Modelling the future impacts of climate change on sediment yield for a semi-arid catchment in South Africa using SHETRAN

Barnardt, Georg Frederik (2021-03)

Thesis (MEng)--Stellenbosch University, 2021.

Thesis

ENGLISH ABSTRACT: Sedimentation (caused by soil erosion and high sediment yields) has become a major problem in South Africa, especially in semi-arid regions like the Karoo, where water scarcity and reduction of reservoir storage capacity can cause social and environmental concerns. The uncertainties regarding the impact climate change may have on the hydrological cycle, and the effect on catchment response increase these concerns. This thesis's main objective was to evaluate the possible future impacts of climate change on sediment yield by incorporating predicted future climate data and a physically-based hydrological and sediment yield model, SHETRAN. From a literature study, background information regarding soil and vegetation properties, soil erosion, sediment yield, physically-based models (focussing on the SHETRAN model), climate change, and climate models were obtained. The Nqweba Dam catchment (3651 km2), located in the semi-arid region of the Eastern Cape of South Africa, was identified for the analysis. All the information and data required to execute a SHETRAN simulation were obtained, which include: Topography; soil distribution and -characteristics; land cover distribution and vegetation properties; streamflow data; and reservoir survey data. The reservoir survey data was used to determine the historical bed sediment densities and average sediment yield for numerous historical periods in the catchment. The SHETRAN model was calibrated against observed streamflow and sediment data for current catchment and climate conditions. The calibration parameters were verified, and high sediment yield areas were identified. Future climate data projected by eleven climate models for two possible future emission scenarios were used to determine climate change signals for numerous future periods. The climate change signals were applied to the current climate data to represent possible future climate conditions. It was determined that climate change would cause an increase in average rainfall and evaporation in the study area. The possibility of vegetation change was evaluated and the calibrated SHETRAN model was implemented for different future scenarios. It was found that climate change will increase sediment yield in relation to the baseline period for the Nqweba Dam catchment. However, the predicted sediment yield is still lower than some historical observations. During the early 1900s, sediment yields higher than 400 t/km2/a have been recorded, while the future predictions range between 90 and 200 t/km2/a. The current sediment yield for the Nqweba Dam is 57 t/km2/a. The historical catchment characteristics were evaluated. It was determined that poor farm management and overgrazing during the early 1900s had a more significant influence on catchment response and the increase in sediment yield than climate change. Improved farm practices and the construction of numerous farm dams that act as sediment traps significantly impacted the decline in historical sediment yields. It was suggested that improved farm management must be maintained. In high sediment yield areas, farmers must be educated on the impact of overgrazing and poor farm management on erosion and the downstream effect. Recommendations for the methodology that can be adopted to model climate change and suggestions for future research were given.

AFRIKAANSE OPSOMMING: Reservoir sedimentasie wat veroorsaak word deur gronderosie en sedimentlewering het 'n groot probleem in Suid-Afrika geword, veral in semi-woestyn streke soos die Karoo, waar waterskaarste en die vermindering van opgaarkapasiteit van damme, sosiale- en omgewingsrisiko’s kan veroorsaak. Die onsekerhede rakende die impak wat klimaatsverandering het op die hidrologiese siklus en die invloed op die opvanggebied se reaksie, verhoog hierdie kommer. Die hoofdoel van hierdie tesis was om die moontlike toekomstige gevolge van klimaatsverandering op sedimentlewering te evalueer deur 'n fisies-gebaseerde hidrologiese en sedimentleweringmodel, SHETRAN te implementeer, en voorspelde toekomstige klimaatdata daarop toe te pas. Uit 'n literatuurstudie is agtergrondinligting rakende grond- en plantegroei-eienskappe, gronderosie, sedimentlewering, fisies-gebaseerde modelle (wat op die SHETRAN-model fokus), klimaatsverandering en klimaatmodelle verkry. Die Nqweba-opvanggebied (3651 km2) wat in die semi-woestyn streek van die Oos-Kaap van Suid-Afrika geleë is, is vir die ontleding gekies. Al die inligting en data wat benodig word om 'n SHETRAN-simulasie uit te voer, is verkry, insluitend: Topografie; grondverspreiding en -eienskappe; verspreiding van plantegroei en plantegroei-eienskappe; stroomvloei data; en reservoiropname data. Die reservoiropname data is gebruik om die historiese bodemsedimentdigtheid en gemiddelde sedimentlewering vir talle historiese periodes in die opvanggebied te bepaal. Die SHETRAN-model is gekalibreer teen die waargenome stroomvloei- en sedimentdata vir huidige opvanggebied- en klimaatstoestande en hoë sedimentleweringstreke is geïdentifiseer. Toekomstige klimaatsdata wat deur elf klimaatmodelle geprojekteer word vir twee moontlike toekomstige emissiescenarios, is gebruik om klimaatsveranderingseine vir talle toekomstige tydperke te bepaal. Hierdie seine is op die huidige klimaatdata toegepas om moontlike toekomstige klimaatstoestande voor te stel. Daar is vasgestel dat klimaatsverandering 'n toename in gemiddelde reënval en verdamping sal veroorsaak in die studie area. Die moontlikheid van plantegroeiverandering is geëvalueer en die gekalibreerde SHETRAN-model is geïmplementeer vir verskillende toekomstige scenario's. Daar is gevind dat klimaatsverandering 'n toename in die sedimentlewering sal veroorsaak in verhouding met die basislynperiode vir die Nqweba-opvanggebied, maar die voorspelde sedimentlewering is steeds laer as sommige historiese waarnemings. Gedurende die vroeë 1900’s is sedimentlewerings van meer as 400 t/km2/a waargeneem, terwyl die toekomstige voorspellings slegs tussen 90 en 200 t/km2/a is. Die huidige sedimentlewering vir die Nqweba Dam is 57 t/km2/a. Die historiese opvanggebiedseienskappe is geëvalueer en daar is vasgestel dat swak boerderybestuur en oorbeweiding 'n groter invloed op die verhooging in sedimentlewering gehad het as klimaatsverandering. Die verbetering van boerderypraktyke en die konstruksie van talle plaas damme, wat sediment opvang, het ‘n beduidede invloed op die vermindering van sedimentlewering vir die Nqweba Dam opvangsgebied gehad. Daar is voorgestel dat verbeterde boerderypraktyke gehandhaaf moet word en in gebiede met hoë sedimentlewerings moet boere ingelig word oor die impak van oorbeweiding en swak boerderybestuur op erosie en die stroomaf-effek. Aanbevelings vir die metodiek wat gebruik kan word om klimaatsverandering te modelleer, asook voorstelle vir toekomstige navorsing is gegee.

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