Determining the water quality ecological reserve for non-perennial rivers: a prototype environmental water assessment methodology
The South African National Water Act adopted in 1998, is implemented by means of the National Water Resource Strategy. The NWRS provides the framework for the management of the water resources. Some of the protective measures are designated Resource Directed Measures such as the establishment of the Reserve. The NWA establishes the ‘Reserve’ consisting of an unallocated portion of water that is not subject to competition with other water uses. It refers to both the quality and quantity of water and is made up from two distinct parts, namely the basic human needs reserve and the Ecological Reserve. The Ecological Reserve describes the quantity, quality and flow variability required to protect and maintain the aquatic ecosystems of the water resource on a sustainable basis. All other water demands are controlled by permits and licenses and met only after the Reserve is secured. The Ecological Reserve has to be set for every major river in the country to be able to comply with the NWA. Most of the rivers, except the largest rivers in the semi-arid west of southern Africa, are non-perennial with variable flow regimes, governed by stochastic events, with the highest variability in intermittent and ephemeral rivers. This variability is a key factor in shaping the biotic community structure of ephemeral or non-perennial systems. The hypothesis for the research was that the current, existing water quality methodology for determining the water quality component of the Ecological Reserve, which was developed for perennial rivers, could be used for non-perennial rivers. This hypothesis was addressed in a phased approach. The existing methodologies were identified through a literature review and from the information collected it was decided to use the holistic approach methodologies. The Proposed method described and approved by the Department of Water Affairs and Forestry for use on the perennial rivers was applied to the Seekoei River, an example of a typical non-perennial river. The existing methodology could be used as it is for the water quality component of the Reserve determination. However, the fish, invertebrate and riparian vegetation components of the existing methodology had severe limitations and an alternative methodology was proposed. Six limitations were identified from the Seekoei River study for all the components and were the following: the establishment of reference conditions; suitable hydrological modeling; understanding pools and the connectivity between pools; the surface water/groundwater interactions and the extrapolation of data. When comparing the DWA Proposed methodology (Eight step method) applied to the Seekoei River and the Prototype Methodology (Eleven phase method) as applied to the Mokolo River there were several similarities for the water quality input into both methodologies: - An understanding of the catchment to be able to identify the water quality constituent that will be important for that specific river is required. - Water quality data, both historical and present day data are required – more data are better and improve the confidence in the output. - Standard water quality methods could be applied to both methodologies. - Both require input into a model where response curves were drawn based on different future catchment development scenarios. The water quality component did not change from the Seekoei River application as the basic steps were the same. The standard methods could be applied to the Mokolo River. The current methodologies were equally usable to determine the water quality component of the Ecological Reserve for non-perennial rivers as the same basic methods were used to determine the water quality component of the Reserve. The limitations identified in the Seekoei River study were also the limiting in the Mokolo River study. The key issue is the hydrological modelling. Without a suitable hydrological model the other the other limitations can also not be addressed. The lack of water quality data remains the single most challenging aspect of determining the water quality status of a river, perennial and non-perennial, especially the lack of historical data. One should be cautious in interpreting once-off sampling data or patchy historical data. The confidence in the data used for the EWA sites were low in many instances as a result of either very little data to no data or patchy historical data. This underlines the importance of systematic monitoring over time, as sampling once is not sufficient to draw credible conclusions. The only way to compensate for a lack of date is to use expert knowledge, local knowledge and catchment information (land use, potential pollution sources, soil types, land cover and geology).
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