1887

oa South African Journal of Geology - Observations on the hydrology and geohydrology of the Okavango Delta

Volume 101 Number 2
  • ISSN : 1012-0750
  • E-ISSN: 1996-8590

 

Abstract

The Okavango Delta is situated within two major grabens, one containing the Panhandle and the other the Delta itself. The main graben is underlain by between 200 m and 300 m of largely unconsolidated medium to fine sand. The Delta is an alluvial fan, which is divided longitudinally into two by horsts which form Chief's Island and the Duba Island cluster. Permanent swamp, sustained by base flow of the Okavango River, is developed in the Panhandle, upper fan, and portion of the fan to the east and northeast of Chief's Island. Seasonal flood water enters the Panhandle via the Okavango River, but because of the permeable nature of channel margins and the elevated water surface in the channels, especially at the apex of the fan, it is rapidly leaked from the channels. Most of the seasonal flood water flows to the west of Chief's Island where the bulk of the seasonal swamps are located. The extent of annual inundation is variable, and is strongly influenced by the magnitude of flood discharge and local rainfall, and to a lesser extent by antecedent conditions and evapotranspiration. Seasonal flooding has created a recharge mound beneath the Delta, but analysis of ground water chemistry and isotopic characteristics indicates that the Delta is essentially hydrologically closed, with no ground water outflow, and only limited surface water outflow. There is no large-scale lateral flow of ground water and water movements in the system are essentially vertical. Recharge occurs through the flood plains, and water is lost to the atmosphere by evaporation and especially by transpiration, particularly from islands. Soluble salts are accumulating in the deep ground water. This study has emphasized the need for more data on the aerial extent of seasonal flooding, on the ground water recharge, and on the contributions of evaporation and transpiration to water loss.

Die Okavango Delta is tussen twee hoofgrabens gelee. Een bevat die Pypsteel en die ander die Delta self. Die hoofgraben word onderle deur tussen 200 en 300 m grootliks ongekonsolideerde medium tot fyn sand. Die Delta is 'n alluviale waaier wat longitudinaal in twee verdeel word deur horste wat Chief's Eiland en die Duba Eiland-tros vorm. Permanente boommoeras. onderhou deur basiese vloei van die Okavangorivier, is in die Pypsteel, die bo-waaier, en gedeelte van die waaier oos en noordoos van Chiefs Eiland, ontwikkel. Seisoenale vloedwater kom die Pypsteel binne via die Okavangorivier, maar as gevolg van die deurlatende aard van die kanaalwande en die opgehewe watervlak in die kanale, veral by die punt van die waaier, lek dit vinnig uit die kanale uit. Die meeste van die seisoenale vloedwater vloei na die weste van Chief's Eiland, waar die meerderheid van die seisoenale boommoerasse gelee is. Die omvang van jaarlikse oorstroming varieer, en word sterk be·invloed deur vloedafsetting en plaaslike reenval, en in ' n mindere mate deur voorafgaande gebeure en evapotranspirasie. Seisoenale oorstromings het 'n aanvullingsheuwel benede die Delta geskep, maar analise van grondwaterchemie en isotoopeienskappe dui daarop dat die Delta hoofsaaklik hidrologies gesluit is, met geen uitvloei van grondwater nie, en slegs beperkte uitvloei van oppervlak water. Daar is geen grootskaalse laterale vloei van grondwater nie, en die beweging van water in die stelsel is hoofsaaklik vertikaal. Aanvulling geskied deur die vloedvlaktes, en verlies van water met betrekking tot die atmosfeer geskied deur verdamping en veral deur transpirasie, veral vanaf die eilande. Oplosbare soute is besig om in die diep grondwater op te hoop. Hierdie studie het die behoefte aan meer data oor die verbreiding van seisoenale oorstromings, oor die grondwateraanvulling, en oor die bydrae van verdamping en transpirasie tot waterverlies, beklemtoon.

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/content/journal/10520/EJC-1abed244bb
1998-06-01
2020-09-19

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