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The Himalayan basins are characterised by severe soil erosion rates and several basins are among the largest sediment dispersal systems in the world. Unsustainable agricultural activities increase the soil erosion rates and influence the overall hydro-geomorphic regime of river basins. Consequently, the water holding capacity of soil reduces, which enhances the flood risk in the lowland regions. In addition, excessive sediment flux severely affects the reservoir capacity in the mountainous regions, thus amplifying the flood hazard in the upland regions. Here, we have analysed two large and hydro-geomorphically diverse Himalayan River basins, namely, the Ganga Basin (GBA) from source to Allahabad in northern India and the Kosi Basin (KB) draining through Nepal and north Bihar plains in eastern India. Based on RULSE and region-specific SDR modelling framework, which includes model calibration, validation and uncertainty assessment, we demonstrate that spatial variation in rainfall, hydrogeomorphic conditions, the presence of hydraulic structures, and large-scale agricultural activities influence the overall pattern of sediment production and transport in these two large river basins. Total soil erosion in GBA and KB are estimated to be ~404 × 10 6 t/y and ~724 × 10 6 t/y respectively, a large part of which comes from the mountainous regions in both basins. Sediment yield at the mountain exits of the GBA and KB are computed as 14.1 × 10 6 t/y and 86.4 × 10 6 t/y respectively, which work out to be ~5% and ~15% of total soil erosion from the respective contributing areas of the KB and GBA respectively. Similarly, sediment yields at outlets in the alluvial plains are estimated to be 32.2 × 10 6 t/y and 37.3 × 10 6 t/y in the GBA and the KB, respectively suggesting that a large part of sediments are accommodated in the alluvial plains of KB. These results have significant implications for sediment management in the Himalayan River basins., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

High soil erosion and excessive sediment load are serious problems in several Himalayan river basins. To apply mitigation procedures, precise estimation of soil erosion and sediment yield with associated uncertainties are needed. Here, the revised universal soil loss equation (RUSLE) and the sediment delivery ratio (SDR) equations are used to estimate the spatial pattern of soil erosion (SE) and sediment yield (SY) in the Garra River basin, a small Himalayan tributary of the River Ganga. A methodology is proposed for quantifying and propagating uncertainties in SE, SDR and SY estimates. Expressions for uncertainty propagation are derived by first-order uncertainty analysis, making the method viable even for large river basins. The methodology is applied to investigate the relative importance of different RUSLE factors in estimating the magnitude and uncertainties in SE over two distinct morphoclimatic regimes of the Garra River basin, namely the upper mountainous region and the lower alluvial plains. Our results suggest that average SE in the basin is very high (23±4.7 t ha-1 yr-1) with higher values in the upper mountainous region (92±15.2 t ha-1 yr-1) compared to the lower alluvial plains (19.3±4 t ha-1 yr-1). Furthermore, the topographic steepness (LS) and crop practice (CP) factors exhibit higher uncertainties than other RUSLE factors. The annual average SY is estimated at two locations in the basin - Nanak Sagar Dam (NSD) for the period 1962-2008 and Husepur gauging station (HGS) for 1987-2002. The SY at NSD and HGS are estimated to be 6.9±1.2×105 t yr-1 and 6.7±1.4×106 t yr-1, respectively, and the estimated 90% interval contains the observed values of 6.4×105 t yr-1 and 7.2×106 t yr-1, respectively. The study demonstrated the usefulness of the proposed methodology for quantifying uncertainty in SE and SY estimates at ungauged basins. [ABSTRACT FROM AUTHOR]

High soil erosion is one of the severe problems in the Himalayan River basins. Significant regional variability in sediment production, transport and deposition in the Gangetic Rivers is a function of hydro-geomorphic diversity across the Ganga basin. For instance, the rivers draining the western Gangetic plains (WGP) are characterised by high stream power and low sediment yield whereas those in the eastern Gangetic plains (EGP) have low stream power and high sediment yield. Therefore, a systematic modelling framework is essential to comprehend the controls (i.e. climate, lithology, topography and human influence) on sediment production and to quantify soil erosion (SE) and sediment yield (SY) for individual river basins. In this work, we have used the Revised Universal Soil Loss Equation (RUSLE) and Sediment Delivery Ratio (SDR) equation to compute the SE and SY in a small basin (Garra River basin, WGP). We then assess the applicability of quantified results in this basin using a statistical framework for uncertainty estimation. We have then upscaled and quantified SE and SY variability for two river basins in different hydro-geomorphic settings viz. Upper Ganga basin (UGB) and the Kosi basin (KB) in the western and eastern parts of the Ganga basin respectively. This study predicts SE of 72 t/ha/year for KB and SY of 85 Mt/year and 75 Mt/year at upstream (Barakshetra) and downstream (Baltara) stations respectively with 26.8% associated uncertainty. Similarly, the UGB shows SE of 43 t/ha/year and SY of 36 Mt/year at Kanpur with 24.6% associated uncertainty. The overall estimated SY are validated using the observed data at gauge stations in both the basins. Further analysis suggests that rainfall erosivity, topographic steepness and agricultural practices have significant influence on SE and SY in both the basins. Finally, these modelled results are important to understand the linkage between river form and processes and to design sustainable river basin management strategies. [ABSTRACT FROM AUTHOR]