Study On The Lag And Uncertainty Of Return Flow In A Large Yellow River Irrigation District

On the basis of consulting a large number of domestic and foreign relevant documents and data,aiming at the problems of large return flow,complex composition and source of return flow,hysteresis of return flow,and low utilization rate of return flow resources in large irrigation areas from the Yellow River in northwest China,the uncertainty of irrigation water demand and hysteresis law of return flow in large irrigation areas from the Yellow River were carried out by adopting the technical route of combining actual monitoring,theoretical research and numerical simulation,and combining with the actual situation of Jingdian Irrigation Area in Gansu Province The research on the prediction method of return flow and the optimal allocation of return flow resources has mainly achieved the following results:(1)The net irrigation water demand model based on the water state of the root layer was established.Combining the advantages of Copula function and Monte Carlo method,the uncertainty analysis method of net irrigation water demand was developed,revealing the change characteristics of net irrigation water demand of different crops in Jingdian Irrigation District and the impact of the root layer on it,and revealing the impact characteristics of the combined driving effect of meteorological factors on the uncertainty of net irrigation water demand.The established net irrigation water volume model can fully consider the difference in the effective precipitation utilization rate of different crops.The precipitation utilization rate of autumn crops is higher than that of summer crops,and the annual net irrigation water demand of different crops varies greatly.The net irrigation water demand of crops with larger root depth decreases with the increase of root depth because of the strong water storage capacity of the soil in the root layer and the high utilization rate of precipitation.The factors that contribute to the net irrigation water demand from large to small are crop water demand,precipitation and root depth;In the year with large precipitation,the effect of root depth on net irrigation water demand cannot be ignored.The optimal distribution functions of precipitation and reference evapotranspiration in Jingdian Irrigation District are P-III distribution and lognormal distribution,respectively.The probability density function of the joint distribution of precipitation and reference evapotranspiration is in the shape of "saddle";When one of the precipitation and the reference evapotranspiration is large and the other is small,the probability of occurrence is large;when the two are large or small at the same time,the probability of occurrence is small.Driven by meteorological factors,the net irrigation water demand increases with the increase of the combined probability of precipitation and reference evapotranspiration.(3)This paper analyzes the lag law of channel return flow during the monitoring period in Jingdian irrigation area,establishes the return flow unit hydrograph model and Hydras numerical model,and reveals the hydrodynamic characteristics of channel return flow.The cross wavelet analysis shows that there are differences in the return flow of the sub-basins in the irrigation area.The results of numerical simulation show that the return flow process has experienced three different stages,namely,soil water infiltration,groundwater recharge and ditch drainage,and the hydraulic characteristics of each stage are different.At the stage of soil water infiltration,the water movement direction is mainly vertical downward,and the water movement rate gradually decreases;In the stage of groundwater recharge,the soil water transport rate in the aeration zone gradually decreases,the groundwater flow rate in the saturated zone increases,and the groundwater recharge rate first increases and then decreases;In the drainage stage of the ditch,the groundwater flow direction is mainly horizontal,and the groundwater flow rate and the drainage velocity of the ditch both increase at first and then decrease.The return flow lag in irrigation area is persistent.Based on the linear reservoir theory,a return flow unit hydrograph model is established,which can describe the persistence of the return flow lag process.The numerical simulation results show that the return flow unit hydrograph has good applicability to different soil textures and heterogeneous soils,and the return flow process under different infiltration intensities and different infiltration time intervals conforms to the principle of multiple superposition.It is verified by the measured return flow data that the return flow unit hydrograph model is accurate and reliable.(4)Based on the hysteresis,the lag time contour model and weighted cross-BP neural network model are established for the prediction of channel return flow,and the advantages and disadvantages of the two models are clarified.The change trend of the return flow prediction value and the monitoring value of the lag time contour model is roughly consistent,but the prediction value is more volatile than the monitoring value,and the fitting accuracy is not high.The weighted cross-BP neural network model can effectively improve the prediction accuracy of return flow.The model is applicable to the prediction of return flow with continuous lag effect.When the training samples are small,the method has limitations.(5)Based on the prediction of the future climate change characteristics of Jingdian Irrigation District,an uncertain optimal allocation model of return flow considering the reuse of return flow is established to analyze the impact of the reuse of return flow on irrigation efficiency,crop planting structure and water resource allocation in the irrigation district.The precipitation of Jingdian Irrigation District in the future planning years is increasing,while the reference evapotranspiration is decreasing.When only the optimal allocation of irrigation water diversion is carried out,the net profit of the current year and the planning year will increase;Crop planting structure changes,the proportion of low water productivity crops decreases,and the proportion of high water productivity crops increases;Restricted by the total amount of irrigation water diversion,the irrigation water resources in the current year and the planning year are in deficit.When optimizing the allocation of channel return flow,the net profit and total planting area of the current year and the planning year will increase,and the irrigation diversion will decrease.Reasonable and optimized use of channel return flow can not only improve the irrigation efficiency of irrigation areas,avoid the impact of return flow discharge on the water quality of the Yellow River,but also reduce the cost of water extraction and save the water resources of the Yellow River.