Priority watershed management areas for groundwater recharge and drinking water protection: A case study from Hawai‘i Island

Worldwide, water utilities and other water users increasingly seek to finance watershed protection and restoration in order to maintain or enhance water quality and quantity important for drinking water supply and other human use. Hydrologic studies which characterize the relative effectiveness of watershed management activities in terms of metrics important to water users are greatly needed to guide prioritization. To address this need, we worked with a local water utility in Hawaiʻi to develop a novel framework for prioritizing investments in native forest protection and restoration for groundwater recharge and applied it in the utility's priority aquifers and recharge areas. Specifically we combined land cover and water balance modeling to quantify the 50-year cumulative recharge benefits of: 1) protection of native forest from conversion to non-native forest, and 2) restoration of native forest in non-native grasslands. The highest priority areas (80th percentile of benefits) for native forest protection are projected to prevent the loss of over 48,600 m³ per hectare of recharge over 50 years. Incorporating land cover change modeling (versus assuming all areas are equally susceptible to invasion) shifts prioritization towards low to mid-elevation mesic forest areas at the highest risk of invasion by invasive canopy species as well as to high elevation, cloud forest areas at high risk of conversion to non-native grassland or bare ground. We also find that, in the highest priority areas with substantial fog interception, native forest restoration is projected to increase recharge by over 88,900 m³ per hectare over 50 years, but that decreases in recharge occur in areas with low fog interception. This study provides a framework for prioritizing investments in forest protection and restoration for groundwater recharge in a way that incorporates both the threat of conversion as well as changes in hydrologic fluxes. The framework and results can be utilized by current managers and updated as new ecohydrological data become available. The results also provide broad insights on the links between watershed management and groundwater recharge, particularly on islands and in other regions where species invasions threaten source watersheds and where groundwater is a primary water source.