Forecasting the spatial extent of the annual flood in the Okavango delta, Botswana

Gumbricht, Thomas; Wolski, Piotr; Frost, P. C.; McCarthy, T. S.

doi:10.1016/j.jhydrol.2003.11.010

Cited by 99 publications

(99 citation statements)

References 7 publications

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“…The predictive models of inundation determined by the GLMSelect procedure exhibit how the flooding in the region is not entirely controlled by one variable. The regression equation determined by Gumbricht et al [46] to predict spatial extent of inundation in the Okavango delta also yielded multiple variables that strongly influence the resulting flooding extent. As determined by Gumbricht et al [46], because evapotranspiration varies very little annually, including it the predictive model provides no improvement to the analysis.…”

Section: Discussionmentioning

confidence: 95%

Thermal Imagery-Derived Surface Inundation Modeling to Assess Flood Risk in a Flood-Pulsed Savannah Watershed in Botswana and Namibia

2016

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Abstract:The Chobe River Basin (CRB), a sub-basin of the Upper Zambezi Basin shared by Namibia and Botswana, is a complex hydrologic system that lies at the center of the world's largest transfrontier conservation area. Despite its regional importance for livelihoods and biodiversity, its hydrology, controlled by the timing and relative contributions of water from two regional rivers, remains poorly understood. An increase in the magnitude of flooding in this region since 2009 has resulted in significant displacements of rural communities. We use an innovative approach that employs time-series of thermal imagery and station discharge data to model seasonal flooding patterns, identify the driving forces that control the magnitude of flooding and the high population density areas that are most at risk of high magnitude floods throughout the watershed. Spatio-temporal changes in surface inundation determined using NASA Moderate-resolution Imaging Spectroradiometer (MODIS) thermal imagery (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) revealed that flooding extent in the CRB is extremely variable, ranging from 401 km 2 to 5779 km 2 over the last 15 years. A multiple regression model of lagged discharge of surface contributor basins and flooding extent in the CRB indicated that the best predictor of flooding in this region is the discharge of the Zambezi River 64 days prior to flooding. The seasonal floods have increased drastically in magnitude since 2008 causing large populations to be displaced. Over 46,000 people (53% of Zambezi Region population) are living in high magnitude flood risk areas, making the need for resettlement planning and mitigation strategies increasingly important.

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Section: Discussionmentioning

confidence: 95%

Thermal Imagery-Derived Surface Inundation Modeling to Assess Flood Risk in a Flood-Pulsed Savannah Watershed in Botswana and Namibia

2016

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“…While these products have very high spatial resolution, they have very low temporal resolution, with up to two weeks between observations. Although with lower spatial resolution, and frequent contamination due to cloud cover, the products from MODIS [37][38][39][40] and the Advanced Very High Resolution Radiometer (AVHRR) are also used to classify flood areas [2,[41][42][43][44]. Validation of MODIS-derived inundation extent mapping is done through comparison with either other remotely sensed data such as Landsat [2,3] or stream gauge measurements associated with flood peak detection [4,21].…”

Section: Data Opportunities and Constraintsmentioning

confidence: 99%

MODIS Inundation Estimate Assimilation into Soil Moisture Accounting Hydrologic Model: A Case Study in Southeast Asia

2014

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Flash Flood Guidance consists of indices that estimate the amount of rain of a certain duration that is needed over a given small basin in order to cause minor flooding. Backwater catchment inundation from swollen rivers or regional groundwater inputs are not significant over the spatial and temporal scales for the majority of upland flash flood prone basins, as such, these effects are not considered. However, some lowland areas and flat terrain near large rivers experience standing water long after local precipitation has ceased. NASA is producing an experimental product from the MODIS that detects standing water. These observations were assimilated into the hydrologic model in order to more accurately represent soil moisture conditions within basins, from sources of water from outside of the basin. Based on the upper soil water content, relations are used to derive an error estimate for the modeled soil saturation fraction; whereby, the soil saturation fraction model state can be updated given the availability of satellite observed inundation. Model error estimates were used in a Monte Carlo ensemble forecast of soil water and flash flood potential. Numerical experiments with six months of data -December 2011 showed that MODIS inundation data, when assimilated to correct soil moisture estimates, increased the likelihood that bankfull flow would occur, over non-assimilated modeling, at catchment outlets for approximately 44% of basin-days during the study time period. While this is a much more realistic representation of conditions, no actual events occurred allowing for validation during the time period.

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“…The major sub-regions identified are indicated in Figure 6. These results have been used to explore secular changes in flood water distribution over the last 15 years (Gumbricht et al, 2004b), derived from historic satellite imagery, and indicate that a gradual shift of seasonal flood water from the Boro to the Xudum channel is taking place.…”

Section: Accuracy Of the Micro-topographic Mapsmentioning

confidence: 99%

“…Local-scale flood prediction. Gumbricht et al (2004b) have developed a statistical model for distributed flood prediction, calibrated using inflow and rainfall records, with the resulting flooded area being estimated using coarse resolution (1 km 2 ) NOAA AVHRR satellite images extending back over 15 years. The model can predict the likely area of flooding three months in advance of the seasonal maximum with an estimated accuracy of better than 90 per cent at that scale.…”

Section: Accuracy Of the Micro-topographic Mapsmentioning

confidence: 99%

The micro‐topography of the wetlands of the Okavango Delta, Botswana

Gumbricht

McCarthy

Bauer

2004

Earth Surf Processes Landf

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The surface of the 40 000 km 2 Okavango alluvial fan is remarkably smooth, and almost everywhere lies within two to three metres of a perfectly smooth theoretical surface. Deviations from this perfect surface give rise to islands in the Okavango wetlands. This microtopography was mapped by assigning empirical elevations to remotely sensed vegetation community classes, based on the observation that vegetation is very sensitive to small, local differences in elevation. Even though empirical, the method produces fairly accurate results. The technique allows estimation of depths of inundation and therefore will be applicable even when high resolution radar altimetry becomes available. The micro-topography has arisen as a result of clastic sedimentation in distributary channels, which produces local relief of less than two metres, and more importantly as a result of chemical precipitation in island soils, which produces similar local relief. The micro-topography is, therefore, an expression of the non-random sedimentation taking place on the fan. Volume calculations of islands extracted from the micro-topography, combined with estimates of current sediment influx, suggest that the land surface of the wetland may only be a few tens of thousands of years old. Constant switching of water distribution, driven by local aggradation, has distributed sediment widely. Mass balance calculations suggest that over a period of c. 150 000 years all of the fan would at one time or other have been inundated, and thus subject to sedimentation. Coalescing of islands over time results in net aggradation of the fan surface. The amount of vertical aggradation on islands and in channels is restricted by the water depth. Restricted vertical relief, in turn, maximizes the distribution of water, limiting its average depth. Aggradation in the permanent swamps occurs predominantly by clastic sedimentation. Rates of aggradation here are very similar to those in the seasonal swamps, maintaining the overall gradient, possibly because of the operation of a feedback loop between the two. The limited amount of local aggradation arising from both clastic and chemical sedimentation, combined with constant changes in water distribution, has resulted in a near-perfect conical surface over the fan. In addition to providing information on sedimentary processes, the micro-topography has several useful hydrological applications. Copyright

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Forecasting the spatial extent of the annual flood in the Okavango delta, Botswana

Cited by 99 publications

References 7 publications

Thermal Imagery-Derived Surface Inundation Modeling to Assess Flood Risk in a Flood-Pulsed Savannah Watershed in Botswana and Namibia

Thermal Imagery-Derived Surface Inundation Modeling to Assess Flood Risk in a Flood-Pulsed Savannah Watershed in Botswana and Namibia

MODIS Inundation Estimate Assimilation into Soil Moisture Accounting Hydrologic Model: A Case Study in Southeast Asia

The micro‐topography of the wetlands of the Okavango Delta, Botswana

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