Sap flow variation in selected riparian woodland species in the Okavango Delta, Botswana

In the tropical Okavango Delta, transpiration by trees is an important process partly responsible for maintaining the basin as a freshwater environment. Quantification of evapotranspiration from terrestrial landforms of the delta, fringed by riparian woodlands, is one of the main contributors to uncertainty in current hydrological modelling. We investigated sap flow of common trees in the distal, mid- and upper delta in July-August 2012, November-December 2012 and February-April 2013 using the compensation heat pulse velocity method. In the distal delta, four Diospyros mespiliformis individuals of different sizes were studied. Four trees of different species were studied in the mid- and upper delta. Sap flow density (SFD; flow per unit cross-sectional area) was used as a common unit to facilitate comparison. Sap flow varied with tree size, species, season and location. It was positively correlated with tree size (r ² = 0.67). Sap flow variation between seasons and across locations in all the species studied indicated two distinct groups. Group 1 transpired the least during the hottest season, November-December, and Group 2 the most. In Group 1, the highest average SFD was 1.17 l cm^-2 day^-1 during July-August; in Group 2, it was 1.07 l cm^-2 day^-1 during November-December. Changes in the hydrology of the delta would negatively affect the riparian woodland.