Evaluating the Management of a Tropical Reservoir: Implications of Climate Change for Water Availability

Most tropical islands have limited water supply due to the small size of their watersheds and aquifers, unique geology, and distribution of rainfall; each affecting the reliability of water resources for meeting agricultural needs. To improve water supply, reservoirs provide critical storage of surface water. However, climate-driven shifts in the distribution, intensity, and amount of rainfall will alter runoff that feeds reservoirs. Mass-balance modeling was used to evaluate the effects of various management constraints on water availability for the Wahiawā Reservoir (O‘ahu, Hawai‘i) as well as the impact of 20% less dry season rainfall and stream flow to assess the effect of projected climate change on maintaining: (1) a minimum downstream flow; (2) a minimum agricultural supply; (3) a minimum and maximum reservoir level; and (4) a combination of minimum downstream flow and minimum agricultural supply. Under current climate conditions, maintaining a minimal downstream water release could be met with minimal consequences for agricultural supply and reservoir level. However, both agricultural supply and downstream flow had to be reduced during dry periods to keep the reservoir level above a minimum level. Maintaining a minimum downstream flow resulted in a 4.2% reduction in mean agricultural supply and 2.2% reduction in mean reservoir elevation compared to baseline conditions. By contrast, a 20% reduction in dry season water inputs resulted in a 17.8% reduction in mean agricultural supply in the minimum downstream flow scenario compared to current rainfall conditions. Furthermore, the number of days the reservoir level dropped below the minimum needed for a fishery increased (range 1.15–25) across scenarios compared to baseline. Successful management of surface water within a social-ecological system will be challenged by the increasing unpredictability of rainfall.