,A Lay G.aid To VVater Crop And A
Consumptive Use Concepts
The Water Crop concept -- so seemingly
complex at first glance is really no more 5' 4
difficult to understand than the idea of
keeping sufficient funds in a checking
account. AVAILABLE FR
Although often couched in hydrologic e l MANS USE, 10"
terms and expressed in algebraic equations Ii ONSUMPTIVE
[P= Et + R], the idea essentially boils 1
down to three fundamental facts: i 1
 Our water resources are limited; a L 'c- H
 Our only source of water in the A
District is the rain that falls here; and C O --
 We cannot afford to use more water [
than Nature provides us annually. It would --
be physically possible, of course, to take i\
more water from the aquifer than Nature I
has "deposited" there in any particular i 3
year, because of our vast reserves in the
Floridan Aquifer, but such "mining of the
aquifer" -- if continued for long -- would (RNOFF TO GULF"
soon do irreparable damage in terms of L PRIMA 5IL A
dried-up lakes and streams, salt water !
encroachment, and greatly lowered yields
If we are to avoid "spending" more F-rLE
water than is available from "current 1 I NF I
income" [annual rainfall], we must first
determine how much rain falls on the
District within a 12-month period and how
much is lost to forces over which we have no .w
The U.S. Geological Survey, and the \ ,21L2/ A/Y P
District, conducted the necessary investi- 1 i i '1
gations to give these answers: f T i
 In a typical year, on a District-wide
average, a total of 52 inches of raim
provides 9.2 trillion gallons of water per
 Of the total 52 inches, 39 inches is
lost to the combined process called 3
evapotranspiration [Evaporation directly I ?.1. I "O f
back into the atmosphere and transpiration '
through plants to the atmosphere.]
 The remaining 13 inches, referred to i
as runoff, represents the amount that flows
either as surface water or ground water into I
District rivers and eventually into the Gulf I i --- .
of Mexico. This 13 inches of rain, 2.3 trillion I
gallons per year District wide, is the total /
potential water crop available for human
use. However, according to Garald G.
Parker, chief hydrologist for the District, we
cannot use consumptively all of this water. /A _p, l R,/' I
To keep our streams flowing, our wetlands CA4j-1 4 ^ ,.,fV / .
in good ecologic health, we must not use all
or even most of this potential water crop.
Parker estimates that our usable water crop exactly one thousand gallons per day per water table, it can be reused by othei
is in the order of 2.38 million gallons a day acre. persons. That is called non-consumptive
[868.7 billion gallons a year]. In computing whether a person is using use.
If we then divide the total District water more than his water crop, District hydrolo- If, on the other hand, the water
crop by the 10,000 square miles that gists must consider not only how much evaporates after use, or is put into the air
institutes the District, we find that a water he is pumping but also how the water through plant transpiration, or is dumped
rson who owns one square mile has an is used. Unlike other natural resources, into the Gulf of Mexico after use, it is no
annual water crop of 233 billion gallons, or a such as oil, liquified petroleum gas, and longer available for re-use. Similarly, it may
daily water crop of 640,000 gallons. For coal, water is not necessarily consumed by be altered chemically to an extent that
those who own less that a square mile, the its one-time use. If it is used and then makes it useless to others. Such uses are
water crop is further divided to an acreage returned to the Floridan Aquifer or the considered to be consumptive.
basis that, conveniently, works out to be