The Loess Plateau Underlying Surface Changes On The Influence Of Rainstorm Floods

Human unreasonable land use patterns have changed the underlying surface of the basin,changed the hydrological characteristics of evaporation,infiltration and runoff,thus changed the hydrological cycle process of the basin,and also affected the regional flood process to a certain extent.The maximum peak discharge of Xiliugou watershed has a significant downward trend,but the precipitation shows a non-significant upward trend,which indicates that the flood law of Xiliugou has changed.Based on the observed precipitation and discharge data of Xiliugou watershed,this paper studies the change of storm flood characteristics before and after the change of underlying surface conditions in Xiliugou watershed,and analyses the variation regularity of flood,and studies the variation regularity of flood characteristics in Xiliugou watershed and the influence degree of underlying surface changes on flood process based on HIMS hydrological model.The main research contents and results are as follows:(1)The annual precipitation series of Xiliugou watershed from 1960 to 2015 showed no significant upward trend,and the proportion of P10,P25 and P50 in annual precipitation was60.52%,28.74% and 8.76%,respectively.The annual maximum one-day precipitation of Longtouguai and Gaotouyao stations showed an upward trend,while that of Chaidongmou stations showed a downward trend,and the maximum extreme precipitation of the three stations appeared at different times.The annual runoff,annual sediment transport and maximum sediment concentration in the basin showed a significant decreasing trend,while the annual maximum peak flow showed a non-significant decreasing trend.The year in which the peak flow abruptly changed was determined in 2005.Compared with the pre-catastrophe series,the annual maximum peak discharge has a smaller recurrence period and a larger probability of occurrence of extraordinary floods.(2)The correlation between flood peak and sediment concentration in Xiliugou watershed is strong,and there is a linear positive correlation.Compared with the rainfall-runoff capacity in1965-1990,the rainfall-runoff capacity in 2006-2015 decreases significantly.Compared with1965-1990,the runoff generated in 2006-2015 tends to decrease under the same rainfall conditions.Among the similar precipitation groups,the total flood volume and peak flow of field floods in 2006-2015 decreased by more than 40% and 70% compared with 1960-1990.Under the similar rainfall intensity,the peak discharge and flood volume of the field floods decrease to varying degrees after the underlying surface condition changes.When the rainfall intensity is 10-25 mm/h,the peak discharge of typical flood decreases steadily,with the attenuation rate of about 60%,while the attenuation rate of flood in 2006-2015 is much smaller than that in 1960-1990.When the rainfall intensity is 2-7 mm/h and 34 mm/h,the attenuation rate of peak discharge and flood volume of typical flood is larger.(3)Using HIMS hydrological model to simulate secondary floods,the calibration and verification results are good.The errors of flood peak and flood volume in most flood simulations are about 30%,and the efficiency coefficient of NSE is above 0.6,which proves that the HIMS model has good applicability in Xiliugou flow.The flood caused by rainstorm,heavy rain and moderate rain was selected to simulate secondary flood under two different model parameters before and after the change of underlying surface.The average peak attenuation rate of rainstorm flood was 15.33%,the average flood attenuation rate was26.22%,the average peak attenuation rate of rainstorm flood was 24.68%,the average flood attenuation rate was 17.46%,and the peak attenuation rate of moderate rainstorm flood was35.81%,and the flood volume was 26.22%.The attenuation rate was 16.71%.The change of underlying surface conditions makes the flood peak and secondary flood attenuate,and the attenuation rate of peak flow decreases with the increase of precipitation level,while the attenuation rate of secondary flood increases with the increase of precipitation level.