THE EFFECTS OF VEGETATION on THE PHYSICAL AND CHEMICAL PROPERTIES OF OKAVANGO RIVER, NORTHWESTERN BOTSWANA
The experiments were conducted at Mohembo, where the Okavango River first enters Botswana, and at Maun, located at the distal end of the Delta. We measured photosynthetically available radiation (PAR), water level, temperature, electrical conductivity, dissolved oxygen (DO), oxidation-reduction potential (ORP), pH, alkalinity and silica. Overall, the values of water quality parameters were lower at Mohembo compared to Maun, consistent with evaporative concentration in the downstream direction. Diel variations in river level were observed at Maun but not at Mohembo, suggesting stronger vegetative effects on the water balance and consequently water chemistry. At both sites, DO remained at or above saturation with no clear diel trend. At Mohembo, ORP and pH were steady with higher ORP values during the day and lower pH values during the day. However, at Maun, the ORP and pH increased and decreased at night and decreased and increased, respectively, during the day. Alkalinity at Mohembo was higher at night than during the day, while at Maun, alkalinity fluctuated with no clear diel trend. Silica at both sites showed decreasing trends at night and increasing trends during the day.
Diel variations in PAR appear to be correlated to changes in redox and alkalinity at Maun, but not at Mohembo. Thus, processes other than photosynthesis must also be considered in any models of water balance in this system. Given the growing concern over the availability of water in the Okavango River, and the need to allocate its resources among several nations, these results will be critical in informing future models for water utilization in this river, and can be applied to other river systems.