Regional review: the hydrology of the Okavango Delta, Botswana—processes, data and modelling (original) (raw)
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The Okavango Delta is one of the world's most fascinating wetland systems. The highly dynamic flooding forms the basis for a multitude of different ecosystems and plant and animal communities. Water scarcity and economical development lead to an increasing pressure on the ecosystem. A hydrological model is being developed to help making management decisions more sustainable by simulating possible impacts. The model can simulate anthropogenic changes such as water abstraction, damming, dredging and climate change. A key input parameter for the hydrological model is the topography, particularly the statistical properties of the topographic variability. These properties can be quantified using aerial remote sensing data.
The role of remote sensing in hydrological modelling of the Okavango Delta, Botswana
Journal of Environmental Management, 2009
A coupled surface water-groundwater model of the Okavango Delta has been built based on the United States Geological Survey software MODFLOW 2000 including the SFR2 package for stream-flow routing. It will provide a new tool for evaluating water management and climate change scenarios. The delta's size and limited accessibility make direct, on the ground data acquisition difficult. Remote sensing methods are the most promising source of acquiring spatially distributed data for both model input parameters and calibration. Topography, aquifer thickness, channel positions, evapotranspiration and precipitation data are all based on remote sensing. Simulated flooding patterns are compared to patterns derived from visible to thermal NOAA-AVHRR data and microwave radar ENVISAT-ASAR data.
Journal of Hydrology, 2006
Changes in hydrological inputs to the flood-pulsed Okavango Delta result in changes in such flooding characteristics as floodplain water depth, inundation duration and frequency. A mathematical model is used to assess impacts of changing hydrological inputs on flooding in the Okavango Delta. Future conditions are simulated by superimposing simulated abstractions, upstream developments and climate change effects on the observed time series of hydrological inputs. The effects of change in inputs are then determined by comparing hydrological characteristics such as inundation duration and frequency derived from the original and modified time series of model outputs. Simulations show that upstream abstractions are likely to have small short-term effects on the flooding pattern in the Delta, while other upstream developments such as damming or deforestation have more pronounced effects. All of these effects are relatively small, however, when compared to changes resulting from existing climatic variability, and those from the possible effects of future climate change. The combined effects of human abstraction and climate change, manifested as increased temperatures, decreased rainfall, and reduction in river flows, may result in significant Delta drying. The simulated hydrological changes affect the Delta floodplain ecosystems, with anticipated changes in the area and proportions of permanent swamp, areas covered by sedge and grass vegetation (seasonal floodplains) and floodplain grasslands (intermittently flooded areas). These will have varied effects on ecological processes in the Delta, in particular vegetative succession, primary production, and relationships of floodplains with the surrounding woodland and savannah. Additional ripple effects up trophic levels can also be expected. There may also be downstream a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j h y d r o l
Water SA, 2018
The natural resources of the Okavango Delta, a large wetland in semi-arid Botswana, form the basis of livelihoods of the local population and support economically important high-end tourism. The hydro-ecological system is dynamic at various time scales, responding to climate variability, and both flood and drought conditions have in the past put pressure on the system's users. Human-induced climate change can potentially exacerbate the effects of existing climate variability. In this paper, we present simulated future hydro-ecological conditions in the Okavango Delta generated by a step-wise modelling procedure. The outputs of three different global climate models are used to drive a suite of hydrological models. Lastly, a rule-based dynamic model relates hydroperiod conditions to vegetation assemblages. The simulated future conditions vary from much drier to much wetter than those recorded in the past. Models indicate that climatic change would result in change in both extent and distribution of the major ecotopes of the Okavango Delta. Importantly, the different ecotopes will be affected to varying degrees. The projected changes will have consequences for the wildlife-based management of the system. They will affect, for example, available grazing and migration/movement patterns of large herbivores, as well as fish. Such consequences can have rapid up-trophic level effects, ultimately leading to potentially substantial impacts on the economy. The main conclusion to be drawn is that management planning and land-use systems should be as flexible as possible.
Environmental hydro-refugia demonstrated by vegetation vigour in the Okavango Delta, Botswana OPEN
Climate shifts at decadal scales can have environmental consequences, and therefore, identifying areas that act as environmental refugia is valuable in understanding future climate variability. Here we illustrate how, given appropriate geohydrology, a rift basin and its catchment can buffer vegetation response to climate signals on decadal timescales , therefore exerting strong local environmental control. We use time-series data derived from Normalised Difference Vegetation Index (NDVI) residuals that record vegetation vigour, extracted from a decadal span of MODIS images, to demonstrate hydrogeological buffering. While this has been described previously it has never been demonstrated via remote sensing and results in relative stability in vegetation vigour inside the delta, compared to that outside. As such the Delta acts as a regional hydro-refugium. This provides insight, not only to the potential impact of future climate in the region, but also demonstrates why similar basins are attractive to fauna, including our ancestors, in regions like eastern Africa. Although vertebrate evolution operates on time scales longer than decades, the sensitivity of rift wetlands to climate change has been stressed by some authors, and this work demonstrates another example of the unique properties that such basins can afford, given the right hydrological conditions. Rift basins and associated wetlands may offer attractive habitats and their role in human evolution has been stressed by several authors 1–4. The Okavango Delta is the largest wetland in southern Africa and renowned for its high floral and faunal biodiversity 5. It covers an area of over 40,000 square km and consists of a smooth, (relief ≤2 m; slope 1:3400) conically-shaped, alluvial fan located on the Cubango-Okavango River which feeds a network of distributary channels and flanking swamps that form the Delta itself 6,7. While more accurately an alluvial megafan (or fan delta) rather than a delta we preserve the current terminology to avoid confusion. Located in the Kalahari Basin, the region has been subject to sedimentation throughout the Cenozoic, and was once at the centre of palaeo-lake Makgadikgadi 8–12 , prior to its drainage as a result of continued tectonic activity associated with the westward propagation of the East African Rift and river capture by the Zambezi River 10,13,14. The Delta is now located in an asymmetric graben, demarcated by a series of sub-parallel faults, including the Thamalakane Fault and is in-filled with at least 300 m of sediment deposited sometime in the last 1 Ma 10,15–17. Podgorski and colleagues 12 report a combination of electromagnetic, borehole and seismic data and suggest that this sediment fill consists of palaeo-megafans in the Okavango Basin overlain, by palaeo-lake Makgadikgadi deposits and the current delta/megafan. Recent renewed movements, post the palaeo-lake, along the Thamalakane and Kunyere faults now separate the Okavango and Makgadikgadi basins 10. A series of topographic cross-sections illustrate the geomorphological features of the Delta and wider basin. The Okavango catchment is dominated by aeolian sands and outcrops of weathered bedrock (Fig. 1). The annual sediment discharge within the Cubango-Okavango River consists mainly of bed and solute loads (bed: 170,000 tonnes; solute: 360,000 tonnes) while suspended sediment forms only a minor component (8 mg/l or 39,000 tonnes) 18,19. As a consequence of the high sediment porosity (very well-sorted with rounded grains) over 80 to 90% 20 of seasonal flood water infiltrates into the Delta from the Cubango-Okavango River with peak flows in the Panhandle in April and early May (Fig. 1). Much of this shallow water is then transpired by Delta vegetation as this slow moving flood wave takes over four months to reach the south eastern extremity of the Delta (i.e., in
Indicators of desiccation-driven change in the distal Okavango Delta, Botswana
Journal of Arid Environments, 2007
This work seeks to determine whether riparian woody plant variables respond to drying and salinity regimes in the semi-arid distal Okavango Delta, northern Botswana. Structural and compositional variables were obtained from 47 field sites. Mapping using satellite imagery illustrated differences in the character of riparian zones in terms of species composition and provided data on flood frequency. Salinity data plots show increases downstream. Results imply that woody plant variables respond to desiccation-driven change due to water-table lowering (reduced recharge) and increased salinization through distinct changes in tree and shrub height, plant density and species richness. In the wetter, intermediate distributaries, key biotic indicators of ecosystem change comprise structural variables such as decreases in canopy cover per cent and tree height and increases of shrub height, which are indicative of mainly ground-water declines. Biotic indicators in the less frequently flooded receiver channels comprise plant density and species richness increases involving mainly brackish ground-water-tolerant and dryland species which are indicative of both ground-water declines and/or salinization. These indicators could provide useful parameters for use in long- and short-term monitoring aimed at assessing desiccation-driven change in different parts of the Okavango Delta and possibly other semi-arid wetlands. The indicators are important as a less-expensive alternative to drilling as a means of verifying ground-water declines and/or salinization.
Geographical Journal, 2009
Though wetlands are vital for the proper functioning of terrestrial ecosystems and provisioning of a wide range of goods and services, their sustainability is being threatened by inappropriate human resource use practices due to our limited understanding of how these systems operate and lack of appropriately informed interventions. We attempt to address these limitations by using historical CORONA photographs of 1967, Landsat imagery of 1989, 1994 and 2001 and information from the literature to investigate the role of natural and human factors in influencing the direction of environmental change in the proximal reaches of Botswana's Okavango Delta. Results of this investigation point to fragmentation of natural habitats, localised degradation of areas close to perennial water supplies, significant increase in woody cover, significant decrease in open grassland, increase in scrub and shrubs, deterioration in the quality of grazing and depletion of specific woody species. With the direction of change pointing to persistent decrease in the environment's supporting potentials, there is urgent need to adopt intervention strategies potentially capable of enhancing sustainable utilisation of natural resources in this sub-region.
Dynamics of floodplain-island groundwater flow in the Okavango Delta, Botswana
Journal of Hydrology, 2006
Surface water-groundwater interactions play a crucial role in the hydrology and ecology of the Okavango Delta. The hydrology of the Delta is dominated by the annual arrival of a flood wave which is distributed over an number of branches. Subsequently, the flood water feeds the phreatic aquifers underlying the Delta islands. In order to evaluate the seasonal and long-term dynamics of the surface water-groundwater interactions between the floodplains and the islands, a network of piezometers located in various locations of the Delta was monitored. Groundwater table fluctuations observed for up to 6 years were analysed and modelled using groundwater flow models. The floodplain-island groundwater flow is in general very dynamic and driven by island evaporation and transpiration. A typical small to medium sized island (width !500 m), appear not to be influenced by long-term antecedent conditions. Only on large islands (width O500 m) and at the perimeter of the flooded area is the influence of long-term antecedent conditions apparent. The knowledge gained during this study will be used for the improvement of the hydrological and hydro-ecological model of the Delta, and can be useful for the description of floodplain dynamics in semi-arid regions in general. q
2017
Wetland goods and services provide a means of survival for many riparian rural communities in developing countries, particularly in the Sub-Saharan Africa. To sustain the provision of goods and services for human use, spatial and temporal variations in the quantity and quality of water in a given wetland should be adequately understood and managed. In this study we monitored the quantity and quality of surface water through the Okavango Delta for 2 years, from June 2008 to June 2010. Specifically we monitored precipitation over the Delta and surface inflow and outflow (via the Okavango and Thamalakane Rivers respectively) for water quantity, nutrients, cation and anion levels in the surface water along a river transect from Mohembo through the Delta to Lake Ngami. The study revealed that the water balance during the study period was similar to earlier reports. Most of the water inflow was via the Okavango River while surface outflow via the Boro River accounted for about 2% of total...