An analysis of plant species distributions on the floodplain of the Okavango River, Namibia, with respect to impacts of possible water abstraction
Abstract:The proposed abstraction of water from the Okavango River in Namibia could potentially result in significant changes in the vegetation of floodplains downstream of the abstraction point. Direct gradient analysis was used to determine the distribution of the most common floodplain plant species in relation to elevation above the water surface and distance from the channel, and therefore to the depth and duration of flooding. Species could be ranked and scored according to their distribution in relation to both elevation and distance from the channel, reflecting a gradient from species intolerant of flooding to those that withstand prolonged flooding. In general, as distance from the channel increased, the elevation at which each of the species occurred decreased, but there was a great deal of variation in the range of elevations over which each species occurred, primarily as a consequence of the irregular topography of these floodplains. Furthermore, several ruderal species were restricted to elevated sites in close proximity to the channel, occurring on recently formed point bars which are the product of fluvial processes. Disturbance in the form of sediment deposition on point bars is thus an important determinant of species distribution on floodplains of the Okavango River. Given these patterns of species distributions it is difficult to predict with accuracy the effect of abstraction on vegetation distribution.
A preliminary analysis of possible changes in the hydrological regime after abstraction has also been attempted as part of this study. This was achieved by comparing mean stage heights before and after abstraction assuming a constant rate of abstraction of 3 cumecs, and based on historical daily records from Rundu and Mukwe. Uniform abstraction would have a greater impact at low flows than at medium and high flows. Should abstraction proceed, it would therefore be more appropriate to remove water during periods of high discharge. However, since the Okavango River in Namibia recharges local and possibly regional groundwater, its discharge declines downstream. Rates of water loss from the river thus need to be determined prior to any decision being made about whether or not abstraction should take place, or where to locate the point of abstraction.