Characteristics Of Erosive Rainfall And Runoff Simulation In The Typical Small Watershed Of Guizhou Mountainous Area

Karst landform in Guizhou is well developed.The rate of soil formation is slow,and the soil layer is shallow.Guizhou is one of the most serious soil erosion regions due to the natural geographical condition of high mountains and steep slopes,abundant rainfall but uneven temporal and spatial distribution.The unique terrain,rainy climate and excessive reclamation of slope land,resulting in serious damage to vegetation and severe soil erosion problems.As ecological barriers of the upper reaches of the Yangtze River and the Pearl River basins,soil erosion occurred in Guizhou not only resulted in negative impact on the local ecology and environment,but also brought about transregional ecological and environmental problems in the middle and lower reaches of the Yangtze River and the Pearl River basins.Rainfall is one of the main driving force of soil erosion,studies on rainfall characteristics are of great significance to the study of runoff generation and soil erosion mechanism.This study utilized rainfall,runoff and sediment data over the period of 2012-2014 obtained from the field runoff plots experimental observation in three small watershed in Guizhou.Analysis of erosive rainfall characteristics,and the impacts of rainfall on runoff and soil erosion were carried out by using of mathematical statistics,regression analysis and geographical comprehensive analysis in this paper.Runoff generated on the slopes were simulated using the SCS model.Conclusions are as follows:(1)The daily rainfall concentration index in the study area is about 0.6,the daily rainfall amount is relatively concentrated,and the diversity is not obvious in different areas and years;rainfall events with large rainfall amount,high rainfall intensity and high concentration index had great contribution to runoff and sediment generation on the slopes.(2)Based on the close relationship between runoff/sediment on the slopes with rainfall concentration index,rainfall events in the study area can be divided into four types according to event-based rainfall concentration index,namely the pre-type(type?):rainfall concentrated in the early period of the rainfall process;medium-type(type ?):rainfall concentrated in the middle period of the rainfall process;back-type(type?):rainfall concentrated in the back period of the rainfall process;and even-type(type?):rainfall evenly distributed in the whole rainfall process.(3)The normalized rainfall duration 1 is divided into 5 equal parts at an interval of 0.2.More than 50%rainfall of type ? aggregates in the early 1/5 of the rainfall duration,more than 80%rainfall of type ? aggregates in the middle of the rainfall duration,more than 40%rainfall of type ? aggregates in the later 1/5 of the rainfall duration.Rainfall of type ? is more balanced during the rainfall duration.I60 or I30 is the representative rainfall intensity index of the experimental small watershed,analysis of the rainfall intensity provides basis for the adjustment of rainfall intensity in SCS model;E60 and EI30 were used to estimate the rainfall energy and rainfall erosivity index in different rainfall types in the study area.(4)Due to the complexity of the natural rainfall and the difference of the underlying surface conditions of the experimental small watersheds,the characteristics of runoff and sediment on the slopes under different rainfall types are various.Runoff and sediment on the slopes should be further studied by accumulating longer time series data.(5)It is closely related between runoff/sediment and the temporal distribution characteristics of rainfall.Runoff and sediment were mainly generated from May to August,and the rainfall amount,rainfall intensity and rainfall erosivity were large in June and July.Sediment yields in the small watersheds in Zunyi and Bijie from June to July accounted for 78.0%and 57.2%of the annual sediments respectively,sediment yields in the small watershed in Longli from June to August accounted for 45.4%of the annual sediment,indicating that June to August is the key time period for water and soil conservation in the study area.(6)The value of the initial loss coefficient(?)is 0.01 or 0.05 in simulating runoff on the slopes in the study area using the SCS model.It indicated that the initial loss of rainfall in the experimental small watersheds is small,and runoff generation conditions are well.The correlation coefficient and the model efficiency coefficient between simulated and measured runoff indicated that the simulation effects are well.(7)The mean CN values of the slopes with different length(5m,10m,15m,20m,25m)are 67,64,69,68 and 61.The CN values of the type ?,type ?and type ?rainfall showed a small change with increasing slop length,the CN value of type ?rainfall tended to increase.The mean CN values of the slopes with different slope gradient(5°,10°,15°,20°and 25°)are 51,46,50,35 and 40.The CN values of the type? and type ? rainfall tended to reduce with increasing slope gradient,the CN values of type ? and type ? rainfall did not obviously increased with increasing slope gradient.(8)When the slope gradient and slope length are unchanged,the CN value of artificial arbor forest was larger than that of fruit forest;the fruit forest could be sorted by descending CN values as follows:bayberry,pear,peach,raspberry;when the slope length is unchanged,land types could be sorted by descending CN values as follows:artificial forest,arable land,grassland for ecological restoration,standard runoff plots and fruit forest land.