RES 061 Draft 01 2010-2012 - Laserfiche WebLink

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temperature regimes. High daytime temperatures seldom reach 90°F and low night temperatures rarely fall below 60° F for most of the island. The exceptions at extreme upper elevations include winter snows and recurring frost. The earliest Hawaiians recognized two seasons—the wet season of cooler temperatures roughly embracing fall and winter, and the dry season of warmer temperatures roughly synonymous with spring and summer, except for the Kona districts, which experience a dry winter and wet summer. Diurnal temperature fluctuation is generally less than 20° F in the warm season and less than 30°F in the cool season. Desiccating winds that are problematic for agriculture producers in other locations are neither common nor long lasting on the island, due to the surrounding ocean. Rainfall varies by site, from annual averages exceeding 200 inches on mid -elevations of the windward side to less than 4 inches on the leeward coastal plain. Precipitation patterns are well - understood and site-specific rainfall seasonality and totals are not greatly variable through time or season. The impact of global warming may, increasingly affect these established historical patterns. The unique combination of ocean surface temperatures between 75° and 82° year-round, trade winds out of the northeast bringing cooling air masses from the Gulf of Alaska, and great distance from any continental landmass thermal loading combine to provide a stable and moderate climate regime that may be unique on Earth. It is this climate that results in the unique potential (and needs) of Hawaii Island agriculture. Water Water, in the form of rain, snowmelt, soil moisture, surface flow or irrigation is a critical need for agricultural production. On Hawaii Island, rainfall is the primary source of water. Fast draining, poorly developed soils and substrates with minimal clay content retain little moisture and support virtually no surface springs. Rapid percolation precludes surface run off in all but the most severe rainfall events, so streams, springs, lakes and other common surface water sources are largely absent. Very few sites for surface storage earthen reservoirs can be found and this common and economical means of storing water in other locations is not possible without expensive construction of water retaining barriers. Although irrigation is important in some regions (discussed more fully later), diversion of surface flow for irrigation use is complicated by a lack of streams or earthen catchments. The same "leakage" limitations affect transport efficiency of the many miles of irrigation supply ditches currently in use on the island. The rainfall that quickly percolates through the island landmass fortunately accumulates in a freshwater lens that floats above the saltwater permeated rocks at or below sea level. This subterranean lens provides the water source that the County of Hawaii Department of Water Supply pumps into storage tanks and pipeline distribution systems. The cost of water on Hawaii Island is therefore driven by the pumping costs of raising the water, as well as the cost of distribution, but the resource remains abundant. Energy costs limit large-scale use of deep -well water for agriculture uses like irrigation, and even livestock water supplied from County wells is cost -prohibitive. 8