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Historical Land Use Change Decrease in Sediment Yield by 300% During the Past Century to Be Reversed by Future Climate Change

Author(s): Gerrit Basson; Kuria Kiringu; Erik Barnardt

Linked Author(s): Gerrit Basson, Kuria Kiringu

Keywords: Sediment yield; Soil erosion; Climate change; Modelling

Abstract: Nqweba Dam is one of the oldest dams in South Africa and was commissioned in 1925 for irrigation. The original reservoir depth was 31 m, but over the years sedimentation has reduced the water depth to 9 m. The original storage capacity of 79 million m³ has decreased to 44 million m³ by 2020. The dam supplies the town of Graaff-Reinet and due to the growing population at 2% pa it was realised in 1995 that the water use should change from irrigation to 100% potable use. For the past 25 years the town has been supplied from the dam when it has water and from ground water when the dam runs empty during droughts. The current water requirement of the town is 3.3 million m³/a and during 2019 the dam ran dry, while the ground water resource only supplied less than 50% of the demand. Urgent short and medium term measures were sought to solve the water crisis. Dam raising was found not to be beneficial, but options to reduce the evaporation are, such as dredging or a new off-channel dam, as well as floating balls/solar panels on part of the reservoir will restore the dam’s firm yield. The historical sediment yield of the catchment has been observed from reservoir surveys to decrease by 300 % over a 95 year period. This is due to a transition from sheep farming for wool almost a century ago associated with significant overgrazing and land degradation, to game farming during the past 4 decades which has become much more profitable. The hydrological model SHETRAN was used to evaluate the future climate change impacts on the long term sediment yield. SHETRAN is a physically-based, spatially-distributed rainfall-runoff-erosion-sediment transport model with an integrated surface/ subsurface model and routes the sediment loads along rivers. The model was calibrated against observed daily rainfall, runoff and sediment yield data. Future precipitation and temperature data was retrieved from an ensemble of the regional and global climate model. For future scenarios the precipitation, land cover and evaporation were simulated separately or combined. The global climate models predicted an increase in precipitation and runoff by the end of this century in the catchment which will lead to slightly improved vegetation conditions, but the simulated long term sediment yield will however increase by 300 % by the end of the century, the same as a century ago. Therefore the benefit of the current game farming land use which reduced the negative sediment yield effects of sheep farming by 300%, will unfortunately be cancelled out by the future climate change impacts causing larger floods, erosion and a higher sediment yield.

DOI: doi:10.3850/IAHR-39WC252171192022498

Year: 2022

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