Modification And Application Of SWAT Model In Canal And Well Combined Irrigation District

Irrigated areas play an important supporting role in China’s social and economic development and food security.The joint utilization of surface water and groundwater in the irrigation area has promoted the development of high quality,high yield and high efficiency agriculture,and is closely related to the virtuous cycle of ecological environment and the sustainable development of economy and society in the irrigation area.The water cycle and the evolution of water resources in the irrigated areas are influenced by the natural water cycle and the water cycle under the action of human activities,which involves many factors such as weather and climate,topography,river system,irrigation canal system,crop planting structure and irrigation system,forming a complex complex water cycle system of "natural and human activities".At present,with the intensification of climate change and the impact of human activities,the water cycle and water resources situation in irrigated areas have undergone great changes,resulting in the intensification of the contradiction between the supply and demand of some agricultural water resources and the issue of water security.Also part of the drainage hole by combining the imperfection of the irrigation area water resources management system and measures,irrigation area caused by surface water and groundwater joint use of lack of reasonable guidance and deployment mechanism,water ratio,surface water groundwater in irrigation water,low reliability,irrigation benefit attenuation,fill mining disturbance major common problems of groundwater,and then make the irrigation area of varying degrees of destruction of ecological environment,seriously restricting the sustainable development of social economy in the irrigation area,and threaten the food security of our country.The distributed hydrological model of irrigated areas is an important tool to solve the water problems in irrigated areas.Therefore,it is of great theoretical significance and academic value to further study the drainage well combined with the water cycle of irrigated areas and on this basis to improve the existing hydrological model of the river basin so as to form a distributed hydrological model of irrigated areas that is more in line with the actual water cycle of irrigated areas.The SWAT model is a widely used distributed watershed hydrological model.However,the simulation results of ditch-well combined irrigation areas need to be further improved,and the current application of SWAT model in irrigation areas has the following problems :(1)the model does not divide hydrologic response units(HRUs)according to crop planting structure,and cannot reflect specific irrigation systems and irrigation water sources of various crops;(2)lack of simulation of hydrological processes(such as canal leakage)in irrigated areas;(3)the SWAT model mainly simulated the surface water and soil water processes,but the groundwater simulation was not sufficient.Based on an in-depth analysis of the combined water cycle of natural and human activities in the irrigation area,this study carefully analyzed the shortcomings of SWAT in the irrigation area combined with canal and well.The SWAT model was improved in view of the key links in the water cycle of the irrigation area,and the following conclusions were obtained:(1)HRU division of irrigated areas.In order to reflect specific irrigation systems and irrigation water sources of various crops,this study reclassified HRUs according to the planting structure of crops in the irrigated areas.Firstly,the planting structure of crops in the irrigation area was determined.From 1988 to 2014,the planting structure of crops in the Jinghuiqu irrigation area gradually changed from field grain to grain and orchard cash crops,and the planting proportion of facility agriculture increased rapidly.Considering the planting structure,the specific process of HRUs division is as follows: the land use layer and soil layer are overlapped with the polygon of the crop planting structure,and a single slope class is used in the HRU definition of Arc SWAT operation platform to cover the three layers to generate a new HRUs diagram.(2)improve and perfect the canal system leakage module of SWAT model according to the canal system water balance equation.According to the canal water balance equation: output water = input water-irrigation water-channel leakage,the daily leakage of the canal system was calculated(m3/day).As the canal system daily leakage volume(m3/day)is calculated by multiplying the channel leakage rate by the channel length(km)and the average leakage depth(mm/day).Therefore,the average daily leakage depth(mm)of canal system can be obtained.In the hydrologic cycle,channel leakage loss needs to be added to the shallow aquifer recharge(Wrchrg,j),and finally channel leakage needs to be added to the shallow aquifer adjacent to HRUs of the channel.In the code implementation section,FORTRAN was used to modify the SWAT model groundwater module(gwmod)to add the canal system leakage depth(mm)to the deep leakage depth(mm)(water flows out from the bottom of HRU soil profile to form deep leakage).This occurs in all HRUs adjacent to the channel.(3)the groundwater simulation of SWAT model is not sufficient,and further coupling with the groundwater numerical simulation model MODFLOW is needed in the ditch-well combined irrigation area.In this study,based on swat-modflow,the improved SWAT was coupled with the MODFLOW,and the coupling simulation model of surface water and groundwater in the drainage well combined irrigated area was constructed,and the calibration of relevant parameters and model verification were carried out.(4)based on the analysis of the precipitation data of Jinghuiqu irrigation area from 1988 to 2010,three typical representative years were selected: 1998(abundant water year,532.7 mm),2002(plain water year,418.9 mm),and 2008(dry water year,353.6 mm).Based on the coupling model of surface water and groundwater,the simulation results of the improved SWAT model and the basic SWAT model are compared and analyzed.The simulation results show that the coupling simulation model of surface water and groundwater with the improved SWAT model can fully simulate the important link of canal system seepage to recharge groundwater in the irrigated area,and can better simulate the water cycle in the irrigated area with the combination of canal system and well.