Environmental Hydrology

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A framework to improve irrigation design and operating strategies in the South African sugarcane industry.
(2009) Jumman, Ashiel.; Senzanje, Aidan.; Lecler, Neil Louis.
The purpose of this study was to develop a framework to assess irrigation design and operating strategies. This objective was achieved successfully and the framework was applied to formulate guidelines to increase farm profitability whilst using scarce resources, such as water and electricity, effectively. The study was targeted at sugarcane irrigated with semi-permanent irrigation systems. “ZIMsched 2.0”, a water balance and crop yield prediction model and the “Irriecon V2” economic assessment model were available at the start of the study. The missing link, however, was a relatively cost effective and efficient method to design and cost irrigation hardware alternatives. Irrigation hardware impacts on both the agronomic and economic performance of systems, for example, through different peak design capacities and associated operating limitations. Thus, a novel, spreadsheet-based irrigation design tool, with an automated costing component, was developed to complete the framework. The framework was used to investigate the costs and benefits of potential design and operating solutions to a selection of irrigation issues, including: over-irrigation on shallow soils, the opportunity to shift electricity use out of expensive peak periods and, the opportunity to demonstrate the benefits of deficit irrigation strategies. For shallow soils, the increase in system hardware costs, needed to better match water application to soils, increased margins due to more effective water use. Innovative deficit designs and operating strategies allowed for reductions in water and electricity costs. The reduced costs, however, did not always offset yield penalties and revenue loss resulting from water stress. The financial benefits of deficit irrigation strategies were shown when water savings were used to convert dry land cane into irrigated cane. This highlighted the differences between the direct and opportunity costs of water. Finally, a field work component, relating to the precise monitoring of irrigation strategies and corresponding crop responses was included in this study. Systems which enabled soil water potential and stalk extension to be monitored remotely via the internet were considered useful for the successful implementation of an optimum irrigation strategy. The easily accessible data allows for effective decision making and more importantly, reassures famers of the current state of their crop.
A simulation study of cane transport system improvements in the Sezela Mill area.
(2009) Giles, R. C.; Bezuidenhout, Carel Nicolaas.
The South African sugar industry is of significant local and international importance and covers an area in excess of 450 000 hectares. This area yields approximately 21 million tons of sugarcane per annum which is transported almost exclusively by road, from farms to the sugar mills. The industry is under increasing economic pressures to improve its productivity and competitiveness and sugarcane transport in the sugarcane supply chain has been identified as one area where large improvements and associated cost reductions can be made. This is mainly due to the excess in number of vehicles in the inbound transport system, the high relative cost of transport compared to other production costs in producing sugarcane, and the high fixed costs associated with truck fleet operations. A simulation case study of the transport system was completed in 2005 in the Sezela Mill area in which approximately 2.2 million tons of sugarcane is transported per annum over an average distance of 29 km by approximately 120 independently managed vehicles owned by a wide range of hauliers and individual growers. This amounts to an estimated cost of R58 million per annum. This study investigated the potential savings that could occur as a result of a central fleet control system with integrated vehicle scheduling. A scheduling software package named ASICAM, which resulted in significant savings in the timber industry (Weintraub et al, 1996), was applied within the Sezela region. Results suggested that the number of trucks in the fleet could theoretically be reduced by at least 50%, providing that a central office controls vehicle movements and that all hauliers serve all growers in an equitable fashion. In addition, investigations towards decreasing loading times, decreasing offloading times, changing vehicle speeds and increasing payloads by reducing trailer tare mass showed further reductions in the number of trucks required.
An adaptation of the SCS-ACRU hydrograph generating technique for application in Eritrea.
(2004) Ghile, Yonas Beyene.; Schulze, Roland Edgar.; Smithers, Jeffrey Colin.
Many techniques have been developed over the years in first world countries for the estimation of flood hydrographs from small catchments for application in design, management and operations of water related issues. However, relatively little attention has been directed towards the transfer and adaptation of such techniques to developing countries in which major hydrological decisions are crucially needed, but in which a scarcity of quality hydrological data often occurs. As a result, hydrologists and engineers in developing countries are frequently unable to alleviate the problems that extreme rainfall events can create through destructive flood flows or, alternatively, they do not possess the appropriate tools with which to design economically viable hydraulic structures. Eritrea is a typical example of a developing country which faces difficulties in regard to the adaptation of an appropriate design flood estimation technique for application on small catchments. As a result, the need has arisen to adapt a relatively simple and robust design flood model that can aid hydrologists and engineers in making economic and safe designs of hydraulic structures in small catchments. One objective of this study was, therefore, to review approaches to hydrological modelling and design flood estimation techniques on small catchments, in order to identify the barriers regarding their adaptation, as well as to assist in the selection of an appropriate technique for application, in Eritrea. The southern African adaptation of the SCS (i.e. Soil Conservation Service) design hydrograph technique, which has become a standard method for design flood estimation from small catchments in that region, was selected for application on small catchments in Eritrea for several reasons. It relies on the determination of a simple catchment response index in the form of an initial Curve Number (CN), which reflects both the abstraction characteristics and the non-linear stormflow responses of the catchment from a discrete rainfall event. Many studies on the use of SCS-based hydrological models have identified that adjustment of the initial CN to a catchment's antecedent soil moisture (ASM) to be crucial, as the ASM has been found to be one of the most sensitive parameters for accurate estimates of design flood volumes and peak discharges. In hydrologically heterogeneous regions like Eritrea, the hypothesis was postulated that simulations using a suitable soil water budgeting procedure for CN adjustment would lead to improved estimates of design flood volumes and peak discharges when compared with adjustments using the conventional SCS antecedent moisture conditions (SCS-AMC) method. The primary objective of this dissertation was to develop a surrogate methodology for the soil water budgeting procedure of CN adjustment, because any direct applications of soil water budgeting techniques are impractical in most parts of Eritrea owing to a scarcity of adequate and quality controlled hydrological information. It was furthermore hypothesised that within reasonably similar climatic regions, median changes in soil moisture storage from the socalled "initial" catchment soil moisture conditions, i.e. LIS, were likely to be similar, while between different climatic regions median LISs were likely to be different. Additionally, it was postulated that climatic regions may be represented by a standard climate classification system. Based on the above hypotheses, the Koppen climate classification, which can be derived from mean monthly rainfall and temperature information, was first applied to the 712 relatively homogeneous hydrological response zones which had previously been identified in southern Africa. A high degree of homogeneity of median values of LIS, derived by the daily time step ACRU soil moisture budgeting model, was observed for zones occurring within each individual Koppen climate class (KCC) - this after a homogeneity test had been performed to check if zones falling in a specific KCC had similar values of median LIS. Further assessment within each KCC found in southern Africa then showed that a strong relationship existed between LIS and Mean Annual Precipitation (MAP). This relationship was, however, different between KCCs. By developing regression equations, good simulations of median LIS from MAP were observed in each KCC, illustrating the potential application of the Koppen climate classification system as an indicator of regional median LIS, when only very basic monthly climatological information is available. The next critical task undertaken was to test whether the estimate of median LIS from MAP by regression equation for a specific Koppen climate class identified in southern Africa would remain similar for an identical Koppen climatic region in Eritrea. As already mentioned, LIS may be determined from daily time step hydrological soil moisture budget models such as ACRU model. The performance of the ACRU stormflow modelling approach was, therefore, first verified on an Eritrean gauged research catchment, viz. the Afdeyu, in order to have confidence in the use of values of LIS generated by it. A SCS-ACRU stormflow modelling approach was then tested on the same catchment by using the new approach of CN adjustment, termed the ACRU-Koppen method, and results were compared against stormflow volumes obtained using the SCS-AMC classes and the Hawkins' soil water budgeting procedures for CN adjustment, as well as when CNs remain unadjusted. Despite the relatively limited level of information on climate, soils and land use for the Afdeyu research catchment, the ACRU model simulated both daily and monthly flows well. By comparing the outputs generated from the SCS model when using the different methods of CN adjustment, the ACRU-Koppen method displayed better levels of performances than either of the other two SCS-based methods. A further statistical comparison was made among the ACRU, the SCS adjusted by ACRU-Koppen, the SCS adjusted by AMC classes and the unadjusted SCS models for the five highest stormflows produced from the five highest daily rainfall amounts of each year on the Afdeyu catchment. The ACRU model produced highly acceptable statistics from stormflow simulations on the Afdeyu catchment when compared to the SCS-based estimates. In comparing results from the ACRU-Koppen method to those from the SCS-AMC and unadjusted CN methods it was found that, statistically, the ACRU-Koppen performed much better than either of the other two SCS based methods. On the strength of these results the following conclusions were drawn: • Changes in soil moisture storage from so-called "initial" catchment soil moisture conditions, i.e. L1S, are similar in similar climatic regions; and • Using the ACRU-Koppen method ofCN adjustment, the SCS-SA model can, therefore, be adapted for application in Eritrea, for which Koppen climates can be produced from monthly rainfall and temperature maps. Finally, future research needs for improvements in the SCS-ACRU-Koppen (SAK) approach in light of data availability and the estimation ofL1S were identified. From the findings of this research and South African experiences, a first version of a "SCSEritrea" user manual based on the SAK modelling approach has been produced to facilitate its use throughout Eritrea. This user manual, although not an integral part of this dissertation, is presented in its entirety as an Appendix. A first Version of the SCS-Eritrea software is also included.
An adaptive operational water resources management framework for the Crocodile River catchment, South Africa.
(2014) Jackson, Brian.; Jewitt, Graham Paul Wyndham.
River catchments are complex STEEP (Social, Technological, Economic, Environmental, Political) systems requiring an integrated and adaptive approach to their water resources management with input from diverse stakeholders to generate a shared understanding of the system, and to engage in consensus-driven decision making and cooperative action towards shared objectives. Furthermore, semi-arid run-of-river dominated and closing river catchments are particularly susceptible to degradation and the current institutional arrangements for integrated water resource management are not generally able to adequately deal with issues of river catchment closure. Nor do they appear to effectively integrate both the technical and social-ecological aspects of integrated water resources management. This research thesis aims to investigate, analyse, develop, implement and evaluate an adaptive operational water resources management framework for the semi-arid run-of-river dominated and closing Crocodile River catchment through a collaborative and participatory action research approach which acknowledges the dual learning pathways of science and management, and that allows for researchers, managers and stakeholders to engage in consensus-driven decision making and cooperative action for effective operational water resources management. The research further aims to evaluate whether the framework developed can enable effective operational water resources management and so test the hypotheses that strategic adaptive management can be effectively used to conduct functional and effective operational water resources management in complex semi-arid run-of-river dominated and closing river catchments.
Agroclimatic response mapping for sugarcane production in southern Africa.
(2008) Hull, Phillip John.; Schulze, Roland Edgar.
As is the case in many other regions in the world, sugarcane production in southern Africa is affected by a wide range of climatic conditions, which can vary considerably from location to location and from year to year. As a result, the season length and growth cycles of sugarcane in southern Africa differ greatly. Such conditions include the hot and dry regions of northern KwaZulu-Natal, Swaziland and Mpumalanga, where sugarcane is mostly irrigated, to the humid sub-tropical coastal belt extending from the far north coast of KwaZulu-Natal to areas in the Eastern Cape, as well as the cool frost prone midlands regions of KwaZulu-Natal. Owing to the wide range of climatic conditions in which sugarcane is grown in southern Africa, there are many different external factors that affect sugarcane production, including a range of pests and diseases, frost occurrences and variations in soil water. The objective of this research was to (1) identify a number of important variables that affect cane production in southern Africa, (2) employ suitable models to reflect these variables, and (3) simulate and map the extent and severity of these variables at a high spatial resolution over southern Africa. Such variables include the Eldana saccharina and Chilo sacchariphagus stalk borers, sugarcane rust fungus, heat units with selected base temperatures, frost, soil water content, soil compaction, irrigation water demand, conducive and non-conducive growing conditions, flowering proficiencies for sugarcane, sugarcane yields and yield increments per unit of irrigation. The distribution patterns of the above-mentioned variables relied greatly upon the various models employed to represent them, as well as the accuracy of the temperature and rainfall databases to which the various models were applied. Although not definitive, the models used to reflect the variables which had been identified were considered to be generally satisfactory. The resolution at which the variables which had been identified in this study were mapped, was also found to be adequate.
An assessment of the potential impacts of climate variability on sugarcane production across Southern Africa.
(2023) Ngcobo, Simphiwe Innocent.; Jewitt, Graham Paul Wyndham.; Hill, Trevor Raymond.; Archer, Emma.
The scale and extent of changes to demographic, economic and environmental systems exacerbated by human activities have been rapid and pervasive enough that it has been established that a new geologic era termed the Anthropocene has already begun. One of the most critical and challenging consequences of the Anthropocene has been the accelerated release of greenhouse gases leading to global warming and, consequently, climate change (CC), which has impacted hydrological responses and available water resources by increasing surface temperatures and altering precipitation patterns across spatio-temporal scales. These changes have exacerbated the vulnerability of various systems that sustain livelihoods, placing them at high risk of collapse. One of these systems is sugarcane production, which is a crucially important agricultural activity in many parts of the world, including southern Africa. There is a consensus that as a region, southern Africa will be subjected to amplified hydrological impacts which will affect the sugarcane production landscape. Further the expansion and intensification of sugarcane production across southern Africa is highly likely due, in part, to the recognition of the economic and social importance of this activity for supporting livelihoods. Sugarcane yields have been declining over the past 25 years in the region because of the increased frequency of climatic extremes. Literature reviews showed that by amplifying precipitation variability, climate change will increase the exposure and vulnerability of sugarcane to water stress and will have a devastating impact on yields. However, knowledge gaps remain regarding climate change impacts on water resources and sugarcane yields. Further, few studies have addressed the vulnerability and adaptation potential of sugarcane production at sufficient spatio-temporal scales. To address these knowledge gaps, an initial review was conducted to understand the dynamics between global change and water resources across southern Africa. The review showed that although global drivers are intricately related, their water resources impacts are highly complex, indirectly coupled and spatially and temporally sensitive. Having established a general perspective of the impacts of global change in southern Africa, the multi-scale drivers of sugarcane production were analysed using of a frequency analysis. This approach allowed the determination of proximate and ultimate drivers in the uMngeni, uMlaas, and Umvoti catchments in South Africa, the Ubombo catchment in eSwatini, the Shire catchment in Malawi and the Kilombero catchment in Tanzania. The frequency analysis provided quantitative descriptions of the water resource impacts of sugarcane production across southern Africa. Applying a relationship between observed sugarcane yields and future low, medium, and high production scenarios, this study developed water use estimates for sugarcane over multiple growing cycles. Results indicated that ultimate drivers play the most dominant role in the expansion of sugarcane production within each catchment. Drawing from this analysis, a methodology of assessing yield declines was developed based on a yield gap analysis using the AquaCrop crop growth model. The results were used to develop recommendations to mitigate yield declines by offering safeguards for the sugarcane industry against climatic extremes. Modelling results suggested that yield trends can be attributed to existing crop management approaches instead of prevailing hydroclimatic regimes. The importance of recognising the vulnerability and adaptation potential in sugarcane production was highlighted in this study. It was concluded that if sugarcane growers are to adapt to the effects of extreme climatic events, they must consider shifting crop management approaches and be proactively included in related research. This research highlighted the importance of addressing the interactions between activities that drive land use change, such as sugarcane production, and the current impacts of climatic extremes on water resources. This is important in rapidly developing regions and climate change hotspots such as southern Africa. The development of innovative adaptation policies that will safeguard the already-pressured water resources and secure the sustainability of sugarcane production will become increasingly important under an altered climate.
An integrated sugarcane supply chain model : development and demonstration.
(2006) Stutterheim, Peter.; Bezuidenhout, Carel Nicolaas.; Lyne, Peter William Liversedge.
The South African sugar industry is a large industry which relies on expensive capital equipment to harvest, transport and process sugarcane. An average of 23 million tons of sugarcane are annually supplied to 14 mills from over 2 000 large-scale commercial growers and 48 000 small-scale growers. Supply chain stakeholders can benefit if operations are successfully streamlined. Computer-based mathematical models have been used in other industries to improve supply chains, especially in forestry, and are expected to play an increasingly important role in future planning and management. Management of sugar supply chains has historically focussed on generating competitive individual supply chain components. However, inter-component optimisation generally disregards many important intra-component interactions. Hence, efficiency improvements may be significantly limited. Integrated supply chain modelling provides a suitable approach for addressing this problem. The aim of this project was to develop and demonstrate, in concept, an integrated supply chain model for the sugar industry. Such a model could be used to address various integrated planning and management problems throughout the supply chain. A review of existing integrated agri-supply chain models was conducted followed by the development of CAPCONN, an integrated sugar supply chain model framework, that incorporate all steps from field to mill back end. CAPCONN estimates sugarcane quality, mill recovery, capacity utilisation and production costs. Bottlenecks are highlighted and the model could contribute towards capacity manipulation for efficiency improvements under different harvesting scenarios. CAPCONN was demonstrated by analysing a number of scenarios in a mechanisation case study at Komati Mill where sugarcane is currently burned and manually cut. A total of twelve scenarios were compared, including variations in cropping system and time of year. The model framework predicted that a decrease in sugarcane quality and sugar recovery would occur under mechanical harvesting scenarios. Estimated production costs were also higher, even though the transport fleet was significantly reduced. A manually cut green (unburned) harvesting scenario showed a further decrease in sugarcane quality and sugar recovery. Mechanical harvesting during wet weather caused a substantial reduction in supply chain capacity and an increase in production costs. CAPCONN output trends compared favourably with measured and observed data, though the magnitude of the trends should be viewed with caution, since the CAPCONN framework is only a prototype. This shows that it may be a suitable diagnostic framework for analysing and investigating the sugarcane supply chain as a single entity. With further development to a model, the CAPCONN model framework could be used as a strategic planning tool although, one drawback is that a relatively large number of technical inputs are required to run the model.
An analysis of baseflow recession in the Republic of South Africa.
(1997) Hughes, Gregory Owen.; Schulze, Roland Edgar.
Demands on the water resources of South Africa are ever increasing owing to population growth and increased development of urban, peri-urban and rural communities. Problems in terms of water quantity and quality are likely to be experienced during baseflow recessions. It is therefore imperative that water resources managers not only understand these baseflow periods of streamflow, but are able to model them with confidence. Research for this study thus included a comprehensive literature survey of the factors which affect baseflow as well as the approaches that previous studies have utilised to analyse and model baseflow recession. The primary aims of this study were to establish a streamflow database, to construct master recession curves (MRCs) for each catchment under consideration, evaluate the assumption that South African rivers recede exponentially, to determine a representative set of catchment characteristics for use in the baseflow recession analysis, to attempt to explain the MRC trends using these catchment characteristics and to investigate the feasibility of establishing a rule based model for baseflow recession. A streamflow database for South Africa was therefore established. This consisted initially of 202 catchments which were deemed to be recording natural streamflow. MRCs were established for 134 of these catchments. Those MRCs which were established indicate that the majority of South African rivers do not conform to an exponential model of recession. In order to account for the trends defined by the MRCs, catchment area, average catchment slope, drainage density, mean annual precipitation, rainfall concentration, rainfall seasonality, two independent estimates of groundwater recharge and a geological index were calculated for each catchment. Limited success was achieved when the data set was divided into subsets in order to group catchments with similar baseflow recession responses. The geological composition of the catchments appeared to provide the best results in that those trends exhibited by the MRCs could be explained by the types and proportions of the lithologies present. Owing to the lack of readily useable results it was concluded that until further results were forthcoming the development of a rule based model for baseflow recession analysis in South Africa would be premature. The establishment of a readily accessible database containing streamflows and associated catchment characteristics lends itself to future research.
Applying the social-ecological systems framework to understand impacts of flooding in the Palmiet River catchment.
(2021) Dlamini, Londiwe Zola.; Stuart-Hill, Sabine Ingrid.; Sutherland, Catherine Grace.
Accelerating urbanization in African cities is impacting the ability of urban ecosystem services to provide services to contribute to the wellbeing of people. Additionally, climate change presents increased urban risks such as the increased frequency and intensity of flooding. This thereby threatens human life and built infrastructure; and challenges the resilience of communities already strained by socio-economic challenges. Ecosystem services in urban catchments are poorly understood which further adds to the lack of understanding the value of natural resources in urban catchments and subsequently how to restore and protect vital natural resources in order to ensure ecosystem services delivery. The aim of the study is to understand how impacts of flooding decrease the resilience of the communities in the Palmiet River catchment located in Durban, South Africa, through applying the social-ecological system (SES) framework. The Palmiet River catchment is a dynamic and heavily urbanized catchment in which the Palmiet River extends 26km through its headwaters at an elevation of 510m flowing through the lower informal settlement at 18m elevation. The SES framework is an interdisciplinary approach to understanding biophysical and social aspects in a relational landscape – both of which can no longer be studied in isolation. The methodology of the study uses data collected from public community engagement forums to identifyspecific issues occurring within the catchment and understanding the roles of interested and affected stakeholders. Further, aerial photography images of the Palmiet River catchment from 1981 to 2016 were used to identify the rate of urbanization and terrestrial impacts; this data was additionally supported by drone images. A SES framework was applied for sub-sections of the Palmiet River catchment in order to develop a narrative for the total river catchment to improve understanding of societal actions of urbanization that impact the functionality of the Palmiet River. The findings of the study reflect that: 1) Flood events are occurring more frequently, and more peopleare at risk as the influx of people within the catchment increases and the land use/cover changes. 2) A collaborative social system with a strong governance unit exists within the Palmiet catchment. This has facilitated conversations amongst resources users and actorsin the rehabilitation of the resource system. This could potentially serve as a springboard for identifying viable areas for ecological infrastructure investments. 3) The social system has increased resilience within the catchment – however, this may change as flood events continue to increase in intensity and frequency. 4) The Palmiet River is a dynamic social-ecological system that presents challenges as well as opportunities for sustainable and integrative catchment management. The SES framework provided a tool to evaluate the social and ecological systems through which to assess thecurrent limitations for the Palmiet River to regulate flood events. 5) It was lastly necessary to identify ways in which sustainable urban design systems and ecological infrastructure could be used as a part of catchment management strategies to rehabilitate and enhance ecosystem services. It was concluded that the ecosystem services once offered by the Palmiet River catchment have been compromised byunprecedented rates of urbanisation, particularly impacts of growing informal settlements in the lower parts of the catchment as well as industrial areas in the upper parts of the catchment.
Approaches to modelling catchment-scale forest hydrology.
(2002) Roelofsen, Aukje.; Jewitt, Graham Paul Wyndham.
South African commercial plantations occupy an estimated 1.5 million hectares of the country and as the demands for timber products increase, this area is expected to increase. However, further expansion is limited, not only by the suitability of land, but also by the pressures from other water users. As a result the need has arisen for simulation models that can aid decisionmakers and planners in their evaluation of the water requirements of forestry versus competing land uses at different spatial scales. Different models exist to perform such tasks and range from simple empirical models to more complex physically-based models. The policies of the National Water Act (1998) relating to forestry serve to highlight the requirements of a model used for the assessment of afforestation impacts and these are discussed in this document. There is a perception that physically-based distributed models are best suited for estimation of afforestation impacts on a catchment's water yield since their physical basis allows for extrapolation to different catchments without calibration. Furthermore, it is often stated that the model parameters have physical meaning and can therefore be estimated from measurable variables. In this regard, a review of physically-based modelling approaches and a comparison of two such hydrological models forms the main focus of this dissertation. The models evaluated were the South African ACRU model and the Australian topography-based Macaque model. The primary objective of this research was to determine whether topography-based modelling (Macaque model) provides an improved simulation of water yield from forested catchments, particularly during the low flow period, compared to a physically-based model (ACRU model) that does not explicitly represent lateral sub-surface flow. A secondary objective was the evaluation of the suitability of these models for application in South Africa. Through a comparison of the two models' structures, the application of the models on two South African catchments and an analysis of the simulation results obtained, an assessment of the different physically-based modelling approaches was made. The strengths and shortcomings of the two models were determined and the following conclusions were drawn regarding the suitability of these modelling approaches for applications on forested catchments in South Africa:• The ACRU model structure was more suited to predictive modelling on operational catchments, whilst the more complex Macaque model's greatest limitation for application in South Africa was its high input requirements which could not be supported by the available data. • Despite data limitations and uncertainty, the Macaque model's topography-based representation of runoff processes resulted in improved low flow simulations compared to the results from the ACRU simulations, indicating that there are benefits associated with a topographically-based modelling approach. • The Macaque model's link to the Geographic Information System, Tarsier, provided an efficient means to configure the model, input spatial data and view output data. However, it was found that the ACRU model was more flexible in terms of being able to accurately represent the spatial and temporal variations of input parameters. Based on these findings, recommendations for future research include the. verification of internal processes of both the ACRU and Macaque models. This would require the combined measurement of both catchment streamflow and processes such as evapotranspiration. For the Macaque model to be verified more comprehensively and for its application in operational catchments it will be necessary to improve the representation of spatial and temporal changes in precipitation and vegetation parameters for South African conditions.
Assessing the effects of site preparation treatments on erosion processes and sediment yield on a commercial Eucalyptus plantation: case study at Two Streams, KwaZulu-Natal.
(2020) Bull, Jordan Michael.; Hill, Trevor Raymond.; Everson, Colin Stuart.
Soil erosion monitoring and modelling is critical in the face of climate change, as erosion is detrimental to environmental and human health. It reduces soil productivity through degradation, compromises water quality through nutrient loading freshwater sources, and decreases reservoir capacity through sedimentation. This is a global challenge which is being amplified by increased levels of soil erosion on cultivated lands (e.g. commercial forestry), the combined effects of which hinder the success of several United Nations Sustainable Development Goals. However, commercial forestry is essential for human survival, providing a host of resources for human consumption, and expanding in its global coverage each year; although, this expanding need for commercial forestry creates a paradox, as it has the potential to damage environmental health and biodiversity (systems which humans rely on for survival), particularly through soil erosion and sedimentation of freshwater systems. Afforestation of plantations has been widely considered a land-use activity which reduces soil erosion; although, this is dependent on the management of the commercial plantations, where certain commercial forestry management techniques exacerbate soil erosion, such as the well-used site preparation technique of burning. Therefore, an investigation into the effect of commercial forestry site preparation techniques such as burning (at different severities) and mulching on soil erosion and the modelling thereof is required, as only a paucity of research has reported on this. Soil erosion measurements were conducted on a newly planted Eucalyptus dunnii stand, which consisted of three different site preparation techniques, namely a hot burn, a cold burn and a mulch treatment, in the Two Streams catchment, Kwa-Zulu Natal, South Africa. Micro-runoff and runoff plots were used to respectively measure splash and rill erosion of sediment, nitrogen, phosphorous, dissolved organic carbon, particulate organic carbon loss and runoff on each treatment at different slopes. In addition, soil erosion and runoff of this catchment and treatments were modelled using the ArcSWAT model, and the observations were used to validate the simulated outputs. The mulch treatment had the most consistent reduction in runoff and erosion, while the burn treatments of different severities generated greater respective runoff and erosion quantities through different erosional processes (splash vs rill). The ArcSWAT model over-simulated runoff on the hot burn and mulch treatment, while under-simulating on the cold burn treatment; however, the model consistently over-simulated sediment and nutrient loss on all treatments, indicating the model’s inability to simulate soil erosion on the defined land-use treatments. The reduced runoff and erosion produced by the mulch treatment is attributed to the protection that the mulch provides to the soil from splash erosion and the resistance posed to overland flow reducing rill erosion. The burn treatments generating more erosion and runoff through different erosional processes was attributed to the differing nature of debris produced by each burn severity. This research will contribute towards the data sets necessary to refine the land-use management tools of the ArcSWAT model to better model soil erosion on different land-use treatments. Furthermore, this research demonstrates the erosion processes that differing site preparation treatments are susceptible to, and what this means for future research and protecting soil and downstream water quality in the face of climate change.
Assessing the performance of regional flood frequency analysis methods in South Africa.
(2015) Nathanael, Jermaine Jonathan.; Smithers, Jeffrey Colin.; Horan, Mark John Christopher.
In engineering and flood hydrology, the estimation of a design flood refers to procedures whereby the magnitude of a flood is associated with a level of risk at a given site (Pegram and Parak, 2004). The use of a Regional Flood Frequency Analysis (RFFA) approach improves the accuracy and reliability of estimates of design floods. However, no RFFA method is currently widely used in South Africa, despite a number of RFFA studies having been undertaken, that include South Africa. Hence, the performance of the current RFFA approaches needs to be assessed in order to determine the best approaches to use and to determine if a new RFFA approach needs to be developed for use in South Africa. Through a review of the relevant literature it was found that the Meigh et al. (1997) Method, the Mkhandi et al. (2000) Method, the Görgens (2007) Joint Peak-Volume (JPV) Method, which uses a K-Region regionalisation, as well as a Veld zone regionalisation, and the Haile (2011) Method are most suitable for application in a nationwide study. Each regional approach was assessed by comparing their design flood estimates with those estimated from an at-site flood frequency analysis of the observed flood data, using both the General Extreme Value (GEV) and Log Pearson Type 3 (LP3) distributions. However, due to the LP3 distribution producing inconsistent design flood estimates, it was removed from further analysis and only the GEV distribution was assessed. Annual Maximum Flood (AMF) data were obtained from the Department of Water and Sanitation (DWS) for 1458 stations across the entire country. In addition to these datasets, 89 synthesised dam inflow records were obtained from the DWS and incorporated into the study. Due to a thorough data screening process, the final number of stations and dam inflow records analysed was reduced to 407 stations. In order to determine the overall accuracy of the RFFA methods, Relative Errors (RE) (%) were calculated at each station. Box plots and frequency plots were utilised to represent the distribution of relative errors and the degree of bias was measured using a ratio of the estimated and observed design floods. The results of the study show that the Haile Method generally performs better than the other RFFA methods, however it also consistently under-estimates. The Mkhandi Method generally over-estimates. The Meigh Method generally performs the worst, consistently over-estimating. For the JPV Methods, the K-Region regionalisation generally performs better than the Veld zone regionalisation; however, they both consistently over-estimate design floods. The poor overall performance of the RFFA methods are due to a number of reasons. In the case of the Mkhandi et al. (2000) Method, the tests for homogeneity that were developed were too lenient, which may have incorrectly defined regions as being homogeneous. In the case of the Meigh et al. (1997) Method, the regionalisation of homogeneous flood regions were too broad, where only two flood regions have been identified for South Africa. For the Haile (2011) Method, the logarithmic regressions developed for a number of regions were not able to determine index floods for all catchment areas. Therefore, power regressions were developed in this study. In the case of the JPV Methods, the Kovacs K-Regions and Veld zone regions were used, which have not been updated in the past several years. In response to the generally poor performance of the RFFA methods assessed in this study, it has been recommended that a new method be developed for application in design flood practice in South Africa.
Assessing the performance of techniques for disaggregating daily rainfall for design flood estimation in South Africa.
(2020) Ramlall, Ryshan.; Smithers, Jeffrey Colin.
Design Flood Estimation (DFE) and other hydrological modelling methods are used to limit the risk of failure and ensure the safe design of infrastructure and for the planning and management of water resources. The temporal distribution of rainfall has a significant impact on the magnitude and timing of flood peak discharges. Rainfall temporal distributions are therefore an important component of DFE approaches. In order to improve DFE methods which are based on event or continuous simulation rainfall-runoff models, it is generally necessary to use sub-daily time step rainfall hyetographs as input. However, the number of recording raingauges which provide sub-daily timesteps in South Africa is relatively scarce compared to those which provide daily data. Rainfall Temporal Disaggregation (RTD) techniques can be used to produce finer resolution data from coarser resolution data. Several RTD approaches have been applied in South Africa. However, application of RTD approaches locally is relatively limited, both in terms of diversity of approaches and cases of application, compared to those developed and applied internationally. Therefore, a need exists to further assess the performance of locally applied approaches as well update the list of available approaches through inclusion of internationally developed and applied RTD techniques. A pilot study was performed in which selected locally applied and internationally applied approaches were applied to disaggregated daily rainfall data. Some approaches were applied in their original form while others were modified. Temporal distributions of rainfall were represented by dimensionless Huff curves, which served as the basis for comparison of observed and disaggregated rainfall. It was found that for daily rainfall, the SCS3, SCS4 and Knoesen model approaches performed considerably better than the other approaches in the pilot study. The RTD approaches were further assessed using data from 14 additional rainfall stations. For the additional stations, the Knoesen model disaggregated depths provided the most realistic temporal distributions overall, followed by the SCS-SA approach. In additional, an adapted form of the Triangular distribution was found to show potential for disaggregation when a generalised value for the timing of the peak was utilised.
Assessment of agro-ecosystem sustainability across varying scales in South Africa.
(2005) Walker, Nicholas James.; Schulze, Roland Edgar.
Maize production plays an important socio-economic role in rural communities of the Highveld region of South Africa, yet it is becoming increasingly difficult to produce maize economically with current agricultural policy conditions and existing management systems. This has direct socio-economic impacts for both commercial farmer and small-scale farmer. Sustainable commercial maize production is not only a question of yields, but also of protection of the environmental resource base, social welfare, and the livelihoods of farmers per se as well as the surrounding rural and urban communities. Sustainability for the small-scale farmer, on the other hand raises questions of equity, economic viability and household food security. Therefore, information is required to ascertain whether an existing agro-ecosystem can be identified as sustainable, and what facets of that system make it sustainable or unsustainable. To begin to answer these key questions it is important to state, and to some extent attempt to standardise, the definitions of agricultural sustainability. Agro-ecosystem sustainability with regard to maize production was assessed at the regional scale of the Highveld of South Africa as well as at, the Quaternary Catchment scale and the smallholder farm scale. Von Wiren-Lehr's (2001) goal orientated system was considered an appropriate and practical system by which agro-ecosystem sustainability across a range of scales could be investigated. At the regional scale, optimum management strategies for each of the 497 Quaternary Catchments in the Highveld region were devised, based on present climatic conditions and using an index which was based on mean yields and yield variability. Economic returns and their impact on sustainability were then also assessed under plausible future climate scenarios. At the Quaternary Catchment scales optimum management strategies were ascertained by using a sustainability index. These strategies were then modelled under present and plausible future climate scenarios. The results from the sustainability modelling showed that a maize crop will benefit, especially with respect to mean grain yields, from an effective doubling of atmospheric CO2 concentrations. However, this benefit can be counteracted when there is a concurrent increase in temperature, particularly of 2°C or more. At the smallholder scale, a range of management options was assessed. These options included several types of tillage practices in combination with applications of either inorganic fertiliser or manure. The management strategies were modelled under present climate conditions and under plausible climate change scenarios for southern Africa. The conventional tillage type (disc) was ranked highest under most of the climatic conditions modelled, including present climate conditions. This was in contrast to actual yields from smallholder farmers (-1 ha field size) in the Potshini area, near Bergville in the KwaZuluNatal province of South Africa, who have experienced an increase in yield when conservation tillage practices have been used on their land (Smith et al., 2004). The sustainability of agro-ecosystems depends on the maintenance of the economic, biophysical and social components that make up the system (Belcher et al., 2004). The modelling performed for the Highveld region built on previous work and for the first time incorporated daily temperatures and ISCW soil information into CERES-Maize. The intention was to incorporate other agro-ecosystem functions, as well as yield, into the sustainability assessment. Only limited research has previously been carried out in South Africa with respect to modelling smallholder agro-ecosystems and sustainability. This research sought to model the smallholder system along with the impacts that climate change would have on sustainability and associated food security.
An assessment of canopy and litter interception in commercial and indigenous forests in the KwaZulu-Natal Midlands, South Africa.
(2011) Bulcock, Hartley Hugh.; Jewitt, Graham Paul Wyndham.
Understanding of the hydrological cycle and processes such as interception span as far back as the times of the Renaissance, when Leonardo da Vinci (1452-1519) first described it. However, there remains a gap in the knowledge of both canopy and litter interception in South African forest hydrology. Interception is typically considered to constitute only a small portion of total evaporation and in some models is disregarded or merely lumped with total evaporation, and not considered as a separate process. Interception is a threshold process, as a certain amount of water is required before successive processes such as infiltration and runoff can take place. Therefore an error introduced in modelling interception, especially disregarding it, will automatically introduce errors in the calibration of subsequent models/processes. In this study, field experiments to assess these two poorly understood hydrological processes, viz. canopy and litter interception were established for the three main commercial forestry genera in South Africa, namely, Pinus, Acacia and Eucalyptus as well as an indigenous Podocarpus henkelii stand, thus, accounting for interception of “broad leaf”, “compound leaf” and “needle leaf” trees in order to provide further insight into these processes. The study took place at two locations in the KwaZulu-Natal Midlands over a period of three years. The first site is the Two Streams catchment, located in the Seven Oaks area, about 70km north-east of Pietermaritzburg where the study on the commercial plantation species took place. The second site was the Podocarpus henkelii stand in Karkloof near Howick, 40km north of Pietermaritzburg. From the field data collected (cf. Chapter 2) it was observed that canopy storage capacity, an important parameter governing interception, was not constant and changed with rainfall intensity, with lower intensity events resulting in a higher storage capacity. Building on these findings, a physically based canopy interception model that is based on the well known Gash model was developed, and is referred to herein as the “variable storage Gash model”. While canopy interception is dependent on many factors including the storage capacity, potential evaporation, rainfall intensity and rainfall duration, the litter interception is largely dependent on the storage capacity due to the evaporative drivers under the canopy such as radiation, temperature and wind speed being moderated by the above canopy. From these finding, a litter interception model based on idealised drying curves from litter samples collected at the study sites was also developed (cf. Chapter 3). From the field data, it was found that the canopy interception for Eucalyptus grandis, Acacia mearnsii and Pinus patula was 14.9, 27.7 and 21.4% of mean annual precipitation (MAP) respectively. The simulated canopy interception using the “variable storage Gash model” was 16.9%, 26.6% and 23.3% for E. grandis, A. mearnsii and P. patula respectively. The litter interception measured for E. grandis, A. mearnsii and P. patula was found to be 8.5, 6.6 and 12.1% of MAP respectively, while the simulated litter interception using the idealised drying curve model corresponded well with the measured results and were 10.1%, 5.4% and 13.4% for E. grandis, A. mearnsii and P. patula respectively. The idealised drying curve model is site and species specific and is therefore not transferable to other locations. Conversely, the “variable storage Gash model” is transferable as it is not site and species specific, and relies on readily measureable and available information. Building on field studies, this was then used to simulate the canopy interception for Eucalyptus, Acacia mearnsii and Pinus in South Africa (including Lesotho and Swaziland) for all quinary catchments in which commercial forestry could be grown, i.e. a mean annual precipitation of greater than 600 mm.year-1 (cf. Chapter 4). It was found that, depending on the location and genus, canopy interception loss can be as high as 100 to 300 mm per year or approximately 10% to 40% of MAP. This relates to a mean interception loss of between 1.0 and 3.0 mm per rainday, highlighting the spatial variability of canopy interception. To further investigate the spatial variability of canopy interception, at various spatial scales, remote sensing technology was applied to estimate leaf area index (LAI) for use in modelling/estimating canopy storage capacity and canopy interception (cf. Chapter 6). The NDVI, SAVI and Vogelmann 1 vegetation indices were used in the estimation of the LAI. It was found the Vogelmann 1 index produced the best results. As models to estimate canopy interception typically require LAI and storage capacity, it was calculated that the ability to estimate these parameters over large areas is valuable for water resources managers and planners. An often neglected consideration of canopy and litter interception is its role in determining the water use efficiency (WUE) of a forest stand (cf. Chapter 5). This component of the study was undertaken in an indigenous Podocarpus henkelii stand as well as a commercial Pinus patula stand in Karkloof in the KwaZulu-Natal Midlands. The sap flow (transpiration) was measured in both the P. henkelii and P. patula stands using the using the Heat Pulse Velocity (HPV) technique in order to determine the productive green water use. The canopy and litter interception was measured in the P. henkelii site, but was modelled in the P. patula site using the “variable storage Gash” and idealised drying curve models, in order to estimate the non-productive green water use. It was found that the canopy and litter interception for P. henkelii was 29.8% and 6.2% respectively, while the modelled canopy and litter interception for P. patula was 22.1% and 10.7% respectively. If only the productive green water use (transpiration) is considered, then the water use efficiency of P. henkelii and P. patula was found to be 7.14 g.mm-1 and 25.21 g.mm-1 respectively. However, from a water management perspective it is important to consider the total green water use efficiency (transpiration + interception), which reveals a significantly lower water use efficiency of 3.8 g.mm-1 and 18.8 g.mm-1 for P. henkelii and P. patula respectively. To extend the study to a globally relavent issue, the possible impact of climate change on canopy interception was investigated, as forests growth is critically linked to climate (cf. Chapter 7). To achieve this, the CABALA model was used to model LAI and transpiration of Eucalyptus grandis and Pinus patula under 9 different climate change scenarios, including changes in temperature, rainfall and atmospheric CO2. The simulated LAI values from the CABALA model for all 9 climate scenarios were then used to simulate canopy interception using the “variable storage Gash model”. Results show that LAI may increase by as much as 24% and transpiration may decrease by as much as 13%, depending on the scenario, location and tree species. However, it was found that canopy interception does not change greatly, leading to the conclusion that under climate change conditions, canopy interception may not become a more dominant component of the hydrological cycle than it currently is as the changes under climate change are likely to be less than the natural variability from year to year. However, canopy interception remains an important consideration for water resources management and planning both currently and in the future.
Assessment of satellite derived rainfall and its use in the ACRU hydrological model.
(2017) Suleman, Shuaib.; Chetty, Kershani Tinisha.; Clark, David John.
Many parts of southern Africa are considered water scarce regions. Therefore, sound management and decision making is important to achieve maximum usage with sustainability of the precious resource. Hydrological models are often used to inform management decisions; however model performance is directly linked to the quality of data that is input. Rainfall is a key aspect of hydrological systems. Understanding the spatial and temporal variations of rainfall is of paramount importance to make key management decisions within a management area. Rainfall is traditionally measured through the use of in-situ rain gauge measurements. However, rain gauge measurements poorly represent the spatial variations of rainfall and rain gauge networks are diminishing, especially in southern Africa. Due to the sparse distribution of rain gauges and the spatial problems associated with rain gauge measurements, the use of satellite derived rainfall is being increasingly advocated. The overall aim of this research study was to investigate the use of satellite derived rainfall into the ACRU hydrological model to simulate streamflow. Key objectives of the study included (i) the validation of satellite derived rainfall with rain gauge measurements, (ii) generation of time series of satellite derived rainfall to drive the ACRU hydrological model, and (iii) validation of simulated streamflow with measured streamflow. The products were evaluated in the upper uMngeni, upper uThukela (summer rainfall) as well as the upper and central Breede catchments (winter rainfall). The satellite rainfall products chosen for investigation in this study included TRMM 3B42, FEWS ARC2, FEWS RFE2, TAMSAT-3 and GPM. The satellite rainfall products were validated using rain gauges in and around the study sites from 1 January 2010 to 30 April 2017. The rainfall products performed differently at each location with high variation in daily magnitudes of rainfall. Total rainfall volumes over the period of analysis were generally in better agreement with rain gauge volumes with TRMM 3B42 tending to overestimate rainfall volumes whereas the other products underestimated rainfall volumes. The ACRU model was applied using satellite rainfall and rain gauge measurements in the aforementioned study catchments from 1 October 2007 to 30 September 2016. Streamflow results were generally poor and variable amongst products. Daily correlations of streamflow were poor. Total streamflow volumes were in better agreement with total volumes of observed streamflow. TRMM 3B42 and rain gauge driven simulations produced the best results in the summer rainfall region, whereas the FEWS driven simulations produced the best results in the winter rainfall region.
An assessment of satellite derived total evaporation data as a data source to the ACRU hydrological model.
(2014) Gokool, Shaeden.; Chetty, Kershani Tinisha.; Jewitt, Graham Paul Wyndham.
Hydrological models and tools are often used as decision support systems to inform water resources management. The successful application of these systems is largely dependent on the quality of data being incorporated into them. Accurate information with regards to total evaporation is of paramount importance to water resources managers, as it is a key indicator in determining if water resources are being used for their specific purposes. Due to the inherent spatial limitations associated with conventional techniques to estimate total evaporation, the application of satellite earth observation as a tool to estimate total evaporation is being advocated more frequently. The focus of this Dissertation was to develop an approach which would allow for the incorporation of total evaporation estimates from an existing evaporation model that incorporates satellite earth observation data i.e. the SEBS model, into a hydrological simulation model i.e. ACRU, to simulate streamflow. The SEBS model was first validated in the Komatipoort study site against the surface renewal system. The results of this investigation indicated that the SEBS model over-estimated total evaporation by approximately 47% and produced R2 and RMSE values of 0.33 and 2.19, respectively, when compared to total evaporation estimates obtained from the surface renewal system. Once, the model had been validated, it was then applied to estimate total evaporation for quarternary catchment X23_A for the period 01st December 2011 to 25th November 2012. These estimates were used to create a continuous total evaporation time series, which was used as an input to ACRU to model streamflow. The EVTR3 approach was derived to allow for the incorporation of the aforementioned SEBS total evaporation estimates in ACRU and to estimate streamflow amongst other hydrological parameters. The simulated streamflow for this technique was under-estimated by approximately 10% and produced R2 and RMSE values of 0.41 and 1.05, respectively, when compared to observed streamflow. Although these results appear to be satisfactory at best, similar results were obtained when using the conventional evaporation routine in ACRU to estimate streamflow. This occurrence circuitously highlights the potential of utilizing satellite earth observation data as a data source for a hydrological model.
An assessment of scale issues related to the configuration of the ACRU model for design flood estimation
(2010) Chetty, Kershani Tinisha.; Schulze, Roland Edgar.; Smithers, Jeffrey Colin.
There is a frequent need for estimates of design floods by hydrologists and engineers for the design of hydraulic structures. There are various techniques for estimating these design floods which are dependent largely on the availability of data. The two main approaches to design flood estimation are categorised as methods based on the analysis of floods and those based on rainfall-runoff relationships. Amongst the methods based on the analysis of floods, regional flood frequency analysis is seen as a reliable and robust method and is the recommended approach. Design event models are commonly used for design flood estimation in rainfall-runoff based analyses. However, these have several simplifying assumptions which are important in design flood estimation. A continuous simulation approach to design flood estimation has many advantages and overcomes many of the limitations of the design event approach. A major concern with continuous simulation using a hydrological model is the scale at which should take place. According to Martina (2004) the “level” of representation that will preserve the “physical chain” of the hydrological processes, both in terms of scale of representation and level of description of the physical parameters for the modelling process, is a critical question to be addressed. The objectives of this study were to review the literature on different approaches commonly used in South Africa and internationally for design flood estimation and, based on the literature, assess the potential for the use of a continuous simulation approach to design flood estimation. Objectives of both case studies undertaken in this research were to determine the optimum levels of catchment discretisation, optimum levels of soil and land cover information required and, to assess the optimum use of daily rainfall stations for the configuration of the ACRU agrohydrological model when used as a continuous simulation model for design flood estimation. The last objective was to compare design flood estimates from flows simulated by the ACRU model with design flood estimates obtained from observed data. Results obtained for selected quaternary catchments in the Thukela Catchment and Lions River catchment indicated that modelling at the level of hydrological response units (HRU’s), using area weighted soils information and more than one driver rainfall station where possible, produced the most realistic results when comparing observed and simulated streamflows. Design flood estimates from simulated flows compared reasonably well with design flood estimates obtained from observed data only for QC59 and QCU20B.
An assessment of shallow water tables and the development of appropriate drainage design criteria for sugarcane in Pongola, South Africa.
(2012) Malota, Mphatso.; Senzanje, Aidan.
South Africa, in common with all countries with arid or semi-arid climatic conditions, is facing the consequences of irrigation development without effective subsurface drainage. The quality of irrigation water is also decreasing and hence more water is required for leaching. This is resulting in low irrigation water productivity, as a consequence of shallow water tables, thus limiting crop growth. This study investigated the nature and causes of shallow water table problems in the sugarcane fields of Pongola, South Africa. The DRAINMOD model was also assessed for its reliability to be used as drainage design tool in the area. A water table map of a 32 ha sugarcane field was generated using groundwater table data monitored in 36 piezometers from September 2011 to February 2012. Nearly 12 % of the 32 ha sugarcane field was found to be affected by shallow water tables of less than the 1.0 m Design Water Table Depth (WTD). The inability of the adopted Drainage Design Criteria (DDC) to cope with drainage needs was found to be the cause of the poor drainage problem. On the other hand, analysis of WTDs in a field with a poorly-maintained subsurface drainage system confirmed that the drainage problem is exacerbated by poor drainage maintenance. It was recommended that the subsurface DDC in the area be revisited and that timely maintenance also be provided The DRAINMOD model was calibrated and verified using actual WTD and Drainage Discharge (DD) data. The model evaluation results revealed that the DRAINMOD model can reliably predict WTDs, with a Goodness of fit (R2), Mean Absolute Error (MAE) and Coefficient of Residual Mass (CRM) of 0.826, 5.341 cm and -0.015, respectively. Similarly, the model evaluation results in predicting DDs were also good, with R2, MAE and CRM of 0.801, 0.181 mm.day-1 and 0.0004, respectively. A further application of the validated model depicted that drain pipes installed at depths ranging from 1.4 m to 1.8 m and a spacing ranging from 55 to 70 m, with a design discharge of 2.5 to 4.2 mm.day-1, were adequate in ensuring safe WTDs between 1.0 and 1.5 m in clay-loam soil. On the other hand, drain depths ranging from 1.4 to 1.8 m and spacing between 25 and 40 m, were found to be appropriate in maintaining WTDs between 1.0 and 1.5 m in clay soil, with drainage design discharge ranging from 2.5 to 5.1 mm.day-1. These findings suggest that the current drain spacing needs to be reduced, in order to maintain the 1 m design water table depth. Finally, for the adoptability of the DRAINMOD model in the area, the Rosetta program, a component of the HYDRUS-2D, was tested for its reliability in estimating saturated hydraulic conductivities required by the DRAINMOD model. Results of the investigation revealed that the program can reliably be used to estimate saturated hydraulic conductivities from easily accessed soil data (% sand, silt, clay and soil bulk density), with R2, MAE and CRM of 0.95, 0.035 m.day-1 and -0.031, respectively. Nonetheless, calibration of the DRAINMOD model based on saturated hydraulic conductivity estimated by the Rosetta program was recommended. The findings of this research will form the basis for implementing an agricultural drainage policy that will ensure sustainable rain-fed and irrigation crop production systems in South Africa.
An assessment of the use of remote sensing to estimate catchment rainfall for use in hydrological modelling and design flood estimation.
(2022) Khakhu, Khodani.; Smithers, Jeffrey Colin.
The accurate estimation of catchment rainfall is crucial, especially in hydrological modelling and flood hydrology which is used for the planning and design of hydrological infrastructures such as dams and bridges. Traditionally, catchment rainfall is estimated by making use of ground-based point rainfall measurements from rain gauges. The literature review conducted in this study supports that there is evidence of a decrease in the number of operational groundbased rainfall stations in South Africa which presents a challenge when estimating catchment rainfall for use in hydrological modelling and design flood estimation. Thus, innovative ways are required to estimate catchment rainfall and to improve the estimation of catchment design rainfall. This study investigated the use of remote sensing as an alternative way to estimate catchment design rainfall. To do this, a pilot study was first used to develop and test the methodology using a quaternary catchment that was selected based on the raingauge density. This was followed by the application of a refined methodology in another quaternary catchment which was used to verify the results that were obtained in the pilot study. After a comprehensive review of the literature, the remote sensing product selected for this study was the CHIRPS rainfall product. The methodology adopted first validated the remotely sensed rainfall data using the observed rainfall data and the estimated remotely sensed rainfall values were bias corrected using the observed rainfall data. The statistics that were used for validating are MAE, MBE, RMSE and D. The method that was used for bias correction was empirical quantile mapping Issues encountered, and as documented in the literature, include the unavailability of long periods of observed quality rainfall data and the limited and uneven spatial distribution of rainfall stations. Catchment rainfalls were estimated using observed rainfall, and this was assumed as the best estimate and was compared to the catchment rainfalls that were estimated using the biascorrected remotely sensed rainfalls. The performance of CHIRPS rainfall was varied among the approaches and the selected catchments. Nevertheless, the results from this study still show the potential of the use of remotely sensed rainfall to estimate catchment design rainfalls. At the daily timescale, satellite-derived and observed rainfall were poorly correlated and variable among locations. However, monthly and annual rainfall totals were in closer agreement with historical observations than the daily values. Despite the varied performance , the result of the study shows that CHIRPS rainfall product can be used to estimate catchment rainfall for hydrological modelling and flood frequency analysis. By acknowledging that the performance of remote sensing products is robust, it is of importance to note that the performance of the results presented is strictly for the catchments and stations selected for this project as well as the methods selected to validate and correct the bias in remotely sensed rainfall. The recommendations from the study are that a similar study is conducted in another region where there is even distribution of stations and a long record of quality observed rainfall beyond the year 2000 and consideration of the methods to identify outliers before making any meaningful estimations such as catchment rainfall from rainfall data.

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