Estimation of the preliminary groundwater reserve using numerical models
Abstract
English: The Water Act requires the determination of the Groundwater Reserve to secure the
basic human needs and to protect the environment (aquatic ecosystems in terms of the
Act). In other words, it is necessary to determine the allocatable groundwater
resources and its use may not cause unacceptable negative impacts. As numerical
models have been greatly developed in assessing groundwater resources in recent
years, it is used to estimate the Groundwater Reserve.
An investigation into existing groundwater numerical models has been done in order
to select the suitable numerical model for determining the Groundwater Reserve. The
MODFLOW program is selected for the present work because of the widespread of
applications and the existence of user-friendly graphical interfaces, e.g. Chiang and
Kinzelbach (in press).
The procedure of estimating the allocatable groundwater resources can generally be
divided into two steps:
The first step is to estimate the total accessible groundwater resources by means of the
regional groundwater balance. Groundwater fluxes, recharge, abstraction, and leakage
to/from surface water bodies are balanced over the whole catchment. The
methodology is used for the Pienaars River catchment, where the groundwater
recharge is the only inflow term to the aquifer and its determination will directly
affect the result. The interaction (water exchange) between groundwater and surface
water (rivers and wetland) cannot be quantified without additional data, especially
river flow rate and evaporation measurements in proper positions. To date, most
available computer models are unable to compute the dynamic interaction between
river stages and the groundwater levels. As a result, in most MODFLOW (or other
groundwater models) applications, the river stages are often assumed to be known.
This is, however, not applicable in many cases, where the change of the surface water
profiles (river stages) due to groundwater or surface water abstractions must be
predicted. A computer program which is able to couple the groundwater and surface
water flow models, will be useful for computing and predicting both the groundwater
and surface water levels and their interactions.
The second step is to estimate possible negative environmental impacts due to
groundwater abstractions. The estimated impacts are scales for decision-makers to
give allowances to future abstractions. There are many possibilities to perform the
second step, for example post-auditing or by using numerical models. The former is
often not appreciated, because most groundwater systems are difficult or impossible
to restore once they are damaged or contaminated. The latter is superior due to its
predictive capability and its flexibility at the price of the data requirement.
Because of the complexity of the saltwater intrusion processes, the program
SEAWATER is developed to solve the steady-state salt-/freshwater sharp interface for
using with MODFLOW. SEAWATER calculates the interface by means of an
iterative process, which uses the Ghyben-Herzberg approximation to determine the
location of the interface. The results of the program are compared with several
analytical solutions. Afrikaans: Die Water Wet vereis die bepaling van die Grondwater Reserwe om te verseker dat
aan basiese menslike behoeftes en die bewaring van die omgewing (akwatiese
ekosisteme m.b.t. die Wet) voldoen word. Die verbruikbare grondwaterbron, sonder
onaanvaarbare negatiewe gevolge, moet bepaal word. Numeriese modelle, wat die
afgelope tyd baie verbeter is en baie gebruik word in ondersoeke van grondwater
bronne, is gebruik vir die beraming van die Grondwater Reserwe.
'n Ondersoek na bestaande numeriese grondwatermodelle is gedoen om 'n geskikte
numeriese model te gebruik vir die bepaling van die Grondwater Reserwe.
MODFLOW is gekies vir die studie omrede dit 'n wye toepassingsveld het en daar
gebruikers-vriendelike grafiese koppelvlakke beskikbaar is, soos byvoorbeeld die van
Chiang en Kinzelbach (in druk).
Die metode vir die beraming van die verbruikbare grondwaterbron kan in twee stappe
verdeel word.
Die eerste stap is om die totale beskikbare grondwaterbron te beraam d.m.v. 'n streek
grondwaterbalans. Grondwatervloei, aanvulling, onttrekking en suipelling van en na
oppervlakwater word gebalanseer oor die hele opvanggebied. Die metode is gebruik
vir die Pienaarsrivieropvanggebied, waar grondwateraanvulling die enigste invloei
term na die akwifeer is. Die berekening van aanvulling sal dus die resultaat direk
beinvloed. Die wisselwerking (water uitruiling) tussen grondwater en oppervlakwater
(riviere en vleie) kan nie gekwantifiseer word sonder addisionele data, veral
riviervloeitempos en verdamping by geskikte meetstasie liggings nie. Tot op hede
kan die meeste rekenaarmodelle nie die dinamiese interaksie tussen riviervlakke en
grondwatervlakke bereken nie. As gevolg hiervan word riviervlakke as bekend
aanvaar in die meeste MODFLOW (of ander grondwater modelle) toepassings. Dit is
in baie gevalle nie toepaslik waar 'n verandering in oppervlakwater (riviervlakke)
a.g.v. die onttrekking van grondwater en oppervlakwater voorspel moet word nie. 'n
Rekenaarprogam wat grondwater en oppervlakwater vloeirnodelle koppel sal handig
te pas kom in die berekening en voorspelling van beide grondwater en oppervlak
watervlakke en die wisselwerking tussen die twee.
Die tweede stap IS om die moontlik negatiewe omgewingsimpak a.g.v.
grondwateronttrekkings te beraam. Die geskatte impak kan gebruikword as maatstaf
om toestemming te gee vir toekomstige onttrekkings. Daar is verskeie moontlikhede
om die skatting te doen, bv. deur nabetragting of die gebruik van numeriese modelle.
Eersgenoemde is nadelig in die sm dat dit in die meeste gevalle moeilik of
ontrnoontlik is om beskadigde of besoedelde grondwaterstelsels te herstel.
Laasgenoemde word verkies omdat dit 'n voorspellingseienskap het en buigsaam is
t.o.v. kostes rakende data.
A.g.v. die kompleksiteit van die soutwater indringingproses is die SEAWATER
program ontwikkelom saam met MODFLOW die rustoestand skerp koppelvlak
tussen sout-en varswater te bereken. SEAWATER bereken die koppelvlak deur
middel van 'n herhalingsproses, wat gebruik maak van die Ghyben-Herzberg
benadering om die ligging van die koppelvlak te bepaal. Die resultaat van die
program word vergelyk met verskeie analitiese oplossings.