Analysis of the Relationship Between California Surface Water Deficits and the Evaporative Demand Drought Index

Abstract:

California is in the midst of a historic drought in terms of both duration and severity, and the extreme moisture deficit has led to alarmingly low streamflow and reservoir levels since the last above average water year of 2011. While lack of precipitation is one cause of the depleted surface water, other factors such as record breaking warm temperatures, low near-surface humidity, and wind speed have all exacerbated the recent drought. A better understanding of drought dynamics and feedbacks could potentially improve seasonal drought forecasting and water supply management. In this study we examine the contribution of several near-surface meteorological variables to historical California surface water droughts using the Evaporative Demand Drought Index (EDDI). EDDI measures the physical response of evaporative demand to surface drying anomalies that occur due to land surface-atmosphere interactions. The University of Idaho’s Gridded Surface Meteorological Data are used to drive EDDI, with inputs of temperature, wind speed, humidity, and shortwave radiation at the surface needed to compute a physically based evaporative demand. Streamflow data from several U.S. Geological Survey gauges along the west slope of the Sierra Nevada are accumulated by water-year, standardized, and correlated to EDDI at time scales of 1-, 3-, 6-, 9-, and 12-months to illustrate that EDDI is capable of identifying historical hydrologic droughts based on land surface-atmospheric feedbacks alone. A sensitivity analysis is performed on EDDI to identify the dominant drivers of individual cases. Initial results indicate that EDDI is capable of tracking most major hydrologic droughts, and correlations to water-year streamflow are highest at the 9- to 12-month aggregation periods, and during the summer months. Anomalously high temperatures were the primary driver of EDDI during 2012 through 2014; however other variables drove EDDI during historical periods.