Analysis of soil moisture trends in the Salt River watershed of central Arizona

In this investigation, we generate estimates of soil moisture for the Salt River basin on which the expanding city of Phoenix, Arizona relies upon for nearly half of its water supply. While previous empirical studies have produced mixed results concerning recent drought trends in the southwestern USA, our results support the many numerical climate models which predict that this region will become drier in response to the buildup of greenhouse gases. Over the period 1895-2007, we found that soil moisture levels revealed no significant change. However, if we restrict the study to the period 1980-2007, soil moisture levels have decreased at a highly statistically significant rate. We could find little evidence that the trend is associated with variations in El Niño-Southern Oscillation and the Pacific Decadal Oscillation. Increasing surface pressure over our study area during the latter half of the twentieth century may be a catalyst for the drier conditions.