Hydrologic ecosystem service valuation on Kauaʻi, Hawaiʻi: calculating watershed protection return on investment

Watershed management through native forest protection provides important societal benefits including biodiversity conservation, cultural value, and hydrologic ecosystem services. With growing investment in watershed programs, understanding the spatial variation in return on investment (ROI) for hydrologic services is essential for guiding conservation investments. However, conservation cost and hydrologic service data for these analyses are often lacking. We partner with a watershed conservation partnership on Kauaʻi, Hawaiʻi, to analyze costs and benefits of protecting native forests from high-water-use invasive species, measuring cumulative groundwater recharge protected per dollar spent over 50 years. We combine detailed conservation cost data with land-cover change and groundwater recharge ecosystem service modeling across 16 existing and proposed management units. Using a 3% estimate for annual invasive species spread and a 3% discount rate, existing fenced units average 2244 L of cumulative groundwater recharge saved per dollar invested (range: 322–9935 L/$). When proposed new conservation areas are included, average ROI falls slightly to 2146 L/$ (range: 227–9999 L/$). ROI varies spatially, with the highest benefits occurring in middle elevation areas where evapotranspiration changes with forest composition change are greatest over the 50-year time horizon. Conservation costs are front-loaded due to fence construction and ungulate removal, while annual water benefits grow continuously as avoided recharge loss increases over time. This study demonstrates the importance of considering spatial ROI variation in watershed management, which can both justify the value of conservation efforts and aid spatial prioritization of future investments.