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Characterising inundation conditions for flood-pulsed wetlands is a critical first step towards assessment of flood risk as well as towards understanding hydrological dynamics that underlay their ecology and functioning. In this paper, we develop a series of inundation maps for the Okavango Delta, Botswana, based on the thresholding of the SWIR band (b7) MODIS MCD43A4 product. We show that in the Okavango Delta, SWIR is superior to other spectral bands or derived indices, and illustrate an innovative way of defining the spectral threshold used to separate inundated from dry land. The threshold is determined dynamically for each scene based on reflectances of training areas capturing end-members of the inundation spectrum. The method provides a very good accuracy and is suitable for automated processing. [ABSTRACT FROM AUTHOR]

Abstract: The annual flood cycle in the Okavango Delta is the basis of subsistence of the local population, providing water, arable land and wetland resources. Additionally, it forms the basis of a large tourism industry. Flooding in the Okavango Delta is, however, very variable both at short and long term time scales. Past research suggested the influence of factors such as tectonic activity, sedimentation, vegetation change or human influence on the spatial distribution of floodwaters. Change in the flooding regime at any given site is thus often perceived as an effect of permanent change in the system, and this perception results in unnecessary demands for engineering interventions in the system. However, it is possible that some of the perceived changes in flood regime are apparent transient effects of non-linearity of the system and variation in hydrological inputs. Hydrometric data and satellite-derived flood maps are analysed here using statistical methods (covariance analysis and double mass analysis) to distinguish between temporal and permanent changes in various parts of the Okavango Delta. The analyses reveal that system non-linearity causes the hydrological responses of the Okavango Delta to be non-proportional to the inputs. Periods of one to several consecutive years are present when flood extents and outflows are either considerably higher or lower (flow or flood regimes) than what would be expected considering the magnitude of inflow. Such effects are visible throughout the entire system, or only in some distributaries, and do not, as previously thought, represent permanent change in flood distribution. However, flood regime change that appears to result from a physical change in the system has been detected. This change has a nature of shift in flood distribution between the Thaoge and the Xudum, with the latter receiving more water after 1997 at the expense of the former and causing 2004 re-flooding of Lake Ngami. [Copyright &y& Elsevier]

Summary: 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 impacts on tourist facilities presently on the fringes of the seasonal swamps as a result of reduced or increased flooding. As a result of altered flood regime, some effects are also anticipated with respect to the recharge of aquifers, which are currently used for drinking water supply around the Delta. [Copyright &y& Elsevier]

Abstract: Annual inflows and outflows from the Okavango Delta are analysed for possible changes over time. The magnitude of inflows determines the area inundated annually over the delta, and therefore the delta ecosystem on which the thriving tourism depends. Inhabitants of the delta area greatly depend on harvesting aquatic and terrestrial resources, and the abundance of these resources is affected by flooding. Annual flows, annual maximum flows, and annual minimum flows of the Okavango River are analysed for the presence of abrupt or gradual changes and cycles in order to establish the nature of these perceived changes. Change points have been identified in annual flows of the Okavango River and Zambezi River. Other time series except the annual minimum flows are free of trends. Significant cycles were identified on annual flows, annual maximum flows, and annual minimum flows of both the Okavango and Zambezi Rivers. Residuals obtained after removing the cyclic component from each of these time series are trend free. The apparent decline of inflows into the Okavango Delta is explained by the existence of this cyclic component, rather than gradual or abrupt decline in flows over time. The cyclic component identified on the Okavango River has a regional scale as flows on the neighbouring Zambezi River have similar features. Possible causes of the cyclic behaviour are not known. [Copyright &y& Elsevier]

1. Flood-pulsed wetlands make vital contributions to local and global biodiversity. However, the patterns and controls of spatial and temporal variation in aquatic biodiversity in flood-pulsed wetlands are not well understood. We analysed the relationship between variation in hydrological regime and the patterns of aquatic biodiversity in a large pristine flood-pulsed wetland, the Okavango Delta, Botswana. 2. Surveys of water chemistry, diatoms and macroinvertebrates were conducted over the seasonal phases of the flood pulse. Hydrological variables of flood frequency and hydroperiod class were collated from 16 years of satellite images. Multivariate regression trees and generalised least squares regression were used to determine the chief controls of community composition and taxon richness. 3. Hydroperiod class, phase of the flood and conductivity explained 32% and 43% of the variation in diatom and invertebrate taxon richness, respectively. There was a negative relationship between hydroperiod class and invertebrate taxon richness on the rising, peak and receding flood, whereas at low flood there was no significant relationship. Multivariate regression tree analysis revealed hydroperiod class, phase of the flood and conductivity as the dominant forces shaping invertebrate and diatom community composition. 4. Seasonal and spatial variation in hydrological conditions are the principal drivers of variation in aquatic biodiversity in flood-pulsed wetlands. In pristine flood-pulsed wetlands, increased productivity caused by the arrival of the flood waters appears to override disturbance and connectivity in shaping taxon richness and community composition. Thus, these data suggest that the maintenance of a rich mosaic of habitats covering a broad range of hydroperiod is the key to preserving aquatic biodiversity and natural ecosystem function in flood-pulsed wetlands. [ABSTRACT FROM AUTHOR]

Summary: This paper studies the interactions of density driven flow and geochemical reactions under evapo-concentration. A numerical model that couples the relevant flow, transport and chemical processes was used to analyze density-driven flow on islands in the Okavango Delta, Botswana. Evapo-concentration on the islands leads to steadily increasing solute concentrations until the onset of density-driven flow against the evaporation-induced upward flow. The modelling results suggest that lag times to the onset of density-driven flow are strongly influenced by geochemical reactions. Mineral precipitation and carbon dioxide de-gassing increase the lag time. Carbon dioxide de-gassing is enhanced if humic substances are present at elevated concentrations. Simulation results were shown to be most sensitive to the longitudinal dispersivity. Modelling results are compared to field observations from three islands in the Okavango Delta. A semi-quantitative correspondence between modelled and observed concentration patterns is established for the major chemical constituents. [Copyright &y& Elsevier]