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Ephemeral

Sponsor: Water Research Commission
Leader: Prof DA Hughes
Duration: April 2005 - Nov 2007


 The main objective of this project was to develop and test procedures for generating hydrological information that is appropriate to understanding the ecological functioning of ephemeral river systems. In such systems disconnected channel pools are thought to play a major role in ecological functioning, as well as the frequency with which these are connected by infrequent channel flow events. This periodicity, as well as the source of water that maintains the pools will be important in terms of both the dynamics of pool water storage volume and water quality. The main project team at the University of the Free State had already selected the Seekoei River (a tributary of the Orange River in the north west of the Eastern Cape Province). The project has now been completed and the final report submitted to the Water Research Commission. The water quantity and
quality modelling results have been submitted to an international journal and were presented at the SANCIAHS symposium (September 2007).

During the development of the model setup, it was observed that there are substantial spatial differences in the hydrological response of the catchment, largely related to the presence of a dolerite ridge in the lower
part of the basin (compare the two Google Earth images). Within this area there exist a number of semi-permanent springs that emerge above the regional ground water table and sustain baseflows in the river for longer
periods than occur within the channels of the rest of the basin.

The project concluded that existing water quantity models are appropriate for use in ecological Reserve determinations if they are used with caution and if they are applied in a somewhat unconventional manner. The latter point refers to the need to simulate not only the channel flow but also the condition of the non-connected pools during periods of zero flow. During the project a simple water quality model extension was added
to the water quantity modelling with a focus on the TDS of channel flow and static pools. This model is based on mass balance calculations using the volumes of simulated pool inflows from different sources and the expected TDS concentrations of those water sources. Comparisons with observed water quality data for the Seekoei River suggest that this model has some potential, but requires further testing in other river systems.

While the model results that were generated by the project appear to be largely satisfactory, it should be recognised that the model outputs are restricted to the rather coarse spatial scale of modelling. This scale is very different to the scale at which ecological assessments are typically undertaken (single pools or river cross-sections). Given that there appear to be quite large spatial differences within a single river (individual pools may exhibit quite different quantity and quality characteristics), there remains a question about the value of the model simulations when they are linked with available ecological response information.



Last Modified: Fri, 14 Aug 2015 11:27:27 SAST