Browsing by Author "Wolski, Piotr"

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    Open Access
    Are all wetland models the same? Comparing wetland models and streamflow regulation of catchment-scale hydrological modelling tools under a changing climate
    (2023) Metho, Penisoh; Wolski, Piotr
    Comparing how wetlands are simulated in different hydrological modelling tools is needed to identify their suitability in different contexts. A simulated wetland will result in predictions of streamflow regulation, e.g., storing flood water and reducing high flows and releasing water in drier periods, which may or may not be realistic for a given area. Evaluating wetland models is critical for navigating the different types of physical wetlands with variable influences on streamflow, and the different simulated wetlands conceived in the plethora of modelling tools (i.e. software) available for use. A recent study found that sometimes wetlands are excluded from hydrological models used to inform water resource decisions. When wetlands are included in a hydrological model, few studies identify process similarities between the actual and modelled wetland or the realism of the modelled impacts of the wetland on streamflow before applying the model's output to water resource decisions. This research aims to identify and evaluate wetland characteristics, processes and impacts on catchment streamflow in different modelling tools and models (i.e. setups in a tool). Evaluating wetland models supports wetland-inclusive modelling and ensures that a wetland model is hydrologically sound and suitable. An unchannelled valley-bottom wetland located in the upper Kromme catchment, Eastern Cape, South Africa, was used. Wetland models were compared as independent units conceptually and as functional units within the catchment by modelling. First, using qualitative analysis, a conceptual assessment of wetland model structures in ACRU, WRSM-Pitman, MIKE SHE coupled with Hydro River and SWAT were considered in the context of the case study wetland. Second, using quantitative analysis, model outputs from wetland models in ACRU and WRSM-Pitman were assessed for model performance, behaviour and streamflow regulation during droughts and floods. The predicted impact of the wetland on catchment hydrology was determined from scenarios with and without a wetland and modelled wetland storage fluxes over the whole simulation period, four severe floods and three drought periods. The results from the qualitative and quantitative comparisons suggest that similarities between the physical and simulated wetland improves the likelihood of model suitability, good model performance and streamflow regulation predictions. Additionally, models setup for the same wetland with the same input data simulated potentially acceptable but different streamflow totals: for an observed total of 9.13 Mm3 ; WRSM-Pitman's comprehensive wetland simulated 10.64 Mm3 ; and from ACRU's riparian zone and wetland HRU's simulated 11.31 Mm3 and 8.89 Mm3 , respectively. Modelled actual evapotranspiration was underestimated by the riparian zone wetland (946.08 mm), overestimated in the comprehensive wetland model (2 054.80 mm) and moderately similar in the wetland HRU when compared with remotely-sensed data (1 520.30 mm). During extreme events, all models simulated flood attenuation while drought responses were variable (two wetland models predicted streamflow attenuation). By implication, the results suggest that good model performance does not guarantee the simulation of expected streamflow regulation roles recorded in literature. Furthermore, variable water yields and wetland impacts from the models demonstrated the possibility for different modelling efforts to result in different water supply, use and conservation measures. The study highlights the importance of contextualising model output for catchments with wetlands before applying the simulations to impact assessments or future climate scenarios.
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    Attribution of the 2015-2016 hydrological drought in KwaZulu-Natal to anthropogenic climate change
    (University of Cape Town, 2020) Karlie, Makeya; Wolski, Piotr
    In 2015-2016 Kwa-Zulu Natal (KZN) and other provinces in South Africa suffered from drought conditions. Drought can have negative impacts on the environment, society and the economy. Climate change is predicted to exacerbate extreme events such as droughts that would adversely affect already vulnerable regions such as KZN. The main aim of this study is to implement the attribution procedure, to determine if climate change has contributed to the 2015-2016 hydrological drought in selected KZN catchments. Methodology of the study followed a general framework of implementation of hydrological attribution experiments with climate data obtained from attribution simulations with HadAM3p global climate model. Prior to simulations in attribution mode, QSWAT model was set up for the study area and calibrated using SWAT-CUP and SUFI-2. Calibration results were poor but the model could be applied in the context of this study, under certain constraints. Results of attribution experiments revealed that for all 3 subbasins studied no increase of risk was observed and hence no influence of climate change on the 2015-2016 magnitude of drought for selected catchments was concluded by this study. These results are limited, as they are based on climate attribution experiments with only one climate model, rather than with a multi-model ensemble. Also, QSWAT model, in its implementation with generic climate data is of limited use in attribution (or hydrological) simulations as even after calibration the model performs poorly.
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    Attribution of the risk of extreme flood events to climate change in the context of changing land use and cover: case study of the shire river basin flood of 2015
    (2019) Likoya, Emmanuel; Wolski, Piotr
    The 2015 flood event in the Shire River basin was characterised by Malawi Government’s Department of Disaster Management (DoDMA) as the worst on record. It led to the damage in property worth millions of dollars with recovery still ongoing 3 years later. Over 150 fatalities were confirmed at the time with hundreds of others missing. The extent of the damage of the disaster was perhaps underlined by the swift adoption of the disaster management policy which was still in draft format then and the adoption of the climate change management policy a year later. In the aftermath of the disaster, as with most extreme weather events elsewhere around the world, questions were asked as to whether climate change might have had a hand in the occurrence of such an event and whether, going into a warmer climate, events of that nature of extremity will be the new normal. By using the risk-based event attribution methodology based on dedicated attribution experiments with a global climate model, and focusing on one of the sub-catchments of the Shire River basin, this study explored whether climate change from anthropogenic sources might have influenced the likelihood of such an event occurring. However, given the nature of hydrological events and the land use history of the basin, land use and cover change is another potential flood risk factor which, if overlooked, might affect conclusions with regards to the contribution of external factors to the risk of flooding. To account for both climate change and land use and land change, four sets of rainfallrunoff simulations were run using the Hydrologiska Byrans Vattenbalans-avdelning (HBV) hydrological model which has the ability to simulate the impact of land use and climate change on rainfall-runoff relationships. Each set was a combination of a climate scenario-either “factual” or “counter-factual”- and land use and cover change scenario-either factual (historical) or counterfactual (current). The climate scenarios were based on simulated rainfall and temperature from the HadAM3p model run in two modes-the “factual” and “counter-factual”- simulating the climate with atmospheric conditions closely resembling the atmosphere at the time of occurrence of the event and the climate as it would have been without human emissions of greenhouse gases. The proportion of the risk was calculated to determine how the risk of experiencing a flood of the January-April 2015 magnitude (for 1-day, 10- day, and 30-day maximum flows) changes with climate change only, land use and cover change only, as well as both climate change and land use and cover change. The results demonstrated that the probability of exceeding the 1-day maximum flow of the 2015 magnitude was lower in the factual (current) climate than in the counter-factual. However, changes in land use modify the flood risk such that, when land use change was accounted for, the extent of the reduction in the risk was lower. On the other hand, exceedance probabilities for 10-day and 30-day maximum flows were higher in the factual (current) climate. This was further heightened by changes in land use and cover. The study also established that observational uncertainties typical of the region may influence event attribution results to some extent. The results, which are based on a single attribution method and a single global climate model, do not span the method-model uncertainty range. As a consequence, the results are limited and do not constitute a fully defensible attribution statement.
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    The capacity of the Cape Flats aquifer and its role in water sensitive urban design in Cape Town
    (2017) Mauck, Benjamin Alan; Winter, Kevin; Wolski, Piotr
    There is growing concern that South Africa's urban centres are becoming increasingly vulnerable to water scarcity due to stressed surface water resources, rapid urbanisation, climate change and increasing demand for water. Furthermore, South Africa is a water-stressed country with much of its surface water resources already allocated to meet current demands. Therefore, in order to meet the future urban water supply requirements, countries like South Africa will need to consider alternative forms of water management that focus on moving towards sustainability in urban water management. WSUD is one such approach that aims to prioritise the value of all urban water resources through reuse and conservation strategies, and the diversification of supply sources. This study investigates the capacity of the Cape Flats Aquifer (CFA), assessing the feasibility of implementing Managed Aquifer Recharge (MAR) as a strategy for flood prevention and supplementing urban water supply. The implementation of MAR on the CFA aims to facilitate the transition towards sustainable urban water management through the application Water Sensitive Urban Design (WSUD) principles. The fully-integrated MIKE SHE model was used to simulated the hydrological and hydrogeological processes of the CFA in Cape Town at a regional-scale. Using the results of the regional-scale model, four sites were selected for more detailed scenario modelling at a local-scale. Several MAR scenarios were simulated to evaluate the aquifer's response to artificial recharge and abstraction under MAR conditions. The first objective was to evaluate the feasibility of summer abstractions as a flood mitigation strategy at two sites on the Cape Flats prone to winter groundwater flooding, viz. Sweet Home and Graveyard Pond informal settlements. The second objective of the study was to assess the storage potential and feasibility of MAR at two sites in the south of the Cape Flats, at Philippi and Mitchells Plain. In addition, the migration of solute pollutants from the injected or infiltrated stormwater was simulated and climate change simulations were also undertaken to account for potential fluctuations in rainfall and temperature under climate change conditions. The results indicated that flood mitigation on the Cape Flats was possible and was likely to be most feasible at the Graveyard Pond site. The flood mitigation scenarios did indicate a potential risk to local groundwater dependent ecosystems, particularly at the Sweet Home site. Yet, it was shown that a reduction in local groundwater levels may have ecological benefits as many of the naturally occurring wetlands on the Cape Flats are seasonal, where distinct saturated and unsaturated conditions are required. Furthermore, MAR was shown to improve the yield of wellfields at Philippi and Mitchells Plain through the artificial recharge of stormwater while also reducing the risk of seawater intrusion. MAR was shown to provide a valuable means of increasing groundwater storage, improving the supply potential of the CFA for water supply while aiding the prevention or mitigation of the seasonal flooding that occurs on the Cape Flats. Furthermore, the case was made that MAR is an important strategy to assist the City of Cape Town in achieving its WSUD objectives. MAR and groundwater considerations, in general, are essential for the successful implementation of WSUD, without which, there is an increased risk of overlooking or degrading urban groundwater resources. The findings of this study resulted in a number of recommendation to urban water resources managers, planners and policy makers. First, MAR is an important means for Cape Town to move towards becoming a truly water sensitive city. This study indicated that the CFA should be incorporated as an additional source of water supply for Cape Town especially considering the recent drought conditions and due to its ability for the seasonal storage of water, this would improve the city's resilience to climate change. Furthermore, it was recommended that the application of MAR on the CFA could also be used to reduce groundwater related flooding on the Cape Flats. Second, it was emphasised that urban planning, using WSUD principles is essential for the protection of the resource potential of the CFA. Finally, for the implementation of WSUD and MAR to be successful, there needs to be appropriate policy development alongside the implementation of these strategies to ensure they are achieving their initial objectives and are not causing detriment to the aquifer.
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    Implementation and evaluation of the Pitman model in seasonal hydrological forecasting mode using the Kraai River catchment in Eastern Cape South Africa as a case study
    (2020) Fikileni, Sesethu; Wolski, Piotr
    Seasonal hydrologic extremes such as drought and floods have devastating impacts on human and natural systems (e.g. 2015-2017 Western Cape drought). Sentence has been reworded to: Therefore, the need for a reliable seasonal hydrologic forecast is significant and becoming even more urgent under future climate, as the assimilation of seasonal forecast information in decision making. Hence, SHF becomes part of the short and long-term climate change adaptation strategies in a range of contexts such as energy supply, water supply and management, rural-urban, agriculture, infrastructure and disaster preparedness and relief. This work deals with implementation and evaluation of the Pitman/Water Resources Simulation Model 2012 model (WR2012) in seasonal hydrological forecasting mode. The aim of the study is to improve the understanding of seasonal hydrological forecasting by evaluating the performance of a hydrological model (Pitman Model) in the seasonal forecast mode in Kraai River tertiary catchment (D13) as a case study and the objectives are: To determine steps to be undertaken to implement integration of Pitman in WR2012 configuration with climate forecast to generate seasonal hydrological forecast and to evaluate the performance of the model forced by climate model data in the simulation and forecast mode. Pitman model in the WR2012 version works with a specific rainfall dataset spanning the period of 1920-2009. Operationalizing the seasonal hydrological forecast with Pitman model requires, therefore, updating of the WR2012 rainfall so that it extends to-date. To achieve that, two datasets were evaluated: Climate Hazards Group InfraRed Precipitation with Station Data (CHIRPS), which is a satellite-based gridded rainfall dataset, and rain gauge-based dataset from South African Weather Service (SAWS). The analyses revealed that CHIRPS rainfall data had better correlation and lower bias with respect to the WR2012 data when compared with SAWS rainfall data for the overlap period 1981-2009. The CHIRPS data showed no significant difference from the WR2012 in all the three rainfall zones of the Kraai River catchment. Therefore, CHIRPS data were used to extend the WR2012 data and were used as input to set up Pitman model/WR2012 in the seasonal hydrological forecasting mode. The Pitman/WR2012 model was forced with 10 ensemble seasonal climate forecast from Climate Forecast Systems v.2 which is downscaled using the Principal Components Regression (PCR) approach. The generated seasonal hydrological forecast focused on the summer season, in particular on the Dec-Jan-Feb (DJF) period, which is the rainy season in the catchment. The hydrological forecast showed skills more especially in Dec and Feb (assessed through ROC and RPSS forecast verification methods) with Jan having a poor skill. Importantly, the skill of streamflow forecast was better than that of rainfall forecast, which likely results from the influence of initial conditions of the hydrological model. In conclusion Pitman/WR2012 model can perform realistically when implemented in seasonal hydrological forecasts mode, and it is important that in that model, the model is run with near real time rainfall data in order to achieve good initial conditions. However, the results in terms of forecast skill are specific to the studied catchment and analysed forecast, and skill of forecast in any other catchment has to be investigated separately.
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    Potential impacts of climate change on hydrological droughts in the Limpopo river basin
    (2021) Makhanya, Nokwethaba Zamanguni; Wolski, Piotr; Abiodun, Babatunde
    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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    The Influence of Anthropogenic Climate Change on the 2015-2017 Hydrological Drought in the South-Western Cape, South Africa
    (2021) Hall, Andrew; New, Mark; Wolski, Piotr
    The Western Cape Province in South Africa recently experienced below-average rainfall during the period 2015−2017, this resulted in a three-year compound hydro-meteorological drought event in the Province. The 2015−2017 Western Cape hydro-meteorological drought was the worst drought event since 1904 and caused severe unprecedented water shortages throughout the Western Cape region, with many municipal water supply systems close to failure by the first quarter of 2018; most especially the Western Cape Water Supply System that serves Cape Town. The drought gained a lot of interest from the public, media and climate scientists alike. The main aim of this study was to assess the extent to which human influence on climate from fossil fuel emissions has changed the likelihood of a hydrometeorological drought event with the magnitude of that experienced in 2015−2017 in the SouthWestern Cape. The Pitman hydrological model was set up for the Berg River catchment in a way that enabled multiple simulations with different rainfall inputs so that attribution experiments could be undertaken. The key differences to the standard Pitman model set up included: (i) constant abstractions, return flows, and land use conditions; (ii) reservoir and dam storages were set to reflect current storage volumes; and (iii) extending the observed rainfall inputs to include the drought period. A hydrological model evaluation was then undertaken, using updated streamflow gauging station data, to assess the ability of the Pitman model to realistically simulate runoff in the Berg River catchment. The model was deemed suitable for the purposes of this study in simulating runoff. To generate the climate attribution experiments, Coupled Model Intercomparison Project Phase 5 historical simulations (1861−2010) were merged with the Representative Concentration Pathway 8.5 greenhouse gas scenario simulations (2011−2100) of rainfall from 77 simulations From 42 models to create a long-term (150 years) time series. Attribution experiments were constructed by considering the average conditions in the 31 year period centred on the years of the event, i.e. 2002−2031 to represent current climate conditions and the period 1861−1890 to represent pre-industrial climate conditions. Five 150-year long stochastic time series of rainfall for each individual simulation were then generated conditioned on observed rainfall characteristics this was done to increase the sample size of the models available. These stochastic rainfall time series were then used as input to the Pitman model to generate outputs/realisations of runoff for a pre-industrial and current world; thus generating impact attribution experiments. To determine the role of anthropogenic climate change on the 2015−2017 hydro-meteorological drought in the South-Western Cape the risk-based approach was applied to the rainfall and runoff attribution outputs. The 2015−2017 meteorological/hydrological drought event was defined in terms of three-year mean annual rainfall/runoff received in the Berg River catchment and its individual 12 quaternary catchments. This event definition was used as a rainfall/runoff threshold in the attribution analysis for the 2015−2017 meteorological/hydrological drought in the South-Western Cape. The three-year minimum averages of rainfall/runoff were identified in each of the 150-yearstochastic time series generated from the 77 simulations; resulting in 385 values for both current and pre-industrial climates for rainfall and runoff. A normal distribution was then fitted to the 385 values of the current and pre-industrial rainfall/runoff. From this distribution, the probability of the current rainfall/runoff occurring, based on the defined threshold, was identified and compared to the pre-industrial time series to calculate the risk ratios of the Berg River catchment and its 12 quaternary catchments. Results show that the risk of the meteorological drought event occurring in the Berg River catchment was increased by a factor of 28.5, 95% confidence interval: 26.0−32.4, (but ranged from 11.5−41.0 in the individual quaternary catchments) due to anthropogenic climate change. The occurrence of the hydrological drought event in the Berg River catchment was found to be increased by a factor of 12.9, 95% confidence interval: 11.3−13.5 (2.7−61.0 in the quaternary catchments) due to anthropogenic climate change. The risk ratio for runoff was higher than for rainfall in the wetter southern quaternary catchments, while it was lower than for rainfall in the drier more northern quaternary catchments. Thus, the human influence on meteorological drought appears to have been amplified in those catchments most important to the Western Cape Water Supply System.