Study On Reuse Of Return Flow And Evaluation Of Irrigation Water Use In Irrigation Area Based On Modified SWAT Model

The correct calculation of irrigation efficiency（IE）and water-saving potential（WSP）is the basis of water use evaluation and management in irrigation systems.The traditional irrigation efficiency（η₀）and water-saving potential for irrigation water intake（WSP₀）ignored the reuse of return flow,which cannot truly reflect the water management level of irrigation systems.However,the return flow and its reuse are common phenomena in irrigation systems.Distributed hydrological model is an effective tool to analyze the return flow and its reuse in large scale areas,whereas the current distributed hydrological models are mainly focus on the natural river basin,there are some deficiencies when they are used for simulating the hydrological processes in irrigation systems.Therefore,in this paper,the Soil and Water Assessment Tool（SWAT）was modified according to the characteristics of multi-source irrigation system with paddy rice,and the modified SWAT model was applied to the Yangshudang（YSD）watershed in Hubei Province,China.Then,based on the simulation results with the modified SWAT model,a new method was proposed to calculate the return flow and its reused amount,and the scale effect of the reuse of return flow was analyzed as well as the reuse of return flow under different water-saving scenarios.Finally,a new method was proposed to calculate the irrigation efficiency（IE_r）and water-saving potential（WSP_r）considering the reuse of return flow,which was used to analyze the change rules of IEs and WSPs under different scenarios,and the relationships between the two IEs and the cause thereof,as well as the WSPs,were analyzed and explored.Firstly,the SWAT model was modified through developing the hydrologic cycle structure of the hydrologic response unit,modifying the calculation of water balance elements in paddy fields,adding a multi-source automatic irrigation module to SWAT.And the modified SWAT model was applied to the YSD watershed,the observed daily discharges in the outlet of the YSD watershed,the observed daily evapotranspiration（ET）in typical paddy field and the observed irrigation water consumptions from different water sources were used to calibrate and validate the model.The results showed that the performance of the modified SWAT met the evaluation standard of hydrological model,which was better than the original SWAT model for the simulation of hydrological processes in multi-source irrigation system with paddy rice.Secondly,a new method was proposed to calculate the return flow and its reused amount based on the simulation results with modified SWAT model,which could distinguish rainfall and irrigation return flow.The test data of typical sub-basin was used to verify the rationality of this method.Furthermore,the rainfall&irrigation water reuse rate(η_I+P)and the irrigation water reuse rate（η_I）were introduced to represent the reuse of return flow,and the scale effect of the reuse of return flow and the cause thereof were analyzed and explored.The results indicated thatη_I+P andη_I increased with the increase of scale,the increased amplitude decreased with the increase of scale,after a certain scale was reached,they tended towards stability.The reason for that were due to the fact that the return flow was repeatedly intercepted by downstream paddy fields,farm ponds,and drainage channels at large scales,this reuse rates however reach the upper limit at a scale of 3500 ha,after whichη_I+P andη_Ino longer increase.In addition,η_I under different water-saving scenarios were calculated and analyzed based modified SWAT model,the results revealed thatη_Iincreased with the increase of the drainage area of ponds,but decreased with the improvement of seepage control standards for canal system,additionally,η_I under rice water-saving irrigation scenario was higher than that under traditional rice flood irrigation scenario.Finally,based on the reuse of return flow,a method was proposed to calculate IE_r and WSP_r.And the modified SWAT model was used to simulate the hydrological processes of the YSD watershed under different water-saving scenarios,η₀,WSP₀,IE_r,WSP_r and the resource water-saving potential based on the ET of the YSD watershed（（35）ET）were calculated to analyze their change rules with different water-saving scenarios.The results showed thatη₀,IE_r,WSP₀ and WSP_r increased with the enhancement of water saving,including increasing drainage area of ponds,improving seepage control standards for canal system and adopting the rice water-saving irrigation approaches,and（35）ET existed only when rice water-saving irrigation approaches were adopted,but not in all other water-saving scenarios.Moreover,the relationships among the WSPs and the cause thereof were analyzed,the results indicated that（35）ET had no specific relationship with the other two water-saving potentials;WSP_r was calculated by subtracting the amount of the water saved by the reuse of return flow on the basis of WSP₀.For water-saving approaches that maintain a constant irrigation requirement,WSP_r is higher than WSP₀ if the water-saving approaches can promote the reuse of return flow;but if they bring down the reuse of return flow,WSP_r will be smaller than WSP₀.For the water-saving approaches that can reduce the net irrigation requirement of crops,the relationship between WSP₀ and WSP_r is determined by the changes of the reuse of return flow and the net irrigation requirement.