Reappraisal of contemporary perspectives on climate change in southern Africa's Okavango Delta sub-region

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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

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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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