Doctoral Degrees (Hydrology)
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Item Lidar and satellite observation of aerosals and clouds over South Africa.(2017) Shikwambana, Lerato David.; Venkataraman, Sivakumar.Abstract available in the PDFItem Effects of land use and land cover changes on water quality of the upper Umngeni River, KwaZulu-Natal Province, South Africa.(2017) Namugize, Jean Nepomuscene.; Jewitt, Graham Paul Wyndham.Changes of land use and land cover are important drivers of the quality of water reaching a waterbody. These changes affect the catchment and modify the chemical composition of the atmosphere, and thus altering the cycle of nutrients and the flux of energy. With current developments in Geographic Information Systems (GIS) techniques, hydrological modelling and statistical analyses, one or a combination of many methods can be used to assess the relationships between land use and land cover (LULC) classes and water quality variables. However, all these approaches are reliant on the collection of field measurements, LULC data and water sampling. Typically funding for such long-term information is not generally available in Africa. A three-year study involving analysis of historical data, field work and desktop investigations was conducted in the upper reaches of the uMngeni Catchment (1653 km2), South Africa, to assess the spatial and temporal variation of land use and land cover and its influence on the flux of water, nutrients (nitrogen and phosphorus) and Escherichia coli (E. coli) in the catchment. This involved the analysis of historical land use and land cover information (1994, 2000, 2008 and 2011), analysis and processing of historical datasets of E. coli, electrical conductivity, ammonium, nitrate, soluble reactive phosphorus (SRP), total phosphorus (TP), total suspended solids (TSS), temperature and turbidity. A water quality index based on a long-term data base of water quality emanating from existing monitoring programmes was assessed. In addition, stations were established for river sampling (14) and collection of bulk atmospheric deposition (3) of ammonium, nitrates, SRP and TP, in the Midmar Dam catchment (927 km2). These were consolidated with the application and testing of the Hydrological Predictions for the Environment (HYPE) model in the catchment, in simulating streamflow, transport and dynamic of inorganic nitrogen and total phosphorus, resulting from LULC changes. Results showed that the natural vegetation declined by 17% between 1994 and 2011, coinciding with an increase in cultivated, urban/built-up and degraded lands by 6%, 4.5% and 3%, respectively. This resulted in high variability in the concentrations of water quality parameters, but Midmar and Albert Falls Dams retain over 20% of nutrients and sediment and approximately 85% of E. coli. It was concluded that these dramatic changes in LULC directly affect the chemical composition of water in the catchment. However, these linkages are complex, site-specific and vary from one sub-catchment to another and decision-making regarding water resources management in the catchment must recognise this. The level of E. coli in water is a major issue for human contact during recreational activities in the entire study area. Higher concentrations of E. coli, ammonium, nitrates, SRP and TP were attributed to the poor or lack of sanitation facilities in the informal settlements, dysfunctional sewage systems, effluent discharged from wastewater works, expansion of agricultural activities, as well as a runoff from livestock farming and urban areas. Moreover, water quality in the catchment ranged between “marginal” and “fair”, predominantly “marginal” in 90% of the sites and completely poorer in the Mthinzima Stream, an important tributary of Midmar Dam. A declining monitoring frequency and resultant poorly reporting of water quality in the catchment, led to a recommendation for the establishment of automatic or event-based samplers, which should provide the optimum information on nutrient loadings to the waterbodies. Bulk atmospheric deposition and river inflows into the Midmar Dam studies were conducted under severe drought conditions. Higher concentrations of NH4, NO3 and TP in precipitation samples than those of rivers were found because of the high retention of nutrients in the landscape. In terms of loading, the bulk atmospheric deposition provided significant quantities of NH4, while TP, SRP and nitrates were predominantly from river flows. Specific loads of DIN (nitrate + ammonium) and TP in the catchment were slightly higher that the previously reported values for the catchment and are comparable to the other human-disturbed catchments of the world. HYPE model has successfully simulated streamflow (1961-1999), DIN and TP (1989-1999). For simulations of streamflow NSE values = 0.7 in four out of the nine sites (at a monthly time-step) and NSE > 0 in eight out of nine sites (at a daily time-step). Major floods and drought events were represented very well in the model, with a general over-simulation of baseflow events. The water balance was captured well at calibration sites with over-simulation of streamflow on the Lions River (PBIAS=28%) and their under-simulation in outlet sub-catchments (PBIAS < 0). This is ascribed to the simplification of some processes in the model i.e. evapotranspiration, water release, water abstraction and inter-basin transfer. There has been good fit between the simulations and observations of TP and streamflow with a lagging of the observed values. However, mismatches were noted for DIN. Evaluation of seasonal distribution of DIN suggested that denitrification, crop uptake of DIN and dilution were intensive during the period of rainfall and high temperatures in the catchment, while TP was highly mobilised during rainfall events, due to its strong binding with the soil. The information from this study highlighted the current state of LULC changes, the sub-catchments with the potentiality to export high levels of DIN and TP, the complexity of the relationship between LULC-water quality, the gaps in existing data collection programmes, the catchment responses to LULC changes and the usefulness of hydrological models which may apply beyond the upper reaches of the uMngeni Catchment.Item Evaluating the potential of using satellite earth observation data to quantify the contribution of riparian total evaporation to streamflow transmission losses.(2017) Gokool, Shaeden.; Riddell, Edward Sebastian.; Chetty, Kershani Tinisha.; Jarmain, Caren.Numerous perennial rivers which flow through arid and semi-arid environments in South Africa, have become severely constrained as water resources abstractions are close to exceeding, or have exceeded the available supply and ecosystem resilience. This is a common phenomenon, as river basins are increasingly developed and often over allocated, in order to maximize socio-economic benefits through consumptive water use, often at the expense of the environment. Thus, managing and maintaining environmental water requirement (EWR) flow allocations in these circumstances becomes increasingly important but all the more challenging, especially during periods of water scarcity. The Letaba River situated in the semi-arid north-eastern region of South Africa is a typical example of a river system in which water governance challenges and infrastructural development have resulted in flows within the river no longer resembling the natural flow regime. This situation has improved to some extent after the establishment of river operating rules and an adaptive operational water resources management system. However, one of the major challenges with successfully implementing and managing EWR flows to date has been the uncertainty regarding the magnitude and influence of streamflow transmission losses (TL’s) on flows within the river system. TL’s along the Letaba are thought to be a significant proportion of streamflow during dry periods and this therefore constrains the ability to meet target EWR flows, as it is often the case that specified EWR releases from the Tzaneen dam are not adequately met further downstream at EWR target gauges. To ensure that water provisions and in particular EWR flows can be managed more effectively and efficiently in the future, it is imperative that the hydrological processes contributing to TL’s are quantified at various spatial and temporal scales. Considering this statement as a point of departure, the overall objective of this thesis was to reduce the uncertainty associated with TL’s by attempting to acquire an improved hydrological process understanding of the natural drivers of loss in this system, so that TL’s along the Letaba River can be more accurately quantified. This research involved, conducting detailed characterizations of hydrological processes along a 14 km reach of the Groot Letaba River which has similar land use activities and hydrological characteristics to the broader river system. Particular emphasis was placed upon establishing the influence of riparian total evaporation (inclusive of open water evaporation) on TL’s, as this process is a major contributing factor to the water balance of arid and semi-arid environments, yet has seldom been incorporated or adequately represented into TL’s estimation procedures. These investigations were centred on evaluating the potential of using a satellite-based approach to acquire spatially explicit estimates of evapotranspiration (ET) during the low flow period in this river system (May to October), which typically represents a critical period with regards to water shortages. For this purpose, the satellite-based surface energy balance (SEBS) model and satellite earth observation data acquired from Landsat and Moderate-resolution imaging spectroradiometer (MODIS) were used to estimate ET. However, the trade-off between the spatial and temporal resolution associated with these data sets can limit the reliability of satellite-based ET modelling (except where occasionally correct). Consequently, the SEBS ET estimates from these data sets were used as inputs to two relatively simplistic approaches (actual crop coefficient or Kcact and output downscaling with linear regression or ODLR) to quantify ET at a moderate spatial resolution (30 m) on a daily time step. These ET estimates were compared against in-situ ET estimates using a one sensor Eddy Covariance system to quantify any uncertainties associated with the satellite-derived estimates. To further investigate spatial and seasonal variations in source contributions to plant water uptake during the investigation period, stable isotope analysis (of 18O and 2H) and a Bayesian mixing model were coupled with the satellite derived ET estimates. The insights acquired from these investigations, were then used to derive baseline estimates of TL’s. This involved using the satellite-derived daily ET time series in conjunction with data obtained from a parallel investigation focusing on quantifying the rapport between surface and sub-surface water storage processes. Initial comparisons of ET estimates acquired using the Kcact and ODLR approaches against ECET were fairly poor yielding RMSE values of; 1.88 and 2.57 mm d-1 and 1.10 and 2.39 mm d-1 (for two replicate transects), respectively. The poor performance of these techniques was largely attributed to the SEBS ET estimates used as inputs to these techniques, as SEBS may overestimate evapotranspiration during conditions of water stress. This limitation was overcome using an evaporative calibration factor (termed the environmental stress factor or ESF) into the original SEBS formulation (SEBS0), to correct for the overestimation of the latent heat flux (LE) and the evaporative fraction (EF). The ESF calibration factor was empirically derived and then integrated into SEBS0, so as to better represent the influence of water stress on the EF and consequently LE. The implementation of the modified version of SEBS (SEBSESF) was shown to significantly improve the estimation of energy fluxes, which in turn resulted in an improved correlation and an increase in the percentage of modelled ET estimates within an acceptable accuracy range (± 15 to 30 %) when compared against in-situ observations. Through the application of this modified version of SEBS (SEBSESF), the ability of the ODLR and Kcact approaches to develop a time-series of daily moderate spatial resolution ET estimates could now be demonstrated. The use of SEBSESF ET estimates as inputs to the Kcact approach was shown to compare most favourably to ECET, yielding correlation coefficient and Nash-Sutcliffe efficiency values of 0.79 and 0.60, respectively. With the ability of this satellite-based approach to adequately represent ET within this environment now confirmed. Stable isotope analysis (of 18O and 2H) and a Bayesian mixing model were coupled with the Kcact derived ET estimates, to further investigate spatial and seasonal variations in plant water uptake dynamics. The results of these investigations showed that soil water was the main contributing source to ET. While stream and groundwater use during transpiration was also prevalent within the study area and increased with aridity, the magnitude of the contribution of these sources to transpiration was fairly minimal and not as significant as generally reported in literature. The insights gained from these investigations, as well as those obtained from the quantification of surface and sub-surface water storage processes, assisted in deriving baseline estimates of TL’s along the length of river reach studied. In general, it was found that during the latter stages of the dry season (August to October) TL’s accounted for approximately 5 to 15 % of the flow in the river system, with riparian total evaporation and in particular transpiration the dominant contributing processes to this loss. Through linkages with the recent gazetting of the Letaba Management Class (resource objective setting) and the mandatory implementation of EWR flows, it was shown that flows within the river system were unable to meet low flow targets and are required to be increased in order to fulfil this requirement, whilst simultaneously accounting for TL’s. It should be noted that while the various investigations undertaken in this study enabled the estimation of TL’s and the contribution of processes viz. riparian ET to TL’s, the estimates provided could not be verified due to the lack of reliable upstream (inflow) flow gauge data. Although the investigations and observations detailed in this study provide an understanding of the system for a limited period in time, they would substantially benefit from longer-term monitoring, so that the assumptions and related uncertainties that had to be factored into the analysis could be reduced. Overall the study has detailed key hydrological processes influencing TL’s along the Groot Letaba River, providing invaluable insights on existing knowledge gaps and contributing new knowledge to this research area. It is envisaged that this will enable the establishment of an improved conceptual understanding of the system, which may prove to be beneficial for future hydrological modelling applications in this region.Item Impacts of global changes on a lowland rainforest region of West Africa.(2016) Aduah, Michael Soakodan.; Jewitt, Graham Paul Wyndham.; Toucher, Michele Lynn.Abstract available in PDF file.Item Hydrogeological and three-dimensional numerical groundwater flow modelling of the Lake Sibayi catchment, Northern KwaZulu-Natal, South Africa.(2016) Weitz, Jan Christian.; Demlie, Molla Bekele.Lake Sibayi, a topographically closed fresh water lake in northern KwaZulu-Natal, South Africa, and the coastal aquifers surrounding the lake, are important water resources for the local community and the surrounding ecosystem. A significant decline in lake levels has been experienced over the last decade, dropping from approximately 20 m above mean sea level (amsl) in early 2000 to below 16 m amsl at present. It is believed that this decrease could be attributed to an increase in water abstraction from the lake and surrounding groundwater, the rapidly increasing commercial plantations within the catchment and recent droughts. The effective management of this hydrological system needs a thorough understanding of the interaction of the lake with the surrounding aquifer. In recent years, hydrogeological and numerical groundwater flow modelling have become standard tools with which these interactions are studied. This thesis describes the process of conceptual model design through to the development and calibration of steady-state and transient numerical groundwater flow model for the lake Sibayi system. Through a series of field campaigns, on site measurements of depth to groundwater with surface and groundwater sampling, for hydrochemical and environmental isotope analysis, were undertaken. Hydrochemical parameters and environmental isotopes for the various water sources within the Lake Sibayi hydrological system were determined following standard procedures to study the relationship between these resources. A slight distinction between shallow and deep aquifers appears to be present, where the shallow groundwaters are dominated mainly by a Na-Cl hydrochemical facies, while the deeper boreholes are dominated mainly by a Na-Ca-HCO3-Cl hydrochemical facies. Shallow groundwater samples have relatively low EC values averaging 278 mS/m, while the deeper wells had average EC concentrations of 409 mS/m. Groundwater samples collected along the dune cordon, show a similar hydrochemical and environmental isotope composition as that of the lake. Multivariate statistical analyses including principal component factor analysis and hierarchical cluster analysis (HCA) were undertaken on the hydrochemical data. The HCA grouped the water samples into two clusters, which represented surface and groundwaters. Each of these two clusters were in turn divided into two sub-clusters, representing the shallow and deep aquifers, and stream and lake samples, respectively. As part of the conceptual modelling, the long-term water balance of the lake has been quantified by defining the various inflow and outflow components of the lake. All hydrological information including precipitation, evaporation, surface water runoff, abstraction, as well as geological, hydraulic, hydrogeochemical and environmental isotope information were used to conceptualise the hydrological system of the Lake Sibayi catchment. Local geologic, groundwater head distribution, lake level, hydrochemical and environmental isotope data were used to constrain the link between groundwater and the lake. In the western section of the catchment, groundwater flows to the lake where groundwater head is above lake stage, whereas along the eastern section, the presence of mixing between lake and groundwater hydrochemical and isotopic compositions indicate that the lake recharges the aquifer. Stable isotope signals further revealed the movement of lake water through and below the coastal dune cordon before eventually discharging into the Indian Ocean. Groundwater recharge to the catchment was estimated using the chloride mass balance (CMB) method and the results compared with estimates based on published maps. The CMB recharge estimate resulted in 126 mm/a (12 % MAP) against 95 mm/a (10% MAP), estimated using published maps. The total evaporation and evapotranspiration from the lake and its catchment were estimated at 1 495 mm/a and 1 090 mm/a, respectively. Estimated surface water runoff from the catchment to the lake is about 1% of MAP. Calculated lake water outflow to the sea through the dune cordon opposite the lake, along a 12 km seepage face, is 2.3 x 107 m3/a. The total amount of water abstracted from both surface and groundwater resources within the catchment is about 4.5 x 106 m3/a. The water balance of Lake Sibayi shows that lake levels fluctuate in response to varying amounts of groundwater and surface water inflow into the lake, seepage loss through the coastal dune, abstraction, and evaporation from the lake. Based on the conceptual hydrogeological model, a steady-state and transient numerical groundwater flow model, were developed for the Lake Sibayi system using two independent approaches, namely, the High-K method and Lake Package. Groundwater Modelling Systems (GMS), which runs on the modular finite difference code, MODFLOW 2005 with its several packages were used to characterise the three dimensional flow conditions around the lake. Two layer models were used to simulate the lake stage and aquifer conditions over a forty three year period from January 1970 to September 2014. The simulation period was broken down into 536 monthly stress periods with calibrated parameter values for each of the boundary conditions over the simulation period. The calibrated steady-state model simulation results for the two methods were comparable. While, transient model calibration results show that the Lake Package was more suitable in simulating lake level fluctuations with low calibration errors. The calibrated transient groundwater flow models were further used to evaluate the hydrological response of the lake and the groundwater system to various stress scenarios, including changes in evaporation, precipitation and land use. Once again, the High-K method was very sensitive to changes in model input, simulating rapid changes to the system, while Lake Package simulations results were in line with known changes in the system. Therefore, the High-K technique is most suitable for simple applications, while complex lake-aquifer interactions are better simulated using the Lake Package.Item Exploring the potential for the use of remote sensing technology and GIS to aid the upscaling of rainwater harvesting in Sub-Saharan Africa.(2016) Bulcock, Lauren Michelle.; Jewitt, Graham Paul Wyndham.Increased strain on water resources across the globe, and particularly in Sub-Saharan Africa, has resulted in increased vulnerability of those communities who rely directly on rainfall to sustain their livelihoods, through crop production, water for drinking and domestic purposes and other economic activities. This dynamic interface between people and the environment is central to the current decadal research theme of the International Association of Hydrological Sciences (IAHS) “Panta Rhei” – everything flows, emphasises that greater recognition and understanding of the interconnection between human action and water resources, and how in order for development plan to be sustainable they must take greater cognisance of the dynamic interface between people and the environment. Applying this philosophy to the subject of RWH suggests an alternative approach to the traditional guidelines for assessing RWH suitability approach. A review of the conditions under which RWH currently take pace was done and found that guidelines often only prescribe optimal conditions for RWH which results in many sites which may be suitable being over looked. Results show that RWH is taking place under a much broader range of conditions than those recommended by the guidelines. An alternative approach was investigated which rather aims to assess how much water a selected RWH system can supply in any location, applied at a regional scale across the whole of South Africa, under both present and shifting climate conditions as well as optimising the water storage tank to secure a certain level of supply. Results showed that the eastern portions of South Africa were best suited to RWH with supply being secured for 100 -200 days of the year. However this also highlighted that a multiple source water supply system, which can dynamically adjust to supply water from different sources depending on water supply, will be more sustainable. This will allow water demand for different uses to be satisfied for different supplies, rather than a conventional piped water supply system which provides one quality of water, often drinking water standard, for domestic consumptions where up to 70% of water use is not used for direct consumption. In order to design a dynamic sustainable system, continuous monitoring is needed to understand the constant changes in the system. One such monitoring tool gaining popularity in water resources is remote sensing (RS). RS technology was used to calculate total evaporation (ET) and the normalized difference vegetation index (NDVI) as indicators of the current implementation of RWH. This allows for a census technique to monitor the extent and uptake of RWH systems as well as evaluate the performance of different systems in increasing soil water or water available to plants. Results show that large scale techniques such as the spate irrigation in Tanzania or mass implementation of smaller techniques, such as the “Zai Pits” and contour bunds in Burkina Faso were visible from calculated ET maps. The contour bunds were the most successful in storing water, in the soil profile, for plant use with higher ET being measured from the bunded system compared to the surrounding landscape well into the dry season. However, the fields irrigated by micro-basin plastic storage tank systems in South Africa were not visible from ET maps but were visible from NDVI maps in summer. RS is also used to monitor the extent of less transient factors, such as slope and soil types, which influence the runoff potential that can be generated and then stored. Using RS at a catchment or sub-catchment scale will allow planners to evaluate the runoff potential of a landscape and design a RWH system that can sustainably capture and utilise that runoff. RS can also be used to monitor the impacts of the RWH system on the landscape by continuously monitoring the changes in ET, NDVI, soils and slope over time. RS provides a cost and time effective method for doing this from a remote location.