity standards (Figure 7).
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Figure 7. Map of 303(d) listed impaired streams in Hilo Bay Watersheds. Map reproduced from Figure 6 in USDA,
NRCS, 2022
Minimal freshwater nutrient data exist beyond the nearshore region of the watershed; however,
nutrients and other sources of contamination have been measured in Hilo Bay. Nitrate
concentrations were found at higher concentrations near the mouth of Wailuku and increased
during stormflow conditions (Wiegner et al., 2013). Chlorophyll α concentrations were greater in
the eastern portion of Hilo Bay (dominated by groundwater flow) during low flow conditions
(Lucas et al., 2023), which suggests heavy nutrient loading of groundwater emerging in Wailoa
Pond or as submarine groundwater discharge (SGD). This interpretation is consistent with a
recent NASA DEVELOP report showing consistently high chlorophyll α concentrations at the
mouth of the Wailoa River (Sonobe et al., 2025). A recent study employing multiple tracers in
Keaukaha, east of the Hilo Bay breakwater, documented the presence of OSDS sewage in
coastal waters and measured flow rates of effluent in groundwater to be 130 to 213 m/d (Waiki
et al., 2025). Concentrations of fecal indicator bacteria in coastal springs were similar to
measurements across the state but were orders of magnitude lower than concentrations in Hilo
Bay following storms perhaps due to dilution of effluent in the large groundwater reservoir
(Waiki et al., 2025; Wiegner et al., 2017; Economy et al., 2017).
Across Hilo Bay, 40 – 50 % of samples tested positive for human waste (Wiegner et al 2017;
HDOH, 2018). At high flow, Enterococcus concentrations exceeded HDOH standards throughout
the Bay, while at low flows, standards were exceeded in the Wailuku and Wailoa Rivers.
Concentrations of Enterococcus and Clostridium perfringens within Hilo Bay increased
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dramatically following a high flow event in May 2013 (300 m3/s peak flow, ~ 1.3-year return
interval flood) (HDOH, 2018).
Under dry conditions, groundwater inputs result in decreased salinity along the southern shore
of the bay and turbidity is generally low (Badlowski et al., 2021). During storm events, large
inputs from Wailuku River results in a plume of low temperature, low salinity, and high turbidity
that hugs that western shoreline and extends into the bay (Badlowski et al., 2021).
Hilo Bay is relatively shallow (less than 15 m deep) and despite the breakwater, residence time
is generally low. Mass balance estimates of residence time within Hilo Bay vary from 3.5 days
(dry conditions) to 2.5 days (wet conditions), and only 1.3 days during storms (M&E Pacific
1980). Generally, the combined influence of groundwater inputs and persistent northeast trade
winds result in a ~ 5 cm/s westward surface current (M&E Pacific 1980; USACE 2009).
6.5 Critical Source Areas
6.5.1 Critical Source Areas for Sediment
The recent NWQI report (USDA, NRCS, 2022) mapped critical source areas (CSA’s) where
sediment loss vulnerability (defined by typical rainfall intensity and soil erodibility) overlaps high
transport potential (proximity to streams). The report found CSA’s to be concentrated in
agricultural areas in the northern watersheds, and in areas of stream convergence where
sediment is episodically mobilized (such as mid-Wailuku). The Waiākea watershed has fewer
CSAs because of its highly porous substrate and gentler slope; however, there are potential
sediment source areas in the pasturelands near the middle of the watershed. Throughout the
Hilo Bay Watershed, pasture was found to be the highest-risk land use category for sediment
production. The methodology used, however, was developed for agricultural soils and does not
fully capture sediment production from forests, particularly forests inhabited and modified by
feral ungulates. Studies from other areas of Hawaii island have documented the substantial
impacts of these animals to watershed coverage of vegetation and exposed soil area (Cole and
Litton 2014). Sediment load
data are needed across
different land uses to inform
changes in land use practices
that result in decreasing SSC
below water quality
standards.
6.5.2 Critical Source Areas
for Nutrients
As mentioned above, the
critical source areas for
nutrients are the over 10,000
OSDS within the Hilo Bay
Watershed (Figure 5). We
hypothesize the spatial
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
distribution of nutrient concentrations in streams is correlated to the upslope nutrient loading of
the OSDS. Our results will link upslope loading to groundwater and surface water conveyance, to
ongoing DOH water quality monitoring along the shoreline. Whereas the existing water quality
data on nutrient concentrations in the Hilo Bay region is largely confined to nearshore
monitoring stations (e.g., Figure 8 shows the distribution of nitrate measurements in the
national Water Quality Portal), our monitoring will encompass middle and upper portions of the
stream networks in order to develop a spatially-resolved understanding of nutrient loading
hotspots.
7. FLOODING
● November 1-2, 2000 as ‘flood of record’ for Waiākea.
● Peak rainfall intensity of greater than 80 mm/ h (3.2 in/ h), > 750 mm in 24 h ( 30 in 24 h)
● Waiakea – Palai flood modeling done by USACE (2024).
8. SEA LEVEL RISE
Higher water levels resulting from sea level rise (SLR) is an important factor in determining the
long-term resiliency of coastal regions. Global SLR (GSLR) is driven by climate change, as ocean
volume is increasing due to thermal expansion of warming oceans and due to additional water
from melting ice sheets and glaciers. Global mean sea level has increased by approximately 6.7
inches (17.0 centimeters) over the last 100 years with noted acceleration since about 1970
(Sweet et al. 2022). However, sea level does not increase uniformly around the globe; it is
spatially variable and can change significantly by region. This more locally focused increase is
referred to as relative SLR (RSLR), which can be more or less than the global average (Sweet et
al. 2022). RSLR is a function of vertical land movement (land subsidence / uplift) and changes
in regional ocean currents in addition to GSLR.
The US Sea Level Rise and Coastal Flood Hazard Scenarios and Tools Interagency Task Force
(Interagency Task Force), which is led by the National Oceanic and Atmospheric Administration
(NOAA) and comprised of scientists from several federal agencies and academic institutions,
published a technical report (Sweet et al. 2022) that is widely used to understand SLR. This
technical report provides five SLR scenarios projected to 2150 at global, regional, and local
scales for all US states and territories. These five scenarios cover low, intermediate-low,
intermediate, intermediate-high, and high projections of SLR, each with varying likelihood of
occurring going forward.
Hawai’i’s State Climate Commission has developed RSLR guidance based on the 2022
Interagency Task Force technical report (Hawai’i State Climate Commission 2022). The
recommendations from the state are to evaluate two RSLR scenarios depending on the
tolerance of risk for a given project, which are the intermediate and the intermediate-high
scenarios for Hawai’i. This guidance states that RSLR of 4 feet by the year 2100 (intermediate
scenario) should be considered as the minimum for all planning and design, and RSLR of 6 feet
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
by the year 2100 (intermediate-high scenario) should be considered for planning and design of
public infrastructure and other projects with low tolerance for risk (Hawaiʻi Climate Change
Mitigation and Adaptation Commission. 2017, updated 2025). Note that the RSLR projection
values have a baseline year of 2000. The project team will coordinate closely with the County to
determine the risk tolerance for this project and select the appropriate scenario for use. For
more guidance from the state on this, see Guidance for Addressing Sea Level Rise in Community
Planning in Hawai’i (Courtney et al. 2020) and Guidance for Using the Sea Level Exposure Area in
Local Planning and Permitting Decisions (Romine et al. 2020).
Additionally, to build on the State’s guidance, more site-specific RSLR projections from the 2022
Interagency Task Force technical report are available from nearby tidal stations. In this case,
RSLR projections can be pulled from the Hilo Bay, HI tidal station (Station ID: 1617760). The
projections from this station show that sea levels are rising slightly faster in this location
compared to the state’s guidance. For this project, the RSLR projections from the Hilo Bay, HI
tidal station will be used as they are more conservative and regionally appropriate. Under the
intermediate scenario this area is projected to experience a RSLR of 1.16 feet by 2050, 2.01 feet
by 2070, and 4.17 feet by 2100. Under the intermediate-high scenario this area is projected to
experience a RSLR of 1.43 feet by 2050, 2.97 feet by 2070, and 6.17 feet by 2100 (Sweet et al.
2022). Again, note the RSLR projection values have a baseline year of 2000. To provide the most
appropriate representation of RSLR, the project team will shift the baseline year of the RSLR
projection values to 2025 in accordance with Appendix A in Application Guide for the 2022 Sea
Level Rise Technical Report (Collini et al. 2022) provided by the Interagency Task Force.
The project team will use the selected RSLR values to define the boundary conditions of the
watershed hydraulic models and support the development of flood hazard maps that consider
impacts of future climate conditions. The same RSLR values will be used to support the
assessment of coastal hazards. There is sufficient literature and data available to support these
efforts; no data gaps identified.
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
9. DATA GAPS
Below is a list of data gaps along with planned or proposed solution(s) in blue.
1. Unknown current relationship between discharge and SSC for background (upstream)
and impacted (downstream) reaches of three major rivers. Install turbidity probes,
autosamplers, and (where necessary) stream gauges to develop relationships between
flow and total daily load of natural and impaired streams. Determine sediment yield to
Hilo Bay from the three primary watersheds. To date (June 2026), we have installed 2 of
four sampling stations (at the USGS gaging station on Honoli‘i and in lower Honoli‘i).
Because the magnitude of suspended sediment transport occurs disproportionately in
high flow events, we configured our autosamplers to event-based sampling which will
capture suspended sediment concentration across the rising and falling limbs of the
hydrograph (see Lewis and Eads, 2009).
2. Unknown sediment yield from different portions of the watershed and different land
covers. Where are the ‘hot spots’? Measure SSC at moderate to high flows at distributed
sites throughout the watershed. Use watershed scaling to estimate discharge, total
loads, and spatial distribution of sediment yield. Measure infiltration capacity across
different land uses to determine threshold rainfall intensity to mobilize sediment and
estimate frequency and magnitude of sediment yield across watershed. Map watershed
to identify mechanisms of sediment delivery (e.g., overland flow on dirt roads, slumping
from road cuts, agricultural runoff, riverbank erosion, etc.) Use data to constrain
sediment modeling to estimate total annual loads across the watershed. We also will
incorporate EA’s flood modeling to determine thresholds for bank erosion/suspended
sediment concentration as well as flooding of areas susceptible to erosion.
3. Unknown nutrient concentrations and spatial patterns across Hilo Bay Watershed
streams. Measure nitrate + nitrite/phosphate across streams, gulches, and ditches to
identify areas of high nutrient loading (see proposed monitoring plan in Figure 9).
Measurements to be conducted at low flows and at high flows across wet and dry
seasons. Follow EPA guidelines (see below) in order to support the State Department of
Health’s Clean Water Act reporting requirements (HDOH 2004) and its proposed design
for an inland water quality monitoring program.
o Hawaii Administrative Rules (11-54) specify water quality criteria for inland waters
based on five parameters: Total Nitrogen, Nitrate + Nitrite Nitrogen, Total
Phosphorus, Total Suspended Solids, and Turbidity. Numeric criteria specify values
that are not to be exceeded for the geometric mean, 10%, and 2% of measured
values. Numeric criteria are higher for wet season concentrations (November 1
through April 30) and lower for the dry season (May 1 through October 31).
o Measurements of relevant parameters from the period October 1, 2022 and
September 30, 2027 should be submitted to the State Department of Health Clean
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
Water Branch by October 1, 2027 to be included for consideration in the State’s
2028 Integrated Report for §303(d) and §305(b) of the Federal Clean Water Act,
along with complete metadata and documentation required to assess the
completeness and accuracy of the data. A minimum of 30 samples per waterbody
over the five-year period is required for assessment. Turbidity will be monitored
continuously using in situ sensors, and water samples will be collected for
measurements of nutrients and total suspended solids. Total suspended solids will
be measured gravimetrically by trained technicians at the ASU GDCS laboratory at
the Institute for Pacific Islands Forestry in Hilo and nutrient concentrations will be
analyzed at the state-certified water quality laboratory at that National Energy
Laboratory of Hawaii Authority (NELHA).
o We also plan to analyze the Department of Water Supply’s water quality data and
conduct additional sampling of groundwater if necessary. Our sampling map
identifies springs (groundwater) to sample in the southern (near the Golf Course),
eastern (Keaukaha), and western (Kaumana) portions of the watershed.
4. Measurements of nitrate/nitrite and phosphate align with water quality control
standards, but do not alone identify the sources of elevated nutrients. To distinguish
between natural sources (such as invasive and native nitrogen fixing plants), agriculture,
feral ungulates, and human waste requires additional analytical techniques beyond the
scope of this study (i.e., stable isotopes, fecal indicator bacteria, caffeine/sucralose
concentrations).
5. Cost-benefit analysis to identify ‘low-hanging fruit’ or low(er) cost initiatives that might
address ‘hot spots’ identified above. For example, sharing data/learning from Mauna Kea
Watershed Alliance and other partners to better inform the location of feral ungulate
exclusion and reforestation projects.
6. Little is known about the suite of ‘other’ harmful substances from agricultural, suburban,
and urban sources transported to the bay in surface and subsurface flows. There are
potential/unknown risks associated with pesticides, especially the extensive historical
use of sodium arsenite on sugarcane plantations (Falinski et al. 2014; Hallacher et al.
1985; Johnson et al. 2024), as well as dumping of hazardous materials and toxic waste
in lava tubes (Halliday et al. 2003). Previous studies have measured high levels of
arsenic trapped in undisturbed sediment layers near original sites of pollution in Waiākea
Pond, however there is little evidence that this arsenic has mobilized into biota
(Hallacher et al. 1985; Falinski et al. 2014). USACE has noted that it is unknown the
extent to which potentially remobilized benthic sediment within Hilo Bay poses risk due
to historical use of industrial chemicals, pesticides, and other toxic contaminants within
the watershed, and recommended that field studies be carried out to characterize the
sediments accumulated within the Bay to identify pollutants of concern (USACE, Feb
2023). We do not have resources to document and model the mobilization, transport,
and fate of these contaminants and instead will rely on sediment and nutrients as
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
indicators of a chemical ‘cocktail’. The flood hazard map will help us begin to assess the
possible transport of other harmful substances to the bay.
7. Unknown how watershed sources affect patterns and trends in water quality monitoring
observed by DOH and Hawai‘i Wai Ola along the Hilo Bay Coast. Design our sampling
efforts to complement ongoing sampling and synthesize data sets to expand scope of
inference of proposed upstream monitoring/sampling.
8. Poorly constrained circulation patterns in Hilo Bay. Although previous studies have
documented the persistence of elevated nutrient and sediment levels in Hilo Bay for
multiple days following storm events, uncertainty remains regarding the extent to which
these water quality problems are exacerbated by reduced circulation, low wave energy,
and persistent water column stratification inside the Hilo Bay breakwater. Potential
structural measures to improve water circulation by modifying the breakwater were
previously investigated using computer modeling (2009 USACE Hilo Bay Water
Circulation and Water Quality Study). USACE reported that modeling results indicated
that all of proposed alternatives would have negative effects on navigation but minimal
effects on water quality relative to the magnitude of inputs from the watershed.
Therefore, they concluded that “as a matter of priority, the initial effort should focus on
watershed management to reduce the conveyance of pollutants into Hilo Bay” (USACE
Hilo Bay Watershed Planning Assistance to States Report, Feb 2023). Field data
collection throughout the watershed is designed to fill this previously identified
knowledge gap in order to identify and characterize the relative magnitude of sediment
and nutrient pollution loads flowing into Hilo Bay. Watershed data, including nutrient and
sediment sample results, will be used to calibrate a spatially-explicit watershed model
such as SWAT+, MODFLOW, or InVEST to estimate sediment and nutrient load
reductions under current conditions and potential scenarios of implementing best
management practices (BMPs).
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10. PROPOSED SAMPLING POINTS FOR SUSPENDED SEDIMENT
CONCENTRATION AND NUTRIENT MONITORING
Figure 9. Map of proposed monitoring (autosampler and turbidity probe) and grab sample locations to document
spatiotemporal patterns in surface and subsurface contaminant loading.
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
11. FIELD DATA COLLECTION PROGRESS REPORT, JUNE 2026
Thus far, we have completed
three grab sample
campaigns (in early
February, April/May, and
early June). The conditions
spanned baseflow to
stormflow. Initial results
show a power law
relationship between
discharge and SSC (shown
as TSS on the plot) (Figure
10). We also observe a
positive relationship
between the density of
upstream OSDS and nitrate
measured in springs and
streams (Figure 11). In
March, we installed the first
monitoring station at the
Honoli‘i USGS station (Figure
12). This location is just
downstream of the forest
edge at 469 m.a.s.l. (1549
feet). The station is situated
on a cliff about 20 ft above
the stream. There is an
autosampler (Teledyne
ISCO) with tube descending
to the stream where a turbidity probe (FTS) is housed within a 4”
PVC pipe bolted to the base of the cliff. The turbidity probe cable
and autosampler tube run through a conduit bolted to the
bedrock/tied to neighboring trees (strawberry guava). The turbidity
probe is programmed to take measurements every 15 minutes and
data is logged on a Campbell Scientific CR1000 Datalogger. The
autosampler is programmed to take 500 ml samples across the
rising and falling limbs of storm events to capture variability in SSC
(see Lewis and Eads, 2009). The instruments are powered by a 12
V marine battery recharged by a 100 W solar panel. The battery,
datalogger, and solar charge controller are housed within a plastic
tote, and the equipment is tied to trees and protected from feral
pigs by a hog wire panel fence.
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
Thus far, turbidity data track precipitation and stormflow responses in Honoli’i stream
(Figure 13).
Figure 13. Top panel shows rainfall measured at the Honoli‘i rain gage (USGS # 194602155091801). Bottom panel
shows discharge (left y-axis) measured at the Honoli‘i station (USGS # 16717000). The brown circles show the
timing and magnitude of our SSC measurements (right y-axis) and the brown line shows initial turbidity data
(right y-axis). Data post May (installation and transition to ‘event-paced’ programming) show better sampling
representation across storm hydrographs.
Initial results from laboratory nutrient analyses of water samples collected from 20 sites
throughout the watershed show high variability among sites (Figure 14). Results are largely
consistent with previously documented impairment patterns, however additional sampling is
needed to document seasonal and event-scale variability and establish a statistically robust
data set.
Samples from Kapue Stream (north of Honoliʻi) and Pūkīhae, which are not listed for nutrient
impairments, had low nutrient concentrations (< 10 µg L⁻¹). Samples from Honoliʻi are below the
nutrient impairment thresholds at upstream locations, however concentrations increase an
order of magnitude at the river mouth and approach the seasonal impairment thresholds (Figure
15). The Wailuku River, which is listed as impaired for dry season nitrate concentrations, showed
high variability among sites. Whereas samples collected along the mainstem had relatively low
concentrations, samples collected from its tributaries flowing from developed land to the south
had highly elevated nitrate concentrations. In the Waiākea watershed, all but one sampling
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
location had nutrient concentrations exceeding seasonal impairment thresholds. These results
are consistent with the nutrient impairment listings for both Waiākea and ʻAlenaio streams
based on historical visual observations.
There is a strong downstream gradient between upstream and downstream sites on ʻAlenaio:
both sites exceeded nutrients standards, but concentrations at the downstream site are, on
average, an order of magnitude higher. Surprisingly, nitrate concentrations at the most upstream
sampling site on the Waiākea stream (outside of the area displayed in Figure 14, shown in
Figure 9) were higher than the seasonal impairment thresholds, despite the lack of upstream
development. However, results from those samples also showed a relatively higher fraction of
organic nitrogen compared to downstream sites more influenced by urban development.
Comparing results from samples collected at an inflow to Wailoa Pond and the outflow at
Wailoa Boat Ramp shows that average nutrient concentrations entering the pond typically
exceed concentrations downstream of the pond. The ratio of inorganic nitrogen to phosphorus
in these samples also indicates inflowing waters are phosphorus limited whereas outflowing
water is nitrogen limited, which could indicate that denitrification activity in the pond is buffering
the amount of nutrients exported to the bay.
Figure 14. Preliminary results of water quality nutrient sampling at grab sample monitoring sites. Shading indicates
median concentrations of NO₃ + NO₂–N (µg L⁻¹).
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Community-Based Hilo Bay Resilience And Watershed Management Plan - Task 2: Literature Review & Data Gap Analysis
Figure 15. Preliminary results of water quality nutrient sampling at grab sample monitoring sites in the three main
watersheds. Boxplots show the median, 25th, and 75th percentile of nitrate concentrations measured at each site
during the wet season (in blue, November through April) and the dry season (in orange, May through October), while
diamonds indicate the geometric mean. Values in parentheses after each site name indicate the number of samples.
Arizona State University | PAGE 42
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