A geohydrological assessment of the behaviour and response of the Zululand coastal plain to both environmental influences and human activity
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Date
1991
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Abstract
The Zululand coastal plain on the eastern seaboard of South Africa contains large quantities of readily available fresh water. Lake St Lucia, located within the plain, is a wetland habitat internationally recognised for- its environmental importance. Yet over the past few decades extensive forest developments have taken place within the lake catchment. The considerably higher transpiration rate of the fast growing trees planted there over those of grassland environments they replaced is well established. The effects on the hydrological balance of the lake and its environs of the greater water loss resulting from this land use modification has however been the subject of much speculation.
This study analyses the hydrological regime of the eastern shores plain over the 17 years from 1973 to 1990. During this period both wet and dry climatic conditions were experienced, and in 1981 a plantation covering 1100 ha was established. In order to clarify the magnitude and variability of hydrological parameters under natural and under altered conditions, comparisons were made between wet and dry periods, and between grasslands and forested areas. The extent to which plantations have modified the hydrological balance could thus be determined and placed in regional perspective.
The study began with reviews of the physiography of the eastern shores catchment area and of the existing data base
(chapter 2). Following a substantial upgrading of the hydrometric network (chapter 3), specific data collection took place. The components of the hydrological cycle were then assessed in order to identify differences in hydrological response both to climatic variability and to land use. The results of these assessments are presented as individual sections on meteorology (chapter 4), surface hydrology (chapter 5), and geohydrology (chapter 6).
It is the conclusion of the study that in this sub-tropical environment, with shallow depths to groundwater, commercial forests are responsible for an additional consumptive use of water of between 150 and 175 mm/year. In the regional perspective of the water balance of Lake St Lucia, the total forested area of 25 000 ha will thus be seen to lower average inflow to the lake by between 10% and 12%. During extreme dry periods this figure increases to approximately 30%.
Although they are greatest during dry climatic cycles, the effects of such a reduction in water yield are at all times significant. In the first place, the water balance of the lake is seriously affected. In the second, with.a reduction in its fresh water input, the salinity balance of the lake cannot but be influenced. In the third, changes in this salinity balance will have a direct impact on the flora and fauna of the lake and its environs. It is to be hoped that following the proposed future removal of the plantations from the eastern shores, the situation will improve. However, the remaining 20 000 ha under commercial forestry on the western
shores will continue to have a significant effect upon the hydrology of Lake St Lucia.
Description
A dissertation submitted to the faculty of Science at the University of Zululand, in partial fulfilment of the requirements for the degree Master of Science in Hydrology, South Africa, 1991.
Keywords
Lake and river levels, Hydrology--Kwa-zulu natal--Lake st lucia