Effects of projected land use change on water resources in the California Central Valley

With the recent California legislation, SB 1168 (Pavley), SB 1319 (Pavley), and AB 1739 (Dickinson) creating the Sustainable Groundwater Management Act (SGMA), (California Groundwater, 2015) groundwater must be managed sustainably in the future. To accomplish this, a great deal of attention is focused on how groundwater is used, how those uses may change in the future, and what impact climate change and climate variation have on groundwater resources. This research provides a first step into coupling groundwater modeling with land use projections to examine how future water requirements may vary and their corresponding impacts to groundwater and surface water resources. Land use projections from the United States Geological Survey (USGS) are incorporated into the Central Valley Hydrologic Model (CVHM) while controlling for climate. Projected land use from 2090 is compared to land use from 2010 with large shifts to urban areas and agricultural areas shifting from annual to perennial crops. The influence of projected land use change is evaluated through changes in water demand and changes in groundwater elevations. In the Sacramento Valley and Delta regions moderate climate variability is introduced to investigate how both land use and climate change could interact in these regions. This study modeled a decrease of 2 7% in Annual Agriculture, 17% increase in Perennial Agriculture, and a 10% increase in Urban areas across the entire Central Valley study area. These land use projections were examined over two different climate scenarios, an average and a slightly dry of average. In an average climate these land use projections decreased the total required water demand by 13%, with reductions of 6% in surface water demand and 25% reduction in groundwater demand. The groundwater level response in an average climate was variable across the Central Valley with small increases in the northern study area and decreases in the southern study area. When controlling for regional changes in groundwater levels, this study shows that urbanization causes the largest decline in groundwater levels and agricultural expansion into native areas causes the largest increase in groundwater levels. When a dry-of-average climate was modeled in the Sacramento Valley, land use projections caused similar responses in groundwater level changes in some areas as the average year while other areas exhibited higher land use change associated groundwater levels.