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SOUTHWEST FLORID IATER MANAGEMENT DISTRICT INTER-Of ICE MEMORANDUM
SDATE: h 17 >
TO: 8(1A4y /LA/A
pOCT 13 '19
OCT 13 1976
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September 17, 1976
TO: R. R. ATCHfTSON, Public Information Officer
FROM: E. 1 VERGARA, Director, Department of Interagency Coordination
RE: Rod Cherry's "Water Crop" paper
1. Please review Rod's paper on water crop with the thought in mind that we want to
publish it for the more-than-casually-interested but none-the-less layman reader.
You may rephrase as you feel necessary but take great care not to change the actual
data or gist of Rod's intentions.
2. Following completion of your review, have it retyped and submit to Bud Holschuh with a
copy of this memo and ask that he verify the accuracy and validity of the use of all
figures, facts and illustrations. He should do this with great care.
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4. Have all comments incorporated, retyped and send under a cover letter with a copy
of this memo to Rod Cherry asking him to review and revise as he see's fit and return
to us within one week if at all possible.
5. Upon return of the paper from Rod, please bring It in and let's go over it together.
Bring with you a suggested "Introduction" explaining that the paper was produced at the
District's request by Rodney Cherry while serving with Southwest Florida Water Management
District as Director of the District's Water Resources Division. (Provide the exact dates
of his tenure with us. At this time we'll also discuss the format and layout.)
6. Have 1000 copies printed (retain the master paste-ups) by the end of October.
cc: Donald R. Feaster.-/
L. M. Blain
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Water Crop is a concept which describes the amount of water that is
annually available in a given area for man's use. It is defined as the total
amount of precipitation less the amount of evapotranspiration (P-Et), or.as
the water yield of an area (Langbein and Iseri, 1960).
Water resource investigations in west-central Florida conducted from
1964-1966 indicate that the water crop for a 3500-square mile study area is
18 inches. In this particular area, water-inflow consisted entirely of
rainfall (57 inches total). Outflow or discharge from the area consisted of
runoff (17 inches); evapotranspiration (39 inches); and ground-water flow
(1-inch) (Cherry, Stewart and Mann, 1970).
For water management purposes, the use of a conservative estimate of an
area's water crop insures that adequate water will be available for Man's use
in the future as well as that annual groundwater withdrawals do not cause an
extensive reduction in water storage even during periods of prolonged drought.
An Illustration .
The water crop concept can be illustrated by a simple sketch (figure 1).
In the example, a bucket represents the aquifers and lakes within the "entire
District which can store water. This bucket receives 52 inches of precipitation
a year. Of this total amount, 39 inches are lost to evapotranspiration (which
is a combination of the processes of evaporation directly back into the
atmosphere and transpiration through plants to the atmosphere). The remaining
13 inches spill over the bucket's side. The water lost by this overspill re-
presents the amount that could be consumed each year without reducing the amount
of water stored in the bucket. It is, then, the bucket's water crop.
When working with the water crop concept, two important facts must be
noted. The actual amount of evapotranspiration varies with temperature and
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with the availability of water. But, because the rate of evapotranspiration
varies directly with rainfall, the water crop tends to remain constant.
Also, man's demand for water does not occur at the same time and rate as
our rainfall. Consequently, to capture the entire 13 inches of overspill from
the bucket, the water level in the bucket must be lowered. This wayistorage
areayin which to retain the 13-inch surplus till it's needed .+-pi~WidAed
Water Crop and the SWFWMD .
With these two facts in mind, it's easy to compare the surface area of
the bucket to the land area of the SWFWMD. In recent years, the average annual
rainfall over the entire District has been( inches. The average evapo-
transpiration rate is about 39 inches, and the average discharge from the area--
mostly through surface-water runoff--is approximately 13 inches. This 13
inches is the District Staff's conservative estimate of the water crop of the
entire SWFWMD, and was adopted by the District Governing Board as a guideline
for water management decisions on January 1, 1975. That 13 inches of rainfall
a year is equal to the amount of water available annually for man's use within
When Water Is Used .
If water is removed from the District "bucket" (that is, from ground-water
storage within the District), and used consumptively (lost to evapotranspir-
ation, dumped into the Gulf of Mexico after use, or otherwise made unavailable
for Man's re-use), then the amount of water discharged from the.area is reduced.
If the withdrawals exceed the water crop for the area, then discharge will cease
altogether and water will be removed from storage with consequent lowering of
Decreases in discharge and in storage are reflected in many ways. When
the amount of waterin storage is reduced, water levels in streams, lakes and
aquifers are lowered. A decrease in discharge is reflected by a reduction in
streamflow and ground-water outflow.
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In the SWFWMD, where the planned limit of the quantity of water to be
consumptively used is the water crop (which is equal to the amount naturally dis-
charged from the area), the amount of water in storage should not be allowed to
decline for long periods of time. Annual declines should occur only during per-
iods when the water crop is less than the 13 inches.
It is quite likely that major gains in the available-water supply will,
occur if the water crop is increased and if water is reused. The water crop can
most readily be increased if the area's amount of evapotranspiration is decreased.
However, reducing the evapotranspiration will no doubt result in changes in the
area's biota as well as in increases in its water crop. Reuse of water -- such
as for irrigation purposes -- has the net effect of increasing the recharge to the
As a tool for water management, the water crop concept considers only the
inflow or recharge to an area due to climatological processes. It does not con-
sider ground-or surface-water inflow or outflow. Under the concept, if water in
one area is allocated or consumed, there would be no outflow from that area to
another. If water is to be made available for consumptive use in all areas, then
only the water crop should be considered as available in each area and inflow from
one area to another must be excluded for allocation purposes.
APPLICATION OF THE WATER CROP CONCEPT
The Hydrologic System .
Successful application of the water crop concept as a management tool
requires a general understanding of the hydrologic system. This system conveys
all water from where it falls as rain to either the Gulf of Mexico or to the
atmosphere. All streams, lakes, springs, sinkholes and aquifers are part of It.
From wherever it falls as rain, water moves downgradient through the various
interconnected water-conveying components of the hydrologic system. The im-
portance of these components in the system is not always the same in all areas.
In one place, the principal conveying unit could be streams, while in another it
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may be aquifers; in a third, it could be a combination of the two. The water
may pass from stream to acquifer, or from aquifer to stream, or it may
evaporate into the atmosphere while enroute to the Gulf.
Whatever its route, water enters the conveying system as rainfall and is
temporarily stored in streams, lakes or aquifers while enroute to points of dis-
charge from the area. During periods of heavy rainfall, the rate of recharge (or
replenishment) to an area usually exceeds its rate of discharge and water levels
rise accordingly. When there is little or no rainfall, the discharge rate exceeds
the recharge rate, the volume of water in storage declines, and water levels fall
Within the District .
Within the SWFWMD, the quantity of water stored or in transit within the
hydrologic system varies greatly at any particular time. The characteristics
and importance of the water-conveying components within the District also vary
regionally. Optimum development and management of the water resource depends
to a large degree on an understanding of these variations and changes.
In the northern part of the District, the aquifer system consists of a
water-table aquifer (consisting primarily of sand and the artesian Floridan
Aquifer (made of limestone) (see figure 2). The two are commonly sedated
by clay units. In some places, however, the clay layers are thin, leaky or
altogether absent. In these areas, water is readily transmitted downward
from the water-table to the underlying Floridan Aquifer.
In the southern portion of the District the aquifer system lends to be
more complex. (see figure 3). Here the Floridan Aquifer consists of several
layers of limestone of varying thicknesses and water-transmitting character-
istics. The limestone-zones are separated by clay or other limestone units with
low water-transmitting ability resulting in minimal local recharge to the lower
zones. This minimal local recharge, combined with the tendency in this part of
the District to locate large water-withdrawal facilities in these deeper water-
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bearing zones, has caused widespread declines in water levels in the area.
Some Special Management Concerns. .
Although the water crop for a particular tract of land or area may indicate
that sufficient water is available for withdrawal, provisions must be made to
insure that the zone or acquifer from which water is to be withdrawn can be
recharged with available water. Problems may arise when the water crop -- or
even a lesser, but nevertheless substantial amount of water -- is withdrawn
from a water-conveying component which cannot replenish itself as rapidly as
the water is withdrawn.
Sound water management decisions are concerned not only with the avail-
ability of water in a particular area, but also with the method by which the
water is withdrawn. Consequently, management practices must either:
1. Artificially recharge those zones which cannot recharge naturally at
a sufficient rate due to the ideology of the area; or
2. Restrict ground-water withdrawals to zones or components which can
annually replenish or recharge themselves; or
3. Allow water levels to decline in zones from which large withdrawals
are made but which cannot recharge themselves annually.
Sound water management also requires that the withdrawal of water from an
aquifer must be limited to the water crop in the recharge area. This is neces-
sary to pre ent a reduction in storage or a decline in water levels. Fortunately,
most of the SWFWMD is in a recharge area. The water level of the shallow aquifer
is at a greater elevation than the potentiometric surface of the Floridan Aquifer
throughout most of the District, so recharge occurs in nearly all of the
District's area (although at lower rates in the southern portions than in the
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WATER CROP AS A REGULATORY TOOL
The water within the Floridan Aquifer arrived there primarily via downward
seepage from the shallow aquifer. Where the potentiometric surface of the
Floridan Aquifer is higher than the water level in the shallow aquifer, the
source of this water is some distance upgradient from the actual point of
recharge. Where the potentiometric surface is lower than the water table, re-
charge can take place locally. In either case, the rate of movement of water
from the shallow aquifer to the Floridan Aquifer (or vice versa) depends on
both the water-transmitting characteristics of the clay unit separating the two
and the differences in the elevations of the water level in the shallow aquifer
and the potentiometric surface of the Floridan Aquifer.
When water is withdrawn from an aquifer, the water level is lowered and
a cone of depression is formed in the potentiometric surface around the dis-
charging well. In response to the lowered level in this cone, a second, more
subtle cone of depression forms in the water table of the overlying shallow (see
figure 4). The difference in the elevations of the water level in the shallow
aquifer and the potentiometric surface of the Floridan Aquifer is greatest at
the point of withdrawal; other conditions being equal, leakage from the water-
table aquifer to the Floridan Aquifer is also greatest at the point of with-
drawal. These principals can be effectively applied to well-field regulation.
By monitoring the two elevations, the potentiometric surface can be con-
trolled via pumpage regulation so that the head difference between the two
aquifers (that is, the difference in the water pressures within them) will allow
no more than the volume of the water crop to move downward from the shallow to
the Floridan Aquifer. This in turn would prevent continuing declines in water
levels in the shallow aquifer from occurring outside the vicinity of the well
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In effect, this type of management plan allows the water level in the shal-
low aquifer to be lowered during dry periods to provide storage for the annual
water crop. At the same time, it assures that runoff will occur from all areas
outside the well field. This latter aspect is particularly important, as the
origin of all the water withdrawn from the field is not from just the portion of
the shallow aquifer that underlies the well field but from a much larger,
surrounding area. Nonetheless, the water crop is fully withdrawn from the
portion of the shallow aquifer within the well field's boundaries.
This principal is illustrated in Figure 5 which shows the estimated quan-
tities of water that will leak from the shallow aquifer to the Floridan Aquifer
from a well field withdrawing water from the Floridan Aquifer. The estimates A~E
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based oncalculation that assumes that the well field consists of one well with-
drawing 15,000,000 million gallons per day (mgd); that the transmissivity of
the Floridan Aquifer is 300,000 gallons per day per foot (gpd/ft); and that the
leakance coefficient of the confining layer which overlies the Floridan Aquifer
is .0015 gallons per day per square foot per foot (gpd/ft3). This figure shows
that over 60% of the water is derived from the shallow aquifer within about 4
miles of the well (20,000 feet). Larger quantities are derived from units nearer
the well than from those more distant.
In Other Areas..
In areas where only a water-table or unconfined aquifer exists, water
management regulations should limit water withdrawals to the water crop of the
area. Otherwise, reduction in storage accompanies water-level declines in the
area and mining of water occurs -- that is, more water is taken out of the sys-
tem than Nature puts in. If such practices were allowed to proceed unchecked,
the water supply would eventually be exhausted.
Limited Withdrawals. ..
Even though the calculated water crop of a given area may indicate that
the amount of water requested by an applicant is available for his use, it may
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be necessary to limit his withdrawal to a lesser amount because the method of
withdrawal may induce salt-water encroachment or may result in other damage to
In some coastal areas, for example, the salt-water/fresh-water balance is
so delicate that withdrawal of the water crop could cause the interface to move
landward and result in deterioration of the potable water supply. To avoid or
minimize this imbalance, a quantity of water less than the water crop should be
allocated to the user.
Utilization Of The Shallow Aquifer .
Large volumes of water are most often produced from the Floridan Aquifer
because of its high transmissivity. Yields of as much as 5,000 gallons per
minute (gpm) are not uncommon, and some wells yielding 7,000 gpm are currently
in use. However, through the use of networks of sandpoint wells, large volumes
of water can also be developed from the shallow aquifer.
Tests have shown that the permeability of the sands of the shallow aquifer
is about the same as the permeability of the Floridan Aquifer. Water from the
shallow aquifer can readily be used-fPem lawn irrigation, and, in some areas,
for domestic supply. Large withdrawals would probably necessitate the use of
multiple-well manifold systems. For example, where one Floridan Aquifer well
might yield 5,000 gpm, it would require 200 sand-point wells drawing 25 gpm
to produce the same quantity. Although this might seem prohibitively ex-
pensive the cost of such wells is relatively cheap, and in certain instances
might compete with a single larger well into the Floridan.
Recharge Facilities .
In areas where the water crop is sufficient to meet demands, management
may require certain practices to insure its most effective development. For
instance, if large withdrawals of water are to be made from deeper zones some
form of augmentation -- such as recharge wells -- may be required to increase
their naturally slow recharge capacity. However, even where such practices
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will supply the needed recharge water, the recharge must be closely monitored
insure that the aquifer is protected against contamination.
Planned Declines For A Limited Period .
Provided that adequate safeguards against salt-water encroachment and
damage to other users are assured, water levels may be allowed to decline for
certain periods. In large areas such as the Peace and Alafia Basins, water
level declines may be allowed for certain uses for fixed periods. For instance,
phosphate mines require large quantities of water for the life expectancy of the
mine (generally about 20 years). The withdrawals may be within the limits of
the water crop for the area, but economics dictate that water be withdrawn
from zones which cannot be readily or locally recharged. Based on aquifer
characteristics, withdrawal rates, and recharge potential, the rate of
water-level decline in the area can be projected. If the projections indicate
that no potential salt-water encroachment or damage to other users is likely,
the water-level declines in some areas may be permitted for specified periods
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