Association for Public Policy Analysis & Management

California’s Central Valley region has been called the “bread-basket” of the United States as much of the country’s produce is grown there. Such high levels of agricultural productivity require large amounts of fresh water for irrigation. However, the long-term availability of water required to sustain high levels of agricultural production is being called into question as the current California drought enters its fifth year.

Presumably, the dynamics of water use over the course of a drought are dependent on the legal structure of water rights in the state as well as variations in the natural landscape. California water code prioritizes water allocations based on the stated purposes of water use, the type of water right, and the timing of appropriation. The structure of these prioritizations, e.g. domestic use over irrigation; riparian over appropriated; pre-1914 appropriations over recent appropriations, has the potential to constrain agricultural production, particularly for junior water rights holders.  Interestingly, while California water code for surface waters is well-established, ground water use has gone largely unregulated (the first set of state groundwater regulations are set to roll out in 2017). The unstructured nature of access to groundwater is expected to play a major role in short term mitigation of drought impacts by agricultural producers, offsetting the constraints presented by the highly structured surface water rights.

In this study we use Bayesian multilevel spatiotemporal modeling techniques to examine the influence of the structure of water rights in the California Central Valley on agricultural production during the drought. Total vegetative production (TVP), computed using a remotely sensed metric of vegetation health known as the Enhanced Vegetation Index, is used as a proxy for agricultural production.  We assess the extent to which the legal structure of water rights and the nature of water use affect agricultural production across watersheds from 2005 to 2015. Using R-INLA, we account for spatial processes operating at multiple scales (watershed and farmland parcel) that have the potential to influence the effects of water right structures on TVP.  In addition to presenting the findings regarding the effects of water rights structures on agricultural production, we discuss the benefits and limitations of spatiotemporal modeling and the implications of our study for long-term agricultural and water policy in California’s Central Valley.