The further development, application and evaluation of a sediment yield model (WQSED) for catchment management in African catchments
- Authors: Gwapedza, David
- Date: 2021-04
- Subjects: Sedimentation and deposition -- South Africa , Sedimentation and deposition -- Zimbabwe , Watersheds -- South Africa , Watersheds -- Zimbabwe , Watershed management -- Africa , Water quality -- South Africa , Water quality -- Zimbabwe , Modified Universal Soil Loss Equation (MUSLE) , Water Quality and Sediment Model (WQSED) , Soil and Water Assessment Tool (SWAT)
- Language: English
- Type: thesis , text , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/178376 , vital:42934 , 10.21504/10962/178376
- Description: Erosion and sediment transport are natural catchment processes that play an essential role in ecosystem functioning by providing habitat for aquatic organisms and contributing to the health of wetlands. However, excessive erosion and sedimentation, mostly driven by anthropogenic activity, lead to ecosystem degradation, loss of agricultural land, water quality problems, reduced reservoir storage capacity and damage to physical infrastructure. It is reported that up to 25% of dams in South Africa have lost approximately 30% of their initial storage capacity to sedimentation. Therefore, excessive sedimentation transcends from an ecological problem to a health, livelihood and water security issue. Erosion and sedimentation occur at variable temporal and spatial scales; therefore, monitoring of these processes can be difficult and expensive. Regardless of all these prohibiting factors, information on erosion and sediment remains an urgent requirement for the sustainable management of catchments. Models have evolved as tools to replicate and simulate complex natural processes to understand and manage these systems. Several models have been developed globally to simulate erosion and sediment transport. However, these models are not always applicable in Africa because 1) the conditions under which they were developed are not as relevant for African catchments 2) they have high data requirements and cannot be applied with ease in our data-scarce African catchments 3) they are sometimes complicated, and there are little training available or potential users simply have no time to dedicate towards learning these models. To respond to the problems of erosion, sedimentation, water quality and unavailability of applicable models, the current research further develops, applies and evaluates an erosion and sediment transport model, the Water Quality and Sediment Model (WQSED), for integration within the existing water resources framework in South Africa and application for practical catchment management. The WQSED was developed to simulate daily suspended sediment loads that are vital for water quality and quantity assessments. The WQSED was developed based on the Modified Universal Soil Loss Equation (MUSLE), and the Pitman model is a primary hydrological model providing forcing data, although flow data from independent sources may be used to drive the WQSED model. The MUSLE was developed in the United States of America, and this research attempts to improve the applicability of the MUSLE by identifying key issues that may impede its performance. Assessments conducted within the current research can be divided into scale assessment and application and evaluation assessment. The scale assessment involved evaluating spatial and temporal scale issues associated with the MUSLE. Spatial scale assessments were conducted using analytical and mathematical assessments on a hypothetical catchment. Temporal scale issues were assessed in terms of the vegetation cover (C) factor within the Tsitsa River catchment in South Africa. Model application and evaluation involved applying and calibrating the model to simulate daily time-series sediment yield. The model was applied to calibrated and validated (split-sample validation) in two catchments in South Africa, two catchments in Zimbabwe and three catchments were selected from the USA and associated territories for further testing as continuous daily time-series observed sediment data could not be readily accessed for catchments in the Southern African region. The catchments where the model was calibrated and validated range in size from 50 km2 to 20 000 km2. Additionally, the model was applied to thirteen ‘ungauged’ catchments selected from across South Africa, where only long-term reservoir sedimentation rates were available to compare with long term model simulations converted to sediment yield rates. The additional thirteen catchments were selected from areas of different climatic, vegetation and soils conditions characterising South Africa and range in size from 30 km2 to 2 500 km2. The current research results are split into a) MUSLE scale dependency and b) WQSED testing and evaluation. Scale dependency testing showed that the MUSLE could be spatially scale-dependent, particularly when a lumped approach is used, resulting in simulations of up to 30% more sediment. Spatial scale dependence in the MUSLE was found to be related to the runoff and topographic factors used and how they are calculated. The current study resorted to adopting a reference grid in applying the MUSLE, followed by scaling up the outputs to the total catchment area. Using a reference grid resulted in a general avoidance of the problem of spatial scale. The adoption of a seasonal vegetation cover factor was shown to significantly account for temporal changes of vegetation cover within a year and reduce over-estimations in sediment output. The temporal scale evaluation demonstrated the uncertainties associated with using a fixed vegetation cover factor in a catchment with variable rainfall and runoff pattern. The WQSED model evaluation showed that the model could be calibrated and validated to provide consistent results. Satisfactory model evaluation statistics were obtained for most catchments to which the model was applied, based on general model evaluation guidelines (Nash Sutcliffe Efficiency and R2 > 0.5). The model also performed generally well compared to established models that had been previously applied in some of the study catchments. The highest sediment yields recorded per country were 153 t km-2 year-1 (Tsitsa River; South Africa), 90 t km-2 year-1 (Odzi River; Zimbabwe) and 340 t km-2 year-1 (Rio Tanama; Puerto Rico). The results also displayed consistent underestimations of peak sediment yield events, partly attributed to sediment emanating from gullies that are not explicitly accounted for in the WQSED model structure. Furthermore, the calibration process revealed that the WQSED storage model is generally challenging to calibrate. An alternative simpler version of the storage model was easier to calibrate, but the model may still be challenging to apply to catchments where calibration data are not available. The additional evaluation of the WQSED simulated sediment yield rates against observed reservoir sediment rates showed a broad range of differences between the simulated and observed sediment yield rates. Differences between WQSED simulated sediment and observed reservoir sediment ranges from a low of 30% to a high of > 40 times. The large differences were partly attributed to WQSED being limited to simulating suspended sediment from sheet and rill processes, whereas reservoir sediment is generated from more sources that include bedload, channel and gully processes. Nevertheless, the model simulations replicated some of the regional sediment yield patterns and are assumed to represent sheet and rill contributions to reservoir sediment in selected catchments. The outcome of this study is an improved WQSED model that has successfully undergone preliminary testing and evaluation. Therefore, the model is sufficiently complete to be used by independent researchers and water resources managers to simulate erosion and sediment transport. However, the model is best applicable to areas where some observed data or regional information are available to calibrate the storage components and constrain model outputs. The report on potential MUSLE scale dependencies is relevant globally to all studies applying the MUSLE model and, therefore, can improve MUSLE application in future studies. The WQSED model offers a relatively simple, effective and applicable tool that is set to provide information to enhance catchment, land and water resources management in catchments of Africa. , Thesis (PhD) -- Faculty of Science, Institute for Water Research, 2021
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Modelling water quality : complexity versus simplicity
- Authors: Jacobs, Haden
- Date: 2017
- Subjects: Water quality management -- Mathematical models , Water quality -- Measurement , Water quality biological assessment
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/4754 , vital:20721
- Description: Water quality management makes use of water quality models as decision making tools. Water quality management decisions need to be informed by information that is as reliable as possible. There are many situations where observational data are limited and therefore models or simulation methods have a significant role to play in providing some information that can be used to guide management decisions. Water quality modelling is the use of mathematical equations and statistics to represent the processes affecting water quality in the natural environment. Water quality data are expensive and difficult to obtain. Nutrient sampling requires a technician to obtain ‘grab samples’ which need to be kept at low temperatures and analysed in a laboratory. The laboratory analyses of nutrients is expensive and time consuming. The data required by water quality models are seldom available as complete datasets of sufficient length. This is especially true for ungauged regions, either in small rural catchments or even major rivers in developing countries. Water quality modelling requires simulated or observed water quantity data as water quality is affected by water quantity. Both the water quality modelling and water quantity modelling require data to simulate the required processes. Data are necessary for both model structure as well as model set up for calibration and validation. This study aimed to investigate the simulation of water quality in a low order stream with limited observed data using a relatively complex as well as a much simpler water quality model, represented by QUAL2K and an in-house developed Mass Balance Nutrient (MBN) model, respectively. The two models differ greatly in the approach adopted for water quality modelling, with QUAL2K being an instream water quality fate model and the MBN model being a catchment scale model that links water quantity and quality. The MBN model uses hydrological routines to simulate those components of the hydrological cycle that are expected to differ with respect to their water quality signatures (low flows, high flows, etc.). Incremental flows are broken down into flow fractions, and nutrient signatures are assigned to fractions to represent catchment nutrient load input. A linear regression linked to an urban runoff model was used to simulate water quality entering the river system from failing municipal infrastructure, which was found to be a highly variable source of nutrients within the system. A simple algal model was adapted from CE-QUAL-W2 to simulate nutrient assimilation by benthic algae. QUAL2K, an instream water quality fate model, proved unsuitable for modelling diffuse sources for a wide range of conditions and was data intensive when compared to the data requirements of the MBN model. QUAL2K did not simulate water quality accurately over a wide range of flow conditions and was found to be more suitable to simulating point sources. The MBN model did not provide accurate results in terms of the simulation of individual daily water quality values; however, the general trends and frequency characteristics of the simulations were satisfactory. Despite some uncertainties, the MBN model remains useful for extending data for catchments with limited observed water quality data. The MBN model was found to be more suitable for South African conditions than QUAL2K, given the data requirements of each model and water quality and flow data available from the Department of Water and Sanitation. The MBN model was found to be particularly useful by providing frequency distributions of water quality loads or concentrations using minimal data that can be related to the risks of exceeding management thresholds.
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An investigation of the sources and supply of coarse sediment input to a semi-arid channel reach
- Authors: Frauenstein, Glenn Gavin
- Date: 1988
- Subjects: Sedimentation and deposition -- Arid regions
- Language: English
- Type: Thesis , Masters , MA
- Identifier: vital:4803 , http://hdl.handle.net/10962/d1001903
- Description: This study comprises an investigation of the source and supply of coarse sediment input to a semi-arid channel reach. Despite a growing body of lIterature documentIng research of various aspects of sediment response in semi-arId areas, few studies attempt to integrate processes active in specific source areas wIth sediment supply to the channel. Detailed In the present study is an account of the processes active in the study area, identifIcation of source environments, a discussion of some of the factors affecting supply, a comparison of the effectiveness of gravItatIonal and fluvIal supply processes and an estImatIon of the time sequence of sediment supply to and removal from a channel reach. The above aspects of sediment supply are embodied In the aims set for the study. The study is conceptualIsed withIn the framework of a coarse sedIment supply model. The model is formulated from supporting literature and tested in the light of the results obtained through an investigation of the above aspects of sediment supply in the specIfic study area. The model is prImarily a qualitative one and the data collected intended to strengthen the qualitative nature of the model, while at the same time add at least some measure of quantification. Several reasons for studyIng coarse sediment behavIour in semI-arid areas are identIfied and include the need to improve the present lack of understanding of the relatIonship between supply and removal of sediment, the temporal dIstributIon of sediment discharge and the relatIve contrIbutions of coarse sediment to the overall load of rivers. The study area is located within the semi-arid Ecca basin north-east of Grahamstown. A specifIc channel reach is chosen withIn a sub-catchment (catchment B) of the Ecca catchment area as it has a variety of channel bank environments, is accessible through the entire reach, and the proximity of a raingauge and flow measurIng weir provide the necessary hydrometeorological inputs. The methods of observing sediment response from five data collection sites Include the use of slope or bank base sediment traps, erosion pins, tracer particle monitoring. sequential photographic surveys, and channel bed surface profile surveys. Hydrometeorological data is provided by records drawn from the data bank at the Hydrological Research Unit. Rhodes University. All rainfall records as well as channel flow data are available in the form of continuous records. Rainfall amount and intensity for any period could be extracted from these records. Data collection is confined to a period of two years, during which time the study area was visited on an approximate monthly basis. The index of erosivity (EI₃₀) could also be calculated from the hydrometeorological records and has been used as an integrated measure of rainfall intensity over the monthly period between site visits. The results are presented on a sample day for sample day basis. The sediment response data together with hydrological data is represented graphically for each sample day, of which there were nineteen. Discussion and interpretation of the results is left to a separate chapter. The interpretation of the results are based largely upon graphical representation of data time series and of interrelationships between some of the variables measured. The limited number of sample days together with the assumed auto correlation present in much of the data precluded the use of simple statistical testing procedures. The use of more complex procedures is not considered worthwhile and is unlikely to add to the interpretation of the results. Bedrock weathering is found to be a fairly active producer of coarse sediment on exposed shale bedrock outcrops through which sections of the channel are cut. The transport of the weathered detritus to the channel is attributed to a combination of gravitational and fluvial transport processes, with each process dominating at different times, depending on the magnitude of the climatic input. A tentative comparison of the effectiveness of the two processes reveals that both are capable of transporting similar amounts of sediment but on different time scales. The trends of sediment supply from the various bank environments display remarkable similarity , suggesting a measure of consistency of response to climatic input through the entire reach. Source areas of coarse sediment identified were limited to a small percentage of the total valley area and consisted almost entirely of the immediate channel environment. A tributary gully appears to be an important source of coarse sediment during fluvially dominated supply episodes, while the channel banks supply sediment on a quasi-continuous basis. The total yields for each source environment were extrapolated from the sampled amounts, revealing that channel banks are the predominant source environments. An attempt is made to assess the role of various factors which might affect sediment supply. The factors include rainfall amount and intensity, channel flow, geology/lithology, dip of strata, aspect of channel banks and size of weathered material. The findings, though not conclusive, do give some indication of the role of the above factors. It is suggested though that this particular aspect of sediment supply receive further attention in future research. Discussion on the time sequence of supply to and removal from the channel draws attention to a pulse- like movement of sediment 'waves' through the channel, and two scales of removal-accumulation cycles are identified. Finally the validity of the model is assessed and with the exception of a tributary inflow process not envisaged in the original model, is found to be an accurate representation of sediment supply in semi-arid areas, in both its static and dynamic phases. The suggestion is offered that future research on the sediment supply system, in all climatic regimes, can be conceptualised within the context of the basic model proposed in the present study. Specific components of the model should be quantified by numerous individual research efforts, and in this way, serve to build up the model into a widely applicable tool with which to interpret sediment supply
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