Multiscale assessment of agricultural consumptive water use in California's central valley

Authors: WONG, A. - University Of California; JIN, Y. - University Of California; MEDELLIN-AZUARA, J. - University Of California; PAW, K. - University Of California; KENT, E. - University Of California; CLAY, J. - University Of California; Gao, Feng; FISHER, J. - Jet Propulsion Laboratory; RIVERA, G. - Jet Propulsion Laboratory; LEE, C. - Jet Propulsion Laboratory; HEMES, K. - University Of California; ECHELMANN, E. - University Of California; BALDOCCHI, D. - University Of California; HOOK, S. - Jet Propulsion Laboratory

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/28/2021
Publication Date: 9/15/2021
Citation: Wong, A., Jin, Y., Medellin-Azuara, J., Paw, K., Kent, E., Clay, J., Gao, F.N., Fisher, J., Rivera, G., Lee, C., Hemes, K., Echelmann, E., Baldocchi, D., Hook, S. 2021. Multiscale assessment of agricultural consumptive water use in California's central valley. Water Resources Research. 5. https://doi.org/10.1029/2020WR028876.
DOI: https://doi.org/10.1029/2020WR028876

Interpretive Summary: Evapotranspiration (ET) refers to the water flux evaporated from soil and transpired from vegetation into the atmosphere. In agricultural lands, ET is also known as crop consumptive water use, and is often the largest component of the water balance. This paper uses the semi-empirical Priestley-Taylor (PT) approach to estimate ET from satellite images at 30m spatial resolution. Surface measurements from six crop types were used to calibrate the PT coefficients. Results show that satellite-based ET estimates agree well with field measurements for all crop types during Landsat overpassing dates. The uncertainty is slightly higher during non-Landsat overpassing dates. Accurate and frequent ET estimations at field scales are critical for farmers to manage irrigation for increased water productivity.

Technical Abstract: Irrigation accounts for 80% of human freshwater consumption, and most of it returns to the atmosphere through evapotranspiration (ET). A cost-effective and timely estimate of crop ET with known uncertainty, from the farm to watershed level, is becoming increasingly important, especially in California, where water resources are heavily utilized. The semi-empirical Priestley-Taylor (PT) approach, theoretically based yet relatively simple, has been shown robust for accurate ET estimates across diverse ecosystems, when driven by satellite observations such as those from MODIS and AVHRR. We here further improved the semi-empirical PT approach specifically for agricultural lands at higher spatial resolution, to enable growers and farm managers to tailor irrigation management based on in-field spatial variability and in-season variation. Both Landsat and MODIS thermal data were used to improve the daily net radiation estimate at 30m, with a RMSE of 34.8 Wm-2 when compared with field measurements. We optimized the PT coefficients with ET measurements from eddy covariance towers and surface renewal stations for six crop types (alfalfa, almond, citrus, corn, pasture, and rice) in California. Satellite-based ET estimates agreed well with field measurements over a total of 26 sites, with R2 of 0.75 and RMSE 1.2 mm/day on average for all crop types during Landsat overpassing dates. The uncertainty is slightly higher during non-Landsat overpassing dates and when using a generalized optimization of PT coefficient. This model was applied to estimate spatial ET for water year 2015 and 2016 over the California Sacramento-San Joaquin Delta. The derived ET map shows the spatial variability of crop water consumptive use in the Delta, enabling us to compare monthly and seasonal water use of different crops. Continuous monitoring of the dynamics and spatial heterogeneity of canopy and consumptive water use at field scale will inform decision makers to adaptively manage water to maximize yield per drop of water.