Abstract ID
eflows12
Type
Oral
Theme
eflows
Full title
A thirty-year evaluation of the flooding regime in Chobe Basin, Botswana and Namibia
Abstract text
Various water utilization scenarios for agricultural and urban use along the Chobe River in Botswana are being
currently proposed. However, the unusual nature of the flow regime of the Chobe – both forward and reverse
to and from the Zambezi, sporadic connections through Lake Liambezi with the Linyanti or with Selinda
Spillway, variable contributions from local and regional precipitation, and other surface and groundwater
connections with the Zambezi across the Caprivi Strip – means that the understanding of the hydrology or
annual flow regime is relatively poor. Because some climate change predictions for Southern Africa indicate a
potential reduction in river runoffs associated with changing patterns of precipitation and increasing
temperatures, allocating water between ecological flows and economic and domestic uses will become more
challenging, especially in the context of limited historical flow data. Moreover, interviews with local
stakeholders reveal a change in the flooding extent of the Chobe River in the last thirty years. Fieldwork in the
Chobe basin and research elsewhere in semi-arid flood-dependent ecosystems indicate a strong relationship
between changes in flooding extents and vegetation structure. Also, t has been suggested by the literature and
local stakeholders that there is a link between increasing bush encroachment in the Chobe floodplains in
Botswana and Namibia and temporal and spatial modifications in the area flooded annually by the river. This
paper presents a geo-spatio-temporal approach to estimate the nature of changes in the flooding extent and
surface flow connections in the basin between 1986 and 2009. A time-series analysis of bimonthly Maximum
Value Composite (MVC) NOAA Advanced Very High Resolution Radiometer (AVHRR, before 2004) and
NASA Moderate Resolution Imaging Spectroradiometer (MODIS, after 2004) Normalized Difference
Vegetation Index (NDVI) one-km resolution data was employed to determine flooded vs. non-flooded areas.
The threshold between the water and land pixels was set using a simple intensity threshold technique and a
four-class supervised classification (flooded, partially-flooded, minimally-flooded, and dry classes) was
performed. Climatologic – mean monthly precipitation data between 1945-2008 at several stations in the basin
– and hydrologic records – mean, max, and min monthly discharge data at Katima Mulilo between 19422008 – were subsequently used to validate the remote sensing analysis. Results indicate that between 15% and
38% of the basin is flooded on an annual basis, with the maximum extent of the flooding occurring in 1991. A
thirty-year trend analysis indicates a consistent decline in the mean monthly flooded area in the basin,
significantly correlated with the trend in mean monthly discharge of the Zambezi at Katima Mulilo. The results
of this study will be used to estimate the relative importance of flooding versus other drivers of change in
determining vegetation dynamics through time in the basin and ultimately regional ecosystem resilience. They
may also prove useful in future water utilization feasibility studies and in determining measures for protecting
high flows through minimum flows and levels.
Submission date
2009-09-25
Keywords
flooding regime, Chobe River, remote sensing
Will be submitting paper?
Yes