土壤初始含水率对膜孔灌湿润体特征的影响

为了研究膜孔灌中土壤初始含水率对湿润体特征及累积入渗量的影响,首先通过室内试验验证HYDRUS模拟西安粉壤土膜孔灌湿润体形状以及含水率分布的可靠性,然后基于HYDRUS模型模拟在不同初始含水率条件下膜孔灌湿润体的变化过程。结果表明:基于HYDRUS模型模拟的累积入渗量和湿润锋运移距离与室内试验结果的R~2均接近1,标准偏差绝对值均小于10%,拟合良好,表明HYDRUS模型模拟入渗过程的可靠性。膜孔灌湿润锋形状可采用椭圆方程表示。当初始含水率较小(不大于0.1 cm~3/cm~3)时,湿润体半径的含水率分布可采用椭圆方程表示;从膜孔中心到湿润锋表面,随着初始含水率的增大,湿润体内的含水率梯度减小,湿润体半径的含水率分布曲线由椭圆曲线逐渐转变为平缓曲线。基于湿润体含水率分布规律建立了考虑初始含水率的累积入渗量模型,累积入渗量与湿润体半径的三次方呈正比,湿润体半径可表示为湿润锋水平运移距离和垂向运移距离的几何平均值;对于不同的膜孔半径(1～5cm),模型计算累积入渗量与HYDRUS模拟值的R~2为0.99,标准偏差绝对值小于10%;对于粉土、粉壤土和壤土,当初始体积含水率不大于0.25 cm~3/cm~3算累积入渗量与HYDRUS模拟值的R~2为0.99,标准偏差绝对值小于10%,结果表明该模型对不同土壤质地和膜孔半径的适用性良好;该模型在计算作物灌水需求量方面优于Kostiacov模型等传统的经验模型。该研究揭示了不同初始含水率下的膜孔灌湿润体特征,并建立了累积入渗量模型,可为膜孔灌灌溉水量的计算提供参考依据。

Effect of initial soil water content on wetting body characteristics of film hole irrigation

Abstract: The objective of this study was to explore the effect of different initial soil water contents on the characteristics of the wetting body and to establish the quantitative model of ?lm hole irrigation between cumulative in?ltration and the shape of the wetting body. Laboratory experiments and HYDRUS numerical simulation were carried out. The reliability of the simulation results using HYDRUS software in film hole irrigation was verified by laboratory experiment, and the results showed that the R2 of HYDRUS simulation and laboratory experiment results (wetting front migration distance and cumulative infiltration amount) were both close to 1, and the absolute value of percentage of bias was less than 10%, indicating a good fitness. Based on this, the dynamic characteristics of Xi''an silt loam in film hole irrigation under different initial soil water content were simulated. The experimental results showed that the shape of the wetting body formed by film hole irrigation was approximately half of a rotating ellipsoid, and the shape of wetting front curve could be expressed using an elliptic curve equation. The horizontal and vertical migration distances of the wetting front were semi-major and semi-minor axis of the ellipse, respectively. When the initial soil water content was as low as around 0.1 cm3/cm3, the soil water content on the wetting body radius from the center of the film hole to the surface of the wetting front decreased gradually and the change rate of soil water content increased gradually, reaching the maximum near the wetting front. Moreover, the distribution of soil water content on the wetting body radius could be accurately expressed by using an elliptic curve equation. As the initial soil water content of the wetting body increased gradually, the water content gradient in the wetting body decreased, the water distribution became more uniform and the water content distribution curve of the wetting body radius gradually changed from an elliptic curve to a gentle curve close to a straight line. According to the distribution law of soil water content in the wetting body, the model of cumulative infiltration considering initial soil water content was established. The model showed that the cumulative in?ltration of ?lm hole irrigation was proportional to the third power of the equivalent radius of the wetting body, and the wetting body radius was equal to the geometric mean of the horizontal and vertical migration distances of the wetting body. In order to verify the accuracy and applicable scope of this model, the HYDRUS simulation results of three typical soils (silt, silt loam and loam) and five different film hole radius (1, 2 , 3, 4 and 5 cm) were used to verify the model. For the silt, silt loam and the loam soils, when the initial water content was not higher than 0.25, the R2 of calculated cumulative infiltration and simulated value using HYDRUS were both close to 1, and the absolute value of percentage of bias was less than 10%, which showed that the model had good applicability in silt, silt loam and the loam when the initial water content was not higher than 0.25. In addition, for the different film hole radii, the R2 of calculated cumulative infiltration and simulated value using HYDRUS model were close to 1, and the absolute value of percentage of bias was less than 10%, which suggested that the model had good applicability to the conditions of different film hole radii. Since this model reflected the relationship between the cumulative infiltration and the shape of the wetting body, for crops, this model was superior to the traditional empirical models such as the Kostiacov model in calculating the irrigation demand of crops to a certain extent. This study revealed the characteristics of the wetting body of film hole irrigation under different initial water contents and established a model for calculating cumulative infiltration, which provided valuable information for the precise irrigation of film hole irrigation.