ASSESSING TERRESTRIAL GROUNDWATER DEPENDENT ECOSYSTEMS IN SOUTH AFRICA
REPORT NO: 1090-2/2/03
This report presents an approach to determining the existence of terrestrial groundwater dependent ecosystems. It discusses what is meant by dependency on groundwater, outlines a proposed protocol to assess dependency and describes various tools which may be used to determine groundwater dependency. Two field studies were carried out to test some of the techniques described.
Terrestrial vegetation systems may be afforded the protection of Resource Quality Objectives under the South African National Water Act. To understand what restrictions may be placed on allocable groundwater we need to understand the nature, extent and degree of dependency of vegetation communities on aquifers.
The degree of groundwater dependency may range from total to seasonal. Even seasonal dependency may be critical and could result in the demise of the ecosystem if groundwater was no longer available. The extent may be localised to widespread depending on the nature of the aquifer and the water table. It should be noted that even a localised aquifer may support a keystone ecosystem which has a ecological importance greater than its geographical extent. The nature of the dependency is possibly the most difficult to predict and may only be realised once an ecosystem has been stressed beyond a critical threshold. For most communities the depth to the water table is likely to be the most important hydrogeological parameter. In coastal areas, salinity may be an important control and in other areas, the presence of nutrients.
A preliminary protocol to assess terrestrial ecosystem dependency on groundwater asks the following key questions:
A preliminary national scale map is presented showing the probability of TGDE occurrence based on groundwater levels and moisture growing season duration. This map may be interpreted in the context of hydrogeological terrains and biomes to infer the probability of TGDEs occurring. It may be useful for coarse, catchment level planning.
Simplified South African hydrological terrains, surficial, carbonate, granitic, extrusives, Karoo dykes and sills, and fractured sedimentary terrains, are described with expected type settings for TGDEs.
Section 2 provides a summary of the various techniques available to assess groundwater use by plants. Each tool is considered in terms of its application, what it measures, the costs, environmental constraints and suitability, capacity required, the time required to get meaningful results, the resolution of those results, the format of the outputs, whether it should be used conjunctively with other techniques, the level of previous use world wide and the experience of previous use in South Africa. These aspects are considered most relevant to Catchment Management Agencies (CMAs) and consultants who will need to make decisions about which techniques they should apply to their circumstances.
Section 3 describes two case studies started under this project. The first is in the Kammanassie Nature Reserve and describes a remote sensing application to identify potential groundwater dependent vegetation. Further work is being carried out by DWAF and the Western Cape Nature Conservation Board measuring water stress. This will help to groundtruth the initial results presented here.
The second field study is at Atlantis well field. Water balance studies have been carried out to determine relative water use at sites with groundwater at different depths and different vegetation cover.
It is hoped that this body of work will be useful to hydrogeologists, water managers and ecologists attempting to predict the future impacts of new abstraction schemes and set Resource Quality Objectives to protect groundwater's ecological role in the catchment.
Objectives of the Project
The original and revised objectives of this project were to:
Recommendations for further work and coordination of the evolving knowledge base on TDGE's in South Africa are: