Area Ratio Of Canal To Well Irrigation Areas For Combined Use Of Groundwater And Surface Water In Hetao Irrigation District

Hetao Irrigation District is located in arid and semi-arid climate region,where agricultural development is highly dependent on irrigation.As the water supply and demand conflicts in the Yellow River Basin are becoming more and more intensified,conjunctive use of groundwater and surface water has become an effective measure to cope with the policy pressure to reduce the amount of water drawn from the Yellow River and to ease the contradiction in water use.Well-canal combined irrigation has the dual role of increasing water supply and reducing water waste,which can improve the utilization rate of irrigation water resources,reduce the inefficient evaporation of groundwater,and have a significant effect in promoting efficient water use in agriculture and water salt regulation in farmland.However,improper implementation will lead to over-exploitation of groundwater,resulting in serious damage to ecological environment and crop yield.In order to guarantee the water balance between groundwater recharge and withdrawal,the critical point lies in the determination of reasonable area ratio of well to canal irrigation area.In this study,two analytical models are developed based on the principle of groundwater balance to calculate reasonable area ratio in Hetao Irrigation District in Inner Mongolia.The research results of this paper can provide reference for the implementation of well-canal combined irrigation in Hetao Irrigation District.The main research contents and conclusions are as follows:(1)An analytical model and a simplified model were established based on the principle of groundwater balance,and area ratio of canal to well irrigation area under 108 different scenarios were obtained,which were different in groundwater salinity standards,the frequency of autumn irrigation,channel water supply scenarios and irrigation plans.The two recommended schemes are as follows:the standard of groundwater salinity in well-canal combined area is 2.5 g/L;the frequency of autumn irrigation in well irrigation areas is once in every two years;the canal irrigation area use the water drawed from the Yellow River in both growth period and autumn irrigation period while the well irrigation area use the groundwater water in growth period and use the water drawed from the Yellow River in autumn irrigation period;when calculating the channel water supply,the recommended scheme 1 correspond to ignore the general main canal and main canal water supply,while the recommended scheme 2 correspond to ignore the general main canal,main canal,and sub-main canal water supply.The area ratio of Hetao Irrigation District determined by the recommended schemes ranges from 1.9 to 2.9.According to the principle of ensuring ecological sustainable development of irrigation areas and reducing the risk of over-exploitation of groundwater,it is recommended that the area ratio in Hetao Irrigation District should be 3.0.(2)The influence of the main parameters on the calculation results of the area ratio and the sensitivity of the parameters were analysed,which can deepen the understanding of the reliability of the calculation results.Coefficient of groundwater extraction,groundwater recharge coefficient of field irrigation,groundwater recharge coefficient of canal system,and groundwater recharge coefficient of rainfall are positively correlated with the area ratio,while coefficient of land use is negatively correlated with the area ratio.The sensitivity of coefficient of groundwater extraction is the highest.In order to reduce the risk of over-exploitation of groundwater,the selection of this value should be conservative.(3)Furthermore,in order to test the applicability of the recommended area ratio and investigate the groundwater dynamics under various ratios,a numerical model based on Visual MODFLOW is adopted.When the area ratio is set to 3.0,the corresponding water table depth is 1.03m to 2.51m in canal irrigation area of well-canal combined irrigation area during growth period and autumn irrigation period,1.1Om-2.78m in well irrigation area of well-canal combined irrigation area during growth period and autumn irrigation period.The result demonstrates that the recommended area ratio of 3.0 can guarantee that the water table depth is kept within a reasonable range.(4)Considering the possibility of further water saving needs,water table depth corresponding to different area ratio under different water saving levels were simulated and analyzed.Taking the average water table depth within the year as the criterion for judgment,and using 3m as the appropriate upper limit,the following conclusions are reached:When the water-saving level of canal irrigation area is 100%(current irrigation mode),the area ratio should be greater than 1.8;When the water-saving level of canal irrigation area is 90%,the area ratio should be greater than 2.0;When the water-saving level of canal irrigation area is 85%,the area ratio should be greater than 2.2;When the water-saving level of canal irrigation area is 75%,the area ratio should be greater than 2.7;When the water-saving level of canal irrigation area is 60%,the area ratio should be greater than4.1.Taking the maximum water table depth in growth period and autumn irrigation period as the criterion for judgment,the following conclusions are reached:When the water-saving level of canal irrigation area is 100%(current irrigation mode),the area ratio should be greater than 2.4;When the water-saving level of canal irrigation area is 90%,the area ratio should be greater than 2.9;When the water-saving level of canal irrigation area is 85%,the area ratio should be greater than 3.3;When the water-saving level of canal irrigation area is 75%,the area ratio should be greater than 4.2;the water-saving level of canal irrigation area should not be less than 60%.(5)Reduction of Yellow River water diversion was calculated and analyzed,which can provide data support for assessing the water saving effect of well-canal combined irrigation mode.The greater the groundwater salinity upper limit,the larger the amount of water used in autumn irrigation period,the more canal recharge is considered,the greater the amount of the reduction.Besides,scheme 2>scheme 3>scheme 1.When the area ratio is 3.0,the diversion reduction is 3.4×108m3.