Operational Mapping of Evapotranspiration over Agricultural Land in the California Central Valley
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A key challenge in groundwater modeling in California is the limited historic data on groundwater withdrawals for irrigation. Satellite mapping of evapotranspiration (ET) is an efficient way to quantify consumptive water use and can play an important role in filling this data gap. Recent advances in satellite mapping of ET have made it possible to largely automate the process of mapping ET over large areas at the field-scale. This development allows for the creation of multi-decade timeseries that can be used to constrain groundwater models and improve estimates of groundwater withdrawals in agricultural regions.We present an approach for operational mapping of ET in California that leverages two automated ET mapping capabilities to estimate ET at the field scale over agricultural areas in the California Central Valley. We utilized the NASA Earth Exchange and applied a python-based implementation of the METRIC surface energy balance model and the Satellite Irrigation Management Support (SIMS) system, which uses a surface reflectance-based approach, to map ET over agricultural areas in the Central Valley. Though theoretically and computationally quite different from each other, the combined approach increases data continuity and reduces reliance on a single satellite or sensor. We present a comparison of results from both models, and discuss the strengths of limitations of the combined approach. We also discuss comparisons against ET measurements collected on commercial farms in the Central Valley and discuss implications for accuracy of the two different approaches. The objective of this analysis is to provide data that can support planning for the development of sustainable groundwater management plans, and assist water managers and growers in evaluating irrigation demand during drought events.