Land cover effects on groundwater recharge in the tropics: Ecohydrologic mechanisms

Managing groundwater recharge to ensure sufficient water supply is a growing concern for many communities. We explore land cover effects on recharge processes and rate in Hawai'i, where precipitation on the mountain slopes of leeward Hawai'i Island recharges the aquifer that is the sole source of water for coastal communities. Applying a water balance, we quantify the influences of native forest and cattle pasture on recharge using measurements of precipitation and the drivers of evapotranspiration taken over 18 months. Surface runoff is negligible in these highly permeable basalt watersheds, simplifying fluxes into and out of the recharge area. On the basis of our analysis of measured fluxes, groundwater recharge is 96% of rainfall in pasture, 87% of above-canopy rainfall in open forest, and 106% of above-canopy rainfall in dense forest. Differences in recharge among vegetation types result largely from direct interception of cloud water by native Hawaiian forest. The majority of rainfall occurs in infrequent, large storms, so water moves quickly beyond plant rooting depth, limiting its availability for evapotranspiration. Potential evapotranspiration is low, as its drivers are modest and constant throughout the year. Existing estimates of submarine groundwater discharge support our conclusion that the rainfall-to-recharge ratio is close to one for all sites. Identifying primary influences on the recharge-to-rainfall ratio is key to predicting the types of changes that will have substantial effects on recharge. In any tropical setting with young, porous substrates, precipitation is likely to play a dominant role in the magnitude and variability of groundwater recharge; here, land-use changes that affect local precipitation, such as an increase in fog interception, will play a larger role than vegetation shifts that affect evapotranspiration.