Title: The Water Crop: A Water Management Tool
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 Material Information
Title: The Water Crop: A Water Management Tool
Physical Description: Book
Language: English
 Subjects
Spatial Coverage: North America -- United States of America -- Florida
 Notes
Abstract: The Water Crop: A Water Management Tool, by R N Cherry
General Note: Box 10, Folder 12 ( SF Water Rights-Water Crop - 1973, 1976-77 ), Item 22
Funding: Digitized by the Legal Technology Institute in the Levin College of Law at the University of Florida.
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Bibliographic ID: WL00002258
Volume ID: VID00001
Source Institution: Levin College of Law, University of Florida
Holding Location: Levin College of Law, University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Full Text
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THE WATER CROP

A Water Management Tool

By

R. N. Cherry1


General

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,

1960).

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


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Page 2

General continue

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

District.




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52


Figure 1--Schematic


39


illustrating the Water Crop concept.


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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

outflow.


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

seese


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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Figure 2--Schematic showing aquifers in the orthenpart of the Oistrict.





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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.


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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





Regulatory

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.


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Figure 4--Schematic showing relation of producing wells and regulatory wells.


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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

field.


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.


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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.

Limited Withdrawals

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

be utilized.



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




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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- .



Recharge Facilities



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.

Inv. 56.

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.

Paper 381.


I I




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