San Joaquin Valley - Agricultural Water Stress (Final 2018)

Our assessment of the condition of water resources looked at the three principal sources on which San Joaquin Valley agriculture relies: local surface water from watersheds draining regions in the Valley, imported surface water from from outside regions in the Valley (e.g., Sacramento River water delivered through the Delta, or imported water along the Friant-Kern Canal) and groundwater. The analysis considered both the degree of reliance on the different sources of water, measured as a percentage of total agricultural water use (Figure 6), and the year-to-year variability in the supply of these sources. The underlying assumption was that areas with a high degree of reliance on water supplies that vary significantly from year-to-year are more water stressed. Conversely, areas with diverse water supplies and/or water supplies that were steady from year-to-year would be less water stressed. For groundwater, the assumption was that areas with deeper groundwater tables and those areas that have experienced the greatest increase in depth to groundwater over the drought are more water stressed.

For surface water, we used data from the California Department of Water Resources (DWR) to measure the amount and percentage of total agricultural water supplies that come from both local and imported sources in each C2VSim planning area as well as the variability of those supplies over the longest and most recent period for which comparable data of a recent time period were available (1973-2009). Variability was defined as the extent by which annual agricultural use over the period deviated from the mean annual usage (coefficient of variation) with higher scores representing the greatest deviation, i.e., less reliability and, hence, greater water stress.

Groundwater usage was estimated in the DWR data providing us with the percentage of water for each planning area that can from groundwater, local water supplies, and imported water supplies. The groundwater usage percentage was combined with data reflecting the condition of groundwater resources including both the depth to groundwater in fall of 2017, and the change in depth to groundwater over the recent drought period up to the current year (2011-2017) . Depth to groundwater reflects the cost of pumping irrigation water to the surface, while the change in depth is a direct indicator of how much groundwater is being drawn down and, in some cases, depleted, increasing the risk of wells going dry.

Of the roughly 11 million acre-feet of water used annually by agriculture in the Valley, just over 40% comes from local surface water sources, 27% from imported surface water supplies and the remainder (32%) from groundwater . But there are significant differences in the mix of these sources from planning area to planning area (Figure 7). Local surface water supplies predominate in the northern part of the Valley, while imported serface water becomes more important farther south and on the west side, and reliance on groundwater increases toward the southern end of the region.