Abstract
The evaluation of the hydrological responses of river basins to land-use and land-cover (LULC) changes is crucial for sustaining water resources. We assessed the impact of LULC changes (1990–2018) on three hydrological components (water yield (WYLD), evapotranspiration (ET), and sediment yield (SYLD)) of the Wami–Ruvu Basin (WRB) in Tanzania, using the Soil and Water Assessment Tool (SWAT). The 1990 LULC imagery was used for SWAT simulation, and imagery from 2000, 2010, and 2018 was used for comparison with modelled hydrological parameters. The model was calibrated (1993–2008) and validated (2009–2018) in the SWAT-CUP after allowing three years (1990–1992) for the warm-up period. The results showed a decrease in WYLD (3.11 mm) and an increase in ET (29.71 mm) and SYLD (from 0.12 t/h to 1.5 t/h). The impact of LULC changes on WYLD, ET, and SYLD showed that the increase in agriculture and built-up areas and bushland, and the contraction of forest led to the hydrological instability of the WRB. These results were further assessed with climatic factors, which revealed a decrease in precipitation and an increase in temperature by 1 °C. This situation seems to look more adverse in the future, based on the LULC of the year 2036 as predicted by the CA–Markov model. Our study calls for urgent intervention by re-planning LULC and re-assessing hydrological changes timely.
Highlights
In recent decades, changes in the hydrological processes of a wide range of river basins have been observed with increasing frequency due to global trends in human population, climate change, and underlying surface features at global, regional, and local scales [1,2,3,4]
In developing countries (e.g., Tanzania) where arable lands are found in river basins, such statistics are lower due to lack of land-use planning; agriculture is critical for the livelihood of more than 70% of the general population in such countries [17,18]
The results of this research are presented in three phases: one for the landuse and land-cover changes (LULCC) that are real and simulated based on the CA–Markov model, and two for the identified trends of the basic climatic data and the hydrological components modelled in the Soil and Water Assessment Tool (SWAT) model
Summary
Changes in the hydrological processes of a wide range of river basins have been observed with increasing frequency due to global trends in human population, climate change, and underlying surface features at global, regional, and local scales [1,2,3,4]. Climate, and underlying surface features, landuse and land-cover changes (LULCC) have a potentially large impact on hydrological processes [5,6]. These include changes in water and sediment yields [7], surface runoff, baseflow, streamflow [8,9,10,11], soil water content [12], groundwater recharge [13], and annual river discharge [14]. The influence of LULCCs is highly visible due to the modification of dense vegetation into agriculture that, by 2017, occupied 37% of the global land surface and employed about 50% of the world population [15,16]. Vegetation can have a significant effect on hydrological fluxes due to variations in the physical characteristics of the soil and land surface, such as the roughness, albedo, infiltration capacity, root depth, architectural resistance, leaf area index (LAI), and stomatal conductance [19,20]
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