## Data and Methods  
### Study Area  
The UZRB serves as the headwater catchment of the fourth largest river in Africa, provides essential freshwater resources to arid and semi-arid regions within its boundaries, and recharges the Northern Kalahari Aquifer (Figure 1). This aquifer directly underlies this region and impacts vegetation growth in the floodplains via nonlinear interactions between surface water and groundwater [32]. MODIS Normalized Difference Vegetation Index (NDVI) data show the inter-annual variability in breaks in vegetation (Figure 2). Shallow (\(\sim\)3 m active depth) and clayey soils in the UZRB give way to seasonal waterlogging, especially along drainage lines, favoring the establishment of wetlands [32]. Woodland savanna, grasslands and miombo dominate the UZRB's diverse ecosystem, marking a complex transition zone between the Congo tropical rainforest and the Kalahari Desert that reflects spatial rainfall gradients.  
The UZRB's diverse ecosystems ranges from wet and dry miombo forest in the northern mountains to savanna and grassland in the south, in accordance with the latitudinal transition from humid to semi-arid and arid regional climate. Wet miombo comprises dry evergreen forest, swamp forest, evergreen riparian forest, and wet dambos (shallow wetlands) and occurs where mean annual precipitation exceeds 1000 mm/year, while dry miombo occurs where annual rainfall is below 1000 mm/year. The water water is water,  
1000 mm/year and consists of dry deciduous forest, deciduous riparian forest, and dry dambos [33]. The wet miombo also tends to have higher canopy heights, around 15 m tall. There is high spatial variability in regional vegetation as seen by damp forests and grasslands growing only kilometers away from dry badlands, which reflects large spatial variability in precipitation as well as in hydrogeology and surface-groundwater interactions. Natural and anthropogenic burning are prevalent in this region. The intensity and frequency of fires determines species prevalence as certain types of miombo are more sensitive or resistant to fires, and a dynamic relationship exists in the development of successive tree canopies that then control the effects of fire [34]. The dominant ecosystems in the UZRB exhibit spatial heterogeneity that reflects topographic and rainfall gradients. The MODIS yearly land cover product (MCD12Q1) over the region shows that wetlands develop preferentially in low-lying convergence zones where dynamical interactions among hydrological and ecohydrological processes, including surface-groundwater interactions, are most vigorous.  
The annual rainfall, which increases with altitude and latitude, ranges from around 200 mm in a dry year in the southernmost part of the UZRB to 1400-1600 mm in the northwest around the AHP. The altitude-latitude rainfall patterns are governed by the migration of major climate boundaries. The Benguela ocean current flows northward along the western coast of Africa and creates dry conditions along the coast of Angola. The ITCZ shifts northward during the dry season (July/August) and southward during the wet season (January) and the CABZ separates easterly and westerly flows and corresponds to a moist, thermally unstable air mass that delivers heavy rainfall to the region [35]. The Angola Low pressure center falls over the Angola highlands during the wet season and is associated with heavy rainfall in Southern Africa [36]. During the wet season, the presence of the ITCZ, CABZ, and the Angolan Low in the vicinity of the study region controls rainfall delivery within the UZRB [37]. Inter-annual variability in the relative positions of these boundary zones during the wet season can have huge impacts on spatial and temporal precipitation distributions [36]. Further, the presence of the CABZ extends the rainy season in the area around the AHP once the ITCZ moves northward [38].