Key parameter analysis of ET estimating for maize and sunflower in Hetao Irrigation District based on S-I model

Abstract: It is very important to estimate field crop evapotranspiration rapidly when water resources distribution and irrigation management would be implemented well-off in irrigation district. The simplified S-I model takes into consideration of crop canopy temperature and field micro-meteorology factors and it has been widely used for evapotranspiration estimation. In this study, 2 key crop parameters in S-I model were calibrated for the use of Hetao Irrigation District situated along the Yellow River in Inner Mongolia, the second-largest irrigation project in China. Two-year field experiments were carried out in Jiefangzha irrigation area of Hetao Irrigation District in 2015-2016 in maize and sunflowers fields (the major crop in Hetao Irrigation District). Measurements and system tests were conducted in an experimental field located at the Shahaoqu Experimental Station in the middle of the Jiefangzha Irrigation Area (40(55(8(N, 107(8(16(E, 1 036 m elevation). Continuous field measurements were taken from mid-May to September in 2015 and 2016 when the maize and sunflowers were in their primary growing periods. The experimental sites were equipped with a Canopy Temperature and Meteorology Monitoring System (CTMS) mounted on a stainless steel stand column. The slope and intercept of the S-I model related the difference of canopy temperature and air temperature and the difference of daily evapotranspiration (ETd) and solar net radiation represented two characteristic parameters of model. The parameters were calibrated based on the 2 year experimental results and then validated by using the other year results, respectively. And the related influencing factors of the parameters were also analyzed. The results showed that: 1) The S-I model could be utilized to estimate ETd in maize and sunflower field, and the best calibration and validation was at 13:00 in Hetao Irrigation District based on 2-year results. High accuracy of the S-I model occurred in crop vigorous growth period, for maize in June, July and August, for sunflower in July and August, respectively. 2) The characteristic parameters of S-I model were impacted by wind speed, surface coverage, and surface roughness, but the impacts varied for different crops. At the best performance time of daily 13:00, the characteristic parameters of S-I model were both negative for maize field, but one was positive and the other was negative for sunflower. These could affect the estimating values and its accuracy. Leaf area index (LAI) impacted the model slope of maize in a different way with that on sunflower, but wind speed had the similar influence on these 2 crops. 3) The calibrated values of characteristic parameters of S-I model could be utilized to estimate crop daily evapotranspiration in maize and sunflower fields in Hetao Irrigation District. The estimation based on data at 13:00 was the best for both sunflower and maize. The recommended values of intercept and slope of the S-I model were (0.251 and (0.209 for maize, and 0.655 and (0.358 for sunflower, respectively. These parameters values could be used as a recommendation to ETd estimation in the other areas when the experimental data were lack. However, if the high accuracy of evapotranspiration estimation are required, we suggested to calibrate the parameters based on local field experiments and measurements.