均质土微润灌湿润体模型构建及验证

Establishment and validation of wetting pattern model of moistube irrigation in homogeneous soil

为探明微润灌土壤湿润体运移的影响因素和变化规律,设置了56种微润灌情景(8种土壤质地、3种土壤基质势和10种微润管比流量的不同组合),利用HYDRUS-2D软件,模拟研究土壤饱和导水率、土壤基质势和微润管比流量对微润灌土壤湿润体的影响。综合考虑单位长度总渗水量、土壤饱和导水率、土壤基质势和微润管比流量等影响因素,基于量纲分析方法,建立了一种均质土微润灌湿润体尺寸估算模型,并利用数值模拟结果,定量获取了所建模型的待定参数,最后,采用试验资料评价了估算模型的可靠性。结果表明,土壤饱和导水率、土壤基质势和微润管比流量对微润灌土壤湿润体形状影响较小,且在湿润锋未到达地表前,湿润体形状均为以线源为轴线的近似"椭圆柱体";相同土壤饱和导水率条件下,土壤湿润体水平、垂直向下和垂直向上方向上的湿润锋运移速率均随土壤基质势和微润管比流量增大而增大;当土壤基质势和微润管比流量恒定时,随土壤饱和导水率数值的增大,垂直向上和水平方向的湿润锋运移速率逐渐减小,而垂直向下的湿润锋运移速率存在先减小后增大的现象;所建模型的统计指标平均绝对误差、均方根误差均趋近于0,纳什效率接近1,说明模型估算效果良好,可为微润灌工程的运行及管理提供科学依据。

Moistube irrigation is a kind of underground line source infiltration irrigation technology developed by using the principle of polymer semi permeable membrane.The growth and yield of crops are affected by the shape and size of soil wetting pattern but the wetting pattern is not easy to observe.Therefore,it is very important to understand the shape and size of wetting pattern for the design of economic and efficient moistube irrigation system.In order to facilitate users to quickly evaluate the moistube irrigation wetting pattern,a total of 56 moistube irrigation scenarios(different combinations of eight soil textures,three soil matrix potentials and ten specific discharges of moistube)were set up.By using HYDRUS-2 D software,the dynamic changes of wetting pattern under different combinations of soil saturated hydraulic conductivity,soil matrix potential and specific discharge of moistube were simulated and analyzed.The results showed that before the soil wetting front reached the surface,there was little difference in the shape of the soil wetting pattern,and its contour was almost"elliptical cylinder";after the wetting front reached the surface,the shape of the wetting pattern at the upper part of the moistube changed from"semi ellipse"to"trapezoid",and the shape of the wetting pattern at the lower part of the moistube kept"semi ellipse".In general,soil saturated hydraulic conductivity,soil matrix potential and specific discharge of moistube had little influence on the shape of the moistube irrigation soil wetting pattern;The size of moistube irrigation soil wetting pattern at the vertical downward was larger than that at the horizontal direction and vertical upward,and with the extension of irrigation time,the wetting pattern expanded outward gradually,but the expansion rate decreased gradually.Under the condition of homogeneous soil,the contour of moistube irrigation wetting pattern was divided into two parts:the upper part and the lower part.When the upper part of the wetting boundary did not reach the surface,it could be expressed by semi elliptic equation.Under the same soil matrix potential and specific discharge of moistube conditions,with the increase of soil saturated hydraulic conductivity,at the same time,the vertical upward and horizontal migration distance of the wet front gradually decreased,while the vertical downward migration distance of the wet front first decreased and then increased;Under the same soil saturated hydraulic conductivity and specific discharge of moistube conditions,there was a positive correlation between the migration distance of the wetting front and soil matrix potential in three directions.The higher the soil matrix potential value resulted in the greater the migration distance of the wetting pattern at the same time;Under the same soil saturated hydraulic conductivity and soil matrix potential conditions,the migration distance of wetting pattern was positively correlated with specific discharge of moistube.The larger the specific discharge of moistube value could result in the faster the migration speed of wetting pattern and the larger the size of wetting pattern at the same time.Generally speaking,soil saturated hydraulic conductivity,soil matrix potential and specific discharge of moistube had significant influence on the migration of wetting pattern in moistube irrigation.On this basis,taking into the influence factors such as the total water seepage per unit length,soil saturated hydraulic conductivity,soil matrix potential and specific discharge of moistube,and based on the dimensional analysis method,the estimation model for the wetting pattern of moistube irrigation of homogeneous soil was established,and the undetermined parameters of the model were obtained quantitatively by using the numerical simulation results.Finally,the reliability of the estimation model was evaluated by the test data.The mean absolute error was not more than 2.10 cm,the root mean square error was not more than 2.46 cm,and the Nash efficiency coefficient was not less than 0.83 of statistical index of the model.It showed that the measured value was consistent with the estimated value of the model,and the model estimation effect was well.It provides a scientific basis for the operation and management of moistube irrigation engineering.