Evaluation of the Normalized Seasonal Wetness Index (NSWI) for Seasonalizing Estimates of Groundwater Recharge in Semi-arid Western U.S. Basins from Climatic Data

In the semi-arid Basin and Range Province of the Western U.S., groundwater recharge is the sole natural input of water to aquifers on which large populations often depend for their water supply. Measuring groundwater recharge directly presents a challenge due to the difficulty of instrumentation and the spatial and temporal heterogeneity of recharge processes in Basin and Range systems. The Basin and Range experiences a primarily bimodal precipitation regime, with dry summers and wet winters often characterized by snow accumulation on mountaintops. In the southern Basin and Range, the North American Monsoon also has influence. Stable water isotopic data for basins throughout the region indicate that winter precipitation contributes disproportionately more to annual recharge than summer precipitation. The Normalized Seasonal Wetness Index (NSWI) has been proposed as a method to estimate seasonal recharge volumes for use in hydrologic models that reflect the impact of regional climatology on annual recharge. The NSWI has previously been shown to closely estimate the percent of annual recharge occurring in the summer and winter seasons for the Upper San Pedro Basin in southeastern Arizona, as determined from stable water isotope analysis. This paper applies the NSWI method for seasonalizing recharge to a suite of other basins in the Basin and Range for which similar isotopic studies have been conducted and can be used to validate the NSWI results. Seasonalization of recharge using the NSWI shows promise for incorporating global climate model data into hydrologic models that seek to predict aquifer conditions under future climate scenarios.