Hawaiʻi’s Evapotranspiration Patterns

ET Patterns

The spatial pattern of evapotranspiration (ET) in Hawai‘i is very complex. The complexity results from the interaction among the numerous variables that influence the process. Generally the pattern reflects the underlying patterns of energy and moisture availability. In the driest areas, non-irrigated lands have low ET because of water limitations. Moving toward wetter areas, ET increases, but reaches a maximum in areas with intermediate rainfall amounts. The very high rainfall areas have lower ET because they correspond with areas of high cloud frequency and, hence, low solar radiation.

Finer scale variations are related to land characteristics, such as vegetation cover, leaf area, and land cover type. The effects of vegetation cover can be seen in areas of Hawai‘i Island where young volcanic substrates with no vegetation or very sparse cover dissect an area with older, soil and vegetation covered land. The relatively recent lava flows on the northwestern and southwestern flanks of Mauna Loa, for example, show up clearly as ribbons of low ET within areas of higher ET. ET Patterns, Big Island
ET Patterns, O‘ahu Land cover type is a major factor in determining the fine scale spatial variation in ET. Urban, agricultural, and natural land covers imply differences in vegetation cover, vegetation height, leaf area, species, and irrigation. Effects of urbanization are clearly seen in the high-density urban areas of Honolulu, where ET is sharply lower than surrounding areas.
In other areas, such as along the dry coastal Kihei-Wailea area on Maui, urbanization results in higher ET because of lawn and golf course sprinkling. Irrigated agricultural lands have some of the highest ET, as seen for example in the last remaining sugar plantation in Hawai‘i, the HC&S plantation on Maui. ET Patterns, Maui
ET Patterns, Big Island In areas of natural vegetation, contrasts in ET are evident where high water use non-native plant species have displaced native vegetation. A good example can be seen in the area between Hilo and Volcano on Hawai‘i Island, where a large area has been invaded by non-native tree species including strawberry guava, resulting in 30-40% higher ET.