THE EFFECT OF VEGETATION ON SOIL AND GROUND-WATER CHEMISTRY AND HYDROLOGY OF ISLANDS IN THE SEASONAL SWAMPS OF THE OKAVANGO-FAN, BOTSWANA

Evapotranspiration exceeds rainfall by a factor of three in the Okavan go swamps of northern Botswana, yet saline surface water is rare. Brin es develop in the groundwater beneath islands, however, and very stron g lateral concentration gradients develop. These arise as a result of transpiration by trees which grow around the fringes of islands as wel l as by capillary evaporation of groundwater from the interior of isla nds. Precipitation of calcite and amorphous silica from the groundwate r occurs beneath island fringes. Long-term monitoring of the water tab le and groundwater chemistry beneath an island in the seasonal swamps has revealed that groundwater rises and falls with the seasonal flood and that the saline groundwater remains centred beneath the island. Th e study shows that islands act as sinks for dissolved solids during in tense evapotranspirational loss from the swamps.