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ARS Home » Northeast Area » Beltsville, Maryland (BARC) » Beltsville Agricultural Research Center » Hydrology and Remote Sensing Laboratory » Research » Publications at this Location » Publication #338032
Title: Temporal downscaling of crop coefficients for winter wheat in the North China Plain: A case study at the Gucheng ecological-meteorological experimental station

Author
item WANG, P. - Chinese Academy Of Sciences
item QUI, J. - Guangdong Academy
item HUO, Z. - Chinese Academy Of Sciences
item Anderson, Martha
item ZHOU, Y. - Iowa State University
item BAI, Y. - Chinese Academy Of Sciences
item LIU, T. - Chinese Academy Of Sciences
item REN, S. - Chinese Academy Of Sciences
item FENG, R. - Atmospheric And Environmental Research
item CHEN, P. - China Medical University

Submitted to: Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/16/2017
Publication Date: 2/23/2017
Citation: Wang, P., Qui, J., Huo, Z., Anderson, M.C., Zhou, Y., Bai, Y., Liu, T., Ren, S., Feng, R., Chen, P. 2017. Temporal downscaling of crop coefficients for winter wheat in the North China Plain: A case study at the Gucheng ecological-meteorological experimental station. Water. doi:10.3390/w9030155.

Interpretive Summary: The crop coefficient method is a common way to get a first order estimate of crop water use and requirements. This method uses daily timeseries of a reference evapotranspiration (Etr) computed from standard meteorological data. Reference ET describes the potential amount of water use from a well-watered reference surface (like short-clipped grass) under the prevailing weather conditions (sunshine, temperature, vapor pressure deficit, winds). This reference ET is then adjusted for different crop types using empirically-derived crop coefficients, which may vary through the crop growth cycle. This paper tests standard crop coefficients developed for winter wheat under the growing conditions typically encountered in the North China Plain. The tests, using local measurements of crop water use and weather data, revealed that adjustments to the standard coefficients are required to accurately model water use by irrigated winter wheat. Improved coefficients effective during critical phenological growth stages are presented.

Technical Abstract: The crop coefficient (Kc) method is widely used for operational estimation of actual evapotranspiration (ETa) and crop water requirements. The standard method for obtaining Kc is via a lookup table from FAO-56 (Food and Agriculture Organization of the United Nations Irrigation and Drainage Paper No.56), which broadly treats Kc as a function of four crop-growing stages. However, the distinctive physiological characteristics of overwintering crops, such as winter wheat (Triticum aestivum L.) which is extensively planted in the North China Plain (NCP), were not addressed in this method. In this study, we proposed a stage-wise method that accounts for Kc variations for winter wheat at each critical phenological stage, thereby estimating Kc at finer temporal scales. Compared with the conventional FAO method, the proposed stage-wise method successfully captured the bimodal pattern in Kc time series for winter wheat, which is shown at both ten-day and phenological time scales. In addition, the accuracy of the proposed stage-wise Kc method and the FAO method were evaluated using micrometeorological measurements of actual ET (ETa) collected at the Gucheng ecological-meteorological experimental station in the NCP. Using a leave-one-out strategy, the evaluation revealed that the stage-wise method significantly outperformed the FAO method at both daily and the critical phenological timescales, with root-mean-square errors in ETa for the stage-wise method and the FAO method being 0.07 mm d-1 and 0.16 mm d-1, respectively, at daily scale, and 0.01 mm d-1 and 0.27 mm d-1 at the critical phenological scale. Generally, the FAO method underestimates ETa during the initial stage and overestimates ETa during both the development and mid-season stages. It is shown that the proposed stage-wise method is important for the water-stressed NCP where precision irrigation is highly desirable, especially during the critical phenological stages. Results from this study provide insight into accurate estimation of water requirements for winter wheat at phenological timescales.