Skip to main content

Growing islands and sinking solutes: processes maintaining the endorheic Okavango Delta as a freshwater system

Plant Ecology volume 196, pages 215–231 (2008)Cite this article

Abstract

In the Okavango Delta 98–99% of the water from inflow and rainfall is lost to the atmosphere through evapotranspiration. As a consequence 94% of inflowing solutes are retained within the Delta landscape. This process might be expected to result in an entirely saline environment, but that is not the case: the surface waters have very low salinity, supporting a typical freshwater biota. It has been deduced that the numerous islands in the Delta (about 150,000 within an area of 13,500 km2) have been formed through evapotransporative concentration in the groundwater, of infiltrating solutes, followed by precipitation and volume increase. Evidence of this is the large amount of calcrete in island soils. These islands of 3–10 m thickness with clayey soils are underlain by fine Kalahari sand to a depth of 200–300 m, which also indicates that they are formed through surface processes. The infiltration rate of surface water from floodplains and streams into islands is very high, and is predominantly a lateral process that is unidirectional. Evapotranspiration in the riparian woodland zone cause the ground-waters in the central area of islands—with halophyte grasslands—to have very high salinities. By use of chloride as a conservative element the concentration factor between central island groundwater and surface water is calculated to be 500–1,000. This groundwater is depleted of calcium and magnesium supporting the early deductions that these elements have precipitated as calcrete. There is also a large depletion of silicate and potassium that probably have precipitated as well forming the clayey soils typical of the islands. The central island groundwater is dominated by sodium, bicarbonate and dissolved organic matter. The gradual increase of salinity here causes a periodic let off of this water through a density-driven process to deeper layers. This process together with island growth through precipitation of solutes are the two major sink processes of inflowing solutes and explains why the Okavango Delta is at present a freshwater system. The whole island complex is calculated to be 100,000–400,000 years old while some intensely studied islands may be younger: 80,000–240,000 years. The discrepancy is explained by a biassed selection of islands currently in flooded areas with better growth conditions. The uniqueness of the Okavango Delta and ideas for future research are discussed.

This is a preview of subscription content, access via your institution.

Access options

Buy single article

Instant access to the full article PDF.

USD 39.95

Price excludes VAT (USA)
Tax calculation will be finalised during checkout.

Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6

References

  • Allen RG, Pereira LS, Raes D, Smith M (1998) Crop evapotranspiration. Guidelines for computing crop water requirements. FAO irrigation and drainage paper 56. FAO, Rome

    Google Scholar 

  • Bauer P (2004) Flooding and salt transport in the Okavango Delta, Botswana: key issues for sustainable wetland management. PhD dissertation, Swiss Federal Institute of Technology, Zurich

  • Bauer-Gottwein P, Langer T, Prommer H, Wolski P, Kinzelbach W (2007) Okavango Delta Islands: interaction between density-driven flow and geochemical reactions under evapo-concentration. J Hydrol 335:389–405

    Article  Google Scholar 

  • Bauer P, Thabeng G, Stauffer F, Kinzelbach W (2004) Estimation of the evapotranspiration rate from diurnal groundwater level fluctuations in the Okavango Delta, Botswana. J Hydrol 288:344–355

    Article  Google Scholar 

  • Conley DJ (2002) Terrestrial ecosystems and the global biogeochemical silica cycle. Global Biogeochem Cy 16:1121

    Article  CAS  Google Scholar 

  • Cronberg G, Gieske A, Martins E, Prince Nengu J, Stenstrom I-M (1996) Major ion chemistry, plankton, and bacterial assemblages of the Jao/Boro River, Okavango Delta, Botswana: the swamps and floodplains. Archive fur Hydrobiologie Supplement 107(3):335–407

    CAS  Google Scholar 

  • Eriksson E (1959) The yearly circulation of chloride and sulphur in nature; meteorological, geochemical and pedological implications. Part I. Tellus 11:375–403

    Article  CAS  Google Scholar 

  • Eriksson E (1960) The yearly circulation of chloride and sulphur in nature; meteorological, geochemical and pedological implications. Part II. Tellus 12:63–109

    Google Scholar 

  • Fernkvist P, Liden A (2003) Inorganic ions and nutrients in groundwater of Okavango Delta islands, Botswana. Report. Department of Water and Environmental Studies, Linkoping University, Sweden

    Google Scholar 

  • Gazal RM, Scott RL, Goodrich DC et al (2006) Controls on transpiration in a desert riparian cottonwood forest. J Agr Forest Meteorol 137:56–67

    Article  Google Scholar 

  • Gumbricht T, McCarthy TS, Merry CL (2001) The topography of the Okavango Delta, Botswana, and its tectonic and sedimentological implications. S Afr J Geol 104:243–264

    Article  Google Scholar 

  • Gumbricht T, McCarthy TS (2003) Spatial patterns of islands and salt crusts in the Okavango Delta, Botswana. S Afr Geogr J 85:164–169

    Google Scholar 

  • Gumbricht T, McCarthy J, McCarthy TS (2004) Channels, wetlands and islands in the Okavango Delta, Botswana, and their relation to hydrological and sedimentological processes. Earth Surf Proc Land 29:15–29

    Article  Google Scholar 

  • Gumbricht T, McCarthy TS, Bauer P (2005) The micro-topography of the wetlands of the Okavango Delta, Botswana. Earth Surf Proc Land 30(1):27–39

    Article  Google Scholar 

  • Hargreaves GH, Samani ZA (1982) Estimating potential evapo-transpiration. J Irr Drain Div—ASCE 108(3):225–230

    Google Scholar 

  • Hogberg P, Lindholm M, Ramberg L, Hessen DO (2002) Aquatic food web dynamics on a floodplain in the Okavango Delta, Botswana. Hydrobiologia 470:23–30

    Article  Google Scholar 

  • Langer T, Heusser D (2004) Geochemical groundwater evolution and age estimations for islands in the Okavango Delta. MSc dissertation, Swiss Federal Institute of Technology, Zurich

  • Liebau F, Condie KC, Siever R (1973) Silicon. In: Wedepohl KH (ed) Handbook of geochemistry. Springer Verlag, Heidelberg

    Google Scholar 

  • McCarthy TS, McIver JR, Cairncross B (1986) Carbonate accumulation on islands in the Okavango Delta, Botswana. S Afr J Sci 82:588–591

    CAS  Google Scholar 

  • McCarthy TS, Metcalfe J (1990) Chemical sedimentation in the Okavango Delta, Botswana. J Chem Geol 89:157–178

    Article  CAS  Google Scholar 

  • McCarthy TS, Ellery WN, Ellery K (1993) Vegetation-induced, subsurface precipitation of carbonate as an aggradational process in the permanent swamps of the Okavango (delta) fan, Botswana. Chem Geol 107:111–131

    Article  CAS  Google Scholar 

  • McCarthy TS, Ellery WN (1998) The Okavango Delta. T Roy Soc S Afr 53:157–182

    Google Scholar 

  • McCarthy TS, Bloem A, Larkin PA (1998) Observations on the hydrology and geohydrology of the Okavango Delta. S Afr J Geol 101(2):101–117

    Google Scholar 

  • Persuad N, Sakamoto CM, Joshua WD et al (1990) Coefficients of Penman’s formula for calculating evaporation and potential evaporation in Botswana. Department of Agricultural Research, Gaborone, Botswana

    Google Scholar 

  • Ramberg L Krah M, Wolski P et al (2000) Groundwater recharge from a seasonal floodplain in the Okavango Delta, Botswana, Paper presented at American Society of Limnology and Oceanography, Kopenhagen

  • Ramberg L, Wolski P, Krah M (2006) Infiltration loss from a seasonal floodplain—A major hydrological process in the Okavango Delta, Botswana. Wetlands 26(3):677–690

    Article  Google Scholar 

  • Reeves CV (1978) The gravity survey of Ngamiland 1970–71. Bulletin 11. Geological Survey Department, Gaborone, Botswana

    Google Scholar 

  • Ringrose S, Huntsman-Mapila P, Kampunzu AB et al (2005) Sedimentological and geochemical evidence for palaeo-environmental change in the Makgadikgadi subbasin, in relation to the MOZ rift depression, Botswana. Palaeogeogr Palaeoclimatol Palaeoecol 217:265–287

    Article  Google Scholar 

  • Stumm W, Morgan JJ (1970) Aquatic chemistry. An introduction emphasizing chemical Equilibria in natural waters. Wiley-Interscience, New York

    Google Scholar 

  • Tiercelin J, Lezzar KF (2002) A 300 million year history of rift lakes in Central and East Africa: an updated broad review. In: Odada EO, Olago DO (eds) The East African Great Lakes, Limnology, Palaeolimnology, and Biodiversity. Kluwer Academic Publishers, Dortrecht

    Google Scholar 

  • Water Resource Consultants (2003) Maun Groundwater Development Project, Project Review. Department of Water Affairs, Gaborone, Botswana

    Google Scholar 

  • Weast RC (1975–1976) Handbook of Chemistry and Physics, 56th edn. CRC Press, Cleveland

  • Wolski P, Savenije HHS (2003) Update of conceptual hydrological model of the Delta. Project report, ICA-4-CT-2001–10040, D2.1. Harry Oppenheimer Okavango Research Centre, Maun, Botswana

    Google Scholar 

  • Wolski P, Savenije HHG (2006) Dynamics of floodplain-island groundwater flow in the Okavango Delta, Botswana. J Hydrol 320:283–301

    Article  Google Scholar 

  • Wolski P, Savenije HHG, Murray-Hudson M, Gumbricht T (2006) Modelling of the flooding in the Okavango Delta, Botswana, using a hybrid reservoir-GIS model. J Hydrol 331:58–72

    Article  Google Scholar 

  • Wooding RA, Tyler SW, White I, Anderson PA (1997) Convection in groundwater below an evaporating salt lake: 2. Evolution of finger plumes. Water Resour Res 33(6):1219–1228

    Article  Google Scholar 

  • Zimmermann S, Bauer P, Held R et al (2006) Salt transport on islands in the Okavango Delta: numerical investigations. Adv Water Res 29:11–29

    Article  Google Scholar 

Download references

Acknowledgements

We are indebted to Professor Susan Ringrose for a review of an early version of this article, for stimulating discussions on island formation and growth, and in particular for the insights on silicate geochemistry that this lead to. The final version was critically read by Dr. Phillippa Huntsman-Mapila and Mr. Mike Murray-Hudson who both made many useful comments. The article has also benefited from inputs from Mr. Thoralf Meier, who helped with data processing.

Author information

Authors and Affiliations

  1. Harry Oppenheimer Okavango Research Centre, University of Botswana, P/Bag 285, Maun, Botswana

    Lars Ramberg & Piotr Wolski

Authors
  1. Lars Ramberg
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Piotr Wolski
    View author publications

    You can also search for this author in PubMed Google Scholar

Corresponding author

Correspondence to Lars Ramberg.

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Ramberg, L., Wolski, P. Growing islands and sinking solutes: processes maintaining the endorheic Okavango Delta as a freshwater system. Plant Ecol 196, 215–231 (2008). https://doi.org/10.1007/s11258-007-9346-1

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1007/s11258-007-9346-1

Keywords

  • Okavango
  • Wetland
  • Island growth
  • Density fingering
  • Calcrete
  • Saline groundwater