Water, Energy, and Land Use Planning on Maui Island, Hawaii: Estimating Surface Water Supply

Maui Island in the State of Hawaii faces land use and freshwater allocation challenges associated with a growing population and a changing economic base as plantation agriculture has declined. Debate about whether water should be restored to environmental flows, allocated to new urban development for residents and tourists, or be used to irrigate food or fuel crops has highlighted Maui’s opportunity to make integrated resource decisions that consider land, water, and energy in particular. One major potential water demand on Maui is for irrigation for biofuels crops, such as sugarcane for ethanol. While Maui’s energy system is currently low in water intensity, using irrigated biofuels could increase the need for local water investment in energy systems. This paper aims to characterize surface water supply on Maui in order to draw conclusions about supply adequacy for biofuel irrigation. Narrow-scope empirical equations linking streamflow and precipitation tend to produce more accurate estimates for individual streams: for example, equations based only on northeast Maui streams tend to predict northeast Maui stream flows better than equations based on all of Maui’s streams. However, specific equations do not exist for most regions of Maui. This paper finds that general and specific empirical equations for northeast Maui predict nearly identical aggregate streamflows. Irrigation ditch flow comprises aggregate streamflow from a given region, so it is likely that existing, general equations can predict irrigation ditch flows with acceptable accuracy.