| Enhancing stable development of agricultural production has great importance for economic prosperity of China. One of the major steps is to improve production of the low and medium yielding lands, which covers over 65% of total farmland in China. About half of the low-yield farmland is affected by water logging and salinity hazards, these lands can be initially improved through farmland drainage.Because drainage discharge contains salt and agrichemicals leached through soils, it has been recognized as the primary contributor of agricultural non-point source pollution. In order to reduce or avoid negative impact of drainage on environment while maintaining agricultural production, rational design and management of agricultural drainage system are critical issues that need to be tackled immediately.Operation and management of agricultural drainage system involve multiple factors that range from weather, soil and crops, and existing hydrologic conditions. Drainage research is hard to accomplish through field studies only. Taking advantages of fast computers, field hydrology models have been developed to predict drainage system performance under varying climatic and soil conditions by combining computation theories of field hydrological components. The model simulations results may provide guidance for relevant management sectors to work out corresponding water management measures.Based on the drainage research conducted internationally, considering current status of drainage studies in China, this dissertation presents a case study in the vertisol soil district of Huaibei Plain, China using the widely applied field hydrology model-DRAINMOD. Model testing was based on the experimental results from the national "Ninth Five-Year Plan" scientific and technological project "Agricultural meteorological disaster prevention technology research" in "agricultural water logging disaster prevention technology research". DRAINMOD was used to analyze improved drainage system design for better crop production; the model was also used to analyze drainage water management that will both enhance crop production and environmental protection. The modeling results may provide theoretical basis from sustainable development of agricultural production of the study area. Major findings of the research are as below:(1) According to limited experimental data and model simulation principle, by analyzing model testing using continuous sequence of experimental data, considering representative data collected during rainfall events, a new model calibration method was proposed using discrete data points and the procedure was successfully applied in model calibration of the study area;(2) Based on the calculation methods for crop evapotranspiration (AET) used in DRAINMOD, the potential evapotranspiration inputs were modified from reference crop (PETo) to actual crop (PETcrop). Simulation results showed that changes in PET inputs are sensitive to drainage and yield predictions for dry season crop, the improved PET inputs generated more realistic predictions and the results are more scientifically reliable.(3) Based on the calibration and improvement of the model, the optimum subsurface drainage system design was derived by long term simulation with DRAINMOD considering the multiple objectives such as profit maximization, environment protection, land economization and sustainable development etc. Optimized drainage system design schemes for the study area were presented for local water management reference;(4) For achieving the multiple objectives of eliminating drought and over drainage, saving water resources, decreasing the stress of downstream flooding and water quality degradation etc, the combination of controlled drainage at the main ditch and irrigation scheme at a frequent and small amount mode were found to be the optimum water management practices through model simulations. These practices can significantly increase crop yields and mitigate negative impact of drainage;(5) Referring to local fertilization practices of the study area, DRAINMOD NII was used to simulate nitrogen losses through subsurface drainage in the Huaibei Plain. The results showed that 90% of total nitrogen losses are the nitrate form lost with the subsurface drainage, controlled drainage at the main ditch can reduce total nitrogen losses by 42% and nitrate losses with subsurface drainage by 43%, indicating that controlled drainage has significant importance for eco-environmental protection in the Huaibei Plain. |
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