Measurement of evapotranspiration of irrigated spring wheat and maize in a semi-arid region of north China

Gravimetric lysimeter experiments were conducted to measure the daily evapotranspiration (ETc) of spring wheat in 1998 and maize in 1999 at Naiman, Inner Mongolia, China. The weekly ETc range of spring wheat varied from 12.32 to 45.43 mm per week. The maximum ETc occurred at the 9th week after sowing during the mid-season stage, and the peak value of leaf area index (LAI) also occurred at this time. As for maize, ETc averaged 16.7 mm per week during the initial stage, approached a peak value of 48.45 mm per week at the 12th week after sowing, and declined to its minimum of 13.33 mm per week during the late-season stage. The total ETc of spring wheat and maize were 425.7 and 572.5 mm, and average daily ETc of spring wheat and maize were 4.05 and 4.09 mm during the both growth seasons. The five reference ET methods (i.e. FAO-56 Penman–Monteith (56PM), FAO-24 Penman (24Pn), FAO-24 Blaney–Criddle (24BC), FAO-24 Radiation (24Rd) and 1985 Hargreaves (Harg)) were evaluated for their capabilities to predict ET0 using daily weather data. Statistical analysis showed that the FAO-24 Penman and FAO-24 Blaney–Criddle methods may be instead of the FAO-56 Penman–Monteith method in predicting ET0 value in this region. Crop coefficients (Kc) for spring wheat and maize were calculated from the measured ETc data and the predicted ET0 data by the FAO-56 Penman–Monteith method, and their values were 0.55, 1.03, 1.19, and 0.65 during the initial, crop development, mid-season, and late-season stages for spring wheat, respectively, and the corresponding values for maize were 0.50, 1.02, 1.26 and 0.68.