By Josué Medellín-Azuara, Kyaw Tha Paw U, Yufang Jin, Quinn Hart, Eric Kent, Jenae’ Clay, Andy Wong, Andrew Bell, Martha Anderson, Daniel Howes, Forrest Melton, Tariq Kadir, Morteza Orang, Michelle M. Leinfelder-Miles, J. Andres Morande, William Li, and Jay R. Lund

As California works to improve its official accounting of water for a range of purposes, one major area lacking widely accepted quantification is the consumptive use of water for agriculture, particularly evapotranspiration (ET) from crops.  In the Sacramento-San Joaquin Delta, such estimates are important, along with other hydrologic flows, for a variety of water rights, operational, and regulatory purposes.

Consumptive use is the proportion of water removed that cannot be reused elsewhere in a basin. For crops in the Sacramento-San Joaquin Delta, this is mostly evapotranspiration. In a region’s water balance, consumptive use can become a keystone for estimating groundwater recharge, outflows from a basin, and the availability of water for water exchanges or market transactions. In places like the Sacramento-San Joaquin Delta (the Delta), crop consumptive use estimation may have the additional challenges of adjusting for a collection of localized factors such as fog, canal seepage, evaporation from canals, and widely varying wind conditions.

Figure 1. 2015 Land use in the Sacramento-San Joaquin Delta, by LandIQ.

Models for calculating consumptive use have been researched for many years and have the potential for improved accuracy in estimating Delta outflows and informing water management and diversion in this complex and fragile estuarine system.  Yet, given the many agencies, stakeholders, and institutions relying on this water system hub, self-reporting of water diversions and estimates of consumptive use are often a challenge.

In an effort to reduce the information gap and converge efforts on consumptive use research, estimation and measurement, the State Water Resources Control Board’s Office of the Delta Watermaster convened funding from several state, regional, and local Delta water agencies to do a comparative study on consumptive use for the Delta. Researchers from the Center for Watershed Sciences and the Department of Land, Air and Water Resources at UC Davis partnered with researchers from UC Cooperative Extension, DWR, NASA-Ames Research Center, USDA-Agricultural Research Service, and CalPoly’s Irrigation Training and Research Center to conduct this study. The two-season study covers the 2014-2015 and 2015-2016 water years and includes completion of land use surveys for each year, deployment of field measurement equipment (thanks to the cooperation of farmers and organizations within the Delta), and estimation of consumptive use using seven methods and models such as CalSIMETAW, DETAW, METRIC, Priestley-Taylor and SIMS.

The study includes an initial report on measurement of evapotranspiration in bare soil, and a blind comparison of evapotranspiration estimates using seven different methods, applied independently. The interim report is posted on the project website at: https://watershed.ucdavis.edu/project/delta-et

Preliminary findings show that bare soil evapotranspiration at the end of the irrigation season was close to zero in four locations selected for field measurements during September-October of 2015 (Figure 2).

Figure 2. Daily actual ET (ETa) measured from bare soil stations (surface renewal and eddy covariance) along with daily reference ET (ETo) from the nearby CIMIS stations. Lines are mean values across stations and gray shading represents one standard deviation from the mean.

A more extensive field campaign during 2016 includes more than a dozen measurement stations over corn, pasture and alfalfa fields, and covers more of the growing season. This will greatly improve measurement and enhance future comparisons of ET estimation methods.

Preliminary findings are that the average annual crop evapotranspiration estimated by the ensemble of the models tested in the Delta Service Area is about 1,550 thousand acre feet (TAF) (Figure 3). That ensemble estimate is roughly consistent with the annual estimates used in DWR’s California Water Plan Update 2013. Crops with the highest variation in ET estimates across estimation methods under comparison are are tomatoes, vineyards and potatoes.  All estimates are within about 20% of the median estimate with the higher discrepancies during December and January, months with lower crop evapotranspiration, and September, in which some crops are being harvested.

The fact that all estimation methods do not closely agree is not surprising, and would be highly unusual in comparisons of ET estimation. However, further work should be able to increase the agreement among these estimates. In addition, it should improve understanding of why and how much different estimation methods are likely to differ, and how modeling and remote sensing estimates are likely to differ from field estimates of crop ET.

Figure 3. Total evapotranspiration estimates for selected crops in the Legal Delta and the Delta Service Area (October 2014-September 2015). Estimates are derived from monthly average estimates of daily evapotranspiration for each method. CalSIMETAW and DETAW coverage is limited to the Delta Service Area.

Discrepancies in model estimates are driven by several factors including variation in input datasets and processes, inherent methodological differences, and other varying meteorological conditions. Refined estimates produced by all of the methods during the second year of study will use common input datasets and protocols to enhance the value of comparisons.  In addition, improved algorithm calibration based on analysis of 2015 data is likely to reduce the range of variation in ET estimates across methods and improve the accuracy and credibility of all methods.

A final report on this two-year study will include final estimates of consumptive use from all methods for the 2014-2015 and 2015-2016 water years. The final report will benefit from a longer field measurement campaign and improved common protocols and input datasets. It is expected to be available during late spring 2017.

Improving estimation of consumptive use is a step toward improving the reliability and transparency of data and analysis for water management, with potential for reducing reporting and accounting expenses for farmers and the state.

Josué Medellín-Azuara, Jay R. Lund, and Andrew Bell are affiliated with the Center for Watershed Sciences, UC Davis. Kyaw Tha Paw U, Yufang Jin,  Quinn Hart, Eric Kent, Jenae’ Clay, and Andy Wong are afiilliated with Land Air and Water Resources, UC Davis. Martha Anderson is affiliated with USDA-ARS. Daniel Howes is affiliated with CalPoly Irrigation Training and Research Center. Forrest Melton is affiliated with NASA-Ames, Monterrey. Tariq Kadir and Morteza Orang are afilliated with the Department of Water Resources. Michelle M Leinfelder-Miles is affiliated with UC Cooperative Extension.

Further Reading

Medellín-Azuara, J., Paw U, K.T., Jin, Y., Hart, Q., Kent, E., Clay, J.,Wong, A., Bell, A., Anderson, M., Howes, D., Melton, F., Kadir, T., Orang, M., Leinfelder-Miles, M., and J.R. Lund. (2016). Estimation of Crop Evapotranspiration in the Sacramento San Joaquin Delta for the 2014-2015 Water Year. An Interim Report for the Office of the Delta Watermaster, State Water Resources Control Board. Center for Watershed Sciences, University of California, Davis. Last Access September 28, 2016

Medellin-Azuara, J. and Howitt, R. (2013) Comparing Consumptive Agricultural Water Use in the Sacramento-San Joaquin Delta: A Proof of Concept Using Remote Sensing, Center for Watershed Sciences University of California, Davis. Last Access September 28, 2016.

Siegfried, Lucas J.; Fleenor, William E.; & Lund, Jay R.(2014). Physically Based Modeling of Delta Island Consumptive Use: Fabian Tract and Staten Island, California. San Francisco Estuary and Watershed Science, 12(4). Jmie_sfews_20875. Last access September 28, 2016

Acknowledgements

The authors are thankful for funding and research support provided by the State Water Resources Control Board, California Department of Water Resources, the Delta Protection Commission, the Delta Stewardship Council, the North Delta Water Agency, the Central Delta Water Agency, and the South Delta Water Agency.  The authors also appreciate the assistance provided in various capacities by Nadya Alexander, J. Andrés Morandé, William Li, Cathryn Lawrence and Barbara Bellieu, who made this report possible.