Indicators of desiccation-driven change in the distal Okavango Delta, Botswana (original) (raw)
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Applied Geography, 2003
An analysis is undertaken to develop techniques to remotely sense relative evapotranspiration outputs in the distal Okavango Delta using enhanced thematic mapper and ground based techniques to help quantify water loss. Much of this work focuses on riparian woodlands which, especially in the distal portion of the Delta, are regarded as being significant in terms of groundwater removal by transpiration. This was confirmed as vegetation cover mapping led to the identification of two riparian classes which, by association with high resistivity aeromagnetic data, were found to be rooted in near surface fresh groundwater. This paper indicates that riparian trees which remain green year long, partly sustain their growth as a result of groundwater uptake. A comparison of frequently flooded and dry floodplains with distinct riparian zones was undertaken using spectral techniques (pixel radiance values and leaf water content indices) to determine whether flooding and lateral groundwater flow stimulated growth (and therefore transpiration rate) following dormancy. Results indicate a basic similarity between the two systems with mixed evidence of assumed leaf growth. Related phenological observations in the riparian zone of the dry floodplain show that renewal of leaf growth is primarily related to rainfall, not flood events in the distal Delta. The results of this work should help effect both surface and groundwater management in the vicinity of population centres in the distal Delta.
African Journal of Ecology, 2016
The objective of this study was to determine the influence of distance from surface water on riparian woodland communities in the Okavango Delta. Vegetation sampling was conducted in seven sites within the Okavango Delta in 20 m 9 10 m belted plots placed perpendicular to the river bank. The plots were placed at 0-10 m, 10-20 m, 20-30 m, 30-40 m and 40-50 m distance classes increasing away from the river bank. Tree height, basal area, species richness, canopy cover and diversity were determined for each distance class. Indicator species analysis was used to determine the characteristic species at each distance class. Single-factor ANOVA and Tukey post hoc analysis were used to compare species diversity, mean tree height, cover and basal area between distance classes. Correlation between distance from surface water and vegetation parameters was sought using Spearman regression analysis. All parameters except for species richness varied significantly (P < 0.05) along distance from surface water. Distance from surface water was positively correlated all vegetation parameters except for mean species richness/plot. These results show that distance from surface water influences riparian plant community composition and distribution in the Okavango Delta. This implies that riparian plant species can be indicators of long-term hydrologic conditions in the Delta.
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
Bothalia, 2020
Background and objectives: Species composition and distribution in seasonal floodplain plant communities are influenced by variation in flooding. However, the influence of intra-flooding variation phases on the diversity of seasonal floodplain plant communities has not been studied in the Okavango Delta. The objective of this study was to investigate environmental factors that influence species composition and distribution of seasonal floodplain communities before and after flooding. It was hypothesised that environmental factors that influence the species composition and distribution in seasonal floodplain communities will vary with intra-flooding seasons. Methods: Flooding depth was measured in May (before flooding) and September (flood recession/after flooding) in forty 25 m 2 plots. Flooding duration was recorded as the number of weeks in which the plots were inundated. The soil was sampled before and after floods and analysed for pH, extractable P, K, Mg, Ca and Na. Plant identification and estimation of percentage cover were done in the 25 m 2 plots in which environmental variables were sampled. The relationship between environmental variables and seasonal floodplain plant community composition and distribution was sought using Non-metric Multi-dimensional Scaling. Paired Student's t test was used to compare the means of environmental variables before and after flooding. Results: Factors that influenced the distribution of species before flooding were Na, K, water depth and flooding duration. After flooding, the factors that influenced species composition and distribution were K, Na, Mg, pH, water depth and flooding duration. Conclusion: These results suggest that during flooding K and Mg are deposited in the floodplains due to lateral water flow. Our results also suggest that any water abstraction from the Okavango River Basin should take into consideration the importance of flooding duration and depth in sustaining species composition and distribution of seasonal floodplain plant communities so that such developments do not disturb the ecological functioning of the Delta.
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.
African Journal of Aquatic Science, 2011
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Sustainability
Understanding the effects of droughts on vegetation and ecosystem services (ES) is important for climate change adaptation. However, drought occurrence varies across space and time. We examined drought dynamics and impacts on vegetation and ES in the semi-arid Limpopo Basin of Botswana. Weather station precipitation, remotely sensed normalized difference vegetation index (NDVI) and participatory mapping exercises provided data for the analyses. Results show that between 1980 and 2015, rainfall anomaly indices of potential drought years ranged between −4.38 and −0.12. The longest spell of below-average rainfall occurred between 1992 and 1996. On average, drought events lasted for 1.9 years and recurred every 2.3 years. Although the overall drought frequency was 3.7 times in every 5 years, drought prevalence increased to 50%, 60% and 70% between 1981–1990, 1991–2000, and 2001–2010, respectively. The wet season average vegetation condition index between 2000 and 2015 revealed the occur...
Environmental Risks
Dams have been associated with various impacts on downstream river ecosystems, including a decrease in stream flow, species biodiversity, water quality, altered hydrology and colonisation of the area by invasive alien plant species. The impacts normally interfere with the ecosystem functioning of riparian and aquatic environments, thereby leading to decreased biodiversity. This study aims to assess the impacts of dams on downstream river ecosystems, using data from aerial photographs and orthophotos, supplemented by field work. Five dams in Limpopo Province, South Africa, were selected (Albasini, Damani, Mambedi, Nandoni and Vondo), and photographs from different years were used. The area devoid of trees of certain species both downstream and upstream of the dams was calculated using grids of predetermined square sizes on each available photograph. Aerial photographs and orthophoto data were supplemented by field work. The nearest-individual method was used in the field to determine tree density of particular tree species. The environments downstream of the dams show a loss of obligate riparian vegetation and an increase of obligate terrestrial vegetation (Acacia Karroo, Acacia Ataxacantha and Bauhinia galpinii). Treeless area increased in all cases, especially in the case of Mambedi and Vondo dams, indicating lower resilience and higher fragility there.
2008
Visual and digital analysis of Landsat MSS data resulted in the Okavango delta being subdivided into ecological zones. This was achieved by computer classification techniques followed by contextural analysis. This latter technique provided a finer subdivision of spectrally similar features by comparison of classification results with collateral data. Whereas the inherent causes of spectral similarity in wetlands have been previously described, an additional compleXity was found in savanna woodlands and described as the vegetative "darkening effect." Twelve ecological zones were developed for flood inception and maximum flood periods. The delta was shown to consist of 17,000 km land-based ecological zones and 11,000 km2 water-based zones. Seasonal changes were minimal, even after high inflow, because of the large storage capacity of the delta consequent upon prior low flow and drought conditions. The largest increases were in the area of open water which expanded submerging...