Modeling Agricultural Water Productivity At Muti-Scales Based On Water And Salt Processes In Arid Area With Shallow Groundwater

Water shortage and soil secondary salinization are two main constraints of sustainable development of agriculture in arid region.Quantitative simulations of water and salt balance process and the response of crop production to soil water and salt condition are significant to the improvement of agricultural water productivity.Jiefangzha irrigation district in Inner Mongolia is selected as the study area of this study.In this study,field scale and regional scale agricultural water productivity models were developed in full consideration of soil water and salt processes and the response of crop production to this complex processes.Besides,the response laws of water consumption and irrigation water productivity at field scale to different irrigation scenarios and shallow-saline groundwater conditions were simulated.The influence rule of agricultural water saving to regional agricultural water productivity and the time-space distribution laws of regional scale water and salt processes and agricultural water productivity were simulated.Main study contents and results of the dissertation are as follows:(1)Based on a lumped soil-groundwater water balance model,the impacts of water-saving practices implemented at irrigation district on water balance and agricultural water productivity(RWP)were quantitatively simulated.Results indicated that water saving practices implementation and water diversion reduction resulted in the improvement of RWP at regional scale.The RWP and total water supply amount have a negative correlation between.Compared to the period of 1990-1999,RWP improved from 0.86kg/m3 to 0.96kg/m3 during the period of 2000-2009,which indicates that agricultural water saving practices can reduce the noneffective water loss caused by soil surface evaporation in the regional water cycle and moderately lower the groundwater table to improve RWP.Further study indicates that the groundwater contribution to regional evaportranspiration and agricultural water productivity are significant.(2)Fully considering the soil-groundwater water and salt processes and the different response of crop to water and salt condition in root zone at different growing stages,a field scale irrigation water productivity model coupled with water and salt balance and crop evapotranspiration response to water and salt condition was built.The model was calibrated and validated based on the two-year deficit irrigation field experiment data of sunflower.With the developed model,dynamic change in root growing was highlighted,and calibration results indicate that water and salt processes in crop root zone and the response of the field water consumption and yield to these processes can be characterized well.(3)On the basis of the developed field scale irrigation water productivity(IWP)model,field scale water consumption and IWP were simulated under different irrigation scenarios coupled with different irrigation water depths,groundwater table depths and groundwater salinities.Positive effect of groundwater evapotranspiration contributing to crop water use and negative effect of soil salinization accumulation were quantitatively simulated.The simulation results indicate that groundwater evapotranspiration can effectively contribute to crop evapotranspiration and make up for the lack of irrigation water and improve irrigation water proudictivity.On the other hand,the soluble salt along with the groundwater evapotranspiration to crop root zone will increase the salt accumulation which leads to a decrease in IWP.The negative effect of shallow groundwater on IWP is more obvious especially when groundwater salinity is even higher.Additionally,decreasing irrigation water depth may not result in equivalent yield reduction,and thus IWP will be improved due to the contribution of shallow groundwater to crop water use.(4)A regional distributed scale irrigation water productivity model considering water supply and water consumption and water drainage was developed coupling irrigation and drainage process,field scale water productivity process and groundwater runoff exchange.The built model was calibrated and validated with 8-year hydrologic monitoring data and regional evapotranspiration distribution data obtained from remote sensing inversion of Jiefangzha Irrigation District.The model calibration and validation results indicate that regional water and salt balance process,crop yield and regional irrigation water productivity can be simulated well and reasonablely.On this basis,spatial and temporal change rules of regional water and salt processes and agricultural water productivity were simulated and analyzed.Spatially,the groundwater table depth ranges from 0-6.32m with most of them less than 3m.The soil salinity in crop root zone is relatively high when groundwater table depth is less than 3m.With groundwater table depth increasing,groundwater evaportranspiration and soluble salt it carrying decreases obviously,thus soil salinity in crop root zone also decreases.There is a significant negative correlation between groundwater evaporatnspiration and groundwater table depth.Spatial distribution of crop production and evaportranspiration correspond with each other.Besides,among the irrigation water productivity of three main crops,maize is the highest and wheat is the lowest.