The objectives of this report are (1) to review studies that estimate the relationship between watershed conservation activities and groundwater recharge in Hawai‘i and (2) to estimate the volume of freshwater yield saved per dollar invested in conservation at several sites on Hawai‘i Island. We conclude from the literature review that more work should be done to integrate information from smaller-scale studies of invasive-native water use differences into regional water balance models. This would help to inform decisions related to watershed conservation activities statewide. Using budget information obtained from the Nature Conservancy and the Division of Forestry and Wildlife as well as publicly available landcover and evapotranspiration (ET) data, we estimate the gallons of freshwater yield saved per dollar invested in watershed conservation. Under baseline conditions—a 3 percent discount rate and a 10 percent rate of spread for existing invasive plant species—roughly 400 gallons are saved on average across management sites per dollar invested. In other words, about $2.50 in present value terms is required to protect every one thousand gallons 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.
