Characterisation of riparian woodlands and their potential water loss in the distal Okavango Delta, Botswana

An analysis is undertaken to develop techniques to remotely sense relative evapotranspiration outputs in the distal Okavango Delta using enhanced thematic mapper and ground based techniques to help quantify water loss. Much of this work focuses on riparian woodlands which, especially in the distal portion of the Delta, are regarded as being significant in terms of groundwater removal by transpiration. This was confirmed as vegetation cover mapping led to the identification of two riparian classes which, by association with high resistivity aeromagnetic data, were found to be rooted in near surface fresh groundwater. This paper indicates that riparian trees which remain green year long, partly sustain their growth as a result of groundwater uptake. A comparison of frequently flooded and dry floodplains with distinct riparian zones was undertaken using spectral techniques (pixel radiance values and leaf water content indices) to determine whether flooding and lateral groundwater flow stimulated growth (and therefore transpiration rate) following dormancy. Results indicate a basic similarity between the two systems with mixed evidence of assumed leaf growth. Related phenological observations in the riparian zone of the dry floodplain show that renewal of leaf growth is primarily related to rainfall, not flood events in the distal Delta. The results of this work should help effect both surface and groundwater management in the vicinity of population centres in the distal Delta.