河套灌区玉米和向日葵ET的S-I估算模型关键参数分析

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

农田作物蒸散的快速评估对于灌区水资源最优调配和灌溉实时管理至关重要。简化S-I模型综合考虑作物冠层温度、田间气象参数和作物特征参数,可以进行实时农田作物ET的精确估算。该文利用河套灌区解放闸灌域2015—2016年2 a田间试验观测资料,对主要农作物玉米和向日葵的S-I模型中2个特征参数分别进行了率定和验证,并分析了模型蒸散估算的相关影响因素。结果表明:1)利用S-I简化模型可以对玉米和向日葵田间进行作物日蒸散量(daily evapotranspiration,ET_d)的估算,在该地区以13:00时率定和验证结果最优。在玉米主要生育期(6—8月),利用S-I模型估算ET_d可以达到较高的精度;而在7—8月,采用模型估算向日葵地ET_d也可以达到很高的精度;2)S-I模型中特征参数值受风速、地表覆盖度、表面粗糙度等因素的影响,不同作物其值不同。13:00时玉米S-I模型中特征参数值皆为负值,而向日葵中特征参数值为1正1负,进而影响模型估算精度。叶面积指数变化对特征参数值大小的影响在玉米和向日葵田块呈相反的趋势,而风速的影响则为一致。推荐13:00时率定的参数值可以在河套灌区玉米和向日葵作物需水量估算时直接应用。

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.