Hydrology

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An assessment of the critical source areas and transport pathways of diffuse pollution in the Umngeni Catchment, South Africa.
(2018) Nsibirwa, Nantale Edith.; Jewitt, Graham Paul Wyndham.
The difficulty in locating and managing diffuse pollution sources and their transport pathways is one of the reasons for the continued degradation of surface water in South Africa. Dealing with this problem is complex, as the sources and transport pathways of the pollutants are often not known because of the diffuse nature of the pollution. This study demonstrates the constraints of conventional diffuse pollution assessment approaches in identifying the Critical Source Areas (CSAs) and transport pathways of diffuse pollution, as applied in the uMngeni Catchment, South Africa. The use of various risk-based modelling approaches are reviewed for identifying the risk of diffuse pollution generation and transportation across a catchment landscape. The Sensitive Catchment Integrated Modelling and Analysis Platform (SCIMAP) Model is a risk-based tool that was developed to give a spatial representation of diffuse pollution sources. In this study, the SCIMAP Model was applied to identify and prioritise the protection and control of nutrient CSAs and transport pathways within the uMngeni Catchment. The results of the study were displayed in a catchment scale web map. The hydrological connectivity risk in the catchment was higher in the high-lying western areas and lower in the middle-eastern areas. The upper and middle parts of the catchment that are dominated by commercial agriculture and built-up urban areas were identified as the most impactful CSAs for intervention. The results are immediately applicable to water managers in the catchment and are strongly linked to the investment efforts in ecological infrastructure. A walkover survey revealed that the SCIMAP Model was able to direct the CSA investigations to the nutrient sources at four out of five locations. The survey also revealed that the accuracy of the modelled transport pathways increased with an increase in the elevation difference. The sensitivity of the SCIMAP Model to input land cover weightings was assessed, using an objective function. A high sensitivity of the modelled high-risk areas was observed on the intermediate diffuse pollution risk map, and a slight sensitivity of the modelled high-risk areas on the final diffuse pollution risk map, when the input landcover weightings were increased and decreased by 5%, 10% and 15%. This implies that caution should be practised in the formulation of the input land cover weightings, as they are a potential source of error in the model outputs. It is concluded that SCIMAP is a valuable tool for identifying the CSAs and transport pathways of diffuse pollution in a catchment. The results of the model can better inform the management of diffuse pollution and guide investments in the protection of the ecological infrastructure in the uMngeni Catchment. However, the establishment of input land cover weightings is very important and should receive priority in similar studies in the future.
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.
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.
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.

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