MECHANISMS FOR LAND-ATMOSPHERE MOISTURE RECYCLING AT THE OKAVANGO DELTA, BOTSWANA, AND IMPLICATIONS FOR PALEO-MEGA-LAKE MAKGADIKGADI

The Okavango Delta, Botswana is one of the world's largest continental wetlands, about 15,000 km² at full inundation (~3-4 times the area of the Great Salt Lake). The Delta floods seasonally during austral Fall, after a pulse of remote summer rain has traversed ~800km of the Okavango watershed, resulting in a flood pulse that is delayed by several months relative to the local rainy season. The Delta is an alluvial megafan within the Okavango Rift Zone graben, a young southwestern extension of the East African Rift System. During the Quaternary, paleo- mega-lake Makgadikgadi filled an estimated 90,000 km² of middle Kalahari’s Mababe Depression, extending from the Delta to what are now the Makgadikgadi Pans. A surface water-body of this extent in a tropical-subtropical monsoon environment likely impacted continental precipitation recycling and paleo-rainfall in the region. To begin addressing this we consider the modern land-atmosphere hydrologic interactions that operate to sustain the Delta. The deep overturning circulation of the world’s monsoons are driven by near-surface moist entropy maxima. The southern Africa moist entropy maximum overlies the Okavango Delta, where evapotranspiration (ET) outpaces precipitation. Here, we explore whether the development of moist entropy over the Delta may be a product of ET, a concept that is uniquely investigable at the Delta due to its unique seasonal pacing of floodwaters vs. precipitation. We first assess the seasonal development of near-surface moist static energy (MSE) and discern the influences of humidity and temperature. We find the summer increase in MSE over the Delta is in phase with changes in humidity and ET, but out of phase and larger in amplitude than temperature changes. This could mean that enhanced rainy season MSE over the Delta and the strength and focusing of monsoonal and ITCZ-driven rainfall is a response to contributions of moisture to the lower troposphere from ET over the Delta. We offer a novel approach to interpretation of monsoon region paleo-archives by aligning the study with modern concepts of these systems as driven by MSE.