Monitoring Water Stress in Riverine Vegetation
By Michele Hofmeyr
A delegation from SANParks, lead by Dr Thomas Gyedu-Ababio, took the long road to Mapungubwe National Park earlier this year to look at the effects of water being used from the Limpopo River on the riverine vegetation.
Water is being drawn from the Limpopo River for, amongst other uses, mining operations by the Venetia diamond mine. The water abstraction effects are monitored by how “water stressed” the trees along the river are. As one of the water users in the region, the diamond mining operations require water which is drawn from groundwater aquifers at the confluence of the Limpopo and Shashe rivers. In order to better manage the water resource, the mine is particularly interested in collaborating in and working co-operatively on sustainable solutions.
The group met with Gerhardt Debeer, from the Limpopo Department for Environmental Affairs, who is currently measuring the water stress in the Riverine gallery forest trees, namely the Feverberry, (Croton megalobotrys). The Feverberry trees are typical riverine species found growing in alluvial soils along rivers and are shown to be good indicators of a drop in the water table. Gary Brown, an independent consultant who works with the Venetia Mine was also part of the group and gave the SANParks team the background to the monitoring and co-operative management plan.
By using a specialized pressure measuring apparatus, the level of water stress experienced by the vegetation can be accurately measured. The machine, called a pressure bomb, is calibrated to work out the pressure required to push the sap of a small twig of a tree to it’s cut surface. This monitoring is being done to detect if there is a link in the levels of water stress in these trees and the removal of water.
If the trees are seen to be showing signs of water stress, the mining operations are advised accordingly and water abstraction from the underground aquifer is stopped and the system switches to using dam water. The monitoring programme also aims to work out what the appropriate thresholds of water stress for these trees.
The Limpopo valley is a dry, arid area with a low annual rainfall. Declines in the water level of the Limpopo River will have serious consequences for the entire area, not only the trees and plants growing along the banks of the river. Dr Gyedu-Ababio will represent SANParks interests on the recently re-formed Greefswald (water) Operating Committee, which will look at the results of the monitoring programme, and other water uses in the region, and see where the water problems in this area can be resolved. Scientific Services staff from SANParks will be taking over the monitoring of water stress in the gallery tall trees from Gerhardt de Beer.
The Scientific Services offices in Skukuza and Kimberely have combined and are now called the Savanna Unit. The new Scientific Services Savanna Unit are responsible to assisting research and monitoring in Kruger as well as all other savanna parks, including Mapungubwe and Marakele National Parks. The scientists will also be working with the staff from Mapungubwe in developing an adaptive management system that will help the monitoring programme feed information into the water management plan for the park.