Estimating soil moisture, actual evapotranspiration, climatic water deficit, and groundwater recharge during periods of drought for current and future climate conditions in Hawaiʻi

Mid- and end-of-century climate projections for the Hawaiian Islands indicate that rainfall is projected to decrease across large areas. In areas affected by drought or where the future climate becomes drier, reduced groundwater recharge can affect freshwater availability. Reduced rainfall can also reduce soil moisture, which can increase the risk of wildfire. Cloud-water interception, or fog drip, is the process by which cloud-water droplets are captured on the leaves and branches of plants with some of the captured cloud water subsequently dripping to the ground. Studies in Hawaiʻi indicate that fog drip can contribute substantially to total precipitation and may have the potential to lessen the negative effects of drought or a drying climate on freshwater availability and wildfire risk. Wildfire danger assessments in the continental United States have used estimates of soil moisture, evapotranspiration, and climatic water deficit (that is, the evaporative demand that exceeds available water) to improve the identification of areas at risk for wildfires. In this study, water-budget models developed for the islands of Kauaʻi, Oʻahu, Molokaʻi, Maui, and Hawaiʻi were used to quantify the effects of severe drought, future climate conditions, and reducing or eliminating fog drip on groundwater recharge, soil moisture, evapotranspiration, and climatic water deficit. Island-wide summaries of groundwater recharge, soil moisture, evapotranspiration, and climatic water deficit were developed to (1) illustrate changes between recent, drought, and future climate conditions, (2) illustrate the effects of reducing or eliminating fog drip for recent, drought, and future climate conditions, and (3) highlight areas of increased potential risk for wildfire during drought and future climate conditions. The results of these analyses can be used by natural resource managers in Hawaiʻi.