Crop Evapotranspiration in San Joaquin Valley by Landsat Reflectance-based and Energy-balance Estimation Methods

Evapotranspiration (ET) estimates are needed to support agricultural and natural resource management. Satellite based measurements offer the potential to efficiently monitor ET over large areas. In this study, two analysis methods were applied to Landsat-5 Thematic Mapper imagery to estimate crop evapotranspiration (ETc) in California's San Joaquin Valley. The Landsat L1T images (path 42, row 35) were collected monthly during the main growing season (Apr-Nov) in 2009. In the first method, the images were transformed to surface reflectance, and subsequently to NDVI. The NDVI was used to estimate mean fractional cover of several major crop types including almond, orange, grape, cotton, corn, alfalfa, and tomato across a total of 115 fields. Prior relationships developed by weighing lysimeter were used to convert fractional cover to a crop coefficient expressing ETc relative to grass reference evapotranspiration (ETo). Measurements of ETo by the California Irrigation Management Information System (CIMIS) were then used to calculate ETc on each overpass date. These reflectance-based estimates were compared with values retrieved by the Surface Energy Balance Algorithm for Land (SEBAL). SEBAL combined spectral radiances in Landsat optical and thermal bands with CIMIS meteorological data to derive ET as a surface energy budget residual by applying radiative, aerodynamic and energy balance physics in 25 computational steps. Reasonably strong agreement resulted, with mean absolute error (MAE) between the two approaches <1 mm/d, and coefficients of determination ranging from 0.78-0.90, for most of the crop types examined. Stronger agreement was found for fields deemed by SEBAL to contain unstressed crop (observed ET at-or-near potential) during satellite overpass, with MAE reductions averaging about 30 percent and coefficients of determination largely of range 0.90-0.94.