Variability of the slope component of sediment balance for the erosion-channel system in anthropogenically transformed basin of a small river

The paper presents the study of spatiotemporal variability of surface sediment flow and the assessment of its influence on suspended sediment yield in the outlet section of a small river basin, i.e. the upper part of the Vorskla River (Belgorod Oblast), with an area of 1,85 thousand km2. Time series of average annual suspended sediment yield from 1960 till 2021 were analyzed according to the data for the Kozinka - Vorskla gauging station (116 km from the river source). High heterogeneity of the sediment yield was found, during the period under review it decreased by 6,7 times (according to average values, from 0,27 to 0,04 kg/s). It was found that the surface (basin) component predominates in the sediment balance, which is predominately soil matter eroded from the arable lands of the agriculturally developed catchment areas. The content of suspended sediment in river runoff has a greater response to changes in the intensity of snowmelt erosion than the rainfall one. However, a critical decrease in the snowmelt runoff layer caused by the climate changes of recent decades has led to a decrease in the contribution of snowmelt erosion to annual soil losses. As a result, suspended sediment yield in the outlet section of the river has decreased. A quantitative assessment of rainfall erosion rates using the WaTEM/SEDEM model showed that the main source of sediment is plowed areas; annually 228,8 thousand tons of soil matter are removed from the arable lands. At the same time, 76 thousand tons enter the river network, i. e. 66% of soil material mobilized by rainfall runoff accumulates within the catchment area. The dynamics of river flow regulation by ponds is considered. To date, sediment yield from 62% of the catchment area is intercepted by ponds. Over the past 40 years, the interception area of solid runoff has increased by 30%, and the volume of sediments entering the unregulated river network has decreased by 47%. The ponds and the upperlying gully network hold 61,4 thousand tons of sediments, which is 81% of the surface erosion in the basin. Thus, the climate-related decrease in snowmelt runoff from the catchment area, combined with the reduction of water yield during floods and the water management measures to regulate river flow by ponds, has resulted in a sharp reduction of suspended sediment discharge since the 1980-s and their further gradual decline.