Simulation Of Irrigated Hydrological Processes And Assessment Of Water Productivity In Inner Mongolia Hetao Irrigation District

Drought and soil salinization are the main factors affecting the sustainable development of agriculture in the arid irrigated area of China. Reasonable irrigation and drainage management is helpful to control the soil water and salt content within the suitable condition for crop growth. The development of mathematical model, which can simulate the soil water-salt dynamics and crop growth process, is useful to make water resources managements of water saving, high efficiency and ecological security. In this paper, a mathematical model was established to simulate soil water-salt dynamic and crop growth process on the field scale. Based on the geographic information system (GIS) and remote sensing (RS) technology, a distributed agro-hydrological model was developed to simulate soil water flow, solute transport and crop growth process on the regional scale. The Jiefangzha Irrigation System (JIS) in Hetao Irrigation District in Inner Mongolia was selected as the study area. With the data collected by field experiments and statistical records, the field and regional scale models were calibrated and validated. Then the calibrated models were used to simulate the agro-hydrological and crop growth processes in the study area. And the impacts of water-saving managements on agro-hydrological processes and crop growth were assessed by these two models. The main research contents and conclusions are as follows:(1) A field experiment was conducted in2012and2013. Fourty observation points were selected according to spatial distribution of soil texture, crop species, and groundwater condition in JIS. Soil moisture, salinity, crop height, leaf area index, crop yields of these points were monitored each month. Combined with long series data of irrigation, groundwater depth and crop yield of JIS, the impacts of water-saving measures on soil water moisture, salt and crop yields were analyzed. Results showed that soil water moisture, salinity and crop yields basically unchanged since the implementation of water-saving measures. Currently, JIS is facing the quick declining Groundwater levels in the well irrigaton areas and new shallow grounder water depth areas in the canal irrigation areas.(2) According to the characteristics of shallow groundwater depth in JIS, HYDRUS-1D model was modified by coupling with the EPIC (Erosion-Productivity Impact Calculator) crop growth module for simulating agro-hydrological processes. The evaporation module of HYDRUS-1D was modified by the dual crop coefficient method. With the dataset of soil moisture, soil solute concentration, plant height, leaf area index (LAI) and crop yield collected from wheat, maize and sunflower fields in2012and2013, the model was calibrated and validated. Results showed that, the new model can simulate water flow, salt transport and crop growth process in the aid irrigated area with shallow groundwater depth.(3) Based on GIS technology, the spatial variability of weather, soil, irrigation, planting structure and groundwater conditions of the study area was considered, and the field scale model was expanded to the regional scale in a distributed manner. The accuracy of the distributed simulation was evaluated by comparing the simulated soil water content and salinity, LAI and crop yields with measured data of40monitoring points. Then the model was applied to assess soil moisture, soil solute concentration, irrigation efficiency and crop water productivity of the study area in present irrigation water management practices. Results showed that soil water could meet crop water consumption needs in most areas.In the areas where groundwater depth (GWD) was less than1.0m, root water uptake was limited due to waterlogging. High soil salinity concentration limited crop production seriously. The average irrigation efficiency was about0.7, and water productivity of wheat, corn and sunflower was1.19,2.17and0.59kg m-3. A high percentage of soil evaporation into crop evapotranspiration (40%for the whole area) was the main factor for low water productivity in JIS.(4) The field scale model was used to assess the impacts of groundwater table and irrigation changes on soil water-salt dynamics and crop yields in the field with typical soil texture, salization condition and crop species. Results show that the groundwater depth of1.5m was suitable for wheat, maize and sunflowers. And wheat was over-irrigated in present irrigation schedule. Regional water conservation scenario analysis, which were predicted by the regional scale model, showed that the irrigation amount of the study area can be reduced by28%and17%respectively without negative effects on crop yields, when the groundwater depth were controlled at1.5m and2.5m.(5) Crop yields under historical (1960-2013) and future climate scenarios was simulated by the field scale model. Results show that potential yields of wheat, maize and sunflower without water or salt stress showed a rising trend in the past decades. And there was no significant effect on crop yield and water productivity when rainfall increased by20%. Crop yields and water productivity was positively correlated with solar radiation. When temperature increased by2℃, water productivity of wheat and maize were decreased by6.1%and8.2%, while water productivity of sunflower increased by3.4%.