Hydrology

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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.
An assessment of the water quality of the Baynespruit River and its linkages to the health of the Sobantu community.
(2016) Govender, Jédine.; Stuart-Hill, Sabine Ingrid.
Worldwide, water quality degradation is rife. Rivers are amongst the most susceptible water bodies to this reality. In South Africa, the use of polluted river water for activities such as crop irrigation, washing clothes and recreation, is a common practice in many rural and urban communities. The Baynespruit River, in the province of KwaZulu-Natal, South Africa, is a typical example as it serves as a vital water source to the Sobantu community. There have been numerous reports of extremely poor water quality in this river and suggestions that this may pose health risks to the community. Thus, the aim of this study was to assess the water quality of the Baynespruit River and its linkages to the health of the Sobantu community. This was achieved through analyses of river water quality, river sediment, soil and crop samples, as well as an investigation of the pathways through which community members are exposed to the polluted river and finally, an analysis of urine from a sample of volunteers who are regularly exposed to the river water. The water quality assessment considered pH, electrical conductivity, As, Cd, Cu, Hg, Pb, Zn and E.coli, while the analysis of river sediment comprised of 23 elements including the aforementioned heavy metals. Using microwave acid digestion (EPA 3052) and Inductively Coupled Plasma – Optical Emission Spectrometry (ICP-OES), soil and crop samples from farming sites in Sobantu were analysed for Cd, Cr, Cu, Pb and Zn, and compared against the South African Water Quality Guidelines for Crop Irrigation. These results showed that E.coli contamination was high, there were extremely low concentrations of the heavy metals apart from infrequent elevated detections of Cu and Pb, as well as infrequent occurrences of acidic water. While the heavy metal concentrations of surface water were low, the sediment analysis suggested elevated concentrations of As, Cd, Cr, Cu, Ni, Pb, Zn, Fe, Mn and Ag. Analyses of soils and irrigated crops showed concentrations of heavy metals in excess of national and international guidelines, respectively. It is suggested that these soil and crop results indicate historical flooding events, which mobilized heavy metals in the river sediments and transferred them onto the floodplain where the farming sites are located. Furthermore, long-term irrigation with low concentrations of heavy metals may have also resulted in the build-up of these contaminants in the soil and eventually the crops. A workshop was held in the Sobantu community which included a questionnaire and separate open-ended conversations conducted with various community members, in order to determine the exposure pathways to the river and the associated health issues of participants. The questionnaire and open-ended conversations indicated that the most common exposure pathways to the river included using river water for crop irrigation, consuming irrigated crops, washing clothes and children swimming in the river. The questionnaire and open-ended conversations also highlighted many cases of skin rashes, as a result of being in direct contact with river water, with one reported case of diarrhoea. The confirmation of the presence of heavy metals in the Baynespruit River and its surrounding environment gave rise to a urine analysis, which used microwave digestion and ICP-OES to determine whether community members who volunteered for the study incurred heavy metal toxicities. However, the analysis did not show any severe cases of heavy metal toxicities to exposed volunteers and the high levels of Pb noted could not be attributed to exposure to the Baynespruit River and/or its surrounding environments, since similar levels of Pb were found in the control volunteers. It was therefore unclear as to whether the health of the exposed people of Sobantu was compromised by heavy metal toxicities. The persistent mention of skin rashes in the questionnaire and open-ended conversations suggests that water-related health issues in the community require further investigation. It was concluded overall that the water quality of the Baynespruit River is severely degraded however, a clear link between this poor water quality and the perceived health issues in the Sobantu community, could not be established. A key recommendation from this study would be for further investigation, i.e. through a detailed health monitoring programme, confirming the health issues that community members have associated with polluted river water.
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.
Detection and attribution of long-term climatic and hydrological trends in the Cathedral Peak catchments.
(2016) Majozi, Sibusisiwe N.; Toucher, Michele Lynn.
It has become accepted that global change is a considerable threat to vulnerable environments such as mountains. Various studies highlight the importance of change detection in long term climatic and hydrological data in understanding the catchment responses to various environmental changes. Long term trend detection of rainfall, temperature and streamflow has shown to be of practical importance to water resources management and planning. More especially in mountainous regions which have highly variable microclimates and are vulnerable to climate change impacts. In mountainous regions, the lack of data as a result of sparse observation networks often leads to a poor understanding of the climatic systems and amplifies the degree of uncertainty in trend detection. Given this need, the area of interest to this study was the intensively monitored Cathedral Peak catchments which are representative of the uKhahlamba Drakensberg region where a significant amount of water is generated for the KwaZulu-Natal and Gauteng provinces. In this context, the aim of the study was to detect trends in the historical hydroclimatic data of the Cathedral Peak catchments and gain understanding about the causes of change. To accurately detect and attribute the hydroclimatic trends in rainfall, temperature and streamflow the study was carried out using three methods. The first method investigated the data for historical trends (1948 - 2000), followed by a comparative analysis which investigated the differences between the historical and current records (2012 – 2015). The third method was an attribution study which investigated the influence of rainfall and land use to determine which of the two considered factors contributed as the cause of change. The Mann-Kendall and Sen’s slope estimator non-parametric tests were used to detect trends in the data and determine magnitude of the trends detected while the Mann-Whitney test was used to detect the difference between the historical and current records. The results across all times scales showed a few statistically significant trends in rainfall. However, the majority of the rainfall analyses showed no statistically significant trends with the expectation of a decline autumn rainfall detected in the seasonal analysis. The comparative analysis showed a few significant differences indicating increased rainfall between the historical and current period. The short current record was seen to have restrained the ability to detected definite differences in the rainfall. The significant positive trends detected in the historical temperature records and the comparative analysis provided more evidence of an increase in the temperature between the historical and current period. Furthermore, the positive trends found in the daily maximum and average temperatures were consistent with those from previous studies, which can be used to establish that there has been a general increase in the temperature between 1955 and 2000. Significant negative trends were detected in both the historical streamflow and the comparative analysis which showed evidence of a distinct decline in streamflow between 1949 and 2000. The results from the attribution study indicated that both land use change and rainfall appear to have a noticeable impact on streamflow. The complexity and highly variable nature of rainfall in the Cathedral Peak area as well as the difference in record length largely contributed to the lack of significant trends detected from the historical records and the inconclusive results obtained from the comparative study. However, despite this shortfall detection and attribution studies remain a useful tool in providing valuable information on the effects of global change in sensitive and vulnerable environments such as mountains.
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.
Lidar and satellite observation of aerosals and clouds over South Africa.
(2017) Shikwambana, Lerato David.; Venkataraman, Sivakumar.
Abstract available in the PDF
Estimating water use and yield of soybean (glycine max) under mulch and fertilizer in rainfed conditions in KwaZulu-Natal.
(2017) Lembede, Lungile Phumelele.; Kunz, Richard Peter.; Mabhaudhi, Tafadzwanashe.
South Africa is classified as a semi-arid country characterized by low and erratic rainfall. This poses major limitations to crop productivity, especially for smallholder farmers who rely on rainfed agriculture. This is worsened by lack of knowledge regarding best management practices that can improve crop yields attained by smallholder farmers. In addition, smallholder farmers lack access to markets and do not participate in the agricultural value chain. The Biofuel Regulatory Framework (DoE, 2014) seeks to include smallholder farmers in the biofuel feedstock value chain. However, a prerequisite to their meaningful participation in the value chain would be to increase their current levels of crop and water productivity. The main aim of this study was to estimate the yield and water use of soybean (Glycine max L.) under rainfed and smallholder farming conditions using the AquaCrop model. Secondary to this, the effect of mulch and fertilizer on soybean water use efficiency was assessed. Lastly, the Soil Water Balance model (SWB) was used to compare simulations made by AquaCrop for the non-mulched, full fertilizer treatment. Thereafter, the water use efficiency of soybean was calculated from crop water use and the final yield. The soybean trial was carried out at Swayimane, KwaZulu-Natal. The model simulations of crop water use and reference crop evapotranspiration were also used to calculate crop coefficients under non-standard conditions. Crop growth and yield parameters were measured to calibrate and evaluate model performance. Soil water content was monitored using Watermark sensors, along with climatic variables. An analysis of variance (ANOVA) was used to detect significant interactions between treatments, while statistical indicators were used to evaluate model performance of AquaCrop and the SWB model. Mulching improved soil water content and reduced soil water evaporation, although the final yield and total water use efficiency was reduced. It is believed the yield reduction in mulched plots was mostly affected by nitrogen immobilization as a result of decaying straw mulch. Increasing soil fertility improved crop yield and water use efficiency in both mulched and non-mulched treatments. The AquaCrop model simulated the final yield and biomass fairly well, except in mulched treatments. The model simulated the highest yield in the mulched, fully fertilized plots, which is contrary to what was observed. This is because the model only accounts for improved soil water content and does not account for the complex interactions between the soil and mulch residue that resulted in nitrogen deficiency. The SWB model simulated fairly similar crop water use and yield to AquaCrop. The water use efficiencies obtained in this study were compared to that derived by Mengistu et al. (2014) for the same cultivar grown in a commercial farming environment at Baynesfield, KwaZulu-Natal. In comparison to commercial farmers, smallholder farmers tend to produce lower water use efficiencies. The modelled water use efficiency reported for Baynesfield was 1.277 kg m-3, compared to 0.359 kg m-3 obtained in this study for the non-mulched, full fertilizer treatment. According to AquaCrop, the mulched, full fertilizer treatment had a water use efficiency of 0.485 kg m-3. It is believed that the latter water use efficiency could have been achieved had enough nitrogen been available to the crop. In conclusion, implementing best management practices can help narrow the yield gap between smallholder and commercial farmers. It was evident from this study and others that agronomic practices have a significant impact on crop yield and ultimately, water use efficiency.
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.
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.
The estimation and evaluation of a satellite-based drought index using rainfall and evapotranspiration.
(2017) Mahomed, Maqsooda.; Chetty, Kershani Tinisha.; Clark, David John.
Abstract available in PDF file.
Towards an improved understanding of the influence of rainguage design, slope and aspect on rainfall measurements : a cross-calibration study.
(2017) Gray, Byron Andrew.; Toucher, Michele Lynn.
Abstract available in PDF file.
The contribution of fog to the water balance along the eastern escarpment of South Africa.
(2017) Aldworth, Tiffany Anthea.; Toucher, Michele Lynn.; Clulow, Alistair David.
Fog is a frequent phenomenon in South Africa, occurring mostly on the west coast and along the mountains forming the southern and eastern escarpments. Fog measurements are, however, neglected in water balance studies, resulting in an underestimate of the precipitation input to catchments that experience frequent fog occurrences. World-wide, tropical montane cloud forest (TMCF) studies have proven that fog deposition, facilitated via the interception of fog droplets by vegetation, can represent a significant fraction of the total hydrological input. In South Africa, limited literature exists on the contribution of fog to the country’s water yielding catchments. In particular, information on fog patterns and its contribution to the water balance is extremely scarce in the mountains forming South Africa’s eastern escarpment, where only one study has been previously conducted. Additionally, no forestry studies in the country have attempted to quantify fog. Thus, the aim of this study was to determine the contribution of fog to the water balance of two research catchments of different land use types and altitudes, situated along South Africa’s eastern escarpment. These sites included the Cathedral Peak research catchments and Two Streams; Cathedral Peak is a high altitude montane grassland catchment, whereas Two Streams is at a lower altitude and afforested by exotic plantations. At Two Streams, fog and the climatic conditions were monitored over a 16-month period (July 2015 to October 2016) and additional measurements of throughfall, stemflow and soil water content were carried out in an Acacia mearnsii plantation, to further determine the fog contribution in a forest plantation. At the Cathedral Peak research catchments, fog and the climatic conditions were monitored at three sites, including Mike’s Pass Meteorological Station, Catchment VI and a High Altitude site. Monitoring was conducted over a 14-month period (September 2015 to October 2016) at Mike’s Pass and over a two-month period (August 2015 to September 2015) at Catchment VI and the High Altitude site. Fog was found to be prevalent, occurring frequently and for long durations, potentially contributing fairly substantial amounts of water to the water balance. It occurred all year round, but was predominantly a summer phenomenon, however, it comprised a greater proportion of the total precipitation during the dry winter season. At Mike’s Pass, fog represented a contribution of almost 30 % during several drier months. At Two Streams, during the driest month of August 2015, fog represented a contribution of approximately 38 % of the total precipitation. Fog increased with altitude as a whole, but changes in other topographic features (i.e. hillslope orientation and slope) over short distances, meant that the delivery of fog was not uniform from one point to another at the same altitude. Fog occurrence and water yield increased with wind speed, although this was not found to be a very significant relationship. A stronger relationship between wind direction and fog was observed, particularly at Mike’s Pass, the higher altitude site, which was better exposed to fog-bearing winds. At Two Streams, fog did not facilitate throughfall of rainfall or contribute to soil water. The indirect effects of limiting wet canopy evaporation and transpiration rates were suggested to be a more relevant effect of fog on the water balance. These findings further the understanding of the contribution of fog to the water balance along the eastern escarpment of South Africa and will assist in future long-term climatological studies of fog and low cloud occurrence in the region.
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.
Modelling the impacts of changes in agricultural management practices on water resources with declining hydrometeorological data in the Uthukela Catchment.
(2018) Shabalala, Mlungisi Maxwell.; Toucher, Michele Lynn.
In order to meet the country’s growing demand for food, and to transform the economy of rural communities, the South African Government aims to develop the agricultural sector in the uThukela Catchment, KwaZulu-Natal Province. Intensification of agriculture will depend on the availability of water resources, with subsequent impacts on the quality and quantity of water resources. Therefore, the aim of this study was to investigate the impacts of proposed agricultural developments on the water flows in the upper uThukela Catchment using the multi-purpose, multi-soil-layered, daily time step ACRU model. The first phase of the study was to confirm the model’s ability to simulate flows in three, relatively small, gauged subcatchments of the uThukela catchment (Quaternary Catchments V11K, V14C and V31F), using current land cover and climate information extending to present day. However, the documented decline in the number of, and quality of data from, hydrometeorological stations, particularly since the year 2000, was concerning. Therefore, the impact of this decline on model performance was investigated in the selected subcatchments by comparing simulated flows to available observed flows in a confirmation study. Configuration of the model to present day conditions was restricted by the unavailability of rainfall stations. In cases where stations were available, there were no nearby stations to patch or compare to, when the record had missing or suspicious values. Given this, the model was set to run from 1960 to the latest record date available for catchments V14C and V31F. For V14C, the model performance decreased when the model was run from 1960 to 2012, compared to 1960-1999. Although a slightly better performance was obtained at V31F, the simulation time period was reduced to 1960-1999 for both catchments due to uncertainties with post 2000 rainfall and streamflow data. However, V14C continued to prove problematic and further investigation using of the Indicators of Hydrological Alteration software revealed a marked change in the flow characteristics between 1980 and 1981. No documentation of developments or substantial changes in the catchment could be sourced. Therefore, Quaternary Catchment (QC) V14C was excluded from further analysis. The ACRU model adequately simulated the flows for V11K and V31F, with the simulated flows being more representative of the observed flows in V31F. With the ability of the ACRU model to simulate the flows in the upper uThukela catchment under various land uses confirmed, the model could be used to investigate the impacts of agricultural land management scenarios on water flows. The agricultural land management scenarios were developed from the national and local government’s plan to expand agriculture to transform the socioeconomic status of the uThukela catchment. To develop scenarios for larger scale modelling, numerous scenarios were tested at QCs V31F and V11K. However, V11K was not responsive to changes in land use; therefore, results from the catchment were not used. For large scale modelling, the Upper uThukela (V1) Secondary Catchment was selected. The scenarios considered were: (i) increasing the fraction of irrigated commercial agriculture into currently dryland commercial fields, (ii) increasing subsistence agriculture through reduction of commercial agriculture (i.e. land reform), (iii) conversion of dryland commercial agriculture into crops with biofuel potential (iv) increased burning, (v) intensified land degradation and (vi) rehabilitation of degraded areas. These were developed from current land cover and compared to a simulation assuming natural conditions. The runoff components of interest were baseflow, quickflow and streamflow, as well as the low, median and high streamflows. Irrigation resulted in the highest flow reductions, with permanent cropping and planting two crops per year resulting in the largest decrease in streamflow at V31F and V1, when compared to natural conditions. These scenarios also had the greates impact on low flows. Plantation of biofuels increased flows, with soya beans having a higher impact on baseflows. Intensified burning and degradation increased quickflow and streamflow, while increasing subsistence agriculture and rehabilitation of degraded areas had little impact on flows. These results were generated from poor climate and land cover input information. Therefore, these results cannot be used at a definite decision-making tool, rather as an indication of the possible impacts of land use change on flows at the uThukela Catchment and similar regions. Efforts should be made to improve and maintain hydrometeorological monitoring stations. In addition, there should be more initiatives to collect land cover and water use data at various catchments in order to improve the quality of input data. Lastly, the current version of the ACRU model requires high computational power for large catchment simulations, lowering the model performance. Investigation into better versions or possible development of the current version should be conducted to enable modellers to finish large projects in allocated time.
Assessing climate change impacts on productivity of sugarbeet and sugarcane using aquacrop.
(2018) Mokonoto, Ofentse.; Kunz, Richard Peter.; Mabhaudhi, Tafadzwanashe.
Globally, the use of biofuels has grown over the years and their importance in helping to reduce a) dependency on fossil-based fuels and b) greenhouse emissions has been widely recognised. Various feedstocks are used for biofuels, viz. sugar-based crops for bioethanol production and oil from vegetable crops for biodiesel production. The research presented in this study focused on sugar crops such as sugarcane and sugarbeet. The sugarcane industry is widely established in South Africa, whereas sugarbeet is still a new crop and hence, there is little information on its water use efficiency (WUE) and potential yields under South African growing conditions. Overall, there is a need to better understand the agricultural potential and water use requirements of these feedstocks, in order to grow the biofuels industry in South Africa in a sustainable manner. Furthermore, climate change poses a threat to global food security as well as to biofuel feedstock production. There are uncertainties regarding the potential impacts of climate change on the yield and WUE of agricultural crops. One of the main objectives of this study was to calibrate the AquaCrop crop model for sugarcane and sugarbeet using experimental datasets. This study then followed a modelling approach to estimate dry yields and WUEs of these two sugar feedstocks to add to the existing knowledge base for potential biofuel production in South Africa. Sugarbeet was planted at the Ukulinga research farm and field equipment was used to collect data for the calibration of the crop model to better estimate attainable yield and WUE. Growth and yield datasets were provided by the South African Sugarcane Research Institute to calibrate the model for sugarcane, as well as validate AquaCrop for both feedstocks. The performance of the crop model was tested using various statistical methods. The model’s performance was satisfactory after calibrating it for sugarcane. However, the calibration process was compromised by the lack of sufficient leaf area index data. For sugarbeet, AquaCrop simulated the canopy cover, yield and WUE well, but tended to over-estimate observations. For the validation process, simulations closely matched the observed yields for both feedstocks. However, the model’s ability to simulate soil water content at Ukulinga was considered unsatisfactory. The calibrated AquaCrop model was used for long term assessments of yield and WUE. Baseline simulations were undertaken using 50 and 30 years of climate data and the results indicated that the 30 years of data could adequately estimate the long-term attainable productivity of sugarcane and sugarbeet. According to the literature, an ensemble approach to climate change modelling reduces uncertainty in long-term assessments. Hence, climate projections from several global climate models (GCMs), that were downscaled using dynamical and statistical approaches, were obtained and used to assess the potential impacts of climate change on yield and WUE of the selected feedstocks. An increase in yield and WUE of both feedstocks is projected in the distant future. The statistically downscaled GCMs projected higher increases compared to the dynamically downscaled GCMs. Increases in future WUE are much higher compared to yields projections. The so-called “CO2 fertilisation” effect largely benefits C3 crops (sugarbeet) with regards to yield improvements. However, the results also show that C4 crops (sugarcane) also benefit from improved WUE. Both sugarcane and sugarbeet will benefit from the anticipated climate change when planted in February and May, respectively. However, it is recommended that other planting dates should be studied for sugarcane.
Determination of below-ground vegetation and water use model parameters for a revised South African hydrological baseline land cover.
(2018) McNamara, Megan Ann.; Toucher, Michele Lynn.
The combination of both natural and anthropogenic activities have caused significant changes to the natural land cover which have impacted on the hydrological responses. The assessment of the magnitude of these land use change impacts on the hydrological response is important for sound water resource management, and is largely dependent on the baseline land cover used. The development of an updated natural vegetation map of South Africa by SANBI (2012), together with improved field based measurements of natural vegetation water use in recent studies, has led to the assessment of this map as a new hydrological baseline for South Africa. The proposed new baseline provides an opportunity to address the concerns raised about the current Acocks’ (1988) baseline used in South Africa. This study has provided estimates of the below-ground related vegetation and water use ACRU parameters for the proposed new baseline. These below-ground parameters estimated include the seasonal variations of the distribution of active roots in topsoil and subsoil horizons (ROOTA and ROOTB), the effective rooting depth (EFRDEP). The new and refined set of below-ground land cover ACRU input parameters will contribute to an improved and reliable baseline against which to assess any changes. As it was impractical to produce field-based measurements for the large number of natural vegetation species, and as it was not possible to form new spatial observations of theses below-ground root structures, the refined parameterisation of the below-ground component in ACRU was based primarily on review of measured values from past literature. The ROOTA values were estimated based on the vertical root distributions for various vegetation growth forms from previous studies together with the A-horizon soil depths of the vegetation clusters that constitute the baseline land cover. The effective rooting depth (EFRDEP) values were estimated by applying a linear regression relationship, relating rooting depths to Mean Annual Precipitation (MAP) for each baseline cluster. The study also involved a sensitivity analysis of the land cover input parameters to the ACRU Agrohydrological Model to determine the parameters to which the model is most sensitive.
An investigation of the impacts of Acacia Mearnsii plantations on secondary aquifer systems within the Two Streams catchment, KwaZulu-Natal, South Africa
(2019) Ngubo, Caiphus Zimise.; Demlie, Molla Bekele.; Lorentz, Simon Antony.
Abstract available in the PDF.
Evaluation of soil moisture estimates from satellite based and reanalysis products over two network regions.
(2022) Naidoo, Kivana.; Chetty, Kershani Tinisha.; Gokool, Shaeden.
The soil is an important variable of the hydrological cycle. It plays a key role in the distribution of water and energy fluxes between the surface and atmosphere. Soil moisture data can be used to develop early warning systems for flood and drought monitoring, improve weather and climate forecasting and provide an indication of crop water requirements. Therefore, the regular monitoring of this variable can prove to be beneficial to various management applications. One of the main issues associated with estimating soil moisture is to adequately account for its spatial and temporal variability as it is influenced by factors such as climate, topography, soil properties and land cover. There are different methods available to derive soil moisture estimations such as in-situ, remote sensing and modelling-based approaches. In-situ methods generally produce reliable soil moisture estimates, however, are only suitable for small scale studies. Alternatively, remote sensing and modelled reanalysis methods can provide soil moisture estimates over a large spatial extent, however, they are generally limited by their coarse spatial resolutions and may not be suitable for localised applications. Therefore, the aim of this study was to implement and evaluate a downscaling technique across two regions (South Africa and USA) to ultimately produce finer scale soil moisture and address the scale mismatch between in-situ methods and coarse resolution products. This procedure was facilitated by two data processing platforms, Google Earth Engine (GEE) and R, which showed significant potential for data processing and analysis. Additionally, satellite-based and reanalysis products were also evaluated to determine which of these methods are more suitable for soil moisture estimation. The soil moisture products and the downscaled products were validated against the CRNS instrument, which was particularly chosen for its performance at an intermediate spatial resolution. The SMAP_25 km product performed best at the Two Streams site and was selected for downscaling, whilst the CFSV2 product performed best at the Mead CSP3 and York Benny catchments and was chosen to be downscaled at both these sites. The results from the study indicated that the downscaled products for the Two Streams and Mead CSP3 sites performed better than the original products when compared to the CRNS data. The data acquired for the York Benny site revealed that the downscaled product performed similarly to the CFSV2 product. Therefore, downscaling does not always result in an improved outcome. However, from the results acquired for the Two Streams and Mead CSP3 study sites, it is evident that downscaling shows significant potential in producing better soil moisture estimates, which could be used to improve planning and management operations for various purposes.
Assessing and improving the simulation of runoff and design flood estimation in urban areas using the ACRU and SCS-SA models.
(2022) Ndlovu, Zama Sibahle.; Smithers, Jeffrey Colin.
Urbanisation is increasing at a rapid rate. Pervious and vegetated land is increasingly being replaced by impermeable surfaces (roads, pavements, driveways, parking lots, etc.) resulting in large portions of total imperviousness in catchments. The expansion of urban areas alters the natural underlying surface condition affecting catchment characteristics. The most common impacts of urbanisation on the hydrology of a catchment are increased runoff volumes, reduced baseflows owing to less infiltration taking place and a decrease in catchment response time. These changes can result in increased flood risk and subsequent damage to urban infrastructure and affect livelihoods. Therefore, accurate modelling of runoff and estimation of design floods of highly urbanised areas is necessary, especially in the often neglected catchments with informal settlements and infrastructure and in peri-urban catchments. Peri urban areas are defined as those areas located adjacent to a city area and have a mix of both rural and urban characteristics. Two rainfall-runoff models, namely the ACRU and the Visual SCS-SA model, were selected for application on catchments with typical South African urban conditions. The models have been developed and tested in urban catchments, however not extensively. The study areas are located in the South African urbanised cities of Tshwane and Pietermaritzburg. ACRU is a daily time step conceptual and physically-based agro-hydrological model that is relatively more data intensive compared to the simpler SCS-SA model. Therefore, information systems such as Remote Sensing (RS) and Geographic Information System (GIS) have been explored to aid as data sources and tools for acquiring model input parameters, at a more accurate level. The ACRU default values by Tarboton and Schulze (1992) and impervious area estimations derived by Loots (2020) were initially used to estimate the ACRU impervious parameters. Additionally, the pixel-based land cover classification method using satellite images was carried out in detail for this study as an attempt to map impervious surfaces and obtain impervious ACRU parameters with improved accuracy. Impervious land use classes were also extracted from the 2018 South African National Land Cover Database (SANLC), 2018 Global Man-made Impervious Surface (GMIS) and the 2010 Global Artificial Impervious Areas (GAIA). In order to use the ACRU and SCS-SA models confidently, the simulated results need to be verified against reliable observed data for each impervious scenario, if observed data is available. QGIS was used to obtain and process data into information required for the selected models. Several model input data such as slope, elevation, and catchment rainfall were estimated through GIS. The models over simulated observed design floods for the urbancatchments. Obtaining reliable observed data (rainfall and runoff), and satellite images with good resolution proved to be a consistent challenge throughout the study and could have contributed to the poor performance of the models. Urban area data dating back to the1990s was extracted from the GAIA method for most of the simulation period and a trend in impervious area expansion linked to urbanisation was detected and analysed against simulated streamflow from the urban catchments.
Assessing the water productivity of sweet potato (Ipomoea batatas (L.) Lam.)
(2022) Mthembu, Thando Lwandile.; Kunz, Richard Peters.; Mabhaudhi, Tafadzwanashe.
In water-stressed countries like South Africa, the reliable quantification of actual crop evapotranspiration (ETA) and yield across a wide range of environments is important for improved agricultural water management. In addition, researchers are shifting their primary focus from well-studied major crops to neglected and underutilised crops. Orange-fleshed sweet potato (Ipomoea batatas (L). Lam.) remains an underutilised root and tuber crop (RTC) in South Africa, despite its potential as being nutrient-dense, high yielding and water use efficient, as reported in local literature. When compared to conventional crops, knowledge is limited on the water use and yield of RTCs under rainfed and precision agricultural production in South Africa. It is therefore important to further investigate whether the water use of orangefleshed sweet potato (OFSP) will hinder its production at the commercial scale. This study attempted to contribute towards the limited research on the crop water productivity (CWP) of OFSP. A rainfed field trial with optimum fertilisation was conducted at Fountainhill Estate (KwaZulu-Natal, South Africa) to estimate seasonal ETA, yield and CWP. The soil water balance method was used to determine ETA accumulated over the growing season from 14 December 2021 to 11 April 2022. Total ETA for OFSP was estimated at 468.13 mm, which was used to calculate fresh and dry CWP values of 7.45 and 2.59 kg m-3 , based on final fresh and dry tuber yields of 34.89 and 12.12 t ha-1 , respectively. Harvested tuber and above-ground biomass samples were sent to a laboratory to analyse nutrient content (NC). The nutritional water productivity (NWP) was determined as the product of CWP and NC, highlighting the potential of OFSP to alleviate malnutrition, especially if grown in rural communities. Field observations were used to partially calibrate the Soil Water Balance (SWB) and AquaCrop models. These models were used to simulate ETA, yield and biomass accumulation, from which CWP and NWP were calculated. Compared to observations, AquaCrop provided a better estimate of CWP (2.55 kg m-3 ) relative to the SWB model (1.16 kg m-3 ). However, AquaCrop simulated higher soil water content relative to measurements from volumetric soil water content sensors. This study showed that under suitable management practices, OFSP has the potential to be grown commercially, since the crop can produce high yields and nutrient contents under rainfed agricultural production. However, to improve production, future studies need to conduct research to improve tuber yield and biomass accumulation. Furthermore, the AquaCrop and SWB models should be calibrated and validated across different agroecological zones in South Africa.

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