| The scarcity of water resources for agriculture restricts the improvement of agricultural production in arid and semi-arid areas of China,which to a certain extent constrains the economic growth of the region.Water-saving irrigation is one of the key measures to solve this problem.Drip irrigation system is simple,efficient and inexpensive,and it is a common way to save water for irrigation,which is widely used at home and abroad.The study of soil water transport patterns and related models under drip irrigation mode is of great significance for the proper design of drip irrigation systems to improve water use efficiency in the field.This paper uses a combination of numerical simulation and indoor experiments to investigate the water infiltration process of soil under point source drip irrigation mode.The main task can be expressed as the study of the saturation zone radius law,wetting front migration,soil profile water content distribution,and water redistribution characteristics.Firstly,the dominant factors are screened,then the relevant empirical model is constructed,and finally,the reliability of the model is verified by using indoor experiments and the measured data from the literature.And the main research results are as follows.(1)The saturation zone radius is important for the study of soil water infiltration in point source drip irrigation systems.This paper first verifies the realism of HYDRUS-2D simulated saturation zone radius using indoor measured saturation radius results.Based on this,the saturation zone radius lapse process is numerically simulated for a variety of soils.Analysis of the data pattern shows that there is a good power function relationship between it and soil texture,initial soil moisture content,and drip head flow rate.An empirical model of the radius transport distance in the saturation zone was constructed using a nonlinear regression analysis.The results of the error comparison between the measured and simulated values of this radius show that the root mean square error(RMSE)is 1.950 cm,the Nash efficiency coefficient(NSE)is 0.915 and the Relative root-mean-square error(RRMSE)is 0.2,indicating that the prediction model has reliability.Furthermore,the feasibility of setting the calculated value of the model as the initial boundary of HYDRUS software to simulate the water movement of point source drip irrigation was verified by using the indoor measured data of the wetting front in the process of irrigation,the water content of soil profile at the end of irrigation,and the wetting front and water content after redistribution.(2)The infiltration boundary and initial conditions of drip irrigation system are set reasonably to simulate the migration process of soil wetting front,and the movement law under various influencing factors is qualitatively analyzed.An important factor of initial matrix potential is added to the existing dimensional representation empirical model,and a new dimensional empirical model is established.Finally,the performance of the newly constructed dimensional empirical model is evaluated by using the measured data in published literature and laboratory data.The results show that the RMSE of the horizontal wetting radius of the improved dimensional model in this paper is 1.988cm,the NSE is 0.917 and RRMSE=0.1,the RMSE of the vertical wetting depth is 3.452cm,the NSE is 0.877 and RRMSE=0.16.The performance comparison with the Malek and Peters nonlinear empirical model,the Schwartzman and Zur dimensional empirical model and Naglic dimensional empirical model shows that the proposed dimensional model has the best performance.The new dimensional empirical model is simple,and only the initial soil matric potential and saturated hydraulic conductivity can be used to predict the development of soil wetting pattern under drip irrigation.Secondly,the model can accurately predict the horizontal wetting radius,which can provide theoretical guidance for the study of the wetting depth based on the ground visualization radius.(3)The total amount of irrigation(24L)was fixed and the distribution pattern of water content of the soil profile at the end moment of irrigation was analyzed.The results show that large dripper,fine soil texture,and higher initial soil water content all lead to the expansion of the high water content layer within the wetted body;small drip head flow rate,coarse texture,higher initial moisture content resulting in a sparser moisture content contour plot.The water content in the horizontal and vertical downward directions shows an inversely proportional relationship with the wetting distance.The fitting results showed that the relationship between soil water content and wetting front transport distance in both directions conformed to both linear and quadratic functions,but the quadratic function had a higher fitting accuracy(R~2≥0.936),therefore,on the basis of the practical analysis,prediction models describing the linear and quadratic functions of soil water content in both directions at the end of irrigation were developed respectively.The model was evaluated by using the measured data of published literature and laboratory test data.The horizontal error of the linear function model RMSE=0.001cm~3/cm~3,NSE=0.258,RRMSE=0.0043.The vertical error index is RMSE=0.079cm~3/cm~3,NSE=0.147,RRMSE=0.231;the horizontal and vertical error indexes RMSE of the quadratic function model were 0.001 cm~3/cm~3 and 0.047cm~3/cm~3,NSE values were 0.718 and 0.698,RRMSE values were 0.0036 and 0.137,respectively,indicating that the two forms of function models have good reliability.(4)The change process of wetting front redistribution after the end of irrigation was further investigated on the basis of numerical simulation.The regularity analysis shows that the redistribution process of the eight soils used in this simulation is basically finished within 48 hours,and the wetting front tends to stabilize.The change in horizontal wetting distance is obvious at the beginning of redistribution and gradually decreases with time,while the vertical direction shows the opposite trend,and with the gradual coarsening of soil texture,the earlier the end of the horizontal redistribution process.There is a good power function relationship between the wetting front after redistribution and the wetting front at the end moment of irrigation,redistribution time,soil texture,and initial soil water content.Based on this,a redistributed wetting front prediction model(R~2=0.94)with the above factors was established.The results of the error analysis of the measured and calculated values showed that In the horizontal direction,RMSE=1.774cm,NSE=0.528,RRMSE=0.072;and in vertical direction,the RMSE=2.617cm,NSE=0.829,RRMSE=0.097.(5)Based on the simulation results,the distribution pattern of redistributed water content with redistribution time,initial soil water content,soil texture,and the pre-drip flow rate was plotted,and the changing trend was analyzed qualitatively.The analysis of the law shows that:With the extension of the redistribution time,the initial moisture content of the soil increases the wetting range gradually becomes larger,the moisture content within the wetting body is gradually distributed evenly,the more sparse the contour map.The soil texture increases from fine to coarse,the uniformity of water content gradually increases under the same redistribution time,and the difference of water content contours decreases.The degree of influence on redistribution water content can be ranked as soil texture>redistribution time>initial soil water content>pre-dropper. |
