Post-Drought Crop Changes in the Central Valley and Relationships to Ground Subsidence

Abstract:

The Central Valley is California’s most agriculturally productive region, where nearly 40% of water for irrigation is supplied by groundwater. During the 2012-2016 drought, a 2017 study by Tortajada et al. found that surface water delivery to the region decreased by nearly half, leading to increased extraction of groundwater and a faster rate of ground subsidence. Building on the work of Shivers et al. 2018, this study uses a Random Forest algorithm and county-level crop data grouped according to California Department of Water Resources guidelines to classify imagery from the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS). Random Forest is run based on a 50% minimum green vegetation fraction from AVIRIS as estimated by a multiple-endmember spectral mixture analysis (MESMA). Following the results of Shivers et al., this project found that from 2015 to 2018, there was little evidence that farmers prioritized water-efficient crops: there was an increase in cotton, almonds, and pistachios, which are all water-intensive. Furthermore, drought intensity has been strongly correlated with ground subsidence, which can contribute to permanent loss of groundwater storage, reduced soil porosity, and the release of trapped mobile arsenic and selenium. These factors have the potential to influence crop planting decisions in the Central Valley based on rooting depth and differential uptake of contaminants. By using Sentinel-1 synthetic aperture radar (SAR) to map changes in ground surface elevation at a field-level resolution, we found a non-random relationship between crop type and the level of subsidence.