Utility of precipitation indices as indicators of drought in Nevada

Throughout Nevada snow-dominated watersheds prevail and are critical for water supply for both human and natural environmental demands. Therefore, understanding characteristics of water shortages, such as length and severity, is extremely important to both scientists and the general public. Forecasting extended periods of dry conditions in the western United States will become more crucial in the future, especially if temperatures continue to rise in this region as predicted by most global circulation models (GCMs). This study examines the effectiveness of two precipitation indices, the standardized precipitation index (SPI) and the standardized precipitation evapotranspiration index (SPEI), to identify drought periods and consider the role of evapotranspiration processes in drought conditions. Whereas the SPI reflects solely precipitation deficit or surplus, the SPEI takes into account the effects of both precipitation and temperature. Using the temperature data, potential evaporation is computed and a basic water balance is incorporated into the final SPEI calculation. We have chosen several weather stations throughout Nevada and eastern California, at both irrigated and non-irrigated locations, and computed SPI and SPEI over their entire period of record. Preliminary results suggest that overall the SPEI is an improvement from the SPI in identifying severe and extended drought periods, but results varied from station to station. Drought conditions in the region are affected by both temperature and precipitation. SPI does not always account for droughts in which above normal temperatures play a significant role. To assess future changes in drought length and severity, we used bias-corrected and statistically downscaled GCM projections of monthly temperature and precipitation as input into the SPI and SPEI calculations. Initial results reveal that the wet and dry patterns vary greatly among the GCMs.