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EARTH SCIENCES CENTREGÖTEBORG UNIVERSITYB328 2002

A MINOR FIELD STUDY OF THEINFILTRATION RATE ON A FLOODPLAININ THE OKAVANGO DELTA, BOTSWANA

Louise Björkvald Monica Boring

Department of Physical GeographyGÖTEBORG 2002

GÖTEBORGS UNIVERSITETInstitutionen för geovetenskaperNaturgeografiGeovetarcentrum

A MINOR FIELD STUDY OF THEINFILTRATION RATE ON A FLOODPLAININ THE OKAVANGO DELTA, BOTSWANA

Louise Björkvald Monica Boring

ISSN 1400-3821 B328 Projektarabete

Göteborg 2002

Postadress Besöksadress Telefo Telfax Earth Sciences CentreGeovetarcentrum Geovetarcentrum 031-773 19 51 031-773 19 86 Göteborg UniversityS-405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg

SWEDEN

Royal Institute of TechnologyInternational Office

AddressKTHMFSSE-100 44 Stockholm

Visiting addressInternational OfficeDrottning Kristinas väg 30Stockholm

TelephoneNat 08 7906000Int +46 8 7906000

FaxNat 08 7908192Int +46 8 7908192

Internethttp://www.kth.se/student/utlandsstudier/[email protected]

Preface

This study has been carried out within the framework of the Minor FieldStudies (MFS) Scholarship Programme, which is funded by the SwedishInternational Development Cooperation Agency, Sida.

The MFS Scholarship Programme offers Swedish university students anopportunity to carry out two months of field work in a Third Worldcountry. The results of the work are presented in a report at the Master'sdegree level, i.e. a graduation thesis work or similar in-depth study.Minor Field Studies are primarily conducted within subject areas that areimportant from a development perspective and in countries supported bySwedish international development assistance.

The main purpose of the MFS programme is to enhance the knowledgeand understanding of Swedish university students regarding thesecountries and their problems and opportunities. An MFS should providethe student with initial experience of conditions in such a country. Afurther purpose is to widen the Swedish human resources cadre forengagement in international cooperation.

The International Office at the Royal Institute of Technology, KTH,Stockholm, administers the MFS programme for the faculties ofengineering and natural sciences in Sweden.

Sigrun SantessonProgramme OfficerMFS Programme

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A Minor Field Studyof

the Infiltration rate on a floodplain in

the Okavango Delta, Botswana

Louise Björkvald & Monica BoringDepartment of Earth Sciences

Göteborg UniversityGöteborgSweden

2002

Master’s Thesis in Physical Geography, Göteborg UniversityJune 2002

Supervisors:Professor Lars Ramberg Senior Lecturer Björn HolmerDirector of the Okavango Research Center Department of Earth SciencesUniversity of Botswana Göteborg UniversityP. Bag 285, Maun S-405 30 GöteborgBotswana Sweden

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THE OKAVANGO DELTA

Sunrise over the land of disappearing rivers.The Okavango Delta, a nature in its pristine state – one of

Africa’s last refuges.

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Abstract

The Okavango Delta, situated in the northeastern parts of Botswana, is considered to be oneof Africa’s last refuges for wildlife species. The seasonal flooding of the Okavango River, fedby the rains in the mountains of Angola, spreads water and sediment out into an alluvial fan,thus creating the Okavango Delta, the world’s largest inland delta. The floodplains of theDelta are of primary importance for the flora and fauna in the area. The dynamic features ofthe floodplains are tributed to the seasonal floods, which are crucial for the ecosystem. Theseasonal fluctuations of the water level on the floodplains make them the most productivearea in the Delta since essential nutrients are released during flooding (Ramberg 1997a). Thisinland Delta, which in geomorphological terms is an alluvial fan, is unique due to the fact thatalmost all of the inflowing water is lost to the atmosphere through evapotranspiration (Wilsonet al 1976, Dincer et al 1987, Ellery & McCarthy 1994).

This work focuses on the processes of groundwater refill on a seasonally inundated floodplainsituated in the margin of Chief’s Island. The fringe areas of the floodplains are crucial for theinfiltration of groundwater into the system. The importance of the these areas for theinfiltration of groundwater was investigated in this report by studying its spatial variationacross the floodplain and in different vegetation zones.

The infiltration rate was measured by a method using double ring cylinders (Landon 1991).By studying the grain size distribution of the soils and also the amount of organic matter, itsrelationship to the infiltration rate is estimated. In order to conduct this study, a topographicand a vegetation map were made of the floodplain.

The infiltration rate was found to be highest in the fringe area (zone 3) mostly due to thehigher content of coarser sand, root systems and burrowing animals. The results from thisstudy indicate that the infiltration rate range from 2.9 m/day in the central parts of the floodplain to 3.9 m/day in the fringe area. Silt, on the other hand decreases the infiltration rate.However, there was no correlation between the organic content and the infiltration rate.

The scarceness of water resources in this semi-arid environment makes the Delta veryinteresting for water exploitation. Withdrawal of water from the area can have devastatingeffects on the system, even if the outtake is only a few percent. The effects are due to the lowrelief and the high rate of evapotranspiration. The low relief makes a certain volume of waterto spread laterally over a wider area. If this volume of water decreases, large areas will not beflooded which will alter the infiltration of groundwater. Therefore, it is very important to haveknowledge about where on the floodplain that the main part of the infiltration takes place,since an outtake of water will affect the recharge of groundwater in the fringe area first.

Key words: Okavango Delta, infiltration rate, organic matter, particle size distribution

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Acknowledgements

First of all we would like to thank SIDA, for giving other students and us the opportunity toperform Minor Field Studies.

Our long journey brought us to the big continent of Africa, into the pristine nature of theOkavango Delta, one of Africa’s last wildlife areas. We came to face an extraordinary wildlifeand wonderful, loving and caring people. This adventure has forever changed us as humanbeings.

First of all, we would like to thank Sigrun Santesson, Programme Officer, MFS Programme,for making this trip possible by believing in our project. Also, for just being her, in otherwords helpful, supportive and caring.

In Botswana, we had the opportunity to work with Professor Lars Ramberg, OkavangoResearch Center, Maun, our supervisor to whom we send our most sincere gratitudes forbeing enthusiastic and helpful. We would also like to thank the staff at the OkavangoResearch Centre, especially Billy Mogojwa, Ineelo Mosie and Electricity Motho for theirinvaluable assistance and guidance in our fieldwork.

At the Department of Physical Geography, Göteborg University, Sweden we would like tothank our supervisor Senior Lecturer Björn Holmer for his incredible ability to guide studentsthrough projects. Also we would like to thank Senior Lecturer Lars Franzén for his helpregarding field equipment and our fieldwork. Finally we thank Senior Lecturer Mats Olvmofor his encouraging e-mails.

Last but not forgotten we thank George, Pat and Dan at Gunn’s Camp, the Okavango Delta,for their knowledge about the nature, but most for being the family we left at home. We thankour families for their ever-lasting support, especially Börje Björkvald for making some of ourfield equipment. Finally, we would like to thank Cindy Mackin for checking the language inour report.

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1 INTRODUCTION ............................................................................................................................................. 1

2 GENERAL DESCRIPTION OF THE OKAVANGO DELTA......................................................................... 2

2:1 Geomorphology ......................................................................................................................................... 32:1.1 Topography .......................................................................................................................................... 32:1.2 Tectonic events..................................................................................................................................... 32:1.3 Sediment and Soils ............................................................................................................................... 32:2.4 Classification of sub aerial fans............................................................................................................ 42:1.5 Climate and Hydrology ........................................................................................................................ 52:1.6 Groundwater Levels ............................................................................................................................. 6

2:2 Organic Matter and Infiltration .............................................................................................................. 7

3 THE STUDY AREA .......................................................................................................................................... 8

4 METHODS ...................................................................................................................................................... 10

4.1 Topographic and vegetation mapping ................................................................................................... 10

4.2 Soil survey and soil analysis.................................................................................................................... 104.2.1 Organic matter ...................................................................................................................................... 114.2.2 Particle size distribution ....................................................................................................................... 11

4.3 Infiltration measurements and infiltration rate.................................................................................... 12

4.4 Sources of errors in the methods............................................................................................................ 13

5 RESULTS ........................................................................................................................................................ 14

5:1 Topography.............................................................................................................................................. 14

5:2 Vegetation zones ...................................................................................................................................... 15

5:3 Infiltration rate........................................................................................................................................ 165:3.1 Infiltration rate, organic content and particle size distribution ........................................................... 18

5.4 Cross-sections........................................................................................................................................... 22

5.5 Comparable analyses of infiltration rate vs. particle size .................................................................... 24

6 DISCUSSION .................................................................................................................................................. 28

7 CONCLUSIONS.............................................................................................................................................. 30

8 FUTURE WITHDRAWAL OF WATER? ...................................................................................................... 31

9 REFERENCES................................................................................................................................................ 32

Appendix ...................................................................................................................................................................

1

1 INTRODUCTION

River deltas of all kinds are very valuable water resources and also very unique environments.In Africa, there are a number of deltas but the Okavango Delta, situated in the northeasternpart of Botswana, is extraordinary due to the fact that almost all of the inflowing water is lostto the atmosphere through evapotranspiration (Wilson et al. 1976, Dincer et al. 1987, Ellery& McCarthy 1994). The Delta is one of the few regions in the world where nature is still in itspristine state and the influence of mankind is limited. A major part of the Okavango Delta is awildlife conservation area, lying within the borders of the Moremi Game Reserve. However,the Delta is also considered as a major economical resource with great potential foragricultural development and tourism (Dincer et al. 1987).

The Okavango Delta is a dynamic system, where large-scale changes of the flow take placeon relatively short time periods. The flood events can change from one year to another. Thesystem is very vulnerable and studies of the hydrogeology of the area are therefore veryimportant. In addition, the Delta is situated in a semi–arid part of the world, where only a fewwater resources are available. Therefore, the Okavango Delta has attracted national as well asinternational interest for water exploitation.

This work focuses on the processes of ground-water refill and the assumption that the highestinfiltration is within the fringe area of the floodplain. Withdrawal of water from the area canhave devastating effects on the system, even if the outtake is only a few percent. This is due tothe low relief and the high rate of evapotranspiration.

The purpose of this study is to estimate the infiltration rate of the water into the soils and howthe rate varies in different vegetation zones within a seasonally inundated floodplain situatedin the fringe area of Chief’s Island. By studying the grain size distribution of the soils and alsothe amount of organic matter, its relationship to the infiltration rate is estimated. In order toconduct this study a topographic and a vegetation map were made of the floodplain.

The fringe areas of the floodplains are crucial for the infiltration of groundwater into thesystem. The importance of these areas is investigated in this study by evaluating theinfiltration rate on different sites of the floodplain. In order to estimate the consequences of anouttake of water it is important to have knowledge about where on the floodplain the majorinfiltration takes place. If, as planned by some, more water is withdrawn from the Delta, thefringe areas will be affected first whereby the infiltration of groundwater into the system willbe reduced and altered.

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2 GENERAL DESCRIPTION OF THE OKAVANGO DELTA

At present the Okavango Delta represents the terminal depository for the Okavango River,which drains the wet Angolan highlands. By the deposition of sediments from the OkavangoRiver, a large alluvial fan has developed, upon which the wetlands of the Okavango Delta aresituated.

The Okavango Delta is situated in the northeastern parts of Botswana at approximately 20°S;23°E (figure 1). The size of the Delta is approximately 22,000 km2, corresponding to aboutthree-quarters of the size of the Nile Delta (Watson 1991). The swamp, covering a large areaof the Delta, has a mean propagation of 10,000 km2. However, the size of the swamp variesseasonally between 6,000 and 12,000 km2 (Dinçer et al. 1987).

Source: McCarthy et al. 1993b.

Figure 1. The Okavango Delta in northeastern Botswana.

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The Okavango Delta is divided into two major regions, the upper Panhandle region and thelower alluvial fan. The Panhandle is characterised by relatively narrow meandering channelssurrounded by permanent swamps. These permanent swamps comprise an area of 6000 km2

(McCarthy & Metcalfe 1990, McCarthy et al. 1993b) and are interspersed with islandsdominated by woodland and/or grassland. Only the upper part of the Okavango Delta ispermanently inundated with permanently flowing channels. The depth of the surface watermeasures up to four meters (Watson 1991). As the Okavango River meanders downstream itgradually broadens and degenerates into a number of anastomosing and meandering channels.The major channel systems are the Nqoga/Maunachira channels, the Jao/Boro system and theThoge channel. This second region consists of the lower seasonal swamp and floodplainsalong the distal margins of the fan (McCarthy et al. 1993). This area is only flooded duringparts of the year, and the water depth is usually around 0.5 meter. (Watson 1991). Theseasonal swamps consist of small islands during flooding and of dry grassland and woodlandduring the dry spells (McCarthy & Bloem 1998).

2:1 Geomorphology

2:1.1 Topography

The Okavango Delta, situated at 850-1000 m a.s.l. (Watson, 1991), has a conical shape,typical for an alluvial fan. However, there is very little variation in the topographic relief overthe delta. The total fall from the apex of the Panhandle to the distal part in Maun is about 65meters over a distance of approximately 250 km (Ellery and McCarthy, 1994), equivalent to atopographic gradient of only 1:3600 (McCarthy, Green et al. 1993). The terrain on the fanitself is gently undulating with a relief that seldom exceeds two meters (Watson 1991 andMcCarthy & Bloem 1998).

Small irregularities on the fan surface provide the pattern for the numerous channels andislands. The islands range in size from termite mounds to larger islands such as Chief’sIsland, which is considered to be of tectonical origin (McCarthy and Matcalfe, 1990).

2:1.2 Tectonic events

The unusual setting of the delta was caused by tectonic events during the Miocene, whereby aseries of faults obstructed the ancient course of the Okavango River (Dinçer et al. 1987). TheDelta was initiated after the formation of a graben structure, which is considered as asoutherly extension of the East African Rift Valley System (Hutchins et al. 1976). Thedepression is bounded by the Gomare fault in the northwest (figure 1). The Kunyere andThamalakane faults in the southwest constitute the southern boundary of the Delta since theytransverse the flood systems (Hutchins 1976, Wilson 1976, McCarthy 1993).

2:1.3 Sediment and Soils

Waterborne sediments have built up the Delta, which is proved by its conical shape. The Deltais situated above Recent to Tertiary Kalahari deposits consisting of layers of fine sand,calcrete and silcrete with an average thickness of 200–300 m. Under these deposits is theKarroo series, which are underlain by the Precambrian crystalline basement (Dinçer et al.1987).

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The sandy soils in the Okavango Delta are derived from the catchment area in central andsouthern Angola. The catchment area mostly consists of windblown sands derived from aformer desert that once covered the southern central Africa (Ellery and McCarthy 1994). Inaddition to this, aeolian sand from the Kalahari basin covers the Delta area (McCarthy 1993).Most of the sediment that enters the Delta is through bedload and it is estimated that 170,000tons of aeolian sand enters the system this way annually. However, 95 percent of this isdeposited within the Panhandle (McCarthy 1993).

The banks and the levees by the channels mostly consist of peat mixed with fine clasticmaterial stabilised by vegetation (McCarthy & Bloem 1998). The channel margins arepermeable whereby water is leaked to the surrounding swamps. Bedload is dominant and inthe permanent swamps the leaking of water through the channel margins induces bedloaddeposition and therefore also vertical aggradation of the channels (McCarthy et al. 1993).

Since the Delta formerly extended over a greater area and the channels constantly havechanged their route, the sandy alluvial deposits are wide spread in the area. The sand plain inthe apex of the fan constitutes of brownish or greyish medium to fine-grained sands. The soilsin the Delta are often termed Kalahari sand, because of its sandy dominance. However, thesoils in the Okavango Delta comprise a wide range of classes, from pure sand to heavy clays(Watson 1991).

2:2.4 Classification of sub aerial fans

The Okavango Delta has incorrectly been termed inland Delta by geographers for many years,because of its bird-foot pattern (McCarthy 1993). The Delta is however a sub aerial fan,which is geomorphologically controlled by the climate, plate tectonics and the quality of thesource material (Stanistreet & McCarthy 1993).

Sub aerial fans can be classified into three main groups: fans dominated by either debris flow,braided rivers or fans dominated by meandering/low sinuosity rivers (losimean). The latterrepresents the Okavango Fan (figure 2) (Stanistreet & McCarthy 1993).

The sub aerial fans are classified according to slope, size and the percentage of vegetationcover. A losimean fan is characterised by the lowest slope of them all. However, themaximum size of the fan can be up to 150 km for the losimean type, which makes it thelargest in the size spectrum. Fans dominated by debris flow are characterised by the lack ofvegetation and fans dominated by braided rivers have a vegetation cover of 40 percent. Incontrast to this stands the Okavango Fan, which has a vegetation cover of 90 percent. Thestability of the fluvial systems in the Delta is dependent on the plant cover and therefore theDelta must have developed after Devon (Stanistreet & McCarthy 1993).

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Figure 2. The Okavango Delta is classified as a low sinuosity/meandering fluvial fan, characterised by a low slope and a high percentage of vegetation cover.

2:1.5 Climate and Hydrology

The rain in the Delta falls from November to March and the mean annual precipitation isaround 500 mm with large annual variations. Rainfall is entirely out of phase with the annualflood, except at the upper part of the delta (figure 3 and 4). The precipitation over the swampsitself provides about one third of the total input (Wilson et al. 1976). Mean annual inflow tothe delta is 10,000 x 106 m3 (i.e. 320 m3/s) which is twice the amount contributed byprecipitation.

The mean annual evaporation has been estimated to 1860 mm (estimated by the Penmanmethod using meteorological data for Maun) (Dincer et al. 1987). Evapotranspiration exceedsthe precipitation during all months of the year (figure 4) (Ellery and McCarthy 1994). Themean annual temperature is around 30°C (McCarthy 1993). The Okavango graben isconsidered as an endorheic basin (a basin in which little or none of the surface drainagereaches the ocean). This is due to the fact that approximately 96% of the total input to theswamp (i.e. inflow and precipitation) is lost to the atmosphere by evapotranspiration(McCarthy and Metcalfe 1990).

Source: Ellery & McCarthy 1994.Figure 3. The hydrograph for the Okavango Delta, Figure 4. The rainfall and evaporation during the illustrated as the monthly discharge at the year in the Okavango Delta. inlet and outlet of the Delta.

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Most of the water from the catchment area in the Angolan Highlands arrives during thesummer rains (November to March). The flow of water slowly progresses downstream theOkavango River and by March or April it finally reaches the northern parts of the Panhandle.Further down the Panhandle the channels are not able to confine the water whereby it spreadslaterally across the surface of the fan out into the surrounding floodplains, creating theseasonal swamps (McCarthy 1993). Approximately four months after peak discharge at thePanhandle the floodwater reaches the distal end of the alluvial fan, reaching the ThamalakaneRiver by Maun (Dinçer et al. 1987, Ellery and McCarthy 1994). The maximum extent offloodwater is therefore in August, during the mid-winter drought period.

There have been several outlet channels that have been active at different times in the past. Atpresent the main outlet from the Delta is by the Thamalakane River, which flows into theBoteti River (figure 1). Only two percent of the inflow leaves the Delta as surface outflow viathe Boteti River and another two percent enters the groundwater system (Ellery and McCarthy1994, McCarthy et al. 1993).

2:1.6 Groundwater Levels

Satellite images show that the flooding events differ a lot from one year to another. Howeverthe flooding is crucial for the recharge of the groundwater. The groundwater is mainlyrecharged through vertical infiltration by the flood wave or the rain. Past studies have shownthat the water chemistry of the groundwater is laterally nonuniform (Dinçer et al. 1987). Dueto the extensive transpiration from the vegetation on the islands, saline water is concentratedand cored underneath the islands and there will be no lateral movement of this water.However, groundwater or water from the surrounding swamp area will fill up these coresthrough lateral movement. Lateral movement exists, but the main recharge is through verticalinfiltration (McCarthy & Bloem 1998).

After flooding the groundwater levels generally show a rapid increase. However, indeficiency of surface inflow, the groundwater levels recede at a rate of 1.5 to 2 m per year(that is at a depth of 3 m below the surface). The evapotranspiration from the grass areacreates a faster recede of the groundwater close to the surface, immediately after flooding.This also explains why the groundwater levels, on a floodplain that is annually flooded, are 3m below the surface. The receding groundwater levels are due to evapotranspiration andlateral outflow towards the riparian forest. The groundwater levels are low in the forestedareas due to the large tree evapotranspiration. Once the flood arrives, the infiltration rates onthe floodplain are high, approximately 1.5 m/day (Obakeng & Gieske 1997) which results inrapid changes of the groundwater levels.

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2:2 Organic Matter and Infiltration

Although the amount of organic matter usually is minor in comparison to other components ofthe soil, it is still crucial for the physical and chemical properties of the mineral soil. Theorganic matter stabilises the soil structure and increases the infiltration, drainage and the soil’scapacity to retain the water. Also during the mineralisation of organic matter, important plantnutrients such as nitrogen, carbon and phosphorous are released to the soil. The humus, thereworked and decomposed organic matter, is colloidal and has a large specific surface area(i.e. surface area divided by mass) and surface charge. The capacity of humus to hold waterand nutrient ions exceeds that of clay. Large amounts of humus therefore retain the soil openand loose (Koorevaar et al. 1983).

A soil’s infiltration capacity varies from soil to soil and is usually sensitive to the conditionsnear the surface and also to the previous water state. Therefore the infiltration varies uponfactors such as the particle size distribution of the soil, the porosity and organic matter etc(Landon 1991). When the soil is initially dry, the infiltration capacity is high and the moistureis drawn into the soil due to capillary forces. However theses forces diminish with increasingcontent of soil-moisture and the infiltration capacity drops. Also as the moisture contentincreases the colloidal particles in the soil will swell. Eventually the infiltration rate reaches amore or less constant value. Conditions that promote a high infiltration rate are coarse soils,low soil moisture, well-vegetated land and a layer of topsoil made porous by burrowinganimals (Fetter 1994).

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3 THE STUDY AREA

The floodplain system, on which the studies took place, is situated within the Moremi GameReserve on the southern fringes of Chief’s Island. The study area is located in the vicinity ofthe Boro River (23° 10’ E: 19° 32’ S) and the size of the studied floodplain is about onesquare kilometre (figure 5).

Figure 5. The location of the seasonally flooded floodplain subjected to this study.

The study area is situated at approximately 950 meters above sealevel (m a.s.l.). TheOkavango River flows southeast through Namibia’s Caprivi Strip before it reaches Botswana.Between two parallel faults the river creates the floodplain. This unique fresh-water system isa refuge for a multitude of animals. Here live the last great unharassed herds of elephants,Cape buffalo, zebras and antelopes. The Okavango Delta is also an important breeding groundfor many threatened bird species (Lee & Lanting 1990).

The floodplain, which is seasonally flooded with a peak during August, is a closed catchmentwhereby it is suitable for the study of water balances. Satellite images show that the floodingevents differ considerably from one year to another. The floodplain subjected to this studywas completely inundated during the flooding in August 1991. However, the floods of 1997and 1998 were minor in comparison to previous years (Obakeng & Gieske 1997).

The floodplain, which is surrounded by riverine woodland, exhibits a typical, characteristiczonation of the vegetation (figure 6). The well-marked zonation reflects the extent andduration of flooding.

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Areas flooded for the longest time are characterised by sedge - high, dense grass of thespecies Cyperus articulatus and Scirpus corymbosus. This zone is inundated forapproximately five months or more and this part of the floodplain is hardly used by thewildlife in regard of grazing. Areas flooded for shorter periods are typically dominated byshort grass (Pancium repens, Eragrostis inamoena). This zone is annually flooded during twoto three months and Red Lechwe (Kobus leche) and Impala (Aepyceros melampus melampus)heavily graze the grasses. In the fringes of the floodplain tall grasses (Vetiveria nigritana,Cymbopogon excavatus) are growing. This part is rarely flooded and the grasses are barelygrazed. Woody species for example palm trees (P. reclinata) and different species of thegenus Acacia are as numerous as the other species found in areas that never are flooded(Ramberg 1997).

Figure 6. The different vegetation zones and its characteristic species, on a seasonally flooded floodplain, the Okavango Delta, Botswana.

In the deepest parts of the floodplain there are five open pools, which are connected by achannel that is maintained by the tramping of hippopotamus. Usually they are dry duringthree to five months, but occasionally the rain will refill them. At the peak of flooding theopen pool have a maximum depth of about 200 cm, the zone of Cyperus has a depth of 40-100 cm while water depth over the grassland is up to 40 cm (Ramberg 1997).

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4 METHODS

The fieldwork was performed during December 1998 – February 1999 with the assistance ofstaff from the Okavango Research Centre, Maun, Botswana. Laboratory work was carried outat the Okavango Research Centre and also at the Department of Physical Geography,Göteborg University, Sweden.

During the fieldwork there was no surface water present on the studied floodplain.

4.1 Topographic and vegetation mapping

The topography of the floodplain was mapped using a theodolite and a lath. Themeasurements started out from a reference point; a ground water measuring station (P1).From this reference point the theodolite was aimed towards the lath, placed at the next groundwater measuring station. A peg was then placed 50 meters from P1 and a reference line for agrid system of 50×50 m squares was organised whereby the topography was measured atevery 50th meter across the floodplain.

The topographic map was then made in a raster based GIS-programme, Idrisi. The grid valueswere digitised onto a field map and an interpolation program in Idrisi recalculated the valuesbetween the pegs. Also a contour map with 0.5-m equidistant was made manually.

The different zones of vegetation were easily distinguished over most of the floodplain at thetime for this study. The distribution of the species was well marked and easily identified bythe differentiation of species. By field observation three major zones of natural vegetationwere recognised. Vegetation zone one was dominated by high, dense grass. Short grasses,with a weft of fhanerogams dominated zone two. Broad-leaved, tall grasses characterisedzone three. These three zones worked as a base for the soil survey and their boundaries wasmarked on a map. The map of the floodplain was made by dividing it into a grid pattern of50×50 m, where the distribution of each vegetation zone was marked.

4.2 Soil survey and soil analysis

A number of 60 sampling- and measurement sites were randomly placed on the floodplain(figure 7); 21 in vegetation zone 1, 19 in vegetation zone 2, 15 in vegetation zone 3 and onefrom each of the major five pools. The random sampling procedure was composed through adraw of the given grid numbers.

For each sampling site, infiltration measurements were made (see further section 4.3) and soilsamples were collected from a depth of 5-10 cm. The soil samples were subjected to analysisof organic matter and particle size distribution. Also three cross-sections were made acrossthe floodplain in order to illustrate the relation between the soil composition and theinfiltration rate in the different vegetation zones.

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4.2.1 Organic matter

The amount of organic matter in the soilsis determined by combustion. A smallamount of oven-dry soil (40-60 grams) isweighed and put in a crucible. The sampleis then ignited at a temperature of 900°Cfor 20 minutes, whereby there is a loss ofweight due to the complete oxidation ofthe organic matter (Koorevaar et al.1983). The sample is weighed again andthe amount of organic matter, inpercentage, is obtained from the weightloss (formula 1).

(1) O(%) = 100-(O.S (g) / U.S (g)) × 100

O = Organic matter O.S = Oxidized Sample U.S = Unoxidized Sample

4.2.2 Particle size distribution

The soil samples were transferred to thelaboratory in Maun, where the soils weredried in an oven at 125º C for 24 hours.The particle size distribution was analysedthrough mechanical sieving (> 0.074 mm)according to Swedish standard, (i.e. sievesize distribution 0.074, 0.125, 0.250,0.500, 1, 2, 4, 8, and 16 mm). Hydrometeranalyses were used to determine the finerfractions i.e. < 0.074 mm. The pre-treatment for the finer fractions was madethrough wet-sieving to separate the sandfractions down to 0.063 mm. Thesuspension was then dried at 125º C for24 hours and weighed. To disperse thecolloidal particles, a solution of 40 gsodium hexa-metaphosphate in 5 litres ofdistilled water was added to the samples(Landon 1991). The particle sizedistribution is then plotted in a sizedistribution diagram, which separates thefractions from one another.

Figure 7. The measurement and sampling sites on the floodplain subjected to this study.

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Three dominant fractions were distinguished according to international standards (Landon1991); coarser sand (2.0 mm-0.02 mm), fine sand (0.2-0.02 mm), and silt (0.02-0.002 mm).The particle size of 0.2 mm represents the limit of available water-holding capacity. Thecoarse-textured sand retains a smaller proportion of available water volume than finer sand(i.e. coarser sand has a higher permeability). The limit of 0.02 is significant for the rootsystems, which easily penetrates a soil of fine sand but not the silt. The clay fractioncomprises particles smaller than 0.002 and this particle size exhibit suspension, i.e. Brown’smolecular movement (Landon 1991). The distribution by weight percentage was used forcomparable analyses.

4.3 Infiltration measurements and infiltration rate

The infiltration rate was measured at the 60 soil sampling sites on the floodplain. Formeasurements in the field a double ring cylinder infiltrometer was used. It consists of twosteel cylinders, a water bottle with a mm scale on the side and a constant head apparatus. Thediameter of the inner ring is seven cm and the outer is 12 cm, according to standardequipment at the Department of Earth Sciences, Göteborg University. The construction isaccording to Landon (1993) and was made by Börje Björkvald.

The cylinders were hammered 5 cm into the ground and levelled. Each ring was filled withwater up to about 15 cm above the ground surface (Landon 1991). The water bottle was theninverted above the inner cylinder, which then was filled with water. The amount of water inthe outer ring was kept constant. The rate of water loss from the water bottle was measured inorder to estimate the infiltration rate of the soil. The volume entering the soil for each wateroutput from the bottle is constant (approximately 22 mm=22 ml) and the time for each wateroutput was noted.

In order to measure the infiltration rate at as many sites as possible the rate of infiltration wasonly measured for 30 minutes at each study site. Also, there were natural restrictions forlonger measurements due to deficiency of water during the study. Therefore, water had to betransported long distances for the measurements. Consequently the measurements wererestricted to only half an hour. However, two longer measurements (3 h) in each zone werecarried out to obtain a general view of the infiltration rate. These measurements indicatedminor differences between the steady state value and the value obtained after 30 minutes.Another consequence of the lack of water was that the measuring sites could not be prewetted.

The accumulated volume of water entering the soil was converted to the infiltration rate(mm/min) and was plotted against elapsed time whereby a declining slope was obtained. Theaim of the measurements was to obtain a steady-state infiltration rate. This is achieved whenthe amount of infiltrated water is constant in time, i.e. when the infiltration curve(instantaneous infiltration against time) levels out. To estimate the infiltration rate at steadystate, the terminal infiltration rate (i.e. the infiltration rate obtained at the end of theexperiment), is used as an approximation of the steady state infiltration rate.

13

4.4 Sources of errors in the methods

The error in the accuracy of the teodolite is about 5-10% in the horizontal plane and the errorincreases with the distance. Also, due to irregularities on the floodplain the distance of 50 mwas not always accurate whereby the topographic map may have some errors.

The infiltration measurements should normally run until the steady state is reached (about 3 to5 hours), but due to lack of time and water this was not possible. Also, there should have beenat least three repeated measurements on each site. Crusts, cracks and other conditions mayhave affected the infiltration rate on some of the measurement sites.

Mistakes during preparations of the soil samples for the particle size analyses could havecaused errors in the continued measurements. There could also be some inaccuracy in the soilanalysis due to lack of equipment.

14

5 RESULTS

5:1 Topography

The floodplain is a comparatively flat alluvialplain with very small undulations. However, inthe fringes of the floodplain, termite mounds ofvarious heights and sizes do occur. Thetopography of the floodplain varies between946.00 meters above sea level (m a.s.l.) and948.80 m a.s.l., resulting in a maximum localrelief of only 2.80 m (figure 8). For a moredetailed map, see appendix 1. The floodplain hasa southern slope of approximately 12-15 cm per100 m. The lowest areas are within vegetationzone 1, i.e. the central part of the river channeland in some parts of zone 2. The northern branchand the northwestern parts of the floodplain aremore elevated resulting in a less frequentinundation.

Figure 8. Topographic map of the studied floodplain. Alsoshown are the different vegetation zones on the floodplain(for explanation, see figure 9). The thick black linesrepresent the roads.

N

0 50 100 150 200m

15

5:2 Vegetation zones

On the studied floodplain there was a well-marked zonation of the vegetation cover. Themargin between zone 1 and 2 was especiallywell marked. The vegetation in zone 1 wassparse, especially in the northern parts of theflood plain (figure 9.). This could be due tothe fact that the area has not been inundatedfor several years.

The spatial distribution of vegetation on thefloodplain was dominated by vegetation zonetwo.

Descriptions of the cross-sections, marked asprofile 1, 2 and 3 in figure 9, are describedfurther in section 5.4.

Figure 9. The distribution of the different vegetationzones of the studied floodplain. The cross-sections arealso shown.

N

16

5:3 Infiltration rate

The geographical variation of the infiltration rate on the floodplain is shown in figure 10.

The infiltration rate on the studied floodplainranges from 0.22 mm/min to 5.78 mm/min. Theaverage infiltration rate for the entire populationis 2.26 mm/min. However, no infiltration ofwater occurred at the sampling sites situatedwithin pool areas. This was due to a hard,impermeable layer of clay.The infiltration rate varies a lot on the floodplain,although the infiltration rate is higher in zone 2and 3.

Figure 10. The geographical variation of the infiltrationrate on the studied floodplain, in mm/min.

N

0 50 100 150 200m

17

0.0

5.0

10.0

15.0

20.0

0 5 10 15 20 25

Elapsed time (min)

Infil

trat

ion

rate

(m

m/m

in)

Figure 11 represents the infiltration curves for the different vegetation zones. The differencewas that the infiltration rate was higher in the beginning for zone 3. However, at the end ofthe measurements the infiltration rate is within the same range for the different vegetationzones. Only one diagram is shown just to display that the curve has reached steady state. Thecurve has the same characteristic shape as the infiltration curve for the measurements thatlasted for three hours.

Figure 11. This characteristic shape ofthe infiltration curve representing theinfiltration curves for all of thevegetation zones.

18

5:3.1 Infiltration rate, organic content and particle size distribution

The infiltration rate, organic content and particle size distribution of the soil within vegetationzone 1 is shown in table 1. The sampling sites and the results are categorised according toincreased infiltration rate.

Table 1. The infiltration rate, organic content and particle size distribution of soils within vegetationzone 1.

Vegetation zone 1Particle size distribution

Samplingsite

Infiltration rate(mm/min)

Organic content(%)

Coarsersand*(%)

Fine sand**(%)

Silt***(%)

50 0.33 3.00 31.0 59.0 4.053 0.47 6.22 36.0 45.0 6.057 0.66 4.52 50.0 40.0 4.058 1.06 7.92 50.0 40.0 4.024 1.08 6.99 21.0 62.0 9.014 1.23 2.75 28.0 63.0 4.039 1.58 6.02 51.0 40.0 1.060 1.58 1.82 - - -18 1.68 2.29 33.0 60.0 2.02 1.82 11.56 40.0 28.0 7.0

59 1.95 3.51 54.0 40.0 0.120 1.98 2.27 20.0 67.0 4.054 2.31 6.69 50.0 36.0 5.041 2.31 7.33 55.0 39.0 3.049 2.34 3.72 49.0 43.0 1.051 2.49 4.37 48.0 43.0 2.011 2.59 5.51 - - -48 2.99 5.86 48.0 39.0 6.026 3.17 10.32 41.0 44.0 2.047 4.03 3.33 38.0 51.0 4.07 4.16 8.16 50.0 44.0 1.0

Average 1.99 5.44 41.7 46.5 3.64Median 1.95 5.51 48.0 43.0 4.00

Min 0.33 1.82 20.0 28.0 0.1Max 4.16 11.56 55.0 67.0 9.0

* 2-0.2 mm n=21** 0.2-0.02 mm*** 0.02-0.002 mm

The infiltration rate within vegetation zone 1 range from 0.33 mm/min to 4.16 mm/min, withan average of 1.99 mm/min. There is a large variation in the organic content; the average forvegetation zone is 5.44. There are no signs of increased infiltration rate as the organic matterincreases. The amount of particles smaller than fine sand (<0.02mm) is, in general, less than10 percent. The pools are not included in the table due to the fact that it was not possible totake any soil samples due to compaction of the soil. The compaction also affected theinfiltration measurement, which did not give any results in the pool areas.

19

The infiltration rate, organic content and particle size distribution of the soil within vegetationzone 2 is shown in table 2.

Table 2. The infiltration rate, organic content and particle size distribution of soils within vegetation zone 2.


Samplingsite


Organic content(%)

Coarsersand*(%)

Fine sand**(%)

Silt***(%)

38 0.22 1.58 22.0 70.0 3.030 0.59 7.44 23.0 71.0 1.012 0.78 - - - -31 0.85 7.31 45.0 49.0 4.046 0.89 7.87 53.0 - -15 1.18 3.31 50.0 43.0 2.044 1.40 1.93 - - -27 1.62 3.48 19.0 72.0 2.021 1.87 13.7 - - -37 1.87 5.88 51.0 42.0 1.03 2.01 6.02 22.0 68.0 7.0

55 2.12 1.93 32.0 57.0 4.036 2.62 5.23 54.0 34.0 3.032 2.64 6.37 51.0 41.0 0.556 2.71 - 44.0 48.0 2.034 2.76 6.25 50.0 44.0 1.033 3.08 6.43 62.0 32.0 0.142 4.58 2.88 57.0 37.0 1.06 5.78 4.49 75.0 19.0 2.0

Average 2.08 6.04 44.9 47.9 2.2Median 1.87 6.02 50.0 44.0 2.0

Min 0.22 1.58 75.0 71.0 0.1Max 5.78 14.0 22.0 19.0 7.0

* 2-0.2 mm n=19** 0.2-0.02 mm*** 0.02-0.002 mm

The infiltration rate in vegetation zone 2 ranges from 0.22 mm/min to 5.78 mm/min, with anaverage of 2.08. In comparison to zone 1, zone 2 differs in that the highest infiltration valuescoincide in most cases with the high values of the coarser sand fractions and with the lowvalues for the silt fractions. The organic matter content does not correspond to the infiltrationrate and the same is noticed with the silt fraction.

The amount of grain fractions smaller than fine sand (<0.02 mm) is less than 10 percent andconsists of silt, clay and organic matter.

20

The infiltration rate, organic content and particle size distribution of the soil within vegetationzone 3 is shown in table 3.

Table 3. The infiltration rate, organic content and particle size distribution of soils within vegetation zone 3. A, B and C are samples site that are not marked on the map1.


Samplingsite


Organic content(%)

Coarser sand*(%)

Fine sand**(%)

Silt***(%)

45 0.71 7.24 25.0 66.0 3.54 1.24 3.22 57.0 32.0 10.0

23 1.31 2.34 20.0 74.0 1.019 1.33 14.0 28.0 64.0 2.052 1.60 6.60 36.0 57.0 2.040 1.86 2.42 65.0 26.0 1.529 2.42 12.4 47.0 42.0 2.013 2.43 5.47 33.0 - -28 2.57 5.02 48.0 43.0 2.543 2.66 2.05 52.0 39.0 3.535 2.92 1.82 54.0 37.0 3.022 2.94 2.34 58.5 29.0 0.19 3.22 1.43 50.0 45.0 0.1A 3.37 - - - -B 3.83 - - - -C 4.48 - - - -5 4.61 1.87 79.0 12.0 9.0

16 5.53 13.5 39.0 42.0 1.0Average 2.72 4.54 45.5 44.0 2.9Median 2.62 2.82 48.0 42.0 2.0

Min 0.71 1.43 19.0 12.0 0.1Max 5.53 13.5 79.0 74.0 10.0

* 2-0.2 mm n=18** 0.2-0.02 mm*** 0.02-0.002 mm

The infiltration rate in vegetation zone 3 ranges from 0.71 mm/min to 5.53 mm/min with anaverage of 2.72 mm/min. The organic content varies a lot, with a minimum of 1.4 weightpercent to a maximum of 13.5 weight percent.

The infiltration values do not coincide as well as in vegetation zone 2, with higher content ofcoarser sand fractions and lower content of silt fractions. The organic matter content does notcorrespond to the infiltration rate.

The amount of grain fractions smaller than fine sand (<0.02 mm) is less than 10 percent andconsists of silt, clay and organic matter.

1 The infiltration rate was measured during longer intervals (3 hours) in the beginning as a test. These are the sites that are named A, B and C, marked grey in the table. Unfortunately the exact position for these sampling sites are not marked.

21

The soil samples in the study area mainly consist of sand-fine sand (2.0 mm – 0.02 mm). Thematerial underlying/on the floodplain is well-sorted material, with a low content of finerfractions (<0.02) and clay (<0.002 mm).

The difference in particle size distribution within the different vegetation zones is not wellpronounced. However in vegetation zone 3 the amount of coarser sand (2 mm-0.2 mm) issomewhat larger than in the other zones. These areas also showed a lower content of fine sand(0.2-0.02 mm), and even finer particles (<0.002 mm). The highest amount of fine sandfraction could be found in zone 2, which also had the lowest amount of silt fractions (0.02-0.002 mm). The fraction silt was found to be more abundant in zone 1, which was expected.

The average infiltration rate ranges from 1.99 mm/min in vegetation zone 1 to 2.72 mm/minin vegetation zone 3 (table 4). This results in an infiltration rate of approximately 2-4 m perday.

Table 4. The average infiltration rate, organic content and particle size distribution of the soils on thestudied floodplain.


Organiccontent

(%)

Particle size distribution Coarser sand* Fine sand** Silt***

(%)Zone 1 1.99 5.44 41.7 46.5 3.6Zone 2 2.08 6.04 44.9 47.9 2.2Zone 3 2.72 4.54 45.5 44.0 2.9

* 2-0.2 mm ** 0.2-0.02 mm *** 0.02-0.002 mm

The infiltration rate is highest in vegetation zone 3 with an average value of 2.72 mm/min.However the difference compared to the other vegetation zones is not so very prominent. Thehighest infiltration rate corresponds with the highest sand fraction and the finest particledistributions coincide with the lowest infiltration rate.

The organic content ranges from 0.26 to 14.0 weight percent of the total content. Highestorganic content occurs in vegetation zone 2, where the organic content in average is 6.04weight percent. Vegetation zone 3 shows the lowest organic content with an average of 4.54weight percent. For the whole population the average is 5.18 weight percent.

22

No Zone Inf.mm/min

Org.(%)

Coarsesand(%)

Finesand(%)

Silt(%)

5 3 4.6 1.8 79 12 9.06 2 5.8 4.5 75 19 2.07 1 4.2 8.2 50 44 1.01 Pool 1.5 3.5 - - -2 1 2.0 11.6 40 28 7.03 2 2.0 6.0 22 68 7.04 3 3.0 3.2 57 32 10

946,0

946,5

947,0

947,5

948,0

948,5

Sampling site

m a

.s.l.

0,01,02,03,04,05,06,0

Infil

trat

ion

rate

mm

/min

Elevation Infiltration

56 55 54 53 51 50

No Zone Infmm/min

Org(%)

Coarsesand(%)

Finesand(%)

Silt(%)

56 2 2.7 1.9 44 48 2.055 2 2.1 7.8 32 57 4.054 1 2.3 6.7 50 36 4.053 1 0.5 6.2 36 45 6.051 1 2.4 4.4 48 43 2.050 1 0.3 3.0 31 59 4.0

5.4 Cross-sections

In figure 12-14, the results from the cross-sections are illustrated. For location on thefloodplain, see figure 9.

Figure 12. Cross-section 1, showing the topography, infiltration rate, organic matter and particle size distribution across the floodplain. For location of the cross-section see figure 10.

Cross-section 1 is located by the weir, near the inlet of the floodplain. The cross-sectionillustrates the lower elevation of the river channel. The fringes of the floodplain are situatedapproximately 100-150 cm higher in comparison to the central parts of the river channel. Theinfiltration rate is highest at sampling site 6 (zone 2) and site 5 (zone 3), probably due to arelatively high content of coarse sand. At sampling site 7 the infiltration rate is also relativelyhigh, although the content of coarse sand is lower. However, the content of organic matter atthis site is high which also may contribute to a high infiltration rate. Sampling site 2 (zone 1)and 3 (zone 2) show low infiltration rates, possibly due to the relatively low content of coarsesand. The low infiltration rate in the pool area is due to the very impermeable layer of claythat covered the pool area. This made it impossible to collect any soil sample.

The cross-section shown in figure 13 is situated in the northern parts of the study area, in thebranch of the floodplain.

Figure 13. Cross-section 2, longitudinal cross-section of the northern branch of the floodplain, showing the topography, infiltration rate, organic matter and particle size distribution. For location of the cross section, see figure 10

.

946,0

946,5

947,0

947,5

948,0

948,5

Sampling site

m a

.s.l.

0,01,02,03,04,05,06,0

Infil

trat

ion

rate

m

m/m

in


5 6 7 1 2 3 4

23

No Zone Inf.mm/min

Org(%)

Coarsesand(%)

Finesand(%)

Silt(%)

16 3 5.5 13.5 39 42 1.059 1 1.9 3.5 54 40 0.160 1 1.6 1.8 - - -39 1 1.6 6.0 51 40 1.037 2 1.9 5.2 54 34 3.034 2 2.8 1.4 50 44 3233 2 3.1 6.4 62 32 0.1

In comparison to cross-section 1, this area is more flat; the difference in elevation of only100-cm. Vegetation zone 3 is not represented in this cross-section. Note that this profile is alongitudinal cross-section. The infiltration rate is lowest at sampling site 53 and 50, situated invegetation zone 1. This is probably due to the tramping of animals since the sampling sites arelocated in the vicinity of pool areas.

Figure 14, showing cross-section 3, indicates that the local relief across the floodplain, fromsoutheast to northwest, is gently undulating.

Figure 14. Cross-section 3, showing topography, infiltration rate, organic matter and particle size distribution in a cross-section of the floodplain. For location of the cross-section, see figure 10.

This cross-section shows that the infiltration rate is lower in the central parts of the floodchannel and higher in the fringes of the floodplain. The high infiltration rate at site 16, in zone3, may be due to the high amount of organic matter. Other factors are animals burrowing inthe soil and more extensive rootsystems, creating a more loose soil structure, which facilitatesinfiltration. The maximum relief is approximately 120 cm.

946,0

946,5

947,0

947,5

948,0

948,5

Sampling site

m a

.s.l.

0,01,02,03,04,05,06,0

Infil

trat

ion

rate

mm

/min


16 59 60 39 37 34

24

5.5 Comparable analyses of infiltration rate vs. particle size

Figure 15 shows the correlation between particle size and the infiltration rate of sampling siteswithin the entire population.

The correlation between the infiltration rate and the particle size analysis for the entirepopulation are higher for the sand fraction than the silt fractions. The infiltration of water intothe soils is often determined by the particle size distribution. Water will penetrate more easilyinto soils with larger particles than into silt and clay soils. Limitations in permeability ofwater into the fine particled soils are due to the blocking of pores by finer particles.

Figure 15. Correlationsbetween the infiltrationrate and the content ofcoarse sand, fine sand siltrespectively. Results fromsampling sites of theentire population.

r = 0.520

2

4

6

8

0 20 40 60 80 100

Coarse sand content (%)

Infil

trat

ion

rate

(m

m/m

in)

r = - 0.55

0

2

4

6

8

0 20 40 60 80

Fine sand content (%)

Infil

trat

ion

rate

(m

m/m

in)

r = - 0.20

0

2

4

6

8

0 5 10 15

Silt content (%)

Infil

trat

ion

rate

(mm

/min

)

25

Figure 16 shows the correlation between particle size and the infiltration rate of sampling siteswithin vegetation zone 1.

The results of the correlation between the particle size analysis and the infiltration rate arevague in zone 1. However, compared with the other zones the highest correlation is found inthe silt fraction. This could depend on the particle size distribution which gives a higherpercentage by weight of the finer fraction in zone 1 compared to the whole population. Zone 1gives the poorest correlation of the three zones, which may be due to animals tramping whichcompacts the soil. There is a positive correlation for the coarser sand fraction but a negativecorrelation for the fine sand and silt fractions.

Figure 16. Correlationsbetween the infiltrationrate and the content ofcoarse sand, fine sand andsilt respectively. Resultsfrom sampling sites withinvegetation zone 1.

r = 0.30

012345

0 20 40 60


Infil

trat

ion

rate

(m

m/m

in)

r = - 0.20

012345

0 20 40 60 80


Infil

trat

ion

rate

(m

m/m

in)

r =- 0.33

012345

0 2 4 6 8 10

Silt content (%)

Infil

trat

ion

rate

(m

m/m

in)

26

Figure 17 show the correlation between particle size and the infiltration rate of sampling siteswithin vegetation zone 2.

The correlation between the infiltration rate and the different particle sizes are highest for thesampling sites in vegetation zone 2. The values obtained in this zone reveal the importance ofthe particle size distribution. The permeability is higher for the sand fraction, where water willmore easily infiltrate the soil than in a soil with a higher content of the silt fraction.


r = 0.69

0

2

4

6

8

0 20 40 60 80


Infil

trat

ion

rate

(m

m/m

in)

r = - 0.76

0

2

4

6

8

0 20 40 60 80Fine sand content (%)

Infil

trat

ion

rate

(m

m/m

in)

r = - 0.24

0

2

4

6

8

0 2 4 6 8

Silt content (%)

Infil

trat

ion

rate

(m

m/m

in)

27

Figure 18 shows the correlation between particle size and the infiltration rate of sampling siteswithin vegetation zone 3.

Compared with zone two the highest infiltration is again dominated by the sand fraction inzone three. The correlation is not as strong as in zone two, which could be due to the denservegetation cover in this zone. The dense vegetation gives a good protection for the smalleranimal, which burrows in the ground. This in combination with a more extensive root systemenhances the infiltration rate.


r= 0.460123456

0 20 40 60 80 100


Infil

trat

ion

rate

(m

m/m

in)

r = 0.57

0123456

0 20 40 60 80


Infil

trat

ion

rate

(m

m/m

in)

r = 0.00

0123456

0 5 10 15

Silt content (%)

Infil

trat

ion

rate

(m

m/m

in)

28

6 DISCUSSION

There are few field data on the hydrogeology of the Okavango Delta and to be able to modelthe water balance of the river systems, it is crucial to understand the dynamic interactionsbetween the surface water inflow and the groundwater. This study of the infiltration rate hasfocused on the recharge of the groundwater and how the rate varies on a seasonally floodedfloodplain. The highest infiltration rate was assumed to be in the fringe area.

The topography of the floodplain is relatively flat with a maximum relief of 2.80 m. Thelowest areas are situated at 946.0 m a.s.l. and are found within vegetation zone 1, i.e. thecentral parts of the floodplain. The study site lies within the seasonal swamps and is elevatedin comparison to its surroundings. This is particularly true for the northwestern parts of thefloodplain. The consequence of the elevation is that these areas are only inundated duringmore extreme floods.

The fact that the Okavango Delta is an alluvial fan makes it more vulnerable to anywithdrawal of water. The low relief makes a certain volume of water to spread laterally over awider area. If this volume of water decreases, large areas will not be flooded, which will alterthe infiltration of groundwater.

The floodplain is underlain by well-sorted material. Most often the top 5-10 cm of the soilprofiles were greyish in colour, which probably is due to the presence of organic matter. Themajority of the sampling sites consist of coarse-fine grained sand. In dry condition the sandlacks cohesion, although it sometimes was slightly cemented in the upper parts of the soilprofile. This may contribute to the variation of the infiltration rate. Crusts of silica maycontribute to a lower infiltration rate. There is also a low content of fine particles and clay(less than 10%) in the samples, which improves the water retention of the soil (McCarthy etal. 1998). This in turn will decrease the infiltration rate.

The determing factors for the infiltration rate is the particle size distribution, i.e. the amountof coarse sand and silt. It was expected that the organic matter would influence the infiltrationrate. However, there are no significant signs of increased infiltration rate as the organic matterincreases. This can be due to that most of the finer particles consist of organic matter(McCarthy 1993). Finer particles obstruct the waters way in the soil by their high affinity foraggregation or by blocking the pores. For a more accurate infiltration rate result, the soil at themeasuring site should have been prewetted. At the initial state the organic matter and clayparticles absorbs water and may then obstruct the water to percolate through the pores.However, the results from this study represent the initial state of the flooding.

Fractions smaller than 0.002 mm, i.e. clay particles, were analysed through hydrometeranalysis, but due to loss of soil samples and errors in the equipment these results are notcomplete and therefore not considered reliable.

The infiltration rate was found to be highest in vegetation zone 3, the fringe area of thefloodplain, which can be due to the higher content of sand fraction. Another factor is thevegetation cover, which within this zone was more dense and higher, partly due to lessfrequent flooding. In addition to this, burrowing animals and larger root systems contribute toloosen the soil structure, which enhance the infiltration. However, the organic content washigher in vegetation zone 2, which can be explained by a higher re-circulation of thevegetation cover. Fast growing short-lived grasses characterise this vegetation zone and

29

mammals heavily graze the grasses. The grazing is a positive disturbance for the vegetation,which generates a faster turnover of organic matter and nutrients. The flood is a contributor ofnutrients, including organic matter, which feeds zone 2 and 3 during its regression.The lowest infiltration rate is within the central parts of the floodplain, i.e. vegetation zone 1,where the soils contain the highest amount of silt. Another important factor to the lowinfiltration rate within vegetation zone 1 is the tramping of animals, in particularhippopotamus, that during the wet spells wander between the pools, which are located withinthis zone. Other animals such as elephants, buffalo and wildebeests also tramp frequently inthe proximity of the pool areas in search for water. The tramping compacts the soil and hencereduces the infiltration rate. Field observations of the pool areas, where no infiltration tookplace, revealed a high content of clay. Due to the clay content and the tramping of animals ahard impermeable crust covered the pool areas, which prevented soil sampling.

The results from this study indicate that the infiltration rate range from 2.9 m/ day in thecentral parts of the floodplain to 3.9 m/ day in the fringe area. These results can be comparedto the results of Obakeng & Gieske (1997) who calculated a mean infiltration rate of 1.5m/day. However, other methods were used and the study does not tell how the rate variesacross the floodplain.

In this study, the measurements for the infiltration did not take place until steady state wasattained. The results are therefore somewhat overestimated, since the infiltration in its initialstages is higher and then levels out to a constant rate. When this constant rate is reacheddepend on soil type as well as the conditions of the soil at the time for measurements. Theinitial moisture condition is crucial and in this study this factor was not included or measured.

Correlation analyses show that there is a positive correlation between the amount of coarsersand and the infiltration rate, although it is weak. The opposite can be seen for the fractionsfine sand and silt, where a larger amount of these fractions results in a lower infiltration rate.These results apply for all vegetation zones. The correlation between the infiltration rate andthe particle size analysis for the entire population gives a poor estimation over the situation onthe floodplain. The comparable analyses should therefore only be used as broad indicators ofthe field situation. The result shows that there are other major factors that may have a stronginfluence on the infiltration rates on the flood plain. These factors are animals digging, rootpenetration, salt crust formations and compaction of the different soil layers.

The cross-sections show that the infiltration is indeed higher in the fringe areas. An outtake ofwater or a reduced flooding will have devastating consequences on the refill of groundwater.This due to the fact that the vegetation zone 3, i.e. the fringe areas, will not be inundated.

30

7 CONCLUSIONS

The major factor that controls the infiltration rate is the particle size distribution. There is apositive correlation between the infiltration rate and the content of coarse sand while theopposite is observed for silt.

Other important factors enhancing the infiltration rate are extensive rootsystems andburrowing animals. These factors are the most prominent in the fringe area. Animalsfrequently visit the central pool areas in search for water and their tramping in combinationwith a high content of finer particles inhibit the infiltration.

The result from this study indicate that the infiltration rate is higher in the fringe area(vegetation zone 3) of a seasonally flooded floodplain.

Due to the low relief the fringe area will first be affected as a consequence of less flooding.This might result in a reduced recharge of the ground water within the area. In a wider contextthis will affect the vegetation and the wildlife and the ecosystem as whole.

31

8 FUTURE WITHDRAWAL OF WATER?

The Okavango Delta, with its large quantities of water, has for a very long time attractedinterest for exploitation of the water resources. This interest is due to the fact that the Delta isone of very few water resources in this semi-arid environment.

At the moment the Jao-Boro River System is the main supplier of water to the ThamalakaneRiver and also to the city of Maun. However, this has only been the case since the 1950s. Thedistribution of water is changing and the time scale is relatively short, whereby it is difficultto predict where the main supply will be in the future. Several large channels have failed inthe history of the Delta due to sedimentation and/or tectonic activity or a combination of thetwo (McCarthy et al. 1993). Therefore, to aid the local people, the government made attemptsto restore the water flow. Also there have been plans to exploit the water resources fornational and international benefits. Some plans would have been very destructive if they hadbeen set to work. During the 1960-1970-ties the UN made different investigations of waterutilisation, which were made due to the discovery of diamonds in the region and the lowerparts of the Delta were dredged (Ellery & McCarthy 1994).

The important discovery of local groundwater resources to supply Maun and the diamondmining industry resulted in reduced urgency in the need of water withdrawal from theOkavango Delta. However, population growth has resulted in further investigations in how toexplore the water resources of the area again. In 1992 the World Conservation Union made anindependent evaluation of the issue, whereby the proposals were neutralised.

Ellery & McCarthy (1994) claim that a withdrawal of one to two percent of the total inflowwill have a negligible effect when considering the Okavango Delta as a whole system. Thisconsidered in a context where the system has natural variations of the climate. During the lastfew tens of thousand of years the rainfall has fluctuated a lot in the region and still the systempersists. Also there are short-term variations (decades) of the climate. During the period 1950-1976 the inflow has varied by a factor two. In response to these fluctuations the systemchanges the area of permanent and seasonal flooding. However, an outtake of water will causea permanent loss of water from the system, thereby causing more serious consequences incomparison to natural fluctuations. Large-scale anthropogenic influences, such as dredgingand withdrawal of water, might have devastating effects. Therefore, more knowledge isrequired about the hydrology of the area.

People living in the Okavango Delta and in the area around Maun (50,000- 70,000 people)depend on the seasonal flooding for their farming and for the recharge of the ground water.The area has suffered from water shortage since the early 1990s (Ramberg 1997b) and theneed of water in the area continue to increase. Also, years of drought in central Namibia havecontributed to serious plans to construct a pipeline from the Okavango River to provide theregion around Windhoek with water during years of water shortage. It has been suggested that90-million m3 water should be withdrawn per year. This volume corresponds to less than onepercent of the mean annual inflow but the effects can still be devastating for the ecosystem(Ellery & McCarthy 1994).

Any withdrawal of water will result in a loss of flooded area in the order of 1:3 (Ramberg1997b). The fringe areas will most likely be affected first and a loss of these areas will affectthe recharge of ground water. Also the abundant wildlife in the area will be suffer from lessseasonally flooded areas since these areas are the most productive in the Delta.

32

9 REFERENCES

Dinçer, T., Child, S. & Khupe, B., 1987. A simple mathematical model of a complexhydrological system - Okavango swamp, Botswana, Journal of Hydrology 93, pp41-65

Ellery, W. N. & McCarthy, T. S., 1994, Principles for the sustainable utilization of theOkavango Delta ecosystem, Botswana. Biological Conservation 70, pp 159-168.

Fetter, C.W., 1994. Applied Hydrogeology, third edition. USA, 691 p.

Hutchins, D. G., Hutton, S. M. and Jones, C. R., 1976. The Geology of the OkavangoDelta. In: Proceedings of the Symposium on the Okavango Delta and its futureutilisation, pp. 13-20. Botswana Society Gaborone

Koorevaar, P., Menelik, G., Dirksen, C., 1983, Elements of soil physics – developments insoil science 13, The Netherlands, 228 p.

Landon, J.R., 1991. Booker Tropical Soil Manual; A handbook for soil survey and agricultural landevaluation in the tropics and the subtropics. Hong Kong, 474 p.

Lee, B. D., & Lanting, F., 1990. Okavango Delta: Old Africa’s Last Refuge. NationalGeographic 178:6, pp 38-69.

McCarthy, T.S. and Metcalfe, J., 1990. Chemical sedimentation in the semi-aridenvironment of the Okavango Delta, Botswana. Chemical Geology, 89 pp 157-178.

McCarthy, T. S., McIver, J. R. and Verhagen, B. Th., 1991. Groundwater evolution,chemical sedimentation and carbonate brine formation on an island in the OkavangoDelta swamp, Botswana. Applied Geochemistry 6, pp. 577-595

McCarthy, T.S. 1993a. The great inland of Africa. Journal of African Earth Sciences 17, pp 275-291.

McCarthy, T. S., Green, R. W. and Franey, N. J., 1993b. The influence of neo-tectonicson water dispersal in the north-eastern regions of the Okavango Swamps,Botswana. Journal of African Earth Sciences 17, pp 23-32.

McCarthy, T. S., Ellery, W. N., and McCarthy, T. S., 1994. Principles for thesustainable utilization of the Okavango Delta, Botswana. BiologicalConservation 70, pp.159-168.

McCarthy, T.S. & Bloem. A. 1998. Observation on the hydrology and geohydrology of the Okavango Delta. South African Journal of Geology, 101(2) pp. 101-107.

McCarthy, T. S., Ellery, W. N. And Dangerfield, J. M., 1998. The role of biota in the initiation and growth of islands on the floodplain of the Okavango Alluvial Fan, Botswana. Earth Surface Processes and Landforms 23, pp. 291-316.

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Obakeng, O. and Gieske, A., 1997. Hydraulic conductivity and transmissivity of awater-table aquifer in the Boro River system, Okavango Delta. Department ofGeological Survey, Lobatse, Botswana.

Ramberg, L., 1997a. Ecology of Floodplains in the Okavango Delta - Presentation of aResearch Programme. Okavango Research Centre, University of Botswana.

Ramberg, L. 1997b. A Pipeline from the Okavango River? Ambio 26, p.129.

Stanistreet, I.G. & McCarthy, T.S. 1993. The Okavango Fan and the classification of subaerial fan systems. Sedimentary Geology, 85 pp. 115-133.

Watson, J. P., 1991. A Visual Interpretation of a Landsat Mosaic of the Okavango Delta and Surrounding Area. Remote Sensing Environment 35,pp. 1-9.

Wilson, B. H. and Dincer, T., 1976. An Introduction to the Hydrography of the OkavangoDelta. In: Proceedings of the Symposium on the Okavango Delta and its futureutilisation, pp. 33-47. Botswana Society Gaborone.

34

Appendix

The topographic data from the flood plain represented in squares of 50 meter. See map for locations.

Peg No. m a.sl. Peg No. m a.sl. Peg No. m a.sl. Peg No. m a.sl.U.1 948.28 K.56 947.41 F.111 946.80 P.166 946.41U.2 948.25 K.57 F.112 946.72 P.167 947.62U.3 947.97 K.58 947.52 F.113 946.74 Ö.168 947.15U.4 947.42 K.59 947.23 F.114 947.05 Ö.169 946.82U.5 948.80 K.60 947.93 F.115 947.20 Ö.170 946.60I.6 948.13 J.61 947.60 F.116 947.88 Ö.171 946.81I.7 947.99 J.62 947.38 E.117 947.88 Ö.172 946.00I.8 947.43 J.63 947.52 E.118 947.23 Ö.173 947.25I.9 948.13 J.64 947.43 E.119 947.25 Ä.174 947.68

S.10 948.38 J.65 947.27 E.120 946.70 Ä.175 946.79S.11 947.52 J.66 947.18 E.121 946.57 Ä.176 946.94S.12 948.08 J.67 947.63 E.122 947.19 Ä.177 946.35R.13 947.98 J.68 948.01 E.123 Ä.178 946.81R.14 947.90 J.69 947.82 E.124 946.05 Ä.179 947.09Q.15 948.24 J.70 948.44 E.125 946.82 Ä.180 947.80Q.16 947.52 J.71 947.99 E.126 947.26 Å.181 947.24Q.17 948.16 J.72 947.51 E.127 947.60 Å.182 946.41P.18 947.94 J.73 947.13 D.128 947.87 Å.183 946.46P.19 947.18 J.74 948.24 D.129 947.56 Å.184 946.61P.20 948.55 I.75 947.62 D.130 947.25 Å.185 946.72O.21 948.19 I.76 947.52 D.131 947.20 Å.186 947.04O.22 947.31 I.77 947.60 D.132 946.85 Z.187 947.57O.23 947.81 I.78 947.60 D.133 946.23 Z.188 946.88N.24 947.27 I.79 948.26 D.134 947.30 Z.189 946.16N.25 947.84 I.80 947.51 D.135 Z.190 946.33N.26 948.20 I.81 947.25 D.136 947.58 Z.191 946.41N.27 947.92 I.82 947.09 D.137 946.56 Z.192 946.85N.28 948.06 I.83 947.33 C.138 947.43 Y.193 947.55N.29 947.66 I.84 947.28 C.139 946.62 Y.194 946.49N.30 948.16 I.85 946.67 C.140 947.10 Y.195 946.36M.31 947.97 I.86 947.69 C.141 946.86 Y.196 946.29M.32 947.03 H.87 948.23 C.142 946.38 Y.197 946.74M.33 947.19 H.88 947.75 C.143 946.80 Y.198 947.36M.34 947.60 H.89 947.87 C.144 947.59 X.199 947.60M.35 947.34 H.90 947.79 B.145 947.50 X.200 946.29M.36 947.83 H.91 947.16 B.146 946.88 X.201 946.28M.37 948.14 H.92 946.64 B.147 946.78 X.202 946.46M.38 947.56 H.93 947.32 B.148 946.67 X.203 946.56M.39 948.15 H.94 947.20 B.149 947.02 W.204 947.44L.40 947.58 H.95 947.08 B.150 946.33 W.205 946.14L.41 947.21 G.96 947.78 B.151 946.92 W.206 946.32L.42 947.20 G.97 947.68 B.152 947.08 W.207 947.00L.43 947.06 G.98 947.55 A.153 947.02 W.208 947.51L.44 946.99 G.99 947.34 A.154 946.70 V.209 947.52L.45 947.05 G.100 946.93 A.155 946.67 V.210 946.60L.46 947.37 G.101 947.07 A.156 946.52 V.211 946.34L.47 947.75 G.102 947.39 A.157 946.72 V.212 947.27L.48 947.39 G.103 946.97 A.158 946.55 V.213 947.36L.49 947.74 G.104 947.19 A.159 947.10 U.214 946.26K.50 947.65 G.105 947.12 A.160 947.72 U.215 946.49K.51 947.38 F.106 947.83 P.161 947.64 U.216 946.76K.52 947.37 F.107 947.52 P.162 946.66 U.217 947.61K.53 947.31 F.108 947.28 P.163 946.63 U.218 947.42K.54 947.21 F.109 946.76 P.164 946.54 Threshold 946.13K.55 947.11 F.110 946.91 P.165 946.90

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