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THE WATER CROP
A Water Management Tool
R. N. Cherry1
The water crop is a concept which portrays the amount of water annually
available for man's use and is defined as precipitation less evapotrans-
piration (P ET), or the water yield of an area (Langbein and Iseri,
Investigations of the water resources of west-central Florida during the
1964-66 period indicated that for a 3600-square mile area, the water crop
was 18 inches. The annual inflow to the area consisted entirely of
precipitation (57 inches); outflow, discharge from the area, consisted
of runoff (17 inchesI eva transpiration (39 inches) and ground-water
outflow (1 inch) (Cherry, Stunt and Mann, 1970).
WATER CROP RUNOFF + GROUND WATER DISCHARGE
PRECIPITATION EVAPOTRANSPIRATION RUNOFF + GROUND-WATER DISCHARGE
57 = 39 + 17 + 1
Using a conservative estim
purposes should insure tha
that allocated annual with
water storage even during
R. N. Cherry served as
Southwest Florida Water
ate of the water crop for water-management
t adequate water is available for future use and
drawals should not cause extensive reduction in
periods of prolonged drought.
the Director Water Resources Division of the
Management District dujit 1973-197S.p A4 I
J, [E111... i' i il l: ... ... .
N The water crop of the Southwest Florida Water Mangement District was
conservatively estimated by the District Staff to be about 13 inches,
and this estimate was adopted by the District Governing Board January
1, 1975. Actual evapotranspiration varies with temperature and with
the availability of water. Because the rate of evapotranspiration
varies directly with rainfall, the water crop tends to remain constant..
The water crop concept is illustrated by a simple sketch (figure 1) and
an explanation to show how the concept is similar to that which occurs
naturally. For example, a bucket receives 52 inches of precipitation
yearly, of which 39 inches is lost to evapotranspiration, and 13 inches
spills over the side. The water lost by. overspill represents the amount
of water that4c ld be consumed each year without lowering the water in
Sthe bucket. However, to capture the entire 13 inches, the water level in
the bucket must be lowered to provide storage. If the entire 13 inches
is consumed there would be no water spilling over the side of the bucket.
The land area of the Southwest Florida Water Management District can be
compared to the surface area of the bucket. The average rainfall over the
entire District in recent years has been 52 inches, the average evapo-
transpiration rate is about 39 inches, and the average discharge from the
area is approximately 13 inches. Removal of water for consumptive use
reduces both the water in storage and the discharge from the area. If the
withdrawals exceed the water crop then discharge wti ceas and wat r would
be removed from storage, resulting in lower water levels utroug the
I... ~~ ~
illustrating the Water Crop concept.
VI III Illi
I Whenwter UMdis frii-r gi i the irea is
not increased then her:
(1) the discharge from the area is decreased or
(2) water storage is reduced. 7 .
A decrease in discharge from the area is reflected by a reduction in
streamflow on ground water outflow. A decrease in storage is reflected
by lower water levels in stream; lakes and aquifers. Generally when
water is withdrawn from an area the decrease in storage is observed
first and .tfen followed by a reduction in streamflow and ground water
In the Southwest Florida Water Management District where the planned
limit of the quantity of water to be consumed is the water crop (which
is equivalent to the discharge from the area) then water storage should
not decline except for short periods. Annual declines should only
occur during those periods when the water crop is less than the 13 inches.
Major gains in the available water probably will occur by increasing
the water crop and by reuse of water. -he- ~ r warcter p ot
eid3-beLinaeresas by decreasing the evapotranspiration trom the
Increases in the water crop will occur as water levels decline as
a result of withdrawals of water from the aquifers. However, gains
in the water crop by reducing the evapotranspiration will no doubt
result in changes in the biota of the area. Reuse of water such as
for irrigation purposes has the net effect of increasing the recharge
to the ear-th.
i: L 1.1 ii
The water crop cU r cg.'MS -'- -'-to at area
due to climatolo(Nl processes. This differs a water budget in that
the water crop does not consider ground surface-water inflow or ground-
and surface-water outflow. If water in one area is allocated or consumed
then there would be no outflow to another area. 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 areaa4d- infloa fru i e- ar Lu 1nuLlie
must be excluded for allocation purposes.
Application of the Water Crop Concept
Successful application of the water crop as a management tool requires a
general understanding of the hydrologic system. In the Southwest Florida
Water Management District the quantity of water stored or in transit at a
particular time varies greatly. Optimum development and management of the
water resource depends to a large extent F understanding of
these changes and variations. A comprehension of the complex patterns of
water circulation from ocean to atmosphere, to land, and its return by
various routes to the Gulf of Mexico or to the atmosphere is essential
for proper water management.
The hydrologic system conveys all water from where it falls as rain either
to the Gulf of Mexico or to the atmosphere. All streams, lakes, springs,
sinks and aquifers are part of the water-conveying system. Water.moves
from where it falls as rain downgradient through the various interconnected
water-conveying components of the system. The principal conveying component
in the hydrologic system could be streams in one area and aquifers in
another, or a combination of both. The water may pass from stream to aquifer,
m naquifer to atrenm, or may evapotrannpire into the atmosphere while
onrouta to the Gulf.
Water enters the co ying system na rl~i s teporaly otored in
streams, lakes or quifere while enroute to poin f discharge from the
4 area. During periods of heavy rainfall, the rate of recharge to the area
usually exceeds the rate of discharge, and water levels rise accordingly.
When the discharge rate exceeds the recharge rate, the volume of water in
storage declines and water levels fall accordingly. The release of water
stored at high elevations sustains water levels downgradient.
The characteristics of the water-conveying components within the Southwest
Florida Water Management District vary regionally. Aquifers are the
principal conveying components of the system in the northern part of the
District, whereas streams tend to be the principal conveying components in
the southern part of the District.
The aquifer system (figure 2/) in the northern part of the District con-
sists of a water-table aquifer (sand) and the Floridan Aquifer (limestone).
The water-table aquifer commonly is separated from the Floridan Aquifer by
clay units; however, in places the clay layers are thin, leaky or absent
and in such areas water is readily transmitted downward from the
water table aquifer to the underlying Floridan Aquifer. Recharge to the
Floridnn Aquifer in the northern part of the District tends to be much
higher than in the southern part of the District. The aquifer system
in the southern part of the District tends to be more complex (figure 3).
In thin area, the limestones of the Floridan Aquifer consist of several
l lic~otonec of varying thickness and water-transmitting characteristics.
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. '- ',.. 7 Y$ = "
a1 a a1 a.
Fi g u 2. Sc e t c w i' .. C f. p r o t -i' -- '- .. . .
: -.-- -----:----:-:- -:-----:-:- -:--.-:----_ ---
. '. '...- T ..: ..:.--z .j i- ..= ; __ = I .Z _. ;.. __ ._; I ; i .. .
1 1 1 1 Y/-t l Ail l I I II I r
Figure 2--Schematic showing aquifers in the orthenpart of the Oistrict.
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......L~T;' ... .. -- ----- ------ '0
., . ..........'
:iI I ,-;I eu ~l w If f .4 1.
c: I I L-C ......... ...51
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_p..n' I' D It
lrr711 ~ I (fro%'
o m!--~~:~~~S-. -b
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C n the,* pau~rn~pirt of the Dis: tri ct d. t1-iV C P
Management decisions are not only concerned with the availability of water
in an area, but) the method by which the water is withdrawn.
Management practices must either:
1. Provide a means by which water can be made available to replenish
the zones in which the water levels are lowered, or
2. Restrict withdrawal to zones or components that can annually
replenish or recharge themselves, or
3. Allow the water level to decline in this zone.
iliii i i i iiiii ,i
characteriatice r/ recharge to the lower limef)ne is minimal. The
tendency to locate large water withdrawal facilities in these deeper water-
transmitting zones has caused widespread declines in water levels, probably
because of the minimal recharge to these zones.
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 aquifer from which water is withdrawn can be
recharged with available water. Problems may arise when the water crop or a
substantial quantity of water is withdrawn from a water-conveying component
that is unable to replenish itself as rapidly as the water is withdrawn.
*the rechnrre area to Fent a reduction in storage o0 decline in water
level. Fortunately, most of the Southwest Florida Oter Management District
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 ani--recharge-ocrs-,--although at lower rates
The source of water for the Floridan Aquifer, where its potentiometric surface
is lower than the water table, is by downward seepage from the-
aquifer. Where the potentiometric surface is higher than the water-table,
the source of the water is at some distance upgradient where the actual
recharge takes place. The rate of movement of water from the shallow aquifer
to the Floridan Aquifer, or vice versa, depends on the water-transmitting
S characteristics of the clay unit separating the two aquifers and the difference
in the elevation of 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: discharging
well. In response to the lowered levels in the cone of depression a second
more subdued cone of depression is formed in the water table of the overlying
shallow aquifer (figure 4). The difference in the elevation of the
water level in the shallow aquifer and the potentiometric surface of the
Florian Aquifer is greatest at the point of withdrawal; likewise, leakage
from the water-table aquifer io greatest at the point of withdrawal, other
conditions being equal.
Figure 4--Schematic showing relation of producing wells and regulatory wells.
By controlling the ntiometric surface such thatWe head difference
between the shallow aquifer and the Floridan AquifeL will allow no more than
" the water crop to move from the shallow to the Floridan, continuing declines
in water levels in the shallow aquifer should not occur outside the well field.
Such a management plan allows the water level in the shallow aquifer to
be lowered during dry periods to provide storage for the water crop
and assures that runoff will occur from all areas outside the well field.
The origin of all the water withdrawn from the field is not from the
shallow aquifer within the well field but from a much larger area. However,
the water crop is fully withdrawn from the shallow aquifer within the well
Figure shows the estimated qualte4es of water derived from the shallow
aquifer at various distances from a well field withdrawing water from the
Floridan Aquifer. The estimates are based on calculation'that assum that
)the well eld count of one well withdrawing 15,000,000 mgd (million gallons
per dayj he transmisivity of the Floridan Aquifer is 300,000 gpd (gallons
per day) aWthe leakance coefficient of the confining layer which overlies
the Floridan Aquifer is .0015 gpd/ft2 (gallons per day per square foot).
This figure shows at over 60% of the water is derived from the shallow
aquifer within 4 miles of the well (20,000 ft). Larger quantities are
derived per unit area nearer the well than from more distant areas.
S'Io( ~c-.'.0 3i~
4 C" 0o oO.
^ ~,~~00o 0
^ 3, coo 0.
14, oo;. 000
-" I) oC~) 0
i;P (ii ) I .i1IU .i,~l i I~ILIIIUilL
,;,itl., II j ,nlitl J .i .J._ A il Jl Ulll.l L I
Withdrawal of water ibreas where only a watcr-tabli unconfined aquifer
exists should be liCed to the water crop of the aC-a, otherwise reduction
\ in storage accompanies water-level declines and mining of water 6ccurs4tWthat is
more water is taken out of the system than nature puts into the system. If
such practices were allowed to proceed unchecked, eventually the water supply
would be exhausted.
Although the calculated water crop of a given area may indicate that the
amount of water needed is available for a particular user, withdrawals may
be limited to a lesser amount because the method of withdrawal may induct salt-
water encroachment or result in other damage to the resource.
In some coastal areas, for example, the salt water-fresh water balance is so
delicate that withdrawal of the water crop would shift the interface landward
(% and cause deterioration of the fresh-water supply. To avoid or minimize
this imbalance, some quantity of water less than the water crop should
Utilization of the Shallow Aquifer
Large volumes of water are generally produced from the Floridan Aquifer
because'its high transmissivity, peduej t oAnts-fr~1acturPaa he-oCcureCce-
f-en i. es. Yields of as much as 5,000 gpm
or a single Floridan Aquifer well are not uncommon, and some wells yielding
10,000 gpm are in current use. However, large volumes of water can also be
developed from the shallow aquifer through the use of networks of sandpoint wells.
Tests have shown that the permeability of the sand of the shallow aquifer is
about the same as the permeability of the Floridan Aquifer. The high transmis-
slvity of the Floridan Aquifer is due partly to its greater thickness, but
mostly to its aquiferiaractericti ." tr" from ti hallow aquifer can
readily be used for awn irrigation and, in some al 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 250 sand-point wells drawing 25 gpm to produce the same
quantity of water.
. J l l.. II -, i, i i ,l, I i
S'' Advantages in usin ohallio auue;lr ihclt tte.crtaeB ti tt.e
water crop due to reduction in evapotranspira ) rates as the water level
declines. Disadvantages include greater initial costs of the multiple well
system, plus a much greater upkeep cost. f- .
In areas where the water crop is sufficient to meet demands, management may
require certain practices to insure that the crop can be effectively
developed. Large withdrawals of water from deeper zones may require
enhancement of the recharge capabilities of these zones such as through
recharge wells. However, even where such practices will supply the needed
recharge water, the recharge must be closely monitored to insure protec-
tion of the aquifer against contamination.
Planned Declines for Limited Period
Water levels may be allowed to decline for certain periods, provided adequate
safeguards against salt-water encroachment and damage to other users are
provided. Declines of water levels in large areas such as the Peace and
Alafia Basins may be allowed for certain uses for fixed periods. 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 that cannot be readily recharged. The rate of water-level decline
can be projected based on aquifer characteristics, withdrawal rates, and
recharge potential. Water-level declines may be permitted for specified
Periods of time in some areas if the projections indicate no potential salt-
water encroachment or dmnage to other users.
Cherry, R. N., Ste ), J. W., and Mann, J. A., 19 General Hydrology
of the Middle Gulf Area, Flbrida: Florida Geol. Survey Rept.
Langbein, W. B.,-.Seri, K. T., 1960, General Introduction and Hydrologic
Definitions: U. S. Geol. Surv. Water-Supply paper 1540-/.
Parker, G. G., Hely, A. G., Kreighton, W. B., and Olmsted, F. H., 1964,
Water Resources of the Delaware River Basin: U. S. Geol Surv. Prof.