| Manas River Basin is located in the arid area of Northwest China,where evaporation is strong and rainfall is rare.Drip irrigation under mulch is often used in farmland,and flood irrigation exists in a few areas.Most of the irrigation sources are groundwater or river water.Unreasonable development and utilization of water resources will lead to a series of environmental problems.In order to improve the field water use rate in Manas River Basin,it is necessary to analyze the migration law of local field water in the vadose zone.Vadose zone water is the connection between surface water and groundwater,and is the key water to support crop growth.The water movement law of vadose zone can be studied by the measured data,so as to establish the moisture migration model in accordance with the actual situation.Therefore,it is of great theoretical and practical significance to deeply understand the influence of irrigation amount on farmland water budget,and to formulate reasonable irrigation amount,which is of great theoretical and practical significance for improving farmland water use rate and rational development of water resources.Taking the cotton field in Manas Irrigation District of Xinjiang as an example,based on the measured meteorological data and soil moisture data of ground stations,the spatial and temporal variation characteristics of soil moisture elements were analyzed by means of mathematical statistics,spatial interpolation and model simulation,and the influence of irrigation amount on field water income and expenditure was discussed.The main conclusions are as follows:(1).According to the fluctuation state of soil moisture content at different depths,the soil layer is divided into active layer,sub active layer,transition layer and relatively stable layer.Under the influence of root system,the active layer was located at(0,30]cm,and the secondary active layer was at(30,50]cm.The root shallow buried area(0,10]cm is the active layer and(10,50]cm is the secondary active layer.Compared with the change characteristics of soil moisture content in growth period,the influence of plant water consumption,irrigation amount and irrigation method on soil water movement in active layer and sub active layer was the largest,followed by transition layer(50,100]cm,and the weakest effect was relatively stable layer(100,200]cm.The response of water content of transition layer and relative stable layer to irrigation has a certain lag,and with the increase of depth,the response time is longer.(2).By comparing and analyzing the water content,water potential,temperature gradient and hydraulic gradient under different irrigation modes,we can see that:(1)drip irrigation has a greater impact on water transport in the vadose zone than flooding irrigation.Under the drip irrigation mode,the total head of the whole profile is maintained at a high level for a long time,and the soil moisture distribution is uniform.(2)Root depth and aeration zone lithology have great influence on the location of heat flux surface,which is stable in the root dense area;flood irrigation mode has great influence on the distribution of soil temperature and moisture content,but the duration is short,and it is difficult to maintain the stable state of water and heat for a long time.(3)Under the same irrigation mode,when the root depth is different,the hydraulic gradient of the surface layer of the root shallow profile is greater than that of the root depth profile,which results in the leakage at the bottom of the shallow root profile larger than that in the root depth area;for the same profile of irrigation mode,the hydraulic gradient of the surface layer to the bottom layer under the flood irrigation mode is greater than that of the drip irrigation,resulting in the bottom leakage of the flood irrigation greater than that of the drip irrigation.(3).By comparing the supply water source,irrigation amount and irrigation mode,the following conclusions are obtained:1)when the groundwater irrigation amount is 0.2 m~3/m~2and TDS is 421.34mg/l,the change of soil salt concentration at 100-200cm will not be stimulated;when the river water with TDS of 1066.36mg/l is used as the irrigation source,the irrigation amount is 0.4 m~3/m~2.The results show that the soil salinity at 10cm and 30cm has a rapid response and the fluctuation amplitude is the largest when 0.4 m~3/m~2;the soil salinity response at 50cm has a certain lag,and the fluctuation amplitude is smaller than that of the upper layer;the longer the salt response time at 100cm-200cm with the increase of depth,the salt change is the smallest.In other words,when the TDS of water source is higher and the amount of irrigation is increased,the salinity of the whole section will increase;(2)if the water source and irrigation mode are the same,the salt leaching effect of drip irrigation is better than that of flood irrigation under the same irrigation amount of the same water source.(4).Through HYDRUS-1D calculation of the field water budget under different drip irrigation amount in the simulation period,it is concluded that the shallower the root depth is,the greater the bottom leakage is;when the root depth is fixed,the greater the leakage is with the increase of irrigation amount.Considering the root state in the growing period,the results showed that when the root depth was 55cm,the irrigation amount was0.12-0.15m~3/m~2,or the root depth was 35cm,and the irrigation amount was 0.08-0.12m~3/m~2,the field water use efficiency was the highest.At this time,the transpiration ratio,bottom leakage ratio and water retention ratio of cotton field were the best,and the field water holding capacity was reasonable.It can achieve the maximization of field water use efficiency and achieve the goal of saving irrigation. |
