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University of Cape Town. (2021)

Potential impacts of climate change on hydrological droughts in the Limpopo river basin

Makhanya, Nokwethaba Zamanguni

Titre : Potential impacts of climate change on hydrological droughts in the Limpopo river basin

Auteur : Makhanya, Nokwethaba Zamanguni

Université de soutenance : University of Cape Town.

Grade : Master of Science Environmental and Geographical Science 2021

Résumé
Climate change possibly intensifies hydrological droughts and reduces water availability in river basins. Despite this, most research on climate change effects in southern Africa has focused exclusively on meteorological droughts. This thesis projects the potential effect of climate change on the future characteristics of hydrological droughts in the Limpopo River Basin (LRB). The study uses regional climate model (RCM) measurements (from the Coordinated Regional Climate Downscaling Experiment, CORDEX) and a combination of hydrological simulations (using the Soil and Water Assessment Tool Plus model, SWAT+) to predict the impacts at four global warming levels (GWLs : 1.5℃, 2.0℃, 2.5℃, and 3.0℃) under the RCP8.5 future climate scenario. The SWAT+ model was calibrated and validated with a streamflow dataset observed over the basin, and the sensitivity of model parameters is investigated. The performance of SWAT+LRB model was verified using the Nash-Sutcliffe efficiency (NSE), Percent Bias (PBIAS), Root Mean Square Error (RMSE), and coefficient of determination (R2 ). The study also examines the capability of the CORDEX SWAT+ system in reproducing the hydro-climatology and the influence of the quantile delta mapping (QDM) method on bias correction of CORDEX datasets. The Standardized Precipitation Evapotranspiration Index (SPEI) and the Standardized Precipitation Index (SPI) have been used to detect meteorological droughts. The Soil Water Index (SSI) has been used to define agricultural drought, when the Water Yield Drought Index (WYLDI), the Surface Run-off Index (SRI), and the Streamflow Index (SFI) have been used to characterize hydrological drought. The performance of SWAT+ the model simulations over LRB is sensitive to the parameters CN2 (initial SCS runoff curve number for moisture condition II) and ESCO (soil evaporation compensation factor). The best simulation is generally performed better during the calibration period than in the validation period. In calibration and validation periods, NSE is ≤ 0.8, while PBIAS is ≥ ﹣80.3%, RMSE ≥ 11.2 m3 /s and R 2 ≤ 0.9. Although the CORDEX simulations capture the general spatial and temporal distribution of the hydroclimate variables over the LRB, they feature a cold and wet bias across the basin. However, the QDM bias correction reduces the bias and fosters better agreement among the simulations. The simulations project in all hydrological variables is projected over most parts of the basin, especially over the eastern part of the basin. The simulations predict that meteorological droughts (i.e., SPEI and SPI), agricultural droughts (i.e., SSI), and hydrological droughts (i.e., WYLDI, SRI) would become more intense and severe across the basin. SPEI-drought has a greater magnitude of increase than SPI drought, and agricultural and hydrological droughts have a magnitude of increase that is part-way between the two. As a result, this research suggests that future hydrological droughts over the LRB could be more severe than the SPI-drought projection predicts but less severe than the SPEI-drought projection. This research can be used to mitigate the effects of potential climate change on basin hydrological drought.

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Page publiée le 17 mai 2022