Study On The Law Of Water And Salt Transport In Unsaturated Zone Under Irrigation Condition In Peacock River Basin

Peacock River Basin in Tarim Basin is located in the arid region of Northwest China,where evaporation and intense rainfall are rare.Two crops,bergamot pear and cotton,are mainly planted in this area.Drip irrigation and flood irrigation under film are primary irrigation methods,as well as groundwater and river water are mostly used.The area is typical of "no irrigation,no agriculture".The desire for irrigation water is increasing day by day,with the rapid expansion of agricultural scale in Xinjiang,furthermore,the study area is gradually transformed from river water to groundwater.However,the unreasonable exploitation and utilization of groundwater resources will lead to the decline of groundwater level,soil salinization and other environmental geological problems.Vadose zone is the link between surface water and groundwater,and it is of great significance to crop growth.It is crucial to study the mechanism of water and salt movement in vadose zone under irrigation conditions,to establish water and salt movement model in vadose zone,so as to guide the rational development and utilization of water resources in the area.This article selected the typical farmland in Peacock River Basin of Tarim to establish an in-situ test field.Based on the measured data of water content,electrical conductivity and temperature from meteorological station,as well as the soil water characteristic parameters and solute transport parameters obtained from indoor experiments,the model,water and salt transport,of vadose zone was established using hydraus-2d,the field water and salt transport under different irrigation modes was analyzed.The main conclusions are as follows:1.Soil moisture and salt had different responses to irrigation water at different depths of the aeration zone.The deeper the burial depth was,the less the influence of irrigation on water and salt was.With the increase of soil depth,the hysteresis of soil moisture content and electrical conductivity was more significant.The shallow layer(10-50cm)was the main salt accumulation layer due to the influence of evaporation and water absorption by plant roots.The deep part(below 150cm)had no distinct response to irrigation conditions.2.Comparing the effects of different irrigation methods and irrigation quotas on the aerated zone,it was concluded that drip irrigation had a small influence on the water content,salinity and temperature of the vadose zone,but the moisture content,electrical conductivity and temperature gradient of the vadose zone soil could remain stable for a long time after drip irrigation.Nevertheless,compared with drip irrigation,the duration of flood irrigation was shorter,and the moisture content,conductivity and temperature could not keep a stable state for a long time.Drip irrigation was suitable for crops with shallow roots,such as cotton,while flood irrigation was suitable for crops with deep roots,such as pear.3.Through the comparison of different water sources,irrigation quota and irrigation methods on salt migration in the vadose zone,it was concluded that the salt content of irrigation water had the greatest influence on salt accumulation in the vadose zone.When using brackish water to irrigate,both irrigation methods would lead to the increase of salinity in the whole profile,resulting in the accumulation of salt in the surface soil.If fresh water irrigation was adopted,the salinity in the soil above 50 cm from the surface would decrease,and the salt would migrate to the deep of the vadose zone,which played a certain effect of leaching salt.4.The cotton and pear test fields were simulated by HYDRUS-2D model,and the measured values were used to calibrate and verify the reliability of the model.According to the growth characteristics of cotton and irrigation system in the study area,five scenarios were set up,including all-fresh water irrigation,total water irrigation,total brackish water irrigation,saline brackish water-fresh water irrigation,and alternate brackish water-fresh water irrigation,and the following conclusions were drawn through simulation and prediction :(1)Under brackish water irrigation and alternate brackish water-fresh water irrigation,soil salt content in the main root layer of cotton would exceed the salt tolerance threshold,which would affect the growth of cotton.(2)All-fresh water irrigation will not lead in excess salt to the vadose zone.(3)The salt content in the main root layer(0-40cm)of cotton was accumulated by brackish water irrigation,but the short-term irrigation could be controlled within the salt tolerance threshold of cotton,and the crop growth would not be affected.(4)Under the alternate brackish water and fresh water irrigation,fresh water would wash the salt accumulated by brackish water to the deep of vadose zone,the conductivity of cotton main root layer decreased.This irrigation method could keep the salt content in the vadose zone under the salt tolerance threshold of cotton for a long time,which could make rational use of brackish water resources without affecting the growth of cotton.