Subdividing large mountainous watersheds into smaller hydrological units to predict soil loss and sediment yield using the GeoWEPP model

The number of studies using prediction models on measuring soil loss and/or sediment yield has been continuously increasing since these models are considered timely and cost-effective. Similarly, in this study, we used the GeoWEPP model to determine how much soil is being lost and the amount of sediment being yielded from Godrahav Creek Watershed (GCW) located in northeastern Turkey. Because the watershed is large (5,298.21 ha) and has mountainous and steep terrain, it was subdivided into smaller hydrological units (SHUs) so that the model can run easily and give detailed findings. The results revealed that out of 18,596.8 t of soil loss generated from both hillslopes and channels within the whole GCW, approximately 9,854.8 t y⁻¹ reached Borcka Dam reservoir as sediment. The model also predicted annual average soil loss and sediment yield as 1.73 t ha⁻¹y⁻¹ and 1,86 t ha⁻¹y⁻¹, respectively. In addition, with a sediment delivery ratio (SDR) of 0.530, the results indicated that almost half of the detached soil particles were carried away as sediment. Despite the dominant vegetation coverage, relatively high SDR and soil loss – particularly in certain SHUs – can be associated with steep terrain and conversion of natural lands in the watershed.