ARTICLE IN PRESS Reappraisal of contemporary perspectives on

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Journal of Arid Environments 72 (2008) 1709– 1720
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Journal of Arid Environments
journal homepage: www.elsevier.com/locate/jaridenv
Reappraisal of contemporary perspectives on climate change in
southern Africa’s Okavango Delta sub-region
H. Hamandawana a,, R. Chanda b, F. Eckardt c
a
b
c
Department of Geography and Environmental Sciences, North West University, P. Bag X206, Mmabatho, South Africa
Department of Environmental Sciences, University of Botswana, Botswana
Department of Geographical Science, University of Cape Town, Cape Town, South Africa
a r t i c l e in fo
abstract
Article history:
Received 13 May 2006
Received in revised form
22 February 2008
Accepted 5 March 2008
Available online 5 May 2008
This paper provides a reappraisal of contemporary perspectives on climate change in
southern Africa’s Okavango Delta sub-region by drawing on time-line evidence from
historical/archival records, field-compiled information and multi-date remotely sensed
imagery. By using temporal variations in stream discharge and surface and groundwater
distribution as proxies of declining rainfall from the beginning of the 19th century, trends
emerging from this reconstruction suggest that progressive contraction of the Delta’s
permanent floodplains, the desiccation of Lake Ngami in its distal reaches, fossilization of
receiver channels, sustained dewatering of aquifers, and changes in vegetation from
grassland to drought-tolerant woody species are non-transient precursors of increasing
aridity and deteriorating climatic conditions. With evidence pointing to persistent drying
sequences and system failures to revert to moister climate conditions of the recent
historical past, hypotheses that characterize deteriorating rainfall and recurring flood
failures in this environment as isolated singularities in a punctuated equilibrium need to
be reconsidered in order to provide empirically grounded planetary change perspectives
that are consistent with evidence over long-term temporal horizons.
& 2008 Elsevier Ltd. All rights reserved.
Keywords:
Bush-encroachment
Desiccation
Deterioration
Human interventions
Wetland
1. Introduction
Sub-global reconstruction of environmental trends during the recent historical past offers opportunities for enhancing
our understanding of global climate change by providing case–study evidence of persistent environmental stresses. Spatial
simplification of global/continental scale observations into discrete geographical/ecological areas provides for bottom-up
scientific investigations in which the local informs the regional by facilitating exhaustive interrogation of cause and effect
relationships. The major strength of this discretization arises from its enhanced capabilities to accommodate fine-scale
observations that can be upscaled to coarser resolutions in formulating empirically grounded planetary change
perspectives (Hay, 2005). In view of this consideration, we selected the Okavango Delta region in southern Africa as a
case–study area for climate-change investigation because of its sensitivity to external perturbations of variable intensity
and duration. Though commonly referred to as a delta, it is actually a landlocked wet-fan comprising three hydroecosystems that include the permanent, seasonal and intermittent floodplains (Appendix 1, electronic version only),
respectively, distinguished by perennial and seasonal water residence for the former two and occasional inundation of the
latter during periods of exceptionally high floods. Because of flat terrain, with height differences rarely exceeding 5–10 m at
Corresponding author. Tel.: +27 769 288 720; fax: +27 183 925 775.
E-mail address: [email protected] (H. Hamandawana).
0140-1963/$ - see front matter & 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jaridenv.2008.03.007
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any location (SMEC, 1990) over an inundation area that varies between 4000 and 12 000 km2 (Wolski et al., 2005), the
Okavango Delta is typically a shallow inland wetland. Water-depth in the permanent and seasonal floodplains is about 1.5
and 30 cm, respectively (McCarthy, 2006) over gentle gradients around 1:5140 and 1:3610 for the Panhandle and
intermediate swamps (McCarthy et al., 1997) in a semi-arid environment where rainfall averages 477 mm/annum (CSO,
2000). Due to gentle gradients, minor variations in inflow via the Okavango River from its principal catchments in Angola
and Namibia translate into substantial expansion and contraction of surface water distribution. This sensitivity facilitates
long-term change investigation by amplifying fluctuations in rainfall and channel discharge into detectable proxies of
climate change such as temporal variations in surface water distribution. Consequently, long-term hydrological changes in
this environment reliably capture the direction of environmental change with limited human exploitation of water
resources during the recent past (Hamandawana et al., in press) in a closed system characterized by absence of
groundwater outflow (McCarthy et al., 1998), allowing temporal variations in water distribution to be linked to climate
variability. However, little work has to date been carried out to enhance our understanding of regional trends in climate
change by monitoring long-term changes in this area’s hydrology the major constraint being lack of suitable data at
appropriate temporal and spatial scales.
Though recent initiatives show piece-meal attempts to address this limitation, systematic investigation has been
constrained by restricted temporal coverage to the narrow window period covered by remotely sensed Landsat-era
imagery. There is no evidence in the literature of concerted attempts to extend temporal coverage into the historical past
by exploiting pre-Landsat data archives to build temporally robust databases that link historical/archival information to
more recent data from instrumental records. In cases where satellite imagery, aerial photographs and rainfall data
have been used (McCarthy et al., 2000, 2002; Wolski et al., 2003), such use has been undermined by failure to incorporate
data from historical/archival records while investigations based on the latter (Mackenzie, 1946; Shaw, 1984, 1985; Tlou,
1972) have also failed to incorporate information from the former category. This lack of synergy continues to compromise
our ability to formulate unified regional climate-change perspectives. This problem is further aggravated by extreme
variability in the Delta’s surface water distribution which makes it difficult to arrive at consensual perceptions as high
floods allow optimistic asseverations while flood failures prompt pessimistic projections. As a result, there is lack of
agreement on whether recurring flood failures and persistent drying sequences in this environment are inconsequential
anomalies in stable cycles or regional precursors of a sustained shift toward more arid climatic conditions. In attempting
to narrow this gap in perceptions, we decided to reconstruct ‘recent’ trends in climate change in the Okavango Delta region
by tapping on a database of our own that covers about 150 years between 1849 and 2001 (Hamandawana et al., 2005)
and extending its temporal coverage to the beginning of the 19th century by sourcing additional information from
historical records.
2. Materials and methods
The materials that were used to compile our investigation’s seed-database comprise: (1) historical records and sketch
drawings capturing environmental conditions in known localities within the Okavango Delta sub-region, (2) archival maps,
(3) climate and hydrological data, and (4) conventional satellite imagery and hand-held camera photographs. Historical
records include non-spatially referenced written materials from the archives while sketch drawings consist of fairly dated
graphical illustrations. Archival maps include spatially referenced historical maps that were updated by postcripting
geocoded information from different sources. Climate and hydrological data comprise rainfall records for selected stations in
the Delta’s catchment areas and temporal variations in the Okavango River’s discharge at Mohembo. Conventional satellite
imagery comprises CORONA photographs that were acquired by the same satellite in September 1967 and 12 nearanniversary Landsat thematic mapper (TM) and enhanced TM (ETM) scenes providing blanket coverage of the Okavango
Delta over three time slices for the years 1989, 1994 and 2001. Hand-held camera photographs consist of illustrative
selections from field-based acquisitions. The methods that were used in putting these datasets together include:
(a) compilation of historical information from diary records by early travelers and missionaries and scanning sketchdrawing and photo illustrations to provide a graphical backdrop of environmental conditions during the historical past,
(b) capturing archival maps and geocoded information in a geographic information system (GIS), (c) computing seasonal
averages for rainfall distribution and mean annual discharge for the Okavango River, and (d) georeferencing to the same
projection and mosaicking all image coverages of the Okavango Delta at individual time slices. Procedures that were used
in processing CORONA photographs and to compile the entire geo-database are provided elsewhere (Hamandawana et al.,
2005, 2007a, b). Fig. 1 shows the structural layout of this database and variations in temporal coverage by data type. Fig. 2
provides an overview of the methodology that was followed during the database compilation process.
For image-based investigation, four sample sites each covering 900 km2 (sites 1–4 in Appendix 1) were conveniently
selected to monitor temporal variations in surface water distribution and selected biophysical indicators of climate change
with site 1 and, sites 2 and 3, respectively, providing coverage of the Delta’s proximal and intermediate reaches. Because
the latter two receive most of their water as overspill from the permanent floodplain, the extent of seasonal inundation in
these areas is closely correlated to flood magnitude in the permanent floodplain which is largely determined by regional
rainfall in the Delta’s major catchment areas in Angola and Namibia. Site 4 in the distal reaches was considered to be ideal
for climate-change investigation because of Lake Ngami’s high responsiveness to temporal variations in flood magnitude in
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Fig. 1. Geo-database used for climate change monitoring in the Okavango Delta. Source: Adapted and modified from Hamandawana et al. (2005).
the Delta’s permanent floodplain. Details on procedures that were used to classify satellite imagery are provided elsewhere
(Hamandawana et al., 2006).
3. Results
Results of this investigation are presented in the form of: (a) a tabular overview of the surface water situation in
the Okavango Delta sub-region as captured in archival/historical records, (b) a composite map illustration of the
surface and groundwater situation during the first and second half of the 20th century—Fig. 3, (c) image-based trends
in the distribution of surface water and woody cover and grassland (Fig. 4) in the four sample sites shown in Appendix 1,
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Archival/
historical
records
Conventional
Satellite
imagery
Climate and
hydrological
data
Hand-held
camera
photographs
Sketch drawings
and non-spatially
referenced data
Archival maps and
geolocated
information
Corona, Landsat
TM and Landsat
ETM
Rainfall and
discharge data for
selected stations
Colour photos for
vegetation
discrimination
Chronological
tabulation and
scanning of sketch
drawings
Digitizing and
Geo-referencing
into standard
map-outputs
Geo-referencing,
mosaicking and
spatial accuracy
assessment
Tabulation and
periodization for
graphing and
statistical analysis
Key -logging and
GIS conversion to
point themes in
ArcView
Multi-source
geo-database
Fig. 2. Schematic illustration of methodology followed to compile the geo-database.
(d) a map illustration compiled from geolocated historical information and field observations presentation of which is
conveniently deferred to the discussion (Fig. 5), (e) a sketch-drawing depicting surface hydrology in the Delta’s distal
reaches during the first half of the 19th century—Appendix 2; electronic version only, (f) trend-graphs on rainfall
distribution at selected stations, temporal variations in the Okavango River’s discharge at Mohembo and, the decadal
frequency of arid years for Shakawe and Maun in Botswana—Appendix 3; electronic version only and, (g) hand-held
camera photographs acquired during field investigation—Appendix 4; electronic version only.
4. Discussion
The information in Table 1 provides a convenient entry point for reconstructing trends in climate change in the
Okavango Delta sub-region from the beginning of the 19th century. The documented migration of the baYei from Zambia
who canoed into the Okavango Delta via the Selinda spillway (Appendix 1), and southward from the Delta along the
Thamalakane River until they stopped on the shores of a vast stretch of water (Lake Ngami) around 1800 suggests moister
conditions during the recent past. This proposition is supported by use of canoes in the Selinda spillway that was made
possible by outflow from high floods in the Okavango Delta with flood heights of this magnitude confirming high rainfall in
the Delta’s catchments in Angola and Namibia. Locally known as the River people; the baYei were/are expert fishers
renowned for introducing to the Delta sub-region, technological innovations related to canoeing, fishing and net-making.
Their fishing nets acquired international reputation, attracting the attention and interest of Londoners in the 1850s (Tlou,
1972, p. 156). To their hosts in the Delta sub-region, they introduced trawling from canoes and new hippo-hunting
techniques. These adaptive strategies enabled the baYei to permanently settle around Lake Ngami where perennial water
presence supported a resident hippo population and adequate fish to meet their subsistence requirements. Such habitat
conditions could only be sustained by reliable floods from high rainfall. From early travelers’ records, the first important
observation is that estimates for Lake Ngami’s dimensions between 1849 and 1861 are consistent with the widely accepted
view of periodic fluctuations. This periodicity is undisputed and there is general agreement by many (IUCN, 1992; Magole
and Thapelo, 2005; SMEC, 1990) that pulse variability has always been an intrinsic characteristic of natural changes in this
environment. However, there are vital clues on the complexion of this variability that are frequently overlooked. When
Livingstone first saw Lake Ngami in 1849, he described it as a fine sheet of water. Nine years later in 1858, he described it as
a dismal swamp (Livingstone, 1858), a characterization that has been used by many as confirmation of low water levels
similar to what has been observed on numerous occasions during the 20th century. The following observations make this
asseveration contestable.
Though the Lake might have contracted noticeably before 1858; in 1853, Chapman estimated its depth at 6 ft (Chapman,
1886). On a return visit in 1859, he estimated its length and circumference at 36–37 and 100 km, respectively (Table 1).
These dimensions suggest that there was substantial water in the Lake whose full capacity length from our present
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Fig. 3. Current borehole distribution, geolocated event observations and the Okavango Delta’s surface water situation during the 1920s. Source: Adapted
and modified from Hamandawana et al. (2005). (1) Terminal reaches of the Thaoge in the 1930s. (2) Perennial flow in the 1860s, channel depths averaging
7 ft and dense floodplain reed growth. No such growth today. (3) Terminal reaches of the Thaoge in 1910. Flows no further than Gumare today,
(4) Observed reach of the Thaoge in 1884 after ceasing to flow into Lake Ngami in 1883. It never reaches this point today. (5) The Thamalakane’s
dimensions in 1921 were: 90 yards in flood season and 50 yards in dry season but it is now ephemeral, flowing very briefly after the arrival of floods.
(6) Limit of flow toward the Mababe depression in 1921 earlier described by Livingstone as a huge emptiness destitute of water with masses of brown creeper
grass that looked like miniature haystacks (suggesting seasonal flooding) but today it is extensively bush encroached, flooding only in exceptionally good
rainfall years. (7) The Selinda spillway provided a navigable waterway to the baYei around 1800 but has never flooded with any observed regularity since
the 1870s.
knowledge is approximately 55 km. Livingstone’s dream of a major waterway in southern Africa is well known. His search
for Lake Ngami was largely inspired by expectations of a lake comparable in extent and depth to the lakes in East Africa
i.e. Lake Victoria, with the idea of using it for navigation in his missionary work (VanderPost, 2005). However, because of
confinement to a shallow depression, Lake Ngami was naturally incapable of offering any navigability consistent with the
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Gumare
70
50
60
Percentage composition
Percentage composition
Shakawe
60
40
30
20
10
0
50
40
30
20
10
0
*
Water
Woody cover
1967
1989
1994
*
Grassland
*
Water
2001
1967
1989
1994
**
Grassland
2001
Sehitwa
Boro River Site
70
Percentage composition
50
Percentage composition
**
Woody cover
40
30
20
10
60
50
40
30
20
10
0
0
*
Water
**
Woody cover
1967
1989
1994
Grassland
2001
**
Water
*Significant at α = 0.05
*
Woody cover
**
Grassland
**Significant at α = 0.01
Fig. 4. Temporal variations in the distribution of surface water, woody cover and grassland by sample site in the Okavango Delta’s upper, intermediate and
terminal reaches: 1967–2001.
romantic ideas of his evangelical commitments. His description has to be interpreted within the context of a missionary
explorer’s ambitions that were betrayed by the Lake’s natural shallowness. There is compelling contemporaneous evidence
to support the proposition that contrary to his disinclination, Lake Ngami actually had substantial water volumes
compared to later periods when floods became treacherously intermittent. The footage in Appendix 2, with canoes in the
extreme right (note their strategic positioning suggesting the casting of trawling nets) and Livingstone’s personal
observation of reluctance by natives to venture deep into the Lake point to perennial water presence and, resident hippo and
crocodile populations that provide the most plausible explanation of why natives resented canoeing too far into the Lake.
For earlier periods, perennial water residence is supported by oral traditions, with the local inhabitants describing Lake
Ngami in the 1750s as a lake with waves that throw hippos ashore and roaring like thunder (Tlou, 1972, p. 151). Given the
distance of Lake Ngami from the permanent floodplains (100 km), it is unlikely that what the local people described was a
temporary relocation of hippos from the Delta to a non-perennial lake. In view of these considerations, it is not
unreasonable to suggest that Lake Ngami had a sedentary hippo population on account of regular inflow and a sustained
presence of water. This historical information further implies that rainfall too was equally reliable and higher than in more
recent times. Reliable rainfall is confirmed by regular flow in the Thaoge River as noted by Mcabe, Anderssen, Green and
Chapman between 1852 and 1863 (Table 1) and Chapman’s observation (1862) of abundant production of watermelons,
pumpkins, calabashes, maize, beans, earthnuts and sorghum by villagers around Lake Ngami.
While the foregoing observations strongly suggest reliable rainfall during this period, the mere presence of canoes
(Appendix 2) provides compelling evidence of sustained water presence in the Lake during the second half of the 19th
century. The time required to produce one of these (about 5–6 months) suggests local people’s knowledge of a perennial
lake which justified investment of substantial patience and effort to carve, dry and waterproof these canoes. Their present
disappearance from villages around Lake Ngami confirms prolonged absence of ‘permanent’ water residence in the Lake. In
addition, the reluctanceyto venture deepy described by Livingstone once again confirms extensive water-spread, implying
that the Lake (estimated by Oswell to be 14 km wide in 1849) changed little up to 1861 when Baines estimated its width at
10–12 km while the near-constant dimensions given for the period between 1849 and 1861 (Table 1) further suggest
reliable rainfall, regular inflow and higher floods compared to decades thereafter. Higher floods during this period are
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Fig. 5. Observed channel blockages and human interventions to increase down-stream flow in the Okavango Delta during and after the 1930s.
confirmed by the bidirectional flow observed by Livingstone in 1849, with reverse flow occurring up the Lake River from
Lake Ngami (Appendix 1). This phenomenon; corroborated by oral evidence from local people, prompted Livingstone and
Oswell to erroneously conclude that the Boteti River flowed from Lake Ngami. We know from recent work (Shaw, 1983) that
such flow is only possible when the Lake’s water-level rises to 930–931 m asl to inundate an approximate 800 km2.
What Livingstone described as a dismal swamp was therefore a marginal recession of Lake Ngami from high levels in
1849. This evidence confirms substantial flow of the Thaoge and Thamalakane, the former being described by Chapman in
1863 as reed swamps infested by buffaloes and elephants that had to be hunted from boats (Table 1). Hunting from boats is
indicative of active channels, with the Thaoge’s depth being less than five feet only in three places from Anderssen’s survey in
1853 (Table 1) while the mix of game reported by Chapman in 1863 points to sustained productivity in this environment on
account of regular floods from reliable rainfall. These floodplains dried around 1880, with the Thaoge River ceasing to flow
into Lake Ngami around 1883 (Fig. 3). Since then, the active channels and reed swamps of 1853 and 1863 have given way to
a fossil floodplain; suggesting persistent flood failures due to long-term decrease in rainfall, with tectonic tilting failing to
adequately explain this phenomenon on account of simultaneous floodplain desiccation in the east. On reaching the
Mababe depression in 1858, Livingstone was unimpressed and described it as a vast emptiness destitute of water. In oral
traditions; this environment is captured as Lake Mababe, historically flooded by the Mababe River (Appendix 1) which is
described in local people’s histories as a channel that was deep enough to hinder passage (Shaw, 1984). From available
records, the Mababe River flowed briefly in 1957 after a protracted dormancy since 1910 and completely dried around 1960
(Campbell and Child, 1971) leading to the desiccation of Lake Mababe and its subsequent colonization by acacia bush
(Hamandawana, 2006). Synchronous desiccation of the Delta’s western and eastern floodplains strongly suggests the
progressive onset of drying sequences and increasing aridity initiated by persistent flood failures and long-term decrease in
rainfall. This proposition is supported by declining rainfall throughout the second half of the 20th century at Rundu in the
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Table 1
Historical observations of surface water distribution in Lake Ngami and flow regimes of the Thaoge River: 1800– 1935
Historical observations of lake Ngami and the Thaoge River
Comment
Oral histories 1800: The baYei canoed along the Selinda spillway from
Zambia and down the Delta to settle around Lake Ngami following
pioneer exploration by Hankuzi, who had been attracted by the Lake’s
abundance of hippo and fish
Livingstone Aug 1849: Estimated circumference of L. Ngami at
70–75 km, noted use of mekoro, reluctance by inhabitants to venture
into lake and imperceptible flow of the Thaogea
Oswell Aug 1849: Estimated Lake Ngami’s dimensions: Length ¼ 30 km,
Width ¼ 14 km
Mcabe Aug 1852: Estimated width of the Thaoge to be 3.2 km and 1 m
deep a mile upstream from Lake Ngami
Anderssen July 1853: Estimated Lake Ngami’s width at 7–9 km,
circumference 60–70 km. Shape of lake described as resembling pair of
spectacles. Records submerged tree stumps and former hippo territory
colonised by vegetation
Anderssen 1853: Sailed up the Thaoge. In his words, ((The main course of
the river is northwest-, and is so serpentine that in thirteen days of
ascend—I only made 1 degree latitude due northy (The river’s width)
never—exceeds forty yards—but is deep—depth less than five feet only in
three places))
Green, 1857: Tried Anderssen’s route and found rowing difficult against
a current estimated at 3–3.5 miles/h
Chapman 1859: Length of Lake Ngami estimated to be 36–37 km. He
circumnavigated and measured its circumference to be 100 km using
trochometer & observed direct flow of the Boteti into lake Ngami as
noted by Livingstone in June 1858
Baines 1861: Estimated width of Lake Ngami: 10–12 kmb
Chapman 1863: Notes that: ‘all the country on either side of the Teouge
from its junction with the lake upwards is a land of swamp and reeds,
infested by buffaloes and elephants which are constantly in water or
reeds and have to be hunted from boats’
Stigarnd 1922: Recorded local people’s tales of higher lake levels and dry
bed in recent past. His map, produced between 1910 and 1922 shows:
Seasonal flooding of the floodplain to latitude 211S and perennial
flow of the Thaoge to the same latitude
Dense reed growth on both sides of the channel. Aug and Sep 1921,
the Thaoge flowed as far south as Tsau
Evidence of perennial lake before Livingstone’s arrival in 1849 and high
floods in the Delta sustaining flow to the Zambezi via the Selinda
spillway
Kalahari reconnaissance report 1925: Reported prolonged combustion of
peat for months to depths of several feet below the surface in Lake
Ngami’s emergent environment
Brind 1930– 1935: Assembled a papyrus-cutting machine and cleared
channel blockages along the Thaoge east of Gumare describing the area
during this period as a sea of papyrus
Convincing evidence of a flowing Thaoge in the past and substantial
discharge sustaining Lake Ngami
Tenuous evidence suggesting possible flow of the Thaoge
The width and depth of the Thaoge River indicate a fairly active channel
Evidence of channel dynamics (possible avulsion and inception of
drying sequences) suggested by relocation of floodplain areas
Active, low-gradient anastamosing channel and substantial bed-load
transportation building a mouth bar that later plugged the river as
confirmed by Brind (1955)
Confirms reliable flow of the Thaoge, and high floods in the Delta
Reliable measurement of Lake Ngami’s extent. Flow of the Boteti into
Lake Ngami suggests high water levels in the Delta
Decrease in lake size
This observation provides evidence of a productive environment,
reliable flow and abundance of game and further suggests westward
flow of the river’s sub-channels into the desert
Decline documented by Baines also captured by oral traditions.
Stigarnd’s map (produced according standard mapping conventions and
one of the earliest on record) is obtainable from: Department of Surveys
and Mapping, PLAN BP-122
Evidence of perennial water presence before 1925 and substantial
papyrus growth to support peat accumulation
Clear evidence of an active floodplain environment and water flow from
the Delta below this river’s exit point
Sources: Shaw (1985), Tlou (1972), Brind (1955), Stigarnd (1922), Anderssen (1857), and Chapman (1886).
a
Mokoro (singular), mekoro plural: Local term for dug-out canoe/s.
b
See Baines (1864).
Delta’s catchment area in Namibia (Appendix 3(a)) and similar decrease in the proximal and distal reaches around
Shakawe and Maun (Appendices 3(b) and (c)) with persistent decrease in the Okavango River’s discharge at Mohembo
(Appendix 3(d)) providing further evidence of long-term decrease in local rainfall.
In terms of the mainstream direction of change, decadal distribution of arid years provides additional insights that are
not readily discernible from trend-based analysis of rainfall records. Though this area’s climate is widely classified as semiarid (aridity index (AI) ¼ 0.20 to o0.50), analysis of available data shows increasing frequency of arid years (AI ¼ 0.05 to
o0.20) with Shakawe and Maun, respectively, exhibiting noticeable and significant increase in the number of arid years
after the mid-1930s (Appendix 3(e) and (f)). Of immediate interest is the fact that Shakawe’s steeper downward trend in
rainfall is not accompanied by a corresponding increase in the number of arid years. This unexpected situation suggests
that rainfall averages for this area have remained above the critical threshold below which a regime shift from semi-arid to
arid conditions becomes evident though the overall direction of change is still indicative of a persistent shift toward drier
climatic conditions. For Maun in the Delta’s distal reaches, marginal long-term decrease in rainfall translates into a
significant decadal increase in the number of arid years. This counter-intuitive situation can be explained by Maun’s lower
rainfall compared to Shakawe which makes this sub-system more sensitive to minor decrease in rainfall. This proposition is
in agreement with findings by Hamandawana et al. (2007a, b) who report the Okavango Delta’s semi-arid distal reaches
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around Sehitwa (Appendix 1) as being the worst affected area by drying sequences. Though lack of data prohibits
temperature-based assessment of trends in climate change before 1970 for Shakawe, 1965 for Maun and 1989 for Rundu;
and consistent rainfall records for these stations unavailable for years before 1930, historical evidence suggests higher and
more reliable rainfall during the first quarter of the 20th century.
Fig. 3 shows perennial springs south-west of Mohembo and, perennial pans north of the Mababe depression and south
of Lake Ngami. These water-points were mapped by Captain A.G. Stigarnd between 1910 and 1921 (Stigarnd, 1922). This
evidence supports the hypothesis on higher rainfall during the historical past, with the drying up and subsequent
replacement of these water sources by boreholes during the second half of the 20th century (Fig. 3) pointing to sustained
lowering of water tables, persistent decrease in recharge, progressive dewatering of aquifers, increasing aridity and system
failures to revert to the more reliable rainfall conditions of the historical past. For the interlude years between 1921 and
1940, historic bridges and dams that were constructed by colonial authorities during the 1930s confirm higher floods in the
proximal (Appendix 4(a)) and intermediate (Appendices 4(b) and (c)) reaches of the Delta. The former (suggesting
sustained decrease in flood heights in the Panhandle) is located about 2 km south of Shakawe and the latter (confirming
truncation of the Thamalakane’s tributaries in the Delta’s eastern floodplains) are located about 45 km north of Maun
(Appendix 1). Though these structures are indicative of high floods in their respective localities during the 1930s,
observations in the Delta’s distal reaches during the same period confirm different scenarios characterized by persistent
flood and channel failures and the desiccation of Lake Ngami. The prolonged peat fires recorded in the Kalahari
reconnaissance report (Table 1) point to the inception of increased irregularity in flood cycles and short-lived lake-water
residence in successive years immediately before and after 1925. The persistence of these fires for months suggests that
prior to this period; the Lake’s environment was for a long time highly productive, enabling the accumulation of substantial
peat deposits. With the onset of drying sequences and the drying up of the Lake’s major water supply channels especially
the Thaoge, peat accumulations were gradually dewatered as lake-levels receded, facilitating the spontaneous combustion
observed during the early 1920s. Though Lake Ngami flooded occasionally after 1925, absence of peat fires in this
environment during the second half of the 20th century suggests erratic water presence and inadequate papyrus growth to
sustain peat accumulation. These observations are important because contrary to common perceptions of the Delta as a
resilient ecosystem, there is no corresponding convalescence of its emergent floodplains to support the stable-cycles
hypothesis.
By the early 1950s; the Thaoge River had retreated as far north as Gumare, with its floodplains in the vicinities of this
village, described by Brind (1955) as a sea of papyrus, permanently drying up (Appendix 4(d)) and desiccating into one of
active peat fire areas at present (Gumbricht et al., 2002; Hamandawana, 2006). These fire regimes mimic those of the Lake
Ngami environment during the 1920s with recurrence on an annual basis confirming progressive dewatering of
combustible deposits that might take time to be exhausted. In the north-east, the Selinda spillway used by the baYei during
the early 19th century dried up, never flooding with any observed regularity since the 1870s (Fig. 3) while the desiccation
of Lake Mababe and truncation of the Thamalakane’s tributaries alluded to in preceding sections confirm fossilization of
the Delta’s floodplains in the east, with persistent reduction in the magnitude of successive floods providing the most
plausible explanation of this dry-down. In the south-east, swampy conditions along the Boteti’s terminal reaches, described
by Livingstone as a glorious river in 1849 have disappeared (Campbell and Child, 1971). South and south-west of Lake
Ngami, the copious water fountains and numerous shallow wells observed by Anderssen in 1853 have long dried up
(Anderssen, 1857). Worth reiterating with respect to the above observations is that, there has been persistent contraction of
permanent and seasonal floodplains without any recovery to the abundant surface water situation observed during and
before the first quarter of the 20th century. Though quasi-stationary climatic oscillations in southern Africa have been
suggested (McCarthy et al., 2000; Wolski et al., 2005) the point worth mentioning is that these cycles are periodic
fluctuations in a down-trending situation characterized by progressive surface water contraction (Fig. 4). In the Delta’s
proximal reaches around Shakawe, surface water distribution decreased by 10.9% from 12.4% in 1967 to 1.5% in 2001 with
this trend being statistically significant at a ¼ 0.05. This contraction was accompanied by a 14% increase in woody cover
from 37.1% and a 15.9% decrease in grassland from 19.5%, with the latter trend being statistically significant at a ¼ 0.05
(Fig. 4(a)). In the intermediate reaches, surface water distribution during the same period declined by 12.8% from 14.5% in
the Thaoge’s floodplains east of Gumare (Fig. 4(b)) and by 12. 6% from 39.9% in floodplains of the Boro River east of the
Okavango Delta (Fig. 4(c)), with both trends being statistically significant at a ¼ 0.05. As permanent floodplains contracted,
woody cover in Gumare significantly increased by 45% (a ¼ 0.01) from 19% while grassland decreased by a significant 43%
(a ¼ 0.01) from 46% with the Boro River area exhibiting a significant (a ¼ 0.01) two-fold increase in woody cover from 22%
to 44%. In the distal reaches, Sehitwa’s Lake Ngami contracted by 9.8% from 10.2% with this decrease being statistically
significant at a ¼ 0.01 while woody cover significantly increased by 32% (a ¼ 0.05) from 28% as grassland significantly
declined by 5% (a ¼ 0.01) from 9% (Fig. 4(d)). These changes are indaicative of climate-driven responses to progressive
decrease in local rainfall which has tended to selectively facilitate the expansion of woody cover at the expense of droughtsensitive wetland and dryland grasses.
The observed decrease in surface water distribution around Shakawe is consistent with the field observed dry-down of
outflow channels from the Okavango Delta (Appendix 4(a)). Given the narrow profile of the permanent floodplain in this
environment, it is very unlikely that redirection of water to other areas initiated the observed contraction between 1967
and 2001. Progressive decrease in the Okavango River’s discharge (Appendix 3(d)) provides the most plausible explanation
of this downward trend. Trends in the intermediate reaches illustrate the sensitivity of shallow water environments to
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changes in flood magnitude over time. Compared to the Delta’s proximal reaches around Shakawe, the decrease in surface
water distribution in floodplains of the Thaoge and Boro rivers exceeded what was observed in the former. This observation
suggests that shallow water environments are more sensitive to sustained decrease in the magnitude of successive floods
compared to their deeper counterparts in the Panhandle’s permanent floodplains. The synchronous decrease in water
distribution in the Delta’s eastern and western floodplains further confirms the unlikelihood of substantial eastward
redistribution of surface water often invoked by the tectonic and vegetation blockage hypotheses as the major cause of the
Thaoge River’s dry-down and desiccation of its floodplains. Further south in the Delta’s distal reaches around Sehitwa, the
complete dry-down of Lake Ngami by 1989 (Fig. 4(d)) is illustrative of wide system failures and how drying sequences have
tended to be propagated northward from the terminal reaches of the Delta’s outflow channels. The implication of this
phenomenon compounded by floodplain desiccation in both the east and west is that the surface water situation has; at
least during the recent historical past, progressively deteriorated in a manner that strongly suggests the interaction of longterm decrease in the magnitude of successive floods and similar trends in local rainfall over the Delta’s immediate environs.
Though Lake Ngami has occasionally flooded in intervening years between 1967 and 2001, the duration of water residence
was much shorter compared to decades before the 1960s. The flood observed in 1989 (Fig. 4(d)) provides a good example,
with aerial photographs acquired 2 years after this event in 1991 showing virtual absence of water in the Lake. Floods of
similarly short-lived duration were also observed in 2000 and 2004. Further evidence of climate-driven system failures
comes from human interventions during and after the second quarter of the 20th century that were prompted by the need
to increase declining flow to the distal reaches of the Delta (IUCN, 1992). Between 1931 and the late 1980s, several projects
were undertaken by local farmers and colonial authorities to open up dying channels by burning and clearing papyrus
blockages and by dredging and bunding (Fig. 5).
Those worth noting include: (a) papyrus clearing along the Santantadibe River by Martinus Drotsky, and dams across
the Thamalakane’s tributaries by Charles Naus in the 1930s, (b) bypass channels cut by the Witwatersrand Native Labour
Association in the Thaoge’s floodplains between 1940 and 1950, (c) manual and mechanical blockage clearing along
the Thaoge by Brind during the 1950s, (d) navigation cross-links to Moanatshira by Agriculture Department in 1973,
(e) channel manipulations by farmers for floodplain irrigation along the Xaudum in the 1970s, (f) diversion of the Boteti,
and bunding and dredging of the Boro and Lake rivers by Anglo American Corporation between 1971 and 1973 and, (f)
dredging of the Thaoge River by the Department of Water Affairs in the 1980s. The wisdom informing these manipulations
was that vegetation blockages were interfering with the transmission of flow.
Though factors such as channel blockages in upstream sections of the Thaoge River and eastward diversion of flow due
to tectonic tilting have often been invoked to explain channel and floodplain failures, long-term contraction of the
Okavango Delta as a whole strongly argues for diminished flow volumes. As pointed out in preceding sections, the main
shortcoming of the tectonic and channel blockage hypotheses is that they fail to account for simultaneous contraction of
surface water distribution in the east and west. Though channel blockages can admittedly sever down-stream flow, this
severance is counterbalanced by redistribution of water to other areas through channel avulsion (diversion of flow from an
existing channel by initiation of sediment-induced bypasses). In the absence of significant decrease in inflow as suggested
by proponents of the stable-cycles hypothesis, channel blockages alone do not provide adequate explanation of the entire
system’s persistent contraction. As shown in Fig. 5, blockages have occurred along the Okavango’s distributaries in the east
and west with intermediate streams such as the Boro being equally affected. However, without a corresponding increase in
surface water distribution in the Delta’s intermediate reaches, the role of vegetation blockages in initiating widespread
floodplain failure becomes questionable. Though vegetation blockages affect channel dynamics, the same blockages can
also be initiated by a reduction in stream competence arising from decrease in channel flow volumes. The implication of
this observation is that channel blockages often giving the impression that vegetation is blocking water flow might be
symptoms and not the direct cause of channel failures. Though avulsion is common in many parts of the Delta’s active
floodplains, this phenomenon cannot explain widespread system failures because redirection of water within the
floodplain environment amounts to nothing more than internal redistribution. The same logic can be extended to explain
why sedimentation is unlikely to be a major factor behind coextensive floodplain desiccation because progressive
shallowing of channels should translate into spatial spread of water and a corresponding increase in surface area. The fact
that the Delta’s inundated area has been declining instead of increasing strongly suggests that sedimentation per-se is
unlikely to be one of the major causes of floodplain desiccation. Climate driven changes in vegetation, with woody cover
increasing in emergent floodplains and their immediate peripheries are unlikely to have induced substantial reduction of
surface flow into the Delta by facilitating infiltration because the drying up of the permanent springs observed during the
1920s (Fig. 3) is indicative of progressive dewatering of dryland aquifers on account of persistent decrease in local rainfall.
In the Delta’s intermediate catchment in Namibia, rapid population growth along the Okavango River has been
accompanied by increasing demands for arable land which increased by 20% between 1972 and 1996 (el Obeid and
Mendelsohn, 2001). Though this increase might have increased surface flow because of deforestation, the Okavango River’s
discharge at Mohembo did not exhibit a corresponding reversal of the long-term downward trend that might be
interpreted to suggest substantial influence of anthropogenic factors on water discharge into the wetland. Though
dependable figures on sustainable levels of abstraction are not readily available; in 2000, the estimated offtake from the
Okavango River by Angola, Namibia and Botswana was about 0.26% of its mean annual flow (Hamandawana et al., in press).
This level of exploitation appears incapable of adequately explaining the significant decrease in surface water distribution
in different localities of the Okavango Delta between 1967 and 2001 (Fig. 4). Besides, human interventions of the nature
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outlined above are too recent to explain the persistent deterioration evident from the beginning of the 19th century.
Regarding long-term trends in the Delta’s surface hydrology, what counts is not mere floodplain desiccation because of
avulsions and channel blockages but incontrovertible shrinking of the ‘entire’ system irrespective of internal variations in
flow direction.
In view of the failure of channel clearance, dredging and related manipulations to achieve sustained down-stream flow,
it is logical to suggest that persistent decrease in the Okavango River’s discharge has been the main cause of floodplain
desiccation with anthropogenic factors and channel blockages failing to adequately explain coextensive system failures
characteristic of the Delta’s intermediate and distal environs. With recent evidence pointing to sustained dewatering of
aquifers in Lake Ngami’s immediate environs (Appendix 5) it is apparent that episodic floods during the last quarter of the
20th century are aberrations in a trend characterized by progressive deterioration. Evidence in support of this proposition
comes from the drying up of numerous hand-dug wells that provided reliable sources of portable water during the 1960s
and their subsequent replacement by boreholes. As floods continued to become more erratic, shallow boreholes dried up,
with saline yields from deeper replacements confirming reduced recharge and, sustained dewatering and thinning of
aquifers. These observations provide additional evidence of persistent rainfall failures and increasing aridity.
While temporal variations in surface water distribution are indicative of deteriorating climatic conditions, biophysical
evidence in the form of the earlier described trends in the distribution of grassland and woody cover further points to
sustained shifts toward drier climatic conditions evidenced by the tendency for emergent floodplains to undergo
transitions from reed swamps to acacia thickets (Ringrose et al., 2002). Though overgrazing admittedly contributed to
increase in woody cover by facilitating bush encroachment (Hamandawana et al., 2007a, b), declining rainfall appears to
have exerted the greatest influence by triggering flood failures and floodplain desiccation which created a window of
opportunity for the expansion of woody species intolerant of permanent floodplain conditions. In view of the pervasive
nature of drying sequences and increasing aridity in this sub-region, the need for formulating appropriate policies designed
to mitigate the adverse of effects of deteriorating climatic conditions is now overdue. With evidence suggesting that these
trends are likely to persist into the near and distant future, and current climate-change scenarios pointing to mid-continent
drying in southern Africa centred on Botswana (Hulme et al., 2001), deployment of appropriately informed adaptation
strategies designed to enhance human capacities to cope with deteriorating climate conditions are urgently required.
Articulation and effective adoption of such strategies requires formalized acknowledgement of the non-transient character
of the present direction of change.
5. Conclusion
This paper has attempted to provide a 200-year reconstruction of trends in climate change in the Okavango Delta subregion through the collative use of information from disparate sources. The major insights from this investigation suggest
that; progressive contraction of the Delta’s permanent floodplains, the desiccation of Lake Ngami in its distal reaches,
fossilization of receiver channels, sustained dewatering of aquifers, and changes in vegetation from grassland to droughttolerant woody species are non-transient precursors of increasing aridity and deteriorating climatic conditions.
Hydrological changes in the Okavango Delta sub-region from the beginning of the 19th century to the present show
limited prospects for immediate; let alone, gradual near-term recovery to the abundant water situation of the recent
historical past. With projections of climate change pointing to high probabilities of increased mid-continent drying and
greater variability in rainfall, the potential future for this region is unlikely to deviate much from the mainstream direction
of deteriorating climatic conditions. There is therefore an urgent need to enhance our understanding of climate-change
processes by adopting multidisciplinary investigative techniques potentially capable of yielding more information than is
conventionally available from exclusive use of instrumental records. The approach we employed in this paper represents a
hybrid initiative that extends temporal coverage into the historical past by tapping on different types of information from
multiple sources in order enrich and broaden the knowledge base needed to guide the formulation of informed climate
change policy interventions and appropriate human response and adaptation strategies. We hope our reconstruction,
especially for the period before general availability of instrumental records will be helpful in inspiring those interested to
appreciate the complementary relevance and significance of historical records in attempting to formulate unified and
informative climate change perspectives.
Acknowledgements
The authors would like to thank START International, Canon Collins Educational Trust for Southern Africa (CCETSA) and
the Southern Africa Regional Science Initiative (through Prof. Harold Annegarn, Rand Afrikaans University and Prof. Susan
Ringrose, Harry Oppenheimer Okavango Research Centre) for co-funding research work that enabled us to write this paper.
We are also grateful to the anonymous reviewers whose comments helped us to improve this paper.
Appendix A. Supplementary material
Supplementary data associated with this article can be found in the online version at 10.1016/j.jaridenv.2008.03.007.
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