t/r. IN DAY PER SQUARE FOOT
FIouRE 42.-Effect of recharging stream on semilogarithmic plot of a versus t/rs.
nay be suspected, the drawdown (s,) versus time (t) in
.n observation well near the pumped well is plotted on
ogarithmic paper of the same scale as plate 9 (drawdown
increasing upward at left, time increasing to right). The
:lot of observed data is superposed on the family of type
:urves, as in matching the Theis type curve, and a match
pointt is found for values of T W(u) and 1/u, correspond-
.ng to values of s. and t, respectively. Equation 147 can
hen be solved for T, after which equation 148 can be
:oived for S by rewriting equations 147 and 148 for solu-
:ions of T and S, respectively. From the value of K for
:he particular modified curve followed by the observed
data, the value of r, can be computed from equation 150.
If a suspected boundary is absent, and, therefore, the'
aquifer is extensive, the observed data should fit the
heavy parent type curve, which is the Theis type curve. If
a boundary exists, the observed data will follow the parent
curve until the boundary is first "felt," then it will deviate
from the parent curve along one of the modified curves.
Deviations below the parent curve are caused by re-
charging images; those above, by discharging images.
The term "safe yield" has about as many definitions as
the number of people who have defined it. There are
questions as to the validity of the term, but if it is valid
there remains the question as to who should determine it-
ground-water hydrologists or ground-water managers?
Let us review briefly the history, meaning, and limitations
of the term.
The term "safe yield" seemingly was first defined by
Meinzer (1920, p. 330) as "* the rate at which the
ground water can be withdrawn year after year, for
generations to come, without depleting the supply."
Later Meinzer (1932, p. 99) modified his definition to
"The 'safe yield' of an underground reservoir [is the]
practicable rate of withdrawing water from it perennially --
I have a definition which I taught at U.S. Geological
Survey Ground Water Short Courses beginning in 1952,
namely, "The amount of ground water one can withdraw
without getting into trouble.!' "Withdraw"- may mean
from flowing or pumped wells, and it may mean con-
tinuously. as for many industrial or municipal supplies, or
seasonally, as for irrigation. "Trouble" may mean any-
thing under the sun; such as (1) running out of water,
(2) drawing in salt water, or other undesirable water,
(3) getting shot, or shot at, by an irate nearby wellowner
or landowner, (4) getting sued by a less irate neighbor, or
(5) getting sued for depleting the flow of a nearby stream
for which the water rights have been appropriated. My
definition may sound facetious to some, but remembering
that I would not. attempt to put a number on it before
development or in the early stages of development, espe-
:THE SOURCE OF WATER DERIVED
Under the above title Theis (1940) stated conciseiy the
hydrologic principles upon which depend much of our
present quantitative approach to ground-watcr probiem..
The statements that follow are summarized from these
The essential factors that determine the response of
aquifers to development by wells are:
1. Distance to, and character of, the recharge.
2. Distance to the locality of natural discharge.
3. Character of the cone of depression in the aquifer.
which depends upon the values of T i-wich contains
K and b) and S.
~*-"-Y C~;- ~-I-p
-0. -* .: ...... ....
62 -,. ., .-.. GROUND-WATER HYDRAUICS
for human .* *,. A theAl tre was. nothing clly. I did not know where and how the withdraw
wrong with Meinie'si early definition, they seemingly did 'would be made, it actually makes more sense than
not wholly satisfy all ground-water hydrologists, for definitions, does not differ significantly from Msi ern
beginning about 20 years later many began redefining the original definitions, and is very close to the later defiition'
concept in more and more precise terms to suit themselves of Todd (1959, p. 200): "The safe yield of a ground water
or to suit the particular ground-water conditions with basin is the amount of water which can be withdrawn from
which they were concerned. For a rdsum6 of many of these it annually without producing an undesired result."
definitions, see Kasmann (1951, 1956), Hantush (1955, To determine whether or not a desired quantity and V
p. 71), and Todd (1959, p. 200-218). quality of water can be withdrawn from a given ground-
As stated by Kazmann (1956, p. 1103-2), "The thought water reservoir generally requires an adequate knowledge
became current that the 'safe yield' of an aquifer was of the geologic framework and its plumbing system plus
surely determinable in advance of ground-water develop- the application of philosophy, common sense, and knowl.- *
ment-or even after development had begun." I might edge of the proposed type of development that owners or
add that the thought also became current that one could managers have in mind. As problems become more and
put a number on the safe yield of a ground-water reservoir more complex, however, such as those involving large
regardless of its method of development. However, as investments in land and wells, withdrawal of water from
stated by Thomas (1951, p. 262): both streams and wells that tap a common source, or
In a ground-water reservoir where the water is unconfined i conflicts n water rights, then the solution may require .
certain areas (recharge areas) and under artesian pressure in other highly detailed study. The modern approach is for the
places, the safe yield will be a certain quantity if it is specified that hydrologist to acquire sufficient. detail concerning the
all withdrawals must be by flow from artesian wells, a larger quantity combined ground- and surface-water system so aquifer
if it is permissible to pump water from depths as great as 50 feet, response can be predicted by electric-analog or mathe-
and still more if allowable pumping lifts are as much as 500 feet. matical models. Then management, such as state or local
But the safe yield will vary also depending upon the locations of
wells and the type of wells constructed. Assuming that the well water-conservation agencies, within the framework of
construction and spacing of individual wells are suitable for maxi- prevailing laws or regulations, may test the response of the
mum recovery of water, the safe yield will be a certain quantity if system to various assumed stresses and thereby select the
all wells are 40 miles from the recharge area, considerably greater if most. desirable or equitable distribution of available water.
they are about 10 miles away, and still more if they are adjacent. to Thus the role of the hydrologist, is to gather and present the
or within the recharge are.z..
or witn the rechrge ar. acts; the water manager determines who s, hall have how
The multiplicity and looseness of definitions led Kaz- much water and from what source. In so doir.. the
mann to title his 1956 paper "'Safe Yield' in Ground- manager generally requires and obtains considerable corn-
Water Development, Reality or Illusion?." and led rinuing assistance from the hydrologis;. (See Wcd hnA
Thomas (1951: p. 261) to say: 'Safe yield.' This term, Gabrysch. 1965; Walton and Prickertt 1963: Moor. ::d
originated by hydrologists, may well prove awkward for Wood, 1967.)
them because of the variety of interpretations possible. In the sections that. follow, some additional re ercnc-ric
'Safe yield' is an Alice-in-Wonderland term which means will be made to safe yield in discuss:.n examn e. o:
whatever its user chooses." different types of ground-water reservoirs.
'Ba ~C-~ ~3 ~ IC-. -~SIIIB~ I