Browsing by Author "Riddell, Edward Sebastian."

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Characterisation of the hydrological processes and responses to rehabilitation of a headwater wetland of the Sand River, South Africa.
(2011) Riddell, Edward Sebastian.; Lorentz, Simon Antony.
The erosion of headwater wetlands in the Sand River catchment, in the lowveld of north-eastern South Africa has led to a focus on their rehabilitation, both for livelihood security for those that use them for subsistence agriculture, as well as for provision of streamflow regulation services for the Sand River itself. One such wetland, the Craigieburn-Manalana itself undergoing severe erosion was subject to technical rehabilitation using concrete weirs and gabion dams to stabilize the erosion gullies during 2007. Through a series of papers the research discussed in this thesis examined the response of the wetland?s hydrodynamics to the implementation of these measures. Through the installation of a network of hydrometric apparatus the research has shown that the wetlands hydrology is largely controlled by the presence of both horizontal and vertical clay aquicludes within a hydraulically conductive sandy matrix. The sequence of these aquicludes had allowed for artesian phreatic surface phenomena identified in a relatively hydrologically intact region of the wetland. The gully erosion had initiated hydraulic drawdown of the wetland?s water table leading to the desiccation of the system. The construction of a buttress weir within the erosion gully had restored the wetlands hydrodynamics to that typical of conditions upstream of a clay-plug. The research also explored the role that clay plays in terms of controlling the wetland?s hydro-geomorphic setting through geophysical analysis. A conceptual model was then derived that states that these wetlands are held in place by clay-plugs that form through clay illuviation from the hillslopes at regions of valley confinement. This has important implications for the connectivity of wetland process domains. The research also determined the inputs of surface and subsurface flows to the wetland and it was found through detailed examination of soil moisture responses and variably saturated soil physics modelling using the HYDRUS model, that the wetland is hydrologically connected to its contributing hillslope by threshold induced preferential flow pathways, via macropores, that only respond after specific antecedent soil moisture conditions are met. In addition, the thesis describes novel approaches to use information provided by soil scientists for the development of catchment hydrological models. It was shown that the use of this hydropedology information improved the low flow response function of the catchment model, ACRU. This development has important implications for up-scaling of catchment process domains, or hydrological response units by being able to generalize on hillslope hydrological responses based on configuration of their soil type elements. The research also undertook to examine the role that the wetlands play in catchment processes. It was found through water budgeting, supported by hydrological time-series, stable isotope analysis and the quantification of vegetation water use within the wetland and contributing catchment, that these wetlands do not augment baseflows during the dry season. Furthermore, it is only early on during the wet season that these systems may attenuate peak flows, thereafter they act as conduits for high storm flows. Similarities emanated from this research with previous hydrological studies of headwater wetland systems in southern Africa and these are discussed.
The effect of long-term fire frequencies on soil hydraulic properties in semi-arid savannas in Kruger National Park.
(2013) Strydom, Tercia.; Riddell, Edward Sebastian.; Lorentz, Simon Antony.
Soils are vital in supporting healthy and functioning ecosystems. Thus when soils are degraded, important ecosystem services are affected. In African savannas where fire is a key driver controlling ecosystem composition and structure, there is a lack in current understanding regarding the impacts of long-term fire management on soil hydrology. The Experimental Burn Plots (EBPs) in Kruger National Park (KNP), a long-term fire experiment initiated in the 1950‟s, offered a unique opportunity to determine the effects of long-term fire treatments (i.e. annual burn vs. no burn) compared to a “variable” fire regime (VFR) outside of the EBPs on soil hydraulic properties in semi-arid savannas. This study was conducted during October 2012- May 2013 on different soil types stemming from the two dominant geologies in KNP, i.e. granites and basalts. This study revealed that it is rather the time following a fire and not necessarily frequency which resulted in decreased soil infiltration, with slowest infiltration rates immediately after the fire. Findings suggested that fire only affected infiltration rates at the soil surface and that these fire effects would dissipate within approximately two years— suggesting the soil‟s ability to recover; at least in terms of their hydrological function. Soil compaction, which is recognized for impeding soil infiltration, was measured. The research presented in this thesis indicates that surface compaction may be due to soil processes such as raindrop impact and splash but deeper compaction is attributed to high herbivore concentrations trampling the soil. Interestingly, the extent of soil compaction caused by high densities of herbivores does not result in significantly reduced soil infiltration rates. In addition, long-term fire management effects on soil organic matter content and soil water retention was investigated. Besides promoting soil fertility, soil organic matter is considered hydrophilic and aids in soil water retention. Although alluding to greater organic matter on the fire-suppressed plot on the granitic EBPs, there were no statistically-significant differences found across the varying fire frequencies. However on the basaltic EBPs, organic matter content varied between the various fire frequencies. Unlike the granitic plots where it is believed that fire intensities are not substantial enough to transfer heat deep into the soil and consume organic matter, it is thought that the huge contrast in above-ground biomass between the basaltic burn plots is in fact responsible for the contrast in organic matter contents. Consequently, soil water retention was found to be greatest on the fire-suppressed no burn plots. The ability of the soil to retain moisture, especially at low water contents, is crucial in a post-fire environment in order to facilitate re-establishment of vegetation. Fire impacts on soil hydraulic processes ultimately influence soil water balances. These impacts may have cascading effects on large-scale catchment processes. This study provides valuable insight not only into the relationship between water and fire but also how other factors such as soil, vegetation and herbivores all interact within a water-controlled savanna landscape.
Evaluating the potential of using satellite earth observation data to quantify the contribution of riparian total evaporation to streamflow transmission losses.
(2017) Gokool, Shaeden.; Riddell, Edward Sebastian.; Chetty, Kershani Tinisha.; Jarmain, Caren.
Numerous perennial rivers which flow through arid and semi-arid environments in South Africa, have become severely constrained as water resources abstractions are close to exceeding, or have exceeded the available supply and ecosystem resilience. This is a common phenomenon, as river basins are increasingly developed and often over allocated, in order to maximize socio-economic benefits through consumptive water use, often at the expense of the environment. Thus, managing and maintaining environmental water requirement (EWR) flow allocations in these circumstances becomes increasingly important but all the more challenging, especially during periods of water scarcity. The Letaba River situated in the semi-arid north-eastern region of South Africa is a typical example of a river system in which water governance challenges and infrastructural development have resulted in flows within the river no longer resembling the natural flow regime. This situation has improved to some extent after the establishment of river operating rules and an adaptive operational water resources management system. However, one of the major challenges with successfully implementing and managing EWR flows to date has been the uncertainty regarding the magnitude and influence of streamflow transmission losses (TL’s) on flows within the river system. TL’s along the Letaba are thought to be a significant proportion of streamflow during dry periods and this therefore constrains the ability to meet target EWR flows, as it is often the case that specified EWR releases from the Tzaneen dam are not adequately met further downstream at EWR target gauges. To ensure that water provisions and in particular EWR flows can be managed more effectively and efficiently in the future, it is imperative that the hydrological processes contributing to TL’s are quantified at various spatial and temporal scales. Considering this statement as a point of departure, the overall objective of this thesis was to reduce the uncertainty associated with TL’s by attempting to acquire an improved hydrological process understanding of the natural drivers of loss in this system, so that TL’s along the Letaba River can be more accurately quantified. This research involved, conducting detailed characterizations of hydrological processes along a 14 km reach of the Groot Letaba River which has similar land use activities and hydrological characteristics to the broader river system. Particular emphasis was placed upon establishing the influence of riparian total evaporation (inclusive of open water evaporation) on TL’s, as this process is a major contributing factor to the water balance of arid and semi-arid environments, yet has seldom been incorporated or adequately represented into TL’s estimation procedures. These investigations were centred on evaluating the potential of using a satellite-based approach to acquire spatially explicit estimates of evapotranspiration (ET) during the low flow period in this river system (May to October), which typically represents a critical period with regards to water shortages. For this purpose, the satellite-based surface energy balance (SEBS) model and satellite earth observation data acquired from Landsat and Moderate-resolution imaging spectroradiometer (MODIS) were used to estimate ET. However, the trade-off between the spatial and temporal resolution associated with these data sets can limit the reliability of satellite-based ET modelling (except where occasionally correct). Consequently, the SEBS ET estimates from these data sets were used as inputs to two relatively simplistic approaches (actual crop coefficient or Kcact and output downscaling with linear regression or ODLR) to quantify ET at a moderate spatial resolution (30 m) on a daily time step. These ET estimates were compared against in-situ ET estimates using a one sensor Eddy Covariance system to quantify any uncertainties associated with the satellite-derived estimates. To further investigate spatial and seasonal variations in source contributions to plant water uptake during the investigation period, stable isotope analysis (of 18O and 2H) and a Bayesian mixing model were coupled with the satellite derived ET estimates. The insights acquired from these investigations, were then used to derive baseline estimates of TL’s. This involved using the satellite-derived daily ET time series in conjunction with data obtained from a parallel investigation focusing on quantifying the rapport between surface and sub-surface water storage processes. Initial comparisons of ET estimates acquired using the Kcact and ODLR approaches against ECET were fairly poor yielding RMSE values of; 1.88 and 2.57 mm d-1 and 1.10 and 2.39 mm d-1 (for two replicate transects), respectively. The poor performance of these techniques was largely attributed to the SEBS ET estimates used as inputs to these techniques, as SEBS may overestimate evapotranspiration during conditions of water stress. This limitation was overcome using an evaporative calibration factor (termed the environmental stress factor or ESF) into the original SEBS formulation (SEBS0), to correct for the overestimation of the latent heat flux (LE) and the evaporative fraction (EF). The ESF calibration factor was empirically derived and then integrated into SEBS0, so as to better represent the influence of water stress on the EF and consequently LE. The implementation of the modified version of SEBS (SEBSESF) was shown to significantly improve the estimation of energy fluxes, which in turn resulted in an improved correlation and an increase in the percentage of modelled ET estimates within an acceptable accuracy range (± 15 to 30 %) when compared against in-situ observations. Through the application of this modified version of SEBS (SEBSESF), the ability of the ODLR and Kcact approaches to develop a time-series of daily moderate spatial resolution ET estimates could now be demonstrated. The use of SEBSESF ET estimates as inputs to the Kcact approach was shown to compare most favourably to ECET, yielding correlation coefficient and Nash-Sutcliffe efficiency values of 0.79 and 0.60, respectively. With the ability of this satellite-based approach to adequately represent ET within this environment now confirmed. Stable isotope analysis (of 18O and 2H) and a Bayesian mixing model were coupled with the Kcact derived ET estimates, to further investigate spatial and seasonal variations in plant water uptake dynamics. The results of these investigations showed that soil water was the main contributing source to ET. While stream and groundwater use during transpiration was also prevalent within the study area and increased with aridity, the magnitude of the contribution of these sources to transpiration was fairly minimal and not as significant as generally reported in literature. The insights gained from these investigations, as well as those obtained from the quantification of surface and sub-surface water storage processes, assisted in deriving baseline estimates of TL’s along the length of river reach studied. In general, it was found that during the latter stages of the dry season (August to October) TL’s accounted for approximately 5 to 15 % of the flow in the river system, with riparian total evaporation and in particular transpiration the dominant contributing processes to this loss. Through linkages with the recent gazetting of the Letaba Management Class (resource objective setting) and the mandatory implementation of EWR flows, it was shown that flows within the river system were unable to meet low flow targets and are required to be increased in order to fulfil this requirement, whilst simultaneously accounting for TL’s. It should be noted that while the various investigations undertaken in this study enabled the estimation of TL’s and the contribution of processes viz. riparian ET to TL’s, the estimates provided could not be verified due to the lack of reliable upstream (inflow) flow gauge data. Although the investigations and observations detailed in this study provide an understanding of the system for a limited period in time, they would substantially benefit from longer-term monitoring, so that the assumptions and related uncertainties that had to be factored into the analysis could be reduced. Overall the study has detailed key hydrological processes influencing TL’s along the Groot Letaba River, providing invaluable insights on existing knowledge gaps and contributing new knowledge to this research area. It is envisaged that this will enable the establishment of an improved conceptual understanding of the system, which may prove to be beneficial for future hydrological modelling applications in this region.
Hydrological connectivity in selected pristine catchments in the Kruger National Park.
(2014) Fundisi, Daniel.; Lorentz, Simon Antony.; Riddell, Edward Sebastian.
The understanding of interactions between hydrological processes is essential, especially in water limited ecosystems in semi-arid environments. It is through this understanding that informed planning and management decisions for ecosystem conservation are developed. Assessment of groundwater- surface water connectivity at catchment scale provides a holistic view of the abiotic template that sustains life systems within the catchment. Spatial differences in hydrological responses are thus understood since these are characterised by nonlinearities emanating from catchment heterogeneity across spatial and temporal scales. This study involved an assessment of groundwater-surface water interaction across incremental contributing areas which were based on stream orders. The study areas, Southern Granites and Southern Basalts, are located on the two dominant geologies in the Kruger National Park (KNP). At Southern Granites the 1st order, 2nd order and 3rd order contributing catchments have an area of 0.3km2, 0.9km2 and 1.5km2 respectively. At the Southern Basalts site the areas for similar incremental catchments were 15.4km2, 31.6km2 and 47.8km2 respectively. Both study sites had streamflow levelloggers installed at each outlet the 1st to 3rd order contributing areas. The assessment was done through a combination of hydrometric techniques, Electrical Resistivity Tomography (ERT) and tracer analysis methods. Monitoring of water levels and sampling in the stream, riparian boreholes and piezometers was conducted from September 2012 to May 2013. The monitoring network consisted of 28 piezometers and 19 boreholes at Southern Granites while 6 piezometers and 4 boreholes were installed at the Southern Basalts sites. Streambed hydraulic conductivities were determined using slug tests. Hydraulic gradients between the stream, piezometers and groundwater boreholes were calculated and used to determine direction of fluxes. Connectivity mechanisms were determined and contributions of different water sources to streamflow were quantified using two and three component tracer based hydrograph separations. Results showed that rainfall intensity was the major control to connectivity between surface and groundwater resources in these catchments. Contribution of event water to streamflow was estimated between 60% and 86% across the nested spatial scales for two monitored rainfall events (19 January and 20 February 2013) at Southern Granites study site. Although event water emerged as the dominant source at all scales, higher pre-event contributions were noted for lower order subcatchments at this site. A 2nd order stream channel, previously conceptualised as gaining was demonstrated through hydrometry and tracers to be increasingly losing subsequently behaving as an indirect recharge point at Southern Granites site. The study, therefore, revealed that lower order reaches on the granitic geology are important water sources that sustain baseflow at higher order perennial streams. At Southern Basalts study site limited subsurface contribution to streamflow was observed due to very low interfluvial gradients and low aquifer transmissivities that characterise the basalt geology. Assessment of groundwater-surface water interaction at this site was conducted only at the 3rd order catchment due to a limited network of groundwater boreholes. At this reach the contribution of event water was estimated between 51% and 64% for two monitored events (19 January and 20 February 2013). Groundwater contribution to streamflow through localised preferential fractured rock flow ranged between 36% and 49%.
The influence of scale and parent material on hillslope hydrological processes in Kruger National Park.
(2014) Jumbi, Faith Tatenda.; Riddell, Edward Sebastian.; Lorentz, Simon Antony.
Semi-arid savannas are dynamic and complex systems controlled by fire, nutrient and water availability where water is the major driving force controlling the biological activity in these systems. Knowledge of hydrological processes is therefore, crucial for the prediction of ecosystem changes, given the prospect of climate change and the ever-increasing anthropogenic pressure on resources. There have been numerous advances in hydrological studies especially at the hillslope scale. Many of them have however, been site-specific and mostly comprising of a series of point measurements, thereby making it difficult to link processes at different scales in other sites. Some form of classification system would therefore, enable process integration, which is crucial in understanding and linking processes at various scales. For effective management and water resource planning, knowledge of how a system functions is crucial especially in savannas. It also helps in better prediction of behavior in ungauged systems that are in similar settings since many people rely on savannas for various ecosystem goods and services, the sustainability of livelihoods and biodiversity. Meanwhile, ephemeral systems contribute significantly to how savanna systems work. However, hydrological processes operate at different scales even in these ephemeral systems involving controls such as landscape morphology, geology and climatic conditions in different environments. The effect of scale on hydrological processes is one unresolved issue in hydrological studies and knowledge of how processes vary at different scales is fundamental in understanding scale-dependent processes and those that are not. This also helps in understanding the thresholds and controls at which the processes operate. Hydrological processes were observed and quantified at the hillslope scale in contributing areas to 1st, 2nd and 3rd order ephemeral streams in the Kruger National Park (KNP) landscapes on basalt and granite parent materials. Processes include subsurface lateral and vertical flow, overland flow and potential recharge to groundwater. These were focused on the KNP supersites, which are sites that were especially selected for multidisciplinary research. This would allow the description and measurement of processes and patterns at different scales, rainfall gradients and geological settings within the landscapes. Hillslope hydrological processes are dynamic and very complex but they are nonetheless vital, due to their interactions with surface and groundwater. Hillslope mechanisms of flow and subsurface moisture content are highly variable at both spatial and temporal scales. In order to gain insights into this variability, a number of techniques were used, such as measurement of soil water potential at different depths, geophysical surveys and numerical modelling. From the hydrometry data and modelling exercise, fluxes and water balances were derived in order to derive the hillslope response functions and potential connectivity with the underlying aquifers and/or adjacent streams. Since soils and hydrology have an interactive relationship, hillslope soil type responses were characterized through qualitative hydropedological descriptions. Results showed that hillslope hydrological responses, such as flow mechanisms and connectivity are dependent on scale (incremental stream orders). Hydrological connectivity was highly driven by rainfall intensity on hillslopes associated with high order streams (2nd and 3rd). The major driver of hillslope storage was shown to be ET demand rather than soil depth, in these savanna ecosystems. The initial perception was that deeper soils will have greater storage capacity, but results obtained in this study proved otherwise. This also proved the ability of savanna vegetation to extract large amounts of soil water when available in the soil profile. This study showed that flow mechanisms were mainly influenced by topography and soil hydraulic properties and to a lesser extent the parent material.