Partitioning the use of precipitation- and groundwater-derived moisture by vegetation in an arid ecosystem in California

Demands on groundwater supplies in arid regions have increased interest in the importance of groundwater, relative to precipitation-derived moisture, to vegetation. Depth to water (DTW), precipitation (PPT), and vegetation (total perennial cover, TPC) data have been collected from 30 sites in the Owens Valley, California, since the 1980s. We used these data to construct water budgets for each site, and to calculate precipitation-use efficiencies (PUE) and amount of TPC supported by precipitation and by groundwater. We compared these values among five plant communities over different DTW and PPT regimes to determine relative importance of each water source. Our results confirm that both groundwater and precipitation are important sources of water to Owens Valley vegetation. In general, groundwater becomes more important as DTW decreases, but this relationship varies among communities. TPC increased as DTW decreased up to a point, after which TPC remained constant or declined. Some groundwater usage by shrub- and grass-dominated communities continued to DTW of 6–8 m. PUE was highest at low precipitation and decreased as precipitation increased. Our results indicate that vegetation preferentially used precipitation-derived soil moisture, even with abundant groundwater, and that successful management of groundwater-affected arid ecosystems must account for complex interactions among multiple factors.