Dynamics of soil loss and sediment export as affected by land use/cover change in Koshi River Basin, Nepal

How the dynamics in soil loss (SL) and sedimentation are affected by land use/cover change (LULCC) has long been one of the most important issues in watershed management worldwide, especially in fragile mountainous river basins. This study aimed to investigate the impact of LULCC on SL and sediment export (SE) in eastern regions of the Koshi River basin (KRB), Nepal, from 1990 to 2021. The Random Forest classifier in the Google Earth Engine platform was employed for land use/land cover (LULC) classification, and the Integrated Valuation Ecosystem Services and Trade-offs (InVEST) Sediment Delivery Ratio model was used for SL and SE modeling. The results showed that there was a pronounced increase in forest land (4.12%), grassland (2.35%), and shrubland (3.68%) at the expense of agricultural land (10.32%) in KRB over the last three decades. Thus, the mean SL and SE rates decreased by 48% and 60%, respectively, from 1990 to 2021. The conversion of farmland to vegetated lands has greatly contributed to the decrease in SL and SE rates. Furthermore, the rates of SL and SE showed considerable spatiotemporal variations under different LULC types, topographic factors (slope aspect and gradient), and sub-watersheds. The higher rates of SL and SE in the study area were observed mostly in slope gradient classes between 8° and 35° (accounting for 83%–91%) and sunny and semi-sunny slope aspects (SE, S, E, and SW) (accounting for 57%–65%). Although the general mean rate of SL presented a decreasing trend in the study area, the current mean SL rate (23.33 t ha −1 yr −1 ) in 2021 is still far beyond the tolerable SL rate of both the global (10 Mg ha −1 yr −1 ) and the Himalayan region (15 t ha −1 yr −1 ). Therefore, landscape restoration measures should be integrated with other watershed management strategies and upscaled to hotspot areas to regulate basin sediment flux and secure ecosystem service sustainability.