Spatial Variation Characteristics Of Surface Runoff Generation And Flash Flood Risk Assessment In The Chabagou Watershed

The Loess Plateau region is a key area in the layout of ecological protection and high-quality development of the Yellow River Basin,which is also a typical fragile ecosystem area.Affected by climate change and human activities,the regional uneven spatial and temporal distribution of water resources has been further aggravated,and the frequent occurrence of soil erosion and drought-flood has seriously restricted the development of the regional economy.This study was conducted in the Chabagou watershed on the Loess Plateau and focused on the spatial variability of runoff generation characteristics and flash flood risk assessment.The SWAT model based on the Green-Ampt infiltration method was used to simulate daily scale runoff for the flood season and four rainfall runoff events from 2006 to 2013;key drivers of spatial variability in watershed runoff generation were clarified using Geodetector approach;Combined with the model outputs,a flash flood risk assessment index system was established to assess the hazard,sensitivity,vulnerability and overall risk of flash floods in the Watershed.The main conclusions of this research can be divided into the following aspects:(1)The SWAT model based on the Green-Ampt infiltration method was developed in the Chabagou watershed.The NS coefficients for the calibration period and the validation period were 0.75 and 0.74 respectively,and the coefficients of determination(R~2)were 0.75 and 0.76,both of which met the"good"standard in the daily-scale runoff simulations of the flood season.Compared with the daily-scale runoff,the simulation of rainfall runoff events is better with the NS coefficient and R~2 above 0.9 for all events,reaching the’excellent’standard,except for event20130804 which is rated as’good’.In general,the model has low uncertainty with PBIAS values between±25%,and can be used to analyse surface runoff generation patterns for rainfall-runoff events.(2)The key drivers factors of spatial variability in Chabagou watershed runoff generation were clarified.The areas of high values of surface runoff for the four rainfall runoff events are mostly concentrated on the western side of the middle reaches of the river,with lower values in the northern areas,and the surface runoff from erosion gullies on both sides of the watershed is closely related to the distribution of the river.In terms of single factor effects on spatial variation,rainfall characteristics have the strongest explanatory power for spatial variation in watershed yield;in terms of factor interactions,the interaction between soil-related factors(soil erodibility,antecedent soil moisture content)and rainfall has the strongest explanatory power for spatial variation in surface runoff;among the watershed characteristics,the interaction with elevation significantly increases the explanatory power of other watershed characteristics(land use,soil erodibility,slope)for spatial variation in surface runoff.The results of the identification of the land surface characteristics of the high runoff areas show that the surface runoff capacity is strongest when the slope is between 25°-35°,the elevation is within 960-1020m and the soil type is saturated daisy soil;among the land use types,the rural residential land has the strongest surface runoff capacity,followed by Cropland,grassland and Woodland.(3)The spatial variation pattern of flash flood risk in Chabagou watershed were revealed.The distribution pattern of flash flood risk levels in the Chabagou watershed is gradually decreasing from southeast to northwest,with the smallest proportion of high-risk areas concentrated in Caoping village,Gao Jiaqu village and Sanchuankou town,which are located at the junctions of the midstream and the estuary.Based on the results of the indicators,the hazard of flash floods in the watershed gradually increases from the upstream to the downstream,and the high hazard areas are mainly located in the southeast of the watershed,with the highest hazard in Caoping and Gaojiaqu villages near the estuary;Highly sensitive areas are mainly located in the central and southern part of watershed at the junctions of rivers,including Sanchuankou town and the Shegou village;the high vulnerability areas are concentrated in the northern part of the basin,including Xiaochangmao Village,Dongpingyan Village and Guoyaowan Village.