The semi-arid regions of southern Africa cover large portions of settled land where domestic and
agricultural activities depends on isolated groundwater systems replenished by irregularly occurring
rainfall events. Southern African rainfall patterns are regulated by the annual oscillation of
winter-summer weather systems and most of all, abrupt changes in regional atmospheric patterns,
which may result in either wet/dry cycles. Given the highly differential hydro-climatic conditions
and hydrogeological environment in semi-arid regions, effective groundwater recharge
events are episodic in nature and largely occur once in every five years.
Sustainable, medium-term management of local groundwater resources requires dynamic hydrological
information to ensure a healthy supply-demand balance; thus requiring dedicated hydrological
monitoring. High-level monitoring programmes on a few experimental sites have produced
localised hydrological data, which illustrate how erratic groundwater resources are replenished.
For many years, it was postulated that groundwater resources were recharged every
time the total annual rainfall peaks a certain threshold. This postulation may hold in humid regions,
but surely not elsewhere in the drier parts of South Africa.
Semi-arid regions portray a flattish regional landscape with occasionally elevated parent rock
windows and mountainous regions. Soil cover is restricted to low-relief areas, and lacks the
thick mature soils distinctive of the humid areas. Fractured hard rock windows with very little soil
cover represent potential groundwater recharge terrains, allowing recharge-producing surplus
rainfall to infiltrate directly into the underlying aquifer. The hydrogeological conditions of hard
rock terrains in the same-arid environment do vary in terms of the rock types and their response
on weathering processes. Nonetheless, an array of joints and fractures running from ground surface
into the SZ represents fast and effective pathways when episodic high rainfall events occur.
Mature soil/regolith profiles in plain areas enhances surface run-off and support local floods in
rivulets where riparian vegetation and open-water evaporation intercepts most of the available
bank storage and depression recharge.
Atmospheric moisture is, in principal, generated by warm, evaporating maritime waters, and is
therefore marked by its hydrochemical signature. This signature changed abruptly during its
continental migration, and finally manifests as cloud water. Winter rainwater specifically demonstrates
the impact of oceanic aerosols, hence characterized by a prominent NaCl composition.
Summer rainwater is a diluted version due to continental rainfall/evapotranspiration events, and
is transformed by anthropogenic airborne substances peaking during the late-winter months.
The hydrogeochemical composition of rainwater is therefore quite diverse, and needs logic monitoring to understand its seasonal cyclic oscillation. Short-term hyetograph observations report
episodic rainfall events, occurring mostly over the January-March period of wet hydrological
cycles. These are spaced over a period of 4 to 8 days, of which at least one rain event exceeds
~45 mm, associated with a rain-rate intensity of >1.5 mm·h-1.
Extraordinary depleted rainwater hydrochemistry and isotopic compositions are associated with
these rain weeks, which are significantly different from normal seasonal concentrations. Wet
Clâ concentrations during these high rainfall periods are almost an order of magnitude lower
than the average annual values.
Hyetograph-hydrograph sets confirm that extraordinary groundwater recharge occurs as the result
of episodic rainfall events. Hydrogeochemical profiling in the upper section of the unsaturated
zone verify the presence of different compositions which probably indicate different modes
of recharged rainwater percolation in fractured, hard rock terrains. Hard rock profile sections below
the rebound water table interface containing almost 50% less Clâ than country-wide background
values of ~40 mg·H-1.
Isotopic compositions in a typical rain week period report similar depleted concentrations and
resemble a prominent amount effect. Such depleted rainwater is merely linked to specific seasons,
for example the rainfalls of 2003-2004, 2005-2006 and 2007-2008 hydrological years can
be clustered as high rainwater input periods with notable lighter isotopic compositions; around -
7.5â° M18O, -41â° M2H. The fact that most of the two (2) meter vertical profiles reported relatively
negative isotopic compositions (-8â° M18O, -44â° M2H), indicates a high probability of preferential
recharge with pristine rainwater with even more negative isotopic composition.
Direct groundwater recharge estimations from local, short-term rainfall and groundwater rebound
stage hydrochemical data proposes a recharge value <2% in most flat lying, semi-arid
regions. Although fractured hard rock terrains are isolated, it allows in the order of 4%, where as
local mountainous areas are high at 14%; obviously enhanced by orographic rainfall development.
Recharge on dolomitic terrains are highly variable due to diverse ground surface conditions,
and may vary between 6% in flat lying plains to 13% in mountainous regions (Kuruman
Hills at Kuruman).
Groundwater recharge varies significantly spatially. The control by prominent soil/regolith cappings
is that high that establishing realistic recharge figures for a particular area, will require a
dedicated soil mapping programme to identify direct recharge terrains.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-03232011-084237 |
| Date | 23 March 2011 |
| Creators | van Wyk, Ettienne |
| Contributors | Dr PD Vermeulen, Prof GJ van Tonder |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-03232011-084237/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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