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WHEAT

Estimation of Winter Wheat Evapotranspiration under Water Stress with Two Semiempirical Approaches

^a Luancheng Agroecosyst. Stn., Inst. of Geogr. Sci. and Nat. Resour. Res., Chinese Acad. of Sci., Bldg. 917, Datun Rd., Beijing 100101, China
^b Xiying Zhang, Shijiazhuang Inst. of Agric. Modernization, Chinese Acad. of Sci., 286 Huaizhong Rd., Shijiazhuang 050021, P.R. China
^c Inst. of Environ. Sci., Beijing Normal Univ., Beijing 100875, P.R. China

^* Corresponding author (zhangyq{at}igsnrr.ac.cn).

Received for publication October 31, 2002. Winter wheat (Triticum aestivum L.) is one of most important crops in the North China Plain. However, soil water deficit (SWD) often occurs due to lack of precipitation in its growing season. In this study, we introduce two semiempirical approaches, a recharge model and the crop coefficient (K_c)–reference evapotranspiration (ET₀) approach, to estimate wheat actual evapotranspiration (ET_a) under no SWD and slight and severe SWD conditions. The recharge model allocated ET₀ to reference evaporation and reference transpiration as a function of leaf area index. In the model, ET_a is limited by soil water content, and crop water extraction for ET_a is distributed through the soil profile as exponential functions of soil and root depth. The K_c–ET₀ approach regarded ET_a under the SWD condition as a logarithmic function of soil water availability. Under no SWD condition, the recharge model simulated 10-d ET_a with a root mean square error (RMSE) of 5.58 mm and a bias of 0.95 mm compared with measurements from a large-scale weighing lysimeter. The two approaches both estimated seasonal evapotranspiration (ET) well compared with the adjusted ET (from the soil water balance and the recharge model–simulated deep drainage). The recharge model, which simulated the seasonal ET with the RMSE of 27.8 mm and the bias of –8.0 mm, was better than the K_c–ET₀ approach (RMSE = 31.7 mm and bias = –33.1 mm). The seasonal pattern of soil water stress coefficient (K_s) showed that there were faster water losses at grain-filling stage than at other stages.

Abbreviations: A_w, soil water availability • ET, evapotranspiration • ET_a, actual evapotranspiration • ET₀, reference evapotranspiration • K_c, crop coefficient • K_s, soil water stress coefficient • LAI, leaf area index • NCP, North China Plain • RMSE, root mean square error • SWB, soil water balance • SWD, soil water deficit • SWS, soil water storage