Review of information on interactions between vegetation and groundwater

Abstract

Vegetation plays key roles in the interactions between groundwater and surface-water systems, because of its direct and indirect influence on recharge and because of the dependence of vegetation communities on groundwater. Despite this, groundwater and surface water have traditionally been treated as separate legal entities in South Africa and scientific disciplines have also tended to view them as separate, or at least separable, hydrological systems. This situation is beginning to change as South Africa’s new Water Act recognizes them both as inseparable elements of the hydrological cycle. The Act also requires that water resources be managed sustainably and a much greater understanding of these interactions is needed to meet this obligation. This paper provides a review of what is known about groundwater - vegetation interactions based on local and international literature and on information from the “grey” literature and unpublished sources. Changes in vegetation cover and structure, particularly from low vegetation such as grassland to tall vegetation such as a forest can have a significant impact on groundwater recharge by altering components of the hydrological cycle such as interception and transpiration. Recent research has shown that root systems often extend to more than the 1 m maximum used in defining agricultural soils and frequently to more than 10 m deep where the physical conditions permit root penetration. Woody plants have the deepest root systems and are capable of extracting large volumes of water from depths of 10 m or more. In South Africa the impacts of vegetation changes on base flow or groundwater have been documented in both humid and sub-humid catchments but the greatest changes in groundwater levels have followed type conversions in semi-arid savanna and on the coastal plains of Zululand. Transpiration of water by plants accounts for about half of the largest changes in the water balance associated with vegetation type conversions. Many plant communities, particularly those of wetlands and riparian strips are highly susceptible to changes in the depth to the groundwater, both annual and seasonal. The rate of change (positive or negative) in water-table levels may be important but the data are not conclusive. Interactions between groundwater and vegetation appear to be generally more pervasive and important than was believed in the past.