African Journals Online
WaterSA

Volume 27, Issue 3, 2001

Rationale for an ecological risk approach for South African water resource management
Jooste, S.Claassen, M. 283-292

Abstract: The principle of ecosystem protection in the South African Water Act requires that water resource management tools for a multiple stressor environment be tailored to the characteristics of the aquatic ecosystem. The requirements of the Act, the characteristics of aquatic ecosystems as well as co-occurrence of diverse stressors are considered. Although single substance criteria have a useful role, they are not sufficient for resource management within the context of the ecological reserve. It is proposed that an effect-likelihood approach has the potential to address the variability and uncertainty in management of a surface water body subject to multiple stressors. An in-stream receiving water risk objective approach might be considered.

A possibilistic approach to diverse-stressor aquatic ecological risk estimation
Jooste, S. 293-302

Abstract: A possibilistic approach to assess the risk of co-occurring stressors in an aquatic ecosystem based on the use of fuzzy sets is illustrated at the hand of a hypothetical case study. There are two aspects of importance: a fuzzy stressor response relationship where the response may have reference to a lower level end-point, and a rule-based inference model relating the occurrence of low-level stressors to a high-level ecological goal such as sustainability. The stressor-response is expressed as a conditional possibility. The possibility and necessity measures of the disjunctive composition of the stressor-response with the possibility distribution of the stressors yield an estimate of the ecological risk. Such a possibilistic approach may well serve as a screening procedure in multiple stressor resource management when only qualitative risk assessments are needed.

Hydrological modelling of fine sediments in the Odzi River, Zimbabwe
Liden, R.Harlin, J.Karlsson, M.Rahmberg, M. 303-314

Abstract: Siltation of reservoirs is a major concern in Zimbabwe. Therefore, development of prediction tools is of great importance. In the present study a recently developed empirical sediment model (HBV-SED) based on a daily rainfall-runoff model was applied to simulate riverine fine sediment transport in a 2 486 km 2 catchment in eastern Zimbabwe. The model performance was evaluated and changes in the model structure were suggested. The modelling was, however, associated with many uncertainties due to the adopted simplification of transport processes. An analysis of the model structure and a comparison with the rating curve function was done. The required length of data for calibration purposes was evaluated and model validation through split sample and proxy basin comparison was performed. Furthermore, since the empirical model was dependent on monitored runoff and fine sediment concentrations for calibration purposes, a field measurement campaign was conducted to assess the accuracy of observed data at the station studied. The field measurements showed large errors in monitored runoff and fine sediment concentrations for the 1998/99 wet season, which illustrated the uncertainty in predictions of fine sediment transport based on observed data. The HBV-SED model, which was applied over a period when data were believed to be fairly accurate, simulated the fine sediment transport volume well for the validation period if it was calibrated for a minimum of four years. A shorter calibration period led to a significant increase in prediction uncertainty. The model failed to simulate individual high fine sediment peaks accurately mainly due to poor performance of the rainfall-runoff model on a daily time-scale even if the seasonal flow dynamics were described properly. In the studied catchment the HBV-SED model application resulted in equally poor R 2 -values as the rating curve technique, while the estimated fine sediment volume was more accurate.

Flood frequency analysis at ungauged sites in the KwaZulu-Natal Province, South Africa
Kjeldsen, T.R.Smithers, J.C.Schulze, R.E. 315-324

Abstract: Use of the index-flood method at ungauged sites requires methods for estimation of the index-flood parameter at these sites. This study attempts to relate the mean annual flood to site characteristics of catchments in KwaZulu-Natal, South Africa. The ordinary, weighted and generalised least square methods for estimating model parameters are compared and found to perform equally well, with preference given to the generalised least square model. A separation of KwaZulu-Natal (KZN) into two regions was found to improve predictive ability of the models in the western and north-western parts of the study area. The study also revealed problems with the estimation of the mean annual flood in the coastal areas of the study region.

A hydrological perspective of the February 2000 floods : a case study in the Sabie River catchment
Smithers, J.C.Schulze, R.E.Pike, A.Jewitt, G.P.W. .325-332

Abstract: The exceptionally heavy rains which fell over the north-eastern parts of South Africa, Mozambique and Zimbabwe during February 2000 resulted in disastrous flooding, loss of hundreds of lives and severe damage to infrastructure. The objective of the study reported in this paper is to assess the severity, from a probabilistic perspective, and spatial variability of the extreme rainfall and flooding which occurred in the north-eastern part of South Africa during February 2000. This is performed for events ranging from 1 to 7 days in duration using the Sabie River catchment, upstream of the South African/Mozambique border, as an example. The analyses indicate that the floods experienced in the Sabie catchment during February 2000 were the result of rare rainfall with return periods in excess of 200 years in parts of the catchment. The extent of the extreme rainfall increased for longer durations. The magnitudes of the February 2000 floods were such that many gauging stations did not function and numerous gauging structures were inundated. Hence, a modelling approach was adopted to investigate the spatial variability, magnitudes and probabilities of the floods which occurred during February 2000 in the Sabie catchment. The return periods of simulated runoff depths for durations of 1 to 7 days generally exceeded 50 years for the upper and middle portions of the catchment and 200 years in some parts of the Sabie catchment. Hence, some extremely large and rare flow depths were experienced and the spatial variability of the return periods associated with the simulated runoff depths varied substantially within the catchment.

Installed water resource modelling systems for catchment management agencies
Dent, M.C. 333-340

Abstract: Following international trends there are a growing number of modelling systems being installed for integrated water resource management, in Southern Africa. Such systems are likely to be installed for operational use in ongoing learning, research, strategic planning and consensus-building amongst stakeholders in the catchment management agencies (CMAs). These installed systems are poised to change fundamentally, the way modelling is approached in Southern Africa. They are a logical and irreversible response to the enormous forces which have led to the revision of the South African Water Law and the water resource management paradigms which it embodies.
This paper examines the business forces behind this paradigm shift and it explores the evidence of the changes already taking place in terms of the modelling technology and the organisational and individual responses. Such installed modelling systems are essential for the social process of water allocation as well as for dealing with externalities. Given the paucity of observed data in Southern Africa, it follows that in many decision-making situations the model is not required to produce accurate answers, for we would have no way of checking their accuracy. Rather it is a tool to help organise a negotiation or learning process in which its primary function is to provide a framework for thinking by enabling the participants to make their implicit assumptions explicit in a systematic manner. This, in turn, provides a means for stakeholders to visit the possible consequences of their intended actions. The creativity and opportunities for compromise which this process releases is where the real benefit of modelling lies. Recurrent themes in this paper will be the business, technical and human resource issues pertaining to the use of installed modelling systems in the social process of water allocation.

Estimation of recharge using a revised CRD method
Xu, Y.Van Tonder, G.J. 341-343

Abstract: The cumulative rainfall departure (CRD) method, based on the water-balance principle, is often used for mimicking of water level fluctuations. Because of its simplicity and minimal requirement of spatial data, the CRD method has been applied widely for estimating either effective recharge or aquifer storativity, and consequently gained a focus in South Africa. This paper critically reviews this method and proposes expanded algorithm. Validation of the method under typical South African conditions is discussed based on model-generated and known cases. The study is aided with a user-friendly Excel program called Recharge Estimation Model in Excel (REME).

A generalised solution for step-drawdown tests including flow dimension and elasticity
Van Tonder, G.J.Botha, J.F.Van Bosch, J. 345-354

Abstract: Step-drawdown and multi-rate tests present convenient tools for the estimation of the long-term yield of boreholes. However, the analytical methods commonly employed for the analysis of such tests are all based on the assumption that the drawdown in a borehole is a linear function of the discharge rate. Numerous constant rate tests, of which a few are discussed in this paper, has shown that this is not necessarily the case with boreholes drilled in the Karoo formations of South Africa. The drawdowns in these boreholes are not only influenced by the peculiar geometry of the aquifers, but also the non-linear deformation of the aquifers during the pumping of a borehole. The two new non-linear models for the analysis of step-drawdown and multi-rate tests introduced here, tries to account for these factors; in particular the deformation of the aquifer, flow dimension and dewatering of discrete fractures. Although the model proposed for multi-rate tests is still based on constant time steps, the one for step-drawdown tests allows the user to use arbitrary time steps, when performing the test in the field. Non-linearities in drawdown curves should always be treated with caution, especially when used to assign sustainable yields for boreholes. However, the example of a step-drawdown test performed at the Campus Test Site of the University of the Free State, shows that non-linearities can be addressed with an appropriate model.

An improved technique for the determination of oxidised nitrogen in natural waters with a Sequential Injection Analysis (SIA) system
Naidoo, E.B.Van Staden, J.F. 355-360

Abstract: An SIA system is proposed for the determination of oxidised nitrogen (nitrate + nitrite as N) in natural waters. A cadmium reductor, made of cadmium granules closely packed in a glass column reduces the nitrate to nitrite. The reduced nitrate and the nitrite present in the water samples is diazotised in the SIA system with sulphanilamide and coupled with N - (1-napthyl) ethylene diammoniumdichloride to form a highly coloured azo dye which is detected at 540 nm with a UV/Vis spectrophotometer. The proposed system is fully computerised and is able to monitor total oxidised nitrogen as nitrite at a frequency of 36 samples per hour with a standard deviation of < 1.2%. The calibration curve is linear up to 5 mg/l with a detection limit of 0.01 mg/l.

Treatment of pulping effluents by using alum and clay - colour removal and sludge characteristics
Dilek, F.B.Bese, S. 361-366

Abstract: The effect of clay addition during alum coagulation, on the removal of colour from pulp-and-paper industry wastewaters, was investigated. Four types of clay, namely beige-and brown-sepiolites, calcium- and sodium-bentonites of different mesh sizes were used. Different quantities of alum and clay were applied, either singly or in combination, to the effluents of the wood-based pulp-and-paper mill. Colour of the treated wastewater, the sludge volume index (SVI) and sludge cake resistance (SCR) of the sludge produced were monitored. It was observed that the clays tested were not efficient in colour removal when they were applied on their own. When clay was added along with alum, a certain increase in colour removal efficiency was obtained as compared to alum alone. When alum was used in conjunction with clay, settling characteristics of the sludge produced improved substantially, in terms of both SVI and SCR. Acid activation of the clays did not improve the colour removal efficiency further. However, acid activation of sodium bentonite caused the SVI and SCR values to improve considerably compared to virgin sodium bentonite. Use of different mesh sizes of clays did not exert any significant effect on the colour removal. However, it did exert significant effects on SVI and SCR values as such that the increase in mesh size caused the SVI values to decrease and the SCR values to increase.

The impact of irrigation return flow on aspects of the water quality of the upper Gamtoos Estuary, South Africa
Pearce, M.W.Schumann, E.H. 367-372

Abstract: Groundwater quality in the Loerie Flats, and subsurface irrigation return flow to the upper Gamtoos estuary was monitored from November 1992 to April 1994. The nutrient loading of groundwater was highly variable both spatially and temporally, with elevated nitrate-N levels reaching 163 mg·l -1 . The elevated nutrient concentrations in the groundwater were directly related to the timing of land-surface application of fertilisers. The nutrient loading to the estuary, via subsurface irrigation return flow was calculated using Darcy's equation together with the mean concentration of nutrients found in groundwater in the Loerie Flats area. Results indicate that an average of 0.52 t nitrate-N, 0.02 t nitrite-N and 0.10 t of total phosphorus enter the Gamtoos estuary from the 5.5 km 2 Loerie Flats area on an annual basis. Such nutrient loads emanate from less than 20% of the total agricultural area bordering the estuary. Despite the elevated nutrient input to the estuary via groundwater discharge, the nutrient content of the estuary was generally low, but at times elevated concentrations of nutrients were recorded in the upper estuary. It was found that estuarine mixing processes and dilution, with adequate flushing have prevented the periods of low quality water from persisting in the Gamtoos estuary.

Some factors governing the water quality of microtidal estuaries in South Africa
Allanson, B.R. 373-386

Abstract: The role of coastal geomorphology and Man-made alterations, including reduced river flow through dam construction, determines, at least in part, the water quality of South African microtidal estuaries. To offer increased understanding of the manner in which these features may modify water quality, a short description of the biogeochemical processes in estuaries is provided. Comment on the present limitations of modelling some of the estuarine processes in South African investigations is given.

Estimating potential impacts of a change in river quality on the tourism value of Kruger National Park : an application of travel cost, contingent, and conjoint valuation methods
Turpie, J.Joubert, A. 387-398

Abstract: Development and resource allocation decision processes are increasingly under pressure to take environmental values into account in order to reach optimal economic outcomes. In South Africa new techniques will be needed to incorporate environmental values into environmental impact assessment and in the allocation of water resources under the new National Water Act (1998), both of which require the comparison of alternative scenarios with varying impacts on the environment. This study on the tourism value of rivers in the Crocodile Catchment is the first case study to develop methods for incorporating the economic values of the goods and services provided by functioning aquatic ecosystems into such decision processes. Rivers within the Kruger National Park (KNP) will be affected by water usage in the portions of their catchment areas upstream of the park boundary. The current tourism value of these rivers was considered in terms of revenues to KNP (visitors' on-site expenditure), contribution to the economy (visitors' on-site and off-site expenditure) and recreational value, including consumers' surplus. The effect of a change in river quality was determined using a joint contingent valuation - conjoint valuation approach, whereby respondents rated four different scenarios, each containing four attributes at four different levels. It was estimated that the current value of KNP tourism is about R136 m. in terms of on-site expenditure, R267 m. in terms of economic impact, or all expenditure related to visiting the park, and R1 bn. in terms of consumers' surplus. The latter two values can be added to calculate total recreational value. Four methods were used to isolate the value of rivers from the total tourism value stated above, and all yielded similar values of about 30% of the total.
This implies that about 30% of tourism business would be lost if rivers were totally degraded. Thus, rivers within the Crocodile Catchment, which takes 22% of KNP visitor-nights, contribute R9 m. to KNP revenues and have a total annual recreational use value of about R85 m., including off-site expenditure and consumers' surplus. The conjoint analysis generated an equation which is able to predict the change in trip expenditure, or total KNP revenue, associated with changes in levels of any of the four attributes considered. Appearance of the riverscape has the greatest influence on recreational use value, followed by waterbird diversity, aquatic megafauna and riparian tree density. Such models can be used in water allocation decision processes when attribute levels associated with alternative management scenarios are predicted by aquatic ecologists.

Pretreatment of urban wastewaters in a hydrolytic upflow digester
Ligero, P.Vega, A.Soto, M. 399-404

Abstract: Domestic wastewater was fed continuously to a laboratory-scale upflow digester operated at short hydraulic retention times (HRT < 4 h). The digester acts as a system for sedimentation and hydrolysis of suspended solids (SS) and for acidification of solubilised substances. Optimum results were obtained at an HRT of 2.3 h. Over 60% SS are retained in the digester and hydrolysed. Average influent SS content is 230 mg/l, whereas effluent SS is 90 mg/l. Effluent SS concentrations shows very stable behaviour, varying little with influent concentration, as with HRT. Retention and hydrolysis of SS causes an increase in volatile fatty acid (VFA) concentration, from about 20 mg/l in the influent to above 100 mg/l in the effluent, also contributing toward soluble fraction acidification. Thus, VFA reached 25% of effluent chemical oxygen demand (COD), while influent VFA COD was below 4%. During the process, a reduction in domestic wastewater COD of between 30 and 40% was observed.

A review of characterisation requirements for in-line prefermenters : Paper 1 : Wastewater characterisation
Rossle, W.H.Pretorius, W.A. 405-412

Abstract: The impact of wastewater prefermentation cannot be evaluated in isolation, based only on the local prefermenter biodegradable organic matter production rate, as represented by the volatile fatty acids concentration increase across the prefermenter. The nutrients ratio changes and solids removal variations from the raw to the settled sewage must be taken into account when considering the suitability of the prefermented wastewater for downstream biological nutrient removal processes. The raw and settled wastewater must, therefore, be characterised according to component nutrients and solids fractions. This paper reviews related wastewater characteristics required for in-line prefermenters, to establish simple strategies on which in-line prefermenter evaluations could be based.

A review of characterisation requirements for in-line prefermenters : Paper 2 : Process characterisation
Rossle, W.H.Pretorius, W.A. 413-422

Abstract: The operational factors having a significant effect on in-line prefermentation efficiency include the sludge recycle rate and the subsequent sludge elutriation rate, solids concentrations and retention times. The prefermenter configuration employed is a determining factor, which allows for some degree of operational flexibility. Side-stream and multiple tank systems are superior in this regard and outnumber the use of in-line single tank prefermenters, which are mainly employed due to lower space and capital cost requirements. This paper reviews the basic design and monitoring requirements for in-line prefermenters, to establish simple strategies on which prefermenter evaluations could be based.