Week 4
beginning Octobe r 19th
Section 4
Natural and Artificial Recharge
1.4. Introduction
It is necessary to have data on rainfall and evaporation in order to estimate
recharge to an aquifer. An accurate method of assessing rainfall is to draw lines
between each of an array of rain gauges. Then bisect each line with a
perpendicular. Each perpendicular is then extended to form a series of polygons
(Figure 1.4.1). It is assumed that the rainfall for the area defined by each polygon
is equal to that of the rain gauge at its centre. This method is considerably more
accurate than a simple arithmetic mean of all rain gauges in an area.
Figure 1.4.1.Theissen polygons for assessing rainfall over a particular area.
It is necessary to obtain measurements of potential evaporation, that is, the
maximum that could occur rather than actual evaporation. These data of rainfall
and potential evaporation measurements make possible an initial calculation of
annual recharge of an aquifer. The difference between rainfall amounts and
evaporation provides the total potential recharge amount. The potential recharge
value (in mm) is then multiplied by the area of exposed aquifer (in m2). Potential
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recharge will probably percolate into the outcrop areas but only a proportion of
that potential recharge will actually soak through to the aquifer itself, that is,
below the water table in a phreatic aquifer. Consideration of the lithology and
structure of the aquifer will assist in determining what % of the potential recharge
actually recharges the aquifer.
1.4.1. Water Balance
Groundwater recharge to an aquifer cannot be measured directly. It may be
inferred from measurements taken from different parts of the hydrologic cycle. A
water balance assessment is made on the assumption that all water entering an
area is equal to the water leaving that area, plus or minus any change in storage.
This may be summarised in the following equation (Figure 1.4.2):
Inflows
Rainfall + recharge
from surface water +
sea-water intrusion +
inflow from other
aquifers + leakage +
artificial recharge
Outflows
=
Abstraction + spring
flow + base flow
+discharge to the sea
+ flows to other
aquifers +
evapotranspiration
+
-
Change in
Aquifer storage
Figure 1.4.2, Components of the water balance equation (after Brassington, 1990).
Potential rainfall recharge is the total rainfall, minus the losses from
evapotranspiration. The remaining water will either percolate into the ground or
run off to surface water drainage systems. Evapotranspiration is the most difficult
part of the hydrologic cycle to quantify. Penman (1948) carried out an experiment
in England, on the relationship between evaporation losses from open water and
those from bare soil and grassland. The results are shown in Figure 1.4.3.
Type of Surface
Open water
Bare soil
Grassland (winter)
Grassland (summer)
Grassland (autumn/spring)
Grassland (whole year)
% evaporation
100
90
60
80
70
75
Figure 1.4.3. Results of the Penman (1948) experiment.
Penman developed a more detailed approach in 1950, using the concept of soil
moisture deficit. This method allows for the fact that vegetation continues to
transpire water from soil during periods without rain, which creates the ‘deficit’
that takes up part of the next fall of rain, thereby reducing recharge. Grindley
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(1969,1970) developed further Penman’s methods and these form the basis of
modern methods.
Measurements of evaporation losses may be calculated from evaporation tanks.
The two types of evaporation tank in general use are the British Standard
evaporation tank (BSET) and the US class A evaporation pan (US/AEP). These
are illustrated in Figure 1.4.4.
Figure 1.4.4. Two types of evaporation tank in common use; (a) The BSET is
1.83m square and 1.61m deep, is made of galvanized iron, and is set into the
ground with a100 mm rim above ground level. (b) US/AEP is circular with a
diameter of 1.21 m and is 225 mm deep. It is set 150mm above the ground on a
wooden platform so that air can circulate round it. In both cases the tanks are
filled with water, which is kept topped up. The water level in (b) is kept about
50mm below the rim, while the water level in (a) is not allowed to fall lower than
100 mm below the rim. Both types suffer from excessive losses, especially in hot
climates where the water is likely to overheat.
source: Brassington (1990)
Evaporation losses are calculated by carefully measuring the water level in the
tank at the same time each day, and allowing for any water added to the tank to
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Figure 1.4.5. Monthly rainfall and evaporation in Jericho.
Source: Isaac & Qumsieh (1997)
Figure 1.4.6. Monthly rainfall and evaporation in Hebron.
Source: Isaac & Qumsieh (1997)
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keep it topped up and also for any rainfall. Evaporation losses are expressed as
rainfall equivalents, i.e. a depth in mm.
Evaporation data for the West Bank is published in Isaac & Qumsieh (1997:9394). Figures 1.4.5 and 1.4.6 graphically illustrate data for Jericho and Hebron.
Summer evaporation rates are 215.1 mm/month in Hebron and 284 mm/month in
Jericho. There is generally no rainfall at all during these months. Humid sea
breezes in Gaza result in a lower rate of evaporation than in the West Bank. In the
winter months the rate of evaporation for Hebron is 69.4 mm/month and for
Jericho 70.9 mm/month.
In the Hebron Mountains, the principal recharge area for the Eastern Basin of the
Mountain Aquifer, total average annual evaporation is 1606.1 mm, that is, more
than twice the average rainfall. In the Jordan Valley, where average annual
rainfall may be only 100 mm to 200 mm, the annual evaporation rate is 2121 mm,
while in the Dead Sea area, the average annual rate of evaporation is 2600 mm
and rainfall is often much less than 100 mm/year. Once water seeps below ground
level it may transpire through plants in the root layer, but is protected from
evaporation once it infiltrates below this layer.
1.4.2. Artificial Recharge
In some aquifers natural recharge is artificially supplemented by water being
recharged through special lagoons or boreholes. This component of recharge may
be easily quantified from direct readings.
1.4.2.1 The Auja System recharging well in Jordan Valley farm (a case study)
The Jordan Valley Aquifer has been severely depleted for at least the last twentyfive years. It would seem that this aquifer could prove useful as a reservoir for
Jordan Valley farmers. Figure 1.4.7 is a cross-sectional diagram showing the
relationship between the Eastern basin of the Mountain Aquifer and the Jordan
Valley along the line of the Auja Spring and the Kutub Farm. Surplus spring
water discharged from the Auja surface water systems are used to recharge
artificially the Jordan Valley aquifer by well injection. Not only is this a means of
water conservation, but also is a way to upgrade this aquifer. That part of the
Mountain Aquifer which recharges Ein Auja is an area of about 45 km2 to the
north and west of Ein Samia, which receives between 400 and 800 mm/year
rainfall and the area between Samia and Auja, about 12 km2 , which receives an
annual rainfall between 200 and 400 mm. This represents a recharge of about
3
3
17Mm /year. Natural discharge is about 13Mm .
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Figure 1.4.7. Section through Auja Spring and Kutub Farm.
Over-pumping of the Jordan Valley Aquifer has resulted in a drop of 16m in the
water table (Awartani, 1992). If allowed its natural flow from the Sea of Galilee,
the River Jordan would act as an influent stream, recharging this aquifer.
However, the lake is the principal fresh-water reservoir for Israel and massive
amounts of water are removed from the lake to supply the National Water Carrier.
The conglomerates and limestones that make up much of the Quaternary to
Recent deposits of the Jordan Valley aquifer could provide a useful reservoir,
recharged artificially from spring-flow excess from the Mountain Aquifer. The
Kutub Farm illustrates how successful such a technique can be. Ein Auja is
situated on the flank of the Auja monocline, near the base of the Yatta Formation,
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and flows out from the stream gravels at a rate of about 13 Mm /year with a
salinity of 30mg/l chlorine (Begin, 1975).
An area of about 45 km 2 to the north and west of the Samia spring , receiving an
annual rainfall of between 400 and 800 mm and an area of about 12 km2 between
the Samia and Auja springs, with an annual rainfall of between 200 and 400 mm
represent the effective recharge areas. Of this, a volume of about 17 Mm3 filters
down to the aquifer each year. Discharge emerges freely as the Ein Auja,
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(185750/151400) which discharges flows at a rate of about 0.55 m / sec.
Following a good rainy season. The spring is seasonal, diminishing considerably
during the summer and even in dry winters. The Israeli water company, Mekorot
has drilled a deep well, Auja 2 (18736/15115) into the Lower Cenomanian, some
200 m downstream of the Auja Spring. The total well depth is 615 m, reaching the
water table at 123.38 m. Just south of this well are Auja 3 (18695/15010) at a
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depth of 738 m, reaching the water table at 288.44 m, and Auja 4 (18674/14955)
at a depth of 651 m reaching the water table at 240.35 m. Pumpage from these
wells has created draw-down of between 40 and 100 m.: Palestinian farmers claim
that these wells deplete the discharge from the spring. The well water is piped to
Israeli settlements.
Palestinian farmers purchase the channeled water by the hour from the Auja
Spring. The Kutub farm, (1914/1490) hires the channel for 32 hours/week. At the
end of February, 1996, water was flowing in the concrete channel from the spring
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into the farm at a rate of 1,500 m /hour (WSERU Report, 1996). This was after a
February of very low rainfall. During a higher than average winter rainfall, a rate
3
of 2,000m /hour could be expected. The Auja Spring is seasonal so that by July,
3
in the middle of the dry season, the rate will have dropped to 300m /hour. The
water flows into the farm by way of a concrete channel from the spring and fills
two large ponds during the rainy season. The extensive plantation and market
garden produce receives drip irrigation from these ponds. The variable nature of
seasonal rainfall is a strong incentive to store excess water supplies underground.
The loss of 80% of the volume that falls as rain, mainly through evaporation, but
also through run-off, can be collected. Excess spring discharge may be used to
recharge an aquifer suitable for containing this water. Recharge would take place
in winter with a view to recovery when demand increases during the dry summer
season (Assaf, 1994).
The very high evaporation rates make surface storage uneconomic. The water
must be treated to a high quality before it is used to recharge the aquifer so that it
may be retrieved in good quality (Assaf, 1994).
The storage aquifer must allow recharge at an economic rate and movement of
water underground should be within calculable limits. Chemical changes will take
place, for example, salt will be dissolved, but within calculable limits (Assaf,
1994). Great care must be taken to create a bubble of fresh water surrounded by
the more saline water native to the Jordan Valley Aquifer. It will be difficult if
not impossible to recover stored water if the distance between recharge and
recovery exceeds the diameter of the bubble (Pyne, 1995). This is especially the
case in the Jordan Valley, where there is such high salinity in the native water.
Movement of the recharged water away from the well-head must be very
gradual. By the end of January, in an average rainy season, at the Kutub Farm,
any excess water is filtered, chlorinated and goes down the recharge well into the
recent deposits, Figure 1.4.8.
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Figure 1.4.8. Well-log of the recharge well at the Kutub Farm.
The water table of the well at the Kutub Farm, before recharge is at 72.50 m depth
and the water table rises to 66 m below ground level following recharge. The
3
discharge is about 90m /hr., producing a 0.9 m draw-down. After pumping, the
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water returns to its original level of 66.00 m below the surface. The chlorine
content of the water is 932 ppm. During the winter months when the Auja has a
high flow rate, filtered and chlorinated spring water is injected through the
columns and pump of this well by gravity into the recent deposits, Figure 1.4.6, at
3
a rate of 100 m /hour for thirty hours per week for about four weeks at the end of
3
January, i.e., about 12,000 m in total. (Assaf, 1991: 28). The low rainfall of the
winter of 1995-96 meant that there could be no recharge of the aquifer. The
3
recharge well would provide water at a rate of 90m /hour in September and
October, the end of the dry season, even with the lack of this year's artificial
3
recharge and at. the Gate well on the same farm produces about 40m /hour.
It is clear that a significant change takes place in this recharge well water
compared with that of the spring, the pond and with the water in the other
ordinary well on the farm. The dramatic rise in sodium and chloride in particular
can only be attributed to the aquifer environment. Recharging a well artificially
produces an inverted recharge cone as the water table is raised about 10m.
Residual evaporites in the newly saturated part of the aquifer would be dissolved
and thus increase the NaCl concentration of the water. The five samples all
plotted in the Ca-Mg area of the cation triangle and the Cl-HCO3 area of the
anion triangle in a Piper Diagram (see WSERU Report,1996).
1.4.2.2. Artificial recharge in the Gaza Coastal Aquifer.
Artificial recharge in the Coastal Aquifer serves four principal purposes:
1. restoration of groundwater to higher levels and thus counteracting further
sea-water intrusion;
2. balancing present and future withdrawals with the long-term average
annual natural and artificial recharge;
3. seasonal and long-term underground storage of winter surpluses available
from other sources;
4. local improvement of groundwater quality;
5. upgrading of low quality water by mixing with innate groundwater of
superior quality.
Due to the size of the available storage capacity of the coastal aquifer and the
distribution of wells, losses of recharged water through outflow to the sea can be
minimized by careful pumping designed to intercept these outflows.
Water could be made available for recharging the Gaza Coastal Aquifer from the
Western Basin of the Mountain Aquifer, from Lake Kinneret or from storm water
runoff. Recharge is carried out through wells and spreading grounds, mainly
during the rainy season when water demand for irrigation is minimal, surpluses
might be available and idle wells utilized as injection wells. Dual-purpose wells
would seem to be most appropriate. Recharge is injected normally at the pumping
yield rate which may be as high as 250m3/hr in the sand and sandstone parts of the
aquifer (Schwartz, 1990:29-30).
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