Changing Water Resources Under El Nino, Climate Change, and Growing Water Demands in Seasonally Dry Tropical Watersheds

The wet-dry tropics of Central America are characterized by long dry seasons, during which communities often struggle for water. High interannual rainfall variability, driven in parts by the El Nino Southern Oscillation (ENSO), increases these challenges. Further, climate change projections indicate that the region will likely become drier. However, research on impacts on water resources at the watershed scale is limited in the region-yet this information is essential for water managers. Therefore, in this research, we quantified the potential impacts of ENSO and four different climate change scenarios on water resources in two watersheds in the wet-dry tropics of Guanacaste, Costa Rica, using a hydrological model (Water Evaluation and Planning Tool [WEAP]). Given that the watersheds are human used, we also explored different water demand scenarios. Modeling results indicated that an extreme El Nino can reduce groundwater recharge and streamflow by similar to 60% relative to ENSO neutral. For 2075-2100, modeling results indicated that while potential evapotranspiration increases, actual evapotranspiration decreases due to limited water availability. Further, climate change may lead to reductions of mean annual streamflow and groundwater recharge by 40%-45% and 26%-28%, respectively, in comparison to the historical baseline. Importantly, high population growth could further hasten potentially irreversible groundwater storage declines. On the other hand, reduction of per-capita water demand could slow down, or even reverse, the decline of groundwater storage.