| Vol. 31 (2002) | |||
- Hydrogeology of the Kalahari in north-eastern Namibia
with special emphasis on groundwater recharge, flow modelling and hydrochemistry
Heike Klock (2003), 194 p., 109 Fig., 30 Tab.
volume out of print (copy available on request)
Abstract
Hydrogeology of the Kalahari in north-eastern Namibia
with special emphasis on groundwater recharge, flow modelling and hydrochemistry
Heike Klock
Abstract: This
study has focused on hydrogeological and hydrochemical settings of the
Northern Namibian Kalahari Catchment which is the Namibian part of the
Makgadikgadi-Kalahari-Catchment. Recharge has been the subject of process-understanding,
quantification and regionalisation. Within the semiarid study area a bimodal
surface constitution is prominent: hardrocks areas allow for fast infiltration
along karsts and joints, whereas areas covered by unconsolidated sediments
receive minor diffuse recharge and locally some preferred flow path recharge
develops along shrinkage cracks and rootlets. Five substratum classes
have been soil physically studied: Pans and vleis, brown to red soils,
dune sand, soil with an aeolian influence, and calcrete. Aeolian sands
are most promising for the development of direct diffuse recharge. Recharge
by preferred flow might occur in all soil classes either due to joints
in calcrete or structures and rootlets in soils. All soil classes contribute
to indirect recharge because even the dune sand allows, albeit very locally,
the generation of runoff. The occurrences of recharge through the unconsolidated
soil and the hardrocks have been confirmed by hydrograph interpretation
and by a study of hydrochemical data which identified groundwater of flood
water and flood water after soil passage composition. Other prominent
hydrochemical processes in the Kalahari are associated with the carbonate-equilibrium-system,
mixing with highly mineralised water that is either sulphate (central
area) or chloride dominated (fringe area) and development of sodium hydrogencarbonate
water types. The latter is mostly generated by feldspar weathering. Variations
of the hydrochemical compositions were observed for shallow groundwaters.
They do not only reflect the recharge amount but also the recharge conditions,
e.g. a wetter year is allowing more vegetation which increases the hydrogencarbonate
content. Inverse determination of recharge by the chloride mass balance
method gives recharge amounts between 0.2 and locally more than 100 mm/a.
The least favoured recharge conditions are found for Kalahari covered
areas, the largest amount occurs in the Otavi area. The distribution of
recharge areas within the catchment is rather complex and regionalisation
of recharge for the entire catchment was done by a forward approach using
satellite images and by an inverse approach using hydrochemical data.
From the inverse hydrochemical approach a basin-wide balanced recharge
amount of 1.39 mm/a is achieved. The forward approach gave a basin-wide
figure of 0.88 (minimum assumption) to 4.53 mm/a (maximum assumption).
A simplistic groundwater flow model confirmed the results from the minimum
recharge regionalisation by satellite images and the result from the hydrochemical
approach. Altogether a mutually verified basin-wide recharge figure of
ca. 1 mm/a turns out.
Paper language: english
?.- € Order