The Okavango Delta is recognized as one of the famous inland wetlands and its sustainable use
is important for socio-economic development of Botswana. The Okavango delta comprises
permanent swamps, seasonal swamps, and drylands on islands within the delta and the
surrounding areas, sustained by Okavango river inflows from upstream and local rainfall. The
Okavango River splits into several distributary channels within the delta. Areas which are
flooded annually vary in response to varying inflows into the delta. Peak inflows into the delta
occur during the February to May period. Due to the low gradient over the delta, these inflows
move slowly resulting in peak outflows from the delta occurring during the June to August
period. The inundated area over the entire delta increases from May until it reaches maximum in
August and starts to decrease from September, reaching minimum inundated area in the months
of December and January. The incoming flood wave into the delta and maximum inundation is
out of phase with the local rainfall season.
Communities living within and around the delta derive their livelihoods from tourism, hunting,
fishing, livestock rearing, and crop production. Crop production is carried out on drylands and
within floodplains. Some of the households take advantage of the increase in soil moisture
arising from this inundation along floodplains to cultivate their crops as the floods recede. This
practice is locally referred to as molapo farming which highly depends on inundation of
floodplains. The availability of floodplain inundation highly depends on the magnitude of
inflows into the delta and the local rainfall which are highly variable resulting in uncertainty
regarding successful crop production, availability of livestock grazing areas, and uncertainty in
reliance on the wetlands resources such as fishing. The uncertainty experienced in timing of
extreme events which cause flooding of resulting in water reaching areas or floodplains where it
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is not wanted, and also uncertainity in timing of low flows, therefore water not reaching some
parts of the delta.
Several hydrological studies have been carried out with the aim of improving the understanding
of the spatial and temporal dynamics of flows throughout the delta including predicting areas that
are likely to be inundated each year. The significant gap addressed by this research is to improve
the understanding of the spatial and temporal influence of magnitude and timing of flows on
floodplain inundation. Local rainfall on the delta is highly variable over time and space due to its
convective nature. This research also addresses the rainfall temporal and spatial variations and its
implications on floodplain inundation. The knowledge about spatial extent and duration of
floodplain inundation should assist in predicting each year the viability of molapo farming. Three
research site, Shorobe, Tubu and Xobe are selected as case studies to understand the dynamics of
floodplain inundation induced either by inflows or local rainfall.
Local rainfall during the December to March period enables the crops to reach maturity. The
onset of the rainy season is very important in supporting sowing of crop seeds. Local rainfall on
the delta varies considerably. Aerial rainfall interpolation shows a change in rainfall magnitudes
over space in different rainfall months, i.e different parts of the delta receive different rainfall
magnitudes in different months of the rainy season. The spatial variation is mainly associated
with the migration of the ITCZ southwards first through East Africa during October and
November and down over Southern Africa in December to February. The movement of the ITCZ
brings rainfall concentration on the northern and eastern parts of the Okavango Delta during
December to January and bringing rainfall concentration to the northwestern part of the delta
around February. However, rainfall spatial correlation between stations can be poor even within
the first 150 km therefore implying neighboring places do not experience floodplain inundation
by rainfall at the same time. The poor spatial correlation of rainfall between neighboring stations
reflects the erratic nature of rainfall in the Okavango Delta characterised by localized
thunderstorms. Change detection shows change points in rainfall which can be associated with
ENSO episodes. A change point is identified in 1976 and 1977 which can be associated with the
El Nino episodes during those years and two change points identified in 1999 and 2004 which
can be associated with the La Nina episodes, therefore rainfall induced floodplain inundation can
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also be associated with wet and dry ENSO episodes. Rainfall does not show any significant
trends except for an increasing trend on 10th percentile of Shakawe rainfall. Rainfall also does
not show any cyclic behavior. Rainfall over the Okavango Delta can be divided into three unique
homogenious sub-regions; sub-region 1: the northern part following the GEV probability
distribution and being the region with highest rainfall amounts; sub-region 2: the lower northern
and the outlet parts of the Okavango Delta following the GPA distribution with moderate
rainfall; and sub-region 3: the middle part of the delta extending to the western and the eastern
fringes of the delta, following the P3 distribution and having the lowest rainfall.
The main characteristic that defines the Okavango Delta flows at Mohembo is its cyclic
behavior. Three significant cycles are identified, close to 10, 20 and 40 years. No significant
trends are identified, only a decreasing trend in minimum flows. Change points are identified in
1979 and 1988 and these can be explained by the existing cyclicity since no major land use
changes have taken place in the Okavango River Basin upstream before 1989. The existence of
cyclicity in Okavango River flows at Mohembo also explains the periodic wetting and drying of
different floodplains in the delta. A long period of low flows was experienced from 1983 until
2003 and floodplain inundation extent was greatly reduced, more especially during the 1993-
2003. During the 1993-2003 period, flows could no longer reach Maun Bridge along
Thamalakne River, therefore leaving molapo floodplains around Boteti River, Gomoti River and
Thaoge River to dry out. The 10 and 40 year return floods are important as they indicate the
probability of a flood magnitude which has potential to result in major inundation in the
Okavango Delta. Therefore, flood magnitudes with recurrence interval 10 and 40 years have high
probability of occurring and can cause major floodplain inundation as they can be above the
2009 flood of 969 m3
/s, which was the return of major inundation of Okavango Delta floodplains
after a long period of dryness.
The Ngoqa-Maunachira distributary channel of the Okavango River receives 32% of flow
volumes entering the Okavango Delta at Mohembo. 12 % of the Mohembo flow volumes reach
the Jao-Boro distributary whilst 1% is received by the Thaoge distributary. Therefore more
inundation is experienced along the Ngoqa-Maunachira system compared to the other two. Only
about 2% of the Mohembo flow volumes leave the Okavango Delta through Boteti River. Long
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term shifting of flow direction amongst reaches along the Okavango Delta distributaries is
evident more especially along the Ngoqa-Maunachira River system. This results in shifting of
inundation. Sub-surface water respond significantly to local rainfall and inflows with high soil
moisture conditions retained at 60 cm and 100 cm below the ground.