The Central Valley of California is one of the most productive agricultural regions of the United States and irrigated agriculture is one of the heaviest consumers of water resources, so understanding how different crops use water over time is essential for planning and managing water allocation, water rights, and agricultural production. It is especially critical to understand how crop water use responds to prolonged drought conditions such as the 2012–2016 California drought. Scientists at the USGS Earth Resources and Observation Science (EROS) Center estimated crop water use for the Central Valley of California from 2008 through 2018 using actual evapotranspiration (ET) and Landsat imagery.
Remote sensing imagery from Landsat is a cost-efficient and effective dataset for estimating crop water use over large areas and multiple years. Landsat thermal data combined with an efficient method such as the Operational Simplified Surface Energy Balance (SSEBop) model is a powerful tool that can characterize the spatial and temporal trends for the entire Central Valley as well as much of the United States. This study utilized over 4,850 Landsat images covering the Central Valley of California from 2008 through 2018 to estimate the annual total of actual ET and then summarized this by crop classification data derived from the U.S. Department of Agriculture National Agricultural Statistics Service Cropland Data Layers (CDL). The annual total crop water use for the top 10 major crops by area was estimated and analyzed for shifting trends over time. A pixel-based Mann–Kendall trend analysis showed the changing crop type and water use at the level of individual fields. For example, the trend analysis revealed an increase in area cultivated with almonds and their associated water use while, during the same period, alfalfa showed a dramatic decline in cultivated acreage and water use. Similar studies can be conducted at regional and global scales to understand and quantify the relationships between land cover change and its impact on water use.
The Central Valley for 2018 showing (a) the generalized top 10 crops identified by USDA-NASS CDL, (b) the SSEBop annual total actual ET for 2018, and (c) the change in SSEBop ETa from 2008 to 2018 (2018 ETa minus 2008 ETa).