Implications of Climate Change on Water Budgets and Reservoir Water Harvesting of Nuuanu Area Watersheds, Oahu, Hawaii

Abstract

Assessing freshwater availability under changing climate and land-use conditions is critical for tropical islands, where small watershed sizes and unique hydrological features mean that freshwater resources are very sensitive to these changes. The objective of this study is to assess the impact of climate change on the water budgets of the Nuuanu area watersheds (NAW), Oahu, Hawaii, and most importantly, on the potential of water harvesting from Nuuanu Reservoir 4 (NR4). The harvest approach concerns water diversion from the reservoir for artificially supplementing groundwater recharge through injection into the subsurface. Following calibration and validation at multiple streamflow gauging stations across the watershed, the soil and water assessment tool (SWAT) model was applied to assess current and future water availability in the NR4. Multiple statistical evaluation criteria were used to demonstrate that SWAT adequately reproduced the observed daily streamflow hydrographs at all stations. Climate change analyses used the representative concentration pathways (RCP) 4.5 and 8.5 scenarios. The reported analysis suggests that the water-budget components of the NAW will generally be adversely affected by climate change. Compared to the baseline of 35 years, the amount of water available from the NR4 for harvesting will decrease by as much as 27%, and the corresponding outflow values will decrease by as much as 37%. Despite future temperature increases, the actual monthly watershed-scale evapotranspiration will decrease because of the overall decrease in rainfall and soil-moisture availability. Rainfall has the dominant control over the magnitude of water-budget components and harvesting compared with temperature and solar radiation. This study presents a blueprint for surface-water-harvesting scenarios, which may become a necessity across the Pacific and other islands in the future.