The Method For Deriving Distributed Unit Hydrograph And Its Application

In recent years,frequent drought and flood disasters have put forward higher requirements for the accuracy of hydrological simulation and forecasting.The distributed unit hydrograph is the main tool of confluence calculation in watershed hydrological models,and its accuracy directly affects the effect of watershed hydrological simulation.In this thesis,methods for deriving the time-variant velocity field(TVVF)and the distributed unit hydrograph based on TVVF are presented to address the issue of inconsistency between the existing distributed unit hydrographs and the actual characteristics of watershed confluence.Through its application in the Baiqueyuan watershed,the impact of the proposed unit hydrograph calculation method on the model simulation results is studied,as well as the impact of calculation unit division on unit hydrograph calculation methods.The study contents and findings are as follows:(1)The calculation method of TVVF.The concept and calculation method of TVVF are proposed and applied to the subwatershed S1 in the upper reaches of the Bailuhe watershed.The results show that the TVVF derived from the proposed method can obtain the velocity field which is completely consistent with the actual situation.(2)Calculation and analysis of distributed unit hydrograph based on TVVF.Based on the calculation results of TVVF of S1 watershed,the confluence time field is calculated,and the distributed unit hydrograph is derived,which is then compared with the calculation results of the distributed unit hydrograph based on the time-invariant velocity field(TIVVF).The results show that the unit hydrograph based on the TVVF has lower peak value with the same time to peak,and the time to peak of the unit hydrograph based on the TVVF is earlier with the same peak.It can be concluded that the proposed unit hydrograph calculation method can effectively solve the problems existing in the existing distributed unit hydrograph method.(3)The impact of unit hydrograph calculation methods on model simulation results.Taking Baiqueyuan watershed as the study watershed,based on the division scheme of16 calculation units,the original model and the improved model of Xin’anjiang model are constructed using the unit hydrograph based on the TIVVF and the unit hydrograph based on the TVVF as calculation tools for river network concentration separately.The results of historical floods simulation show that,compared with the original model,the mean value of the certainty coefficient simulated using the improved model increases from 0.94 to 0.96,the mean value of error of the time to peak decreases from 1.4 hours to 0.54 hours,and the positive and negative fields of error of the time to peak changes from 18 positive values and 0 negative values to 7 positive values and 7 negative values.It can be concluded that the unit hydrograph method based on TVVF is superior to the unit hydrograph method based on TIVVF in simulating the floods,and effectively solves the problem that the latter results in generally delayed time to peak.(4)The impact of calculation unit division on unit hydrograph calculation methods.Based on the division scheme of 11 calculation units of Baiqueyuan watershed,Xin’anjiang models are constructed using the unit hydrograph based on the TIVVF and the unit hydrograph based on the TVVF as calculation tools for river network concentration separately.Historical floods are simulated and the simulation results are compared with those of the two models under the division scheme of 16 calculation units of Baiqueyuan watershed mentioned above.The simulation results of the model with 16 subwatersheds are significantly better than those of the model with 11 subwatersheds when the unit hydrograph based on TIVVF is used.Using the unit hydrograph based on TVVF,the simulation results of the model with 16 subwatersheds are generally similar to those of the model with 11 subwatersheds.It can be concluded that the unit hydrograph method based on TVVF is much less affected by calculation unit division than the unit hydrograph method based on TIVVF.