Masters Degrees (Environmental Hydrology)

Permanent URI for this collectionhttps://hdl.handle.net/10413/6589

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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.
Development of a geographic data model for hydrological modelling.
(2006) Bollaert, M. J.; Clark, David John.
Hydrology is a multi-disciplinary science, and therefore derives data from diverse sources, with the data often of a spatio-temporal nature. A recent trend has been to combine these data with GIS, due to the data’s geographic origin, and inherently requires an abstraction of reality in order to deal with the multitude of data that would otherwise result. Consequently, data models have been developed for this purpose, and these require a generalisation of processes and variables, while offering a simplified structure for their storage. The purpose of this study was to develop a data model for the storage and dissemination of hydrological variables and associated data used in hydrological modelling. Data would be of a spatial and temporal nature, and thus the design of the new data model needed to provide for this. A number of existing geographic data models were therefore reviewed, including the geodatabase model. This data model and the object-relational database model upon which it was built, were ascertained as being the most suitable for the study, and were therefore included in the design of the new data model. The related Arc Hydro data model was subsequently reviewed, since it offered an established means by which to model geographic features associated with surface hydrology. Following this, an investigation into time series storage methods was carried out, as it was important that the new data model be able to store large time series datasets in an efficient manner. Thus a number of methods were identified and evaluated as to their advantages and disadvantages. A new data model was thereby conceived, using the geodatabase as its foundation, and was developed in order to offer efficient storage of hydrological data. The data model developed was subsequently tested by populating it with data from the Quaternary Catchments database which supports the ACRU model. Finally, additional functionality was added to the data model, in the form of export options.
The effect of spatial resolution in remote sensing estimates of total evaporation in the uMgeni catchment.
(2014) Shoko, Cletah.; Clark, David John.; Bulcock, Hartley Hugh.; Mengistu, Michael Ghebrekidan.
The estimation of total evaporation plays a vital role in water resources monitoring and management, especially in water-limited environments. In South Africa, the increasing water demand, due to population growth and economic development, threatens the long-term water supply. This, therefore, underscores the need to account for water by different consumers, for well-informed management, allocation and future planning. Currently, there are different methods (i.e. ground-based and remote sensing-based methods), which have been developed and implemented to quantify total evaporation at different spatial and temporal scales. However, previous studies have shown that ground-based methods are inadequate for understanding the spatial variations of total evaporation, within a heterogeneous landscape; they only represent a small area, when compared to remotely sensed methods. The advent of remote sensing therefore provides an invaluable opportunity for the spatial characterization of total evaporation at different spatial scales. This study is primarily aimed at estimating variations of total evaporation across a heterogeneous catchment in KwaZulu-Natal, South Africa, using remote sensing data. The first part provides an overview of total evaporation, its importance within the water balance and consequently in the management of water resources. It also covers various methods developed to estimate total evaporation, highlighting their applications, limitations, and finally, the need for further research. Secondly, the study determines the effect of sensor spatial resolution in estimating variations of total evaporation within a heterogeneous uMngeni Catchment. Total evaporation estimates were derived, using multispectral 30 m Landsat 8 and 1000 m MODIS, based on the Surface Energy Balance (SEBS) model. The results have shown that different sensors, with varying spatial resolutions, have different abilities in representing variations of total evaporation at catchment scale. It was found that Landsat-based estimates were significantly different (p < 0.05) from MODIS. The study finally estimates spatial variations of total evaporation from Landsat 8 and MODIS datasets for the uMngeni Catchment. It was found that the Landsat 8 dataset has greater potential for the detection of spatial variations of total evaporation, when compared to the MODIS dataset. For instance, MODIS-based daily total evaporation estimates did not show any significant difference across different land cover types (One way ANOVA; F1.924 = 1.412, p= 0.186), when compared to the 30 m Landsat 8, which yielded significantly different estimates between different land cover types (One way ANOVA; F1.993= 5.185, p < 0.001). The validation results further indicate that Landsat-based estimates were more comparable to ground-based eddy covariance measurements (R2 = 0.72, with a RMSE of 32.34 mm per month (30.30% of the mean)). In contrast, MODIS performed poorly (R2 = 0.44), with a RMSE of 93.63 mm per month (87.74% of the mean). In addition, land cover-based estimates have shown that, not only does the land cover type have an effect on total evaporation, but also the land cover characteristics, such as areal extent and patchiness. Overall, findings from this study underscore the importance of the sensor type, especially spatial resolution, and land cover type characteristics, such as areal extent and patchiness, in accurately and reliably estimating total evaporation at a catchment scale. It is also evident from the study that the spatial and temporal variations in SEBS inputs (e.g., LAI, NDVI and FVC) and energy fluxes (e.g., Rn) calculated by SEBS for the two sensors can affect the spatial and temporal variations in total evaporation estimates. For instance, spatial variations in total evaporation reflected similar spatial variations in Rn. Areas with high NDVI, FVC and LAI (which denotes dense vegetation cover) tend to have higher total evaporation estimates, compared to areas with lower vegetation cover. In addition, the MODIS sensor at 1000 m spatial resolution showed lower estimates of SEBS inputs with less variability across the catchment. This resulted in lower total evaporation estimates, with less variability, compared to the 30 m Landsat 8. In addition, with regard to inputs derived from remote sensing, it was found that the spatial variations in total evaporation are not determined by individual variables (e.g., LST), but are influenced by a combination of many biophysical variables, such as LAI, FVC and NDVI. These findings lay a foundation for a better approach to estimate total evaporation using remote sensing for use in the management and allocation of water.
An evaluation of priority and fractional methods of water allocation in the Sand River catchment, South Africa.
(2014) Winckworth, Ross.; Smithers, Jeffrey Colin.; Clark, David John.
The development and apportionment of water resources is a critical issue, both globally and locally in South Africa. This is particularly true in the development and allocation among states sharing watercourse systems. The competition inherent in access to water resources is increasing. In particular, pressure is being placed on water resources from several activities including irrigation, domestic consumption and industrial requirements. Water allocation mechanisms are therefore critical to sustain the existing allocatable water resources while attempting to combine both efficiency and equity principles. The National Water Act of South Africa (Act 36 of 1998) (NWA (36, 1998)) incorporates both institutional and legal policy which promotes the efficient, equitable and sustainable management of water resources. The aims of the NWA (36, 1998) are achieved by a movement away from a Riparian Rights system (a property adjacent to a water course is allowed reasonable use) to an Administrative System (Hallowes et al., 2008). The inception of an Administrative System for the allocation of water in South Africa is vital given that a number of catchments in South Africa have reached a state of being fully developed and more than 50% of the 19 water management areas in South Africa are water stressed, i.e. the demand exceeds the supply (DWAF, 2004). The NWA (36, 1998) makes allowance for only one right to water; that being the Reserve, which consists of two components, the ecological requirement and basic human needs. The management of the resource is important because the NWA (36, 1998) states that the water resources within South Africa are to be protected, used, developed, conserved, managed and controlled in accordance with the National Water Resources Strategy (DWAF, 2004). The water allocation method currently applied in South Africa is referred to as a Prioritybased River and Reservoir Operating Rule (PRROR) institutional arrangement. Under PRROR, when there is a risk of a reservoir or river failing to meet the supply demanded, restrictions are applied to abstractions. The priority extends not only to those who have the priority of use but which users will relinquish water to the higher priority users and by what quantity. Disadvantages of PRROR include the inability of the Water User to manage their water to meet their needs and are then forced into using it when the water is available. Possible alternate allocation methods include Fractional Water Allocation and Capacity Sharing (FWACS), public water allocation and prior rights systems. The PRROR as currently implemented leads to high priority sectors having dominance over access to water which may lead to those sectors not using water efficiently. The introduction of FWACS creates an atmosphere of water awareness and being responsible for managing water use. In this study, the MIKE BASIN model was used in the simulation of the processes of the PRROR and the FWACS allocation methods. The model routes water based on rules specified for the allocation method under review. The efficiency of each allocation method was evaluated in terms of the reliability of supply to Water Users. In the catchment used as a case study (Sand River Catchment), limited information on Environmental Water Requirement (EWR) was available and the EWRs were set as minimum flows at each reservoir and then set as a minimum flow requirement at a downstream node to prevent Water Users downstream of the dam from immediately abstracting the EWR release. Based on data used in the case study and the rules applied to each scenario, the results from the initial study indicated that PRROR provides a 4% higher reliability of supply in comparison to FWACS in the catchment under investigation. This is true when the supply to a Water User is similar between scenarios. However, if the fractions allocated in FWACS are varied away from this baseline, results indicate that a 50% increase on the original FWACS fractions provides for better reliability of supply. Thus the results show that although PRROR is an alternative method for determining water allocation to water users, FWACS+50 is able to improve on the water reliability of supply within the Sand River Catchment.

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Browsing Masters Degrees (Environmental Hydrology) by Author "Clark, David John."