The Spatial-temporal Characteristics And Influence Factors Analysis Of Runoff And Sediment

Soil erosion is a worldwide environmental problem and the Loess Plateau is one of the most severe soil erosion areas in the world.Since the mid-1980 s,with the increasing human activities and natural climate changes,the rapid reduction of water and sediment in the Yellow River has become a hot spot of concern.It is important to study the spatial and temporal variation of runoff and sediment in the basin and to quantify the influence of natural factors and human activities on runoff sediment at the basin scale.Jihe basin in Tianshui,Gansu Province,a gully area of the Loess Plateau,was selected in this study.Based on the hydrological data of rainfall,runoff and sediment in this basin from 1995 to 2016,the regional soil erosion model was verified and simulated the changes of runoff and sediment under the background of land cover and annual rainfall change according to the measured data in typical years.With the help of multiscale geographically weighted regression and connectivity indices analysis,the key influencing factors affecting the runoff and sediment production in the Jihe basin at multi-year scales and medium-large basin scales were analyzed.The main findings are as follows:(1)The annual runoff and sediment discharge in this basin showed an increasing trend.From 1995 to 2002,the runoff and sediment discharge in Jihe basin were at a relatively low level,and the change was stable.From 2003 to 2016,the runoff and sediment discharge were at a relatively high level and fluctuated frequently.The sudden increase and drastic change of annual precipitation was the dominant factor in the fluctuation of water and sediment.The high annual precipitation was the main reason for the sudden increase of sediment transport in2003.The effect of precipitation change on the runoff and sediment production was greater than the improvement of land cover structure.(2)The improvement of land use weakened soil erosion to some extent.Under the same precipitation scenario,the overall connectivity of the basin decreased year by year,and the runoff production and sediment transport capacity of the basin decreased year by year.Under the same precipitation scenario,the erosion intensity of different land cover types decreased with the improvement of land cover structure,which was a joint effect of hillslope-cropland reformation and the overall environmental management of the watershed in the Jihe demonstration area.Steep slopes and cropland are prone to erosion and water and sediment transport.Therefore,hillslope-cropland transformation and vegetation restoration are still the focus of soil and water conservation management in Jihe Basin.(3)Slope is a key factor affecting soil erosion at the watershed scale,and forestland cover has a stronger ability to reduce runoff and sediment production than grassland cover.The multiscale geographically weighted regression was the best fit among the three regression models,and the variables were more explanatory.Vegetation and topography factors have coupled effects on erosion at different scales,and high vegetation cover can effectively weaken the erodible capacity of steep slopes.The variation of vegetation and topographic factors at different scales caused a complex regional variation in watershed erosion intensity.(4)The modified functional connectivity index is an effective measure of watershed connectivity and water and sediment transport capacity,which can be used to study changes in watershed connectivity under different land use structures.It is important for the study of the erosion hotspots distribution and the optimization of the landscape pattern of soil and water conservation measures.This study explored the spatial and temporal variation of runoff and sediment production at multi-year time scales and medium to large watershed scales in the Jihe basin,and quantitatively analyzed the effects of natural factors and human activities on runoff and sediment at the watershed scale.Moreover,land use patterns and landscape structures that could reduce soil erosion and improve ecological benefits of soil and water conservation were analyzed under the existing socio-economic and natural conditions,which could provide theoretical recommendations for rational erosion control and deployment of soil and water conservation measures in the watershed.