Abstract
The Upper Zambezi River Basin (UZRB) delineates a complex region of topographic, soil and rainfall gradients between the Congo rainforest and the Kalahari Desert. Satellite imagery shows permanent wetlands in low-lying convergence zones where surface–groundwater interactions are vigorous. A dynamic wetland classification based on MODIS Nadir BRDF-Adjusted Reflectance is developed to capture the inter-annual and seasonal changes in areal extent due to groundwater redistribution and rainfall variability. Simulations of the coupled water–carbon cycles of seasonal wetlands show nearly double rates of carbon uptake as compared to dry areas, at increasingly lower water-use efficiencies as the dry season progresses. Thus, wetland extent and persistence into the dry season is key to the UZRB’s carbon sink and water budget. Whereas groundwater recharge governs the expansion of wetlands in the rainy season under large-scale forcing, wetland persistence in April–June (wet–dry transition months) is tied to daily morning fog and clouds, and by afternoon land–atmosphere interactions (isolated convection). Rainfall suppression in July–September results from colder temperatures, weaker regional circulations, and reduced instability in the lower troposphere, shutting off moisture recycling in the dry season despite high evapotranspiration rates. The co-organization of precipitation and wetlands reflects land–atmosphere interactions that determine wetland seasonal persistence, and the coupled water and carbon cycles.
Highlights
Wetlands make up less than 9% of global land area, yet they are the largest terrestrial biological source of carbon and the largest source of methane emissions worldwide [1]
The Upper Zambezi River Basin (UZRB) in Sub-Saharan Africa is an area of complex topography and hydrometeorology where interactions between the Angola High Plateau (AHP), the Inter-tropical Convergence Zone (ITCZ), and the Congo Air Boundary Zone (CABZ) determine the spatial and temporal distribution of water resources, and inter-annual climate variability likely maps to the variability observed in vegetation and wetland density [9]
The MODIS MCD43B4 Aqua and Terra combined Nadir BRDF-Adjusted Reflectance (NBAR) product corrected for sun and view angle effects was used to generate a dynamic wetland dataset for the UZRB [39]
Summary
Wetlands make up less than 9% of global land area, yet they are the largest terrestrial biological source of carbon and the largest source of methane emissions worldwide [1]. Indices constructed from visible and infrared satellite reflectance bands highlight specific surface properties like greenness and water content [28,29,31] While these indices are unable to distinguish effectively between different wetland types or vegetation species, they map the physical surface characteristics that are indicative of wetlands. It is of particular importance to evaluate how seasonal precipitation impacts the persistence of wetlands through the dry season and, in turn, how persistent wetlands support environments favorable to rainfall processes These areas serve as an additional moisture source for vegetation undergoing photosynthesis and evapotranspiration. This manuscript aims to show how changes in wetland extent relates to local precipitation patterns and how ephemeral wetland persistence impacts the coupled water–carbon cycles
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