HU - Hydrogeology and Environment

Journal of the Dept. of Hydrogeology and Environment, University of Würzburg (ISSN 09309-3757)

Editorial office: BGI, Greisingstr. 8, 97074 Würzburg, GERMANY

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Vol. 17 (1999)



Abstract



Vol. 17: 

Grundwasserneubildung in der Übergangszone zwischen Festgesteinsrücken und Kalahari-Lockersedimentüberdeckung (Namibia).

[Groundwater Recharge in the Transition Zone between Solid Rock Ridges and the Cover of Loose Kalahari Sediments (Namibia)]

Holger Mainardy


Abstract:
The equation: PRECIPITATION = RUNOFF + EVAPORATION in semi aride climates may only be used for calculating the groundwater recharge if a clear definition of actual evaporation in contrast to the potential evaporation is possible. Whereas precipitation and potential evaporation can be measured more or less accurately, the evaluation of both runoff and actual evaporation in semi aride climates usually entails some difficulties. The usual method of calculating the groundwater recharge by measuring the runoff during dry weather periods is not possible in Namibia because of the lack of perennial rivers. A study area in central Namibia at the western side of the Kalahari between Grootfontein in the North East and Rehoboth in the South West has been the focus of this thesis. The area of concern contains the sub-catchments of the ephememeral rivers from North to South: Omuramba-Omatako, Eiseb, Nossob, Seeis, Olifants and Skaap. The biggest part of Western and Central Namibia is underlaid by solid rocks of precambrian to cretaceous age. The older precambrian to cambrian formations are either metamorphic of sedimentary origin which were folded and faulted during the Panafrican or older orogenesis, or syn- and post-tectonic intrusive bodies of different sizes. Espcecially in central Namibia there are also outcrops of permocarboniferous to cretaceous sedimentary rocks which are part of the Karoo-Sequence. Whereas in the west of the study area solid rocks are outcropping at or a few decimetres under the surface, the cover of unconsolidated eolian and fluviatile sediments of tertiary to quaternary age becomes thicker towards the east and reaches several hundred meters in the Kalahari-Basin outside the study area.

For the evaluation of the order of groundwater recharge in solid rocks concentration factors such as quotients of chloride contents of rain water and groundwater were computed. This was achieved by collecting several rain water samples from selected farms distributed all over the study area. The chloride content of the rain water samples was less than 2 mg/l while chloride content in groundwater samples was rarely higher than 100 mg/l. After estimating the runoff at the surface, rates of groundwater recharge up to 50 mm/a could be calculated at the Waterberg district. In most cases the groundwater recharge computed by this method is only a few mm/a. Similar results were obtained with the computer modeling programe MODBTG, a modification of MODBIL (UDLUFT, 1992 ff., non-published). This program uses the daily amount of rainfall, the slopes and the hydrophysical parameters at the weathering zone of solid rock. During the field studies non-disturbed and disturbed soil samples were taken and analyzed in the hydrophysical laboratory of the University of Würzburg for measuring the permeability of the weathered rock formations. Permeabilities were found to range between 10-4 and 10-7 m/s. The permeability of solid rocks was evaluated dimensionally from the fracture system measured in the study area and was found to be 5·10-4 m/s in very permeable parts of the Etjo-Sandstone at the Waterberg district and between 10-5 and 10-6 m/s in permeable sandstones, marbles and dolostones mostly in the northern part of the study area. Granites and micashists have got a permeability of less than 10-8 up to 10-7 m/s in some extremely fractured zones. These permeabilities are used for several simplified groundwater flow models based on the topographic and geologic situation in the study area. Groundwater flow systems in these models which showed a great similarity to the natural conditions could be developped with a groundwater rechcharge rate of about 30 mm/a in the permeable rocks of the Waterbergs district maximum. In other permeable rocks groundwater recharge rates of 15 mm/a maximum are possible. Areas favourable to groundwater recharge were calculated by the geographic information system (GIS) IDRISI (CLARK UNIVERSITY, 1997) based on digitalized topographic maps. Thereby zones in the study area with permeable rocks and slopes of less than 10° were focussed and sketched in general maps. Rocks of regional importance for groundwater recharge in the study area are the proterotoic marbles and dolostones and the sandstones of the Etjo-Formation in the Waterberg and Mt. Etjo district. Dykes and meta-sandstones however are only important for groundwater recharge at restricted locations in the study area.


Paper language: german; german abstract





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