Abstract
In the Didessa river basin, which is found in Ethiopia, the human population number is increasing at an alarming rate. The conversion of forests, shrub and grasslands into cropland has increased in parallel with the population increase. The land use/land cover change (LULCC) that has been undertaken in the river basin combined with climate change may have affected the Didessa river flow and soil loss. Therefore, this study was designed to assess the impact of LULCC on the Didessa river flow and soil loss under historical and future climates. Land use/land cover (LULC) of the years 1986, 2001 and 2015 were independently combined with the historical climate to assess their individual impacts on river flow and soil loss. Further, the impact of future climates under Representative Concentration Pathways (RCP2.6, RCP4.5 and RCP8.5) scenarios on river flow and soil loss was assessed by combining the pathways with the 2015 LULC. A physically based Soil and Water Assessment Tool (SWAT2012) model in the ArcGIS 10.4.1 interface was used to realize the purpose. Results of the study revealed that LULCC that occurred between 1986 and 2015 resulted in increased average sediment yield by 20.9 t ha−1 yr−1. Climate change under RCP2.6, RCP4.5 and RCP8.5 combined with 2015 LULC increased annual average soil losses by 31.3, 50.9 and 83.5 t ha−1 yr−1 compared with the 2015 LULC under historical climate data. It was also found that 13.4%, 47.1% and 87.0% of the total area may experience high soil loss under RCP2.6, RCP4.5 and RCP8.5, respectively. Annual soil losses of five top-priority sub catchments range from 62.8 to 57.7 per hectare. Nash Stuncliffe Simulation efficiency (NSE) and R2 values during model calibration and validation indicated good agreement between observed and simulated values both for flow and sediment yield.
Highlights
Hydrological processes in watersheds are affected by multitude of factors
Curve number two (Cn2), soil evaporation compensation factor (Esco), soil available water capacity (Sol_awc), threshold depth of water in the shallow aquifer required for return flow (Gwqmn) and alpha base factor (Alpha_bf) were found to be more sensitivefactors with high effect on river flow
LULC = Land use/land cover; RS = relative sensitivity; CN2 = soil conservation service curve number 2; ESCO = soil evaporation compensation factor; Sol_awc = soil available water capacity; Gwqmn = threshold depth of water in the shallow aquifer required for return flow; Alpha_bf =
Summary
Hydrological processes in watersheds are affected by multitude of factors. Land use/land cover (LULC), climate, soil physico-chemical properties, geology of the land, topography, and spatial patterns of interactions among these factors are the prominent ones [1,2]. Anthropogenic interferences through land use/land cover change (LULCC) modify hydrological processes of a watershed by altering the balance between rainfall, evaporation and runoff response of an area [3,4] This implies that knowledge of the effects of LULCC and climate change on river flow and soil loss is important for effective and sustainable land resource monitoring and planning. A study by [16] attributed the high rate of soil erosion in the country to a certain combination of geomorphologic, population growth, deforestation, inappropriate cultivation practices and overgrazing Climate change is another important factor that affects catchments runoff producing certain characteristics and soil loss. Continued unsustainable resource exploitation in the river basin may result in substantial in-situ and ex-situ impacts on water and soil resources added with the current climate change. This study was designed to assess the impacts of LULCC on Didessa river flow and soil loss under historical and future climate
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