Benefit-cost analysis of watershed conservation on Hawai'i Island

•On average 1487 l are saved per dollar invested in forest restoration at five sites on Hawai‘i Island.•$0.67 in present value terms is required to protect 1000 l of freshwater over a 50-year time horizon.•Water yield benefits of restoration increase over time, while management costs tend to be front-loaded.•Forest restoration may be cheaper than other groundwater alternatives such as desalination.•It is important to consider the long run when comparing the benefits and costs of conservation activities.

In landscapes around the world, growing attention is being paid to the link between forest structure and water resources. More clarity is vital for informed decision making, especially as water scarcity continues to increase in many regions across the globe. The objective of this study is to estimate the volume of freshwater yield saved per dollar invested in forest restoration at several sites on Hawai'i Island. Using budget information and publicly available land cover and evapotranspiration data, we find that under baseline conditions—a 3% discount rate and 10% rate of spread for existing non-native plant species—1487 l are saved on average across management sites per dollar invested. In other words, $0.67 in present value terms is required to protect every 1000 l of freshwater over a 50-year time horizon. Annual benefits increase continuously as the avoided loss of freshwater yield rises over time, while conservation costs tend to be front-loaded, as a result of high fence installation and ungulate removal costs. Thus, it is important to consider the long run when comparing the benefits and costs of conservation activities.