STABLE ISOTOPIC VARIATION OF HYDROGEN AND OXYGEN IN THE OKAVANGO RIVER DELTA

The stable isotopes of hydrogen (δD) and oxygen (δ¹⁸O) were measured for the Okavango River in semi-arid northwester Botswana. Measurements were made for river water from several stations between the proximal and distal Okavango Delta, and over time for both the rainy and dry season. The objective was to use the δD and δ¹⁸O to assess the spatial and temporal extent of evaporation of the Okavango River water. The results showed progressively increasing δD and δ¹⁸O due to progressively increasing evaporation as the river flowed from the distal to the proximal Okavango Delta. The results also show small differences (<10%) in the δD and δ¹⁸O between the rainy and dry season indicating a minor role of rain within the Okavango Delta in the hydrology of the Okavango River. The δ¹⁸O-δD co-varied and is defined by a least squares regression equation: δ¹⁸O = 5.0δD + 11 (R² = 0.95). This δ¹⁸O-δD co-variation defines the Okavango Delta Evaporation Line (ODEL). Furthermore, the ODEL intersects the local meteoric water line (LMWL) for Maun in the Okavango Delta and for Menongue at the source of the Okavango River in subtropical Angola at their most negative end. The intersection of the ODEL at the negative end of the LMWLs is consistent with a predominantly light isotopic rainwater source for the Okavango River and a single seasonal pulse discharge of river water into the Okavango Delta.