Multi-Hazard Mitigation Plan: 14. Droughts

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Chapter 14:Hazard Analysis—Droughts CHAPTER 14 - DROUGHTS 14.1 Description of Hazard 14.1.1 Defining and Measuring Drought Drought can be defined in relative or absolute terms. Relative drought is determined by the fluctuations about average local conditions. Absolute drought refers to a single uniform level. Relative drought recognizes that the severity of drought impact varies according to the adaptations and vulnerability of natural and social systems relative to prevailing normal conditions. Absolute drought is used primarily by researchers interested in large drought- impacted areas. Relative drought is useful to examine drought conditions on this island. Much of the information presented in this section was drawn from portions of a statewide drought study(the"Hawaii Drought Plan") applicable to the Island of Hawai'i.67 Drought differs from other natural hazards in three significant ways. First, a drought's onset and end are difficult to determine since the effects accumulate slowly and may linger even after the apparent termination of an episode. Second,the absence of a precise and universally accepted definition adds to the confusion about whether a drought exists, and if it does, the degree of severity. Third,unlike most other natural hazards, drought impacts are less obvious and are spread over a larger geographic area. These characteristics have hindered the development of accurate,reliable, and timely estimates of drought severity and effects. Of the several indicators used to measure drought, the most common is rainfall. A dense network of rain gauges, and long term collected, collated, and published data provide an excellent record of historic rainfall patterns and spatial variations on each Hawaiian Island. There are two popular drought indices. The Palmer Drought Severity Index (PDSI) was not appropriate for Hawaii and it was decided that the Standardized Precipitation Index (SPI) should be used as an initial drought index for the state. The Standardized Precipitation Index (SPT) was developed by Thomas McKee at Colorado State for use as a drought monitoring tool and has been embraced by agencies such as the National Drought Mitigation Center and the Western Regional Climatic Center. The beauty of this index is its simplicity because it uses only monthly rainfall as its input. This simplicity also makes it ideal for use in Hawaii, where there is a relatively dense network of rain gages. In comparison, the Palmer Drought Severity Index(PSDI),which is in widespread use across the Mainland U.S., is much more complex and requires temperature and soil moisture as additional data inputs. These types of additional data are either sparse or non-existent in Hawaii. Furthermore, the PDSI is more applicable to broad climatic areas and is not suited for representing conditions in the small-scale climatic zones of the Hawaiian Islands. 67 Wilson Okamoto Corp."Hawaii Drought Plan".Prepared for the State of Hawaii,Department of Land and Natural Resources,February 2005 14-1 Hawaii County Multi-Hazard Mitigation Plan