Effect Of Irrigation On Coupled Water,Vapor,and Heat Transport In Vadose Zone

In arid and semi-arid areas of Northwest China,water shortage and unreasonable irrigation not only lead to a large number of water resources leakage loss at the bottom,but also a large number of nitrogen and phosphorus elements necessary for plant growth leak into groundwater through the bottom,which seriously threatens the groundwater environment.Shallow soil water content in Vadose zone is generally low.Due to the large daily temperature difference and the influence of soil temperature gradient,the conversion between liquid water and vapor occurs frequently within the soil,which affects the water transport process.Therefore,it is important to consider the coupled water,vapor and heat transport for developing water-saving irrigation agriculture and protecting groundwater ecological environment.Within this study,farmland in Jinghuiqu Irrigation Distrie was selected as the study area to carry out positioning experiments.Based on the data of soil water content and temperature obtained from in-situ test of Vadose zone,meteorological observation data and soil physical parameters obtained from indoor experiment,a numerical model of coupled liquid water,vapor and heat transport in Vadose zone was established by using Hydrus-1d,and the rule of liquid water and vapor transport under irrigation conditions was analyzed on the basis of the model.The main results are as follows:?1?The soil water content changes most dramatically at the top of the soil?above 100cm?,and with the increase of depth,and the variation gradually decreases.The annual and diurnal variations of soil temperature show two main characteristics:?1?The soil temperature of each layer shows a cyclical change in the annual or daily variation.The inter-annual variation of soil temperature decreases with the increase of depth,and the surface temperature changes most dramatic.For diurnal temperature variation,the soil temperature of 0-20 cm shows obvious diurnal variation,while the soil temperature below 20cm is not obvious.?2?The change of soil temperature has a lag phenomenon,and with the increase of depth,the lag time is longer.?2?Established the numerical model of coupled liquid water,water vapor and heat transport and the numerical model of water transport in vadose zone,those model was calibrated and validated by the measured soil water content and soil temperature.The fitting effect of the model was evaluated by three fitting effect indicators:R²?determinant coefficient?,RMSE?root mean square error?and MAE?absolute error?.Results show that,for coupled water,vapor,and heat transport models,in calibration and validation periods,the determinant coefficients of soil water content were 0.9 and 0.818,respectively,and the variation ranges of root mean square error and absolute error were 0.005⁰.033cm^-3·cm^-3,0.004⁰.024 cm^-3·cm^-3,respectively.The determinant coefficients of soil temperature were0.95 and 0.813,respectively,and the variation ranges of root mean square error and absolute error were 0².032cm³·cm^-3 and 0¹.663 cm³·cm^-3,respectively.For water transport models without considering water vapor and heat,the determination coefficients?R²?of calibration and verification periods are 0.903 and 0.83 respectively,and the variation ranges of root mean square error?RMSE?and mean error?MAE?are 0.0033⁰.0319 cm³·cm^-3 and0.0026⁰.0184 cm³·cm^-3,respectively.It is considered that the fitting effect of these two models is better,and the trend of the simulated and measured values changing with time are close to each other.Nearly,it can basically reflect the physical properties of soil and the characteristics of water transport in Vadose zone.?3?Through sensitivity analysis,it is found that for hydraulic models,the hydraulic parameters that have the greatest influence on the simulation effect are?_s and n.When?_s decreases by-20%,the root mean square error value at 250cm changes by 797.90%;when n increases by 20%,the root mean square error value at 600cm changes by 654.06%;when?_r,?and K_S change by±10%and±20%respectively,the root mean square error does not change significantly.Empirical parameters for determining thermal conductivity,when b₁,b₂,and b₃ are changed by±10%and±20%,respectively,the root mean square error does not change greatly,indicating that the thermodynamic model is not sensitive to empirical parameters for determining thermal conductivity.The change of initial soil water content has a great influence on the simulation of soil water content by Hydrus-1d model,but the most important influence on the simulation temperature of thermodynamic model is the initial soil temperature and air temperature.The net radiation and relative humidity have very little influence on the simulation soil temperature.Increasing or decreasing 10%or 20%can not cause a great change of root mean square error.?4?The moisture transport models with and the numerical model of water transport invadose zone were established.By comparing the two models,it was found that the water transport model neglected the effects of water vapor transport and temperature on water transport,resulting in a relatively small calculated water flux.In the study of water-vapor-heat coupled transport,the effects of Matrix potential and soil temperature on liquid water before and after irrigation during summer maize planting period were studied.The results show that the water vapor transport in soil under temperature gradient is dominant.The maximum water vapor transport can reach-0.0042cm/d by simulation calculation,and the water vapor volume under matrix potential gradient is relatively small,which can be neglected.For the balance of water budget,the evapotranspiration of summer maize and winter wheat during planting period calculated by the two models is close,but the bottom leakage of the two models is quite different.