Assessing the effects of bush encroachment on streamflow rate using remote sensing in the Letaba catchment, South Africa

Abstract

Bush encroachment commonly known as woody plant encroachment (WPE) has been a rangeland problem for over a hundred years. However, the impacts on hydrological processes, especially streamflow, have become increasingly significant, particularly in semi–arid environments. This study provides an assessment of the effects of WPE on streamflow within the Letaba River Catchment, Limpopo Province, South Africa. An overview of existing literature was performed to understand the impacts of WPE on streamflow, focusing on factors such as rain canopy interception, surface runoff, soil moisture, groundwater recharge and evapotranspiration (ET). The review indicated that the impact of WPE differs depending on the type of area and climate, with humid areas showing significant streamflow reductions due to increased ET and canopy interception and semi–arid regions show little to no impact in streamflow because these areas have higher potential ET than rainfall regardless of the type of vegetation. The study further quantified the spatial and temporal distribution of WPE, Landsat satellite imagery (1989–2019) was analysed using a random forest classifier to map land use and land cover (LULC) changes. Results revealed a significant increase in woody plant cover from 36 014 ha in 1989 to 561 493 ha in 2019, accompanied by a 33.7 % reduction in grassland. These findings show a significant change in LULC particularly vegetation structure. To further investigate the nature of the hydrological processes within the study area, the Water Evaluation and Planning (WEAP) model was used to model streamflow responses under different past climate and land use scenarios. The model yielded promising results, with a Nash –Sutcliffe Efficiency (NSE) of 0.82, R² of 0.83, and a Percent Bias (PBIAS) of 1.8 %. Results indicated that an increase in ET especially from woody plants led to decreased surface runoff and streamflow, especially during the dry season. Correlation analysis between LULC and streamflow data (1989, 1998, 2004, and 2019) revealed a negative relationship, implying that streamflow decreases with increasing woody plant density. Overall, this study calls for the need to integrate land and water management strategies to address the impacts of WPE on hydrological processes. Given that WPE is an ongoing ecological phenomenon, targeted management strategies such as targeted clearing of woody plants and re-establishment of herbaceous vegetation are required to improve water availability in the downstream of the Letaba River Catchment area.