Title: Water Table Control on Agricultural Lands
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Permanent Link: http://ufdc.ufl.edu/WL00002909/00001
 Material Information
Title: Water Table Control on Agricultural Lands
Physical Description: Book
Language: English
Publisher: Fla Engineering and Industrial Experiment Station
 Subjects
Spatial Coverage: North America -- United States of America -- Florida
 Notes
Abstract: Richard Hamann's Collection - Water Table Control on Agricultural Lands
General Note: Box 12, Folder 1 ( Materials and Reports on Florida's Water Resources - 1945 - 1957 ), Item 23
Funding: Digitized by the Legal Technology Institute in the Levin College of Law at the University of Florida.
 Record Information
Bibliographic ID: WL00002909
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






Water Table Control on Agricultural Lands

by
LUTHER C. HAMMOND*


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More than one-half of the land area of Florida has
a water table near enough to the soil surface to be of
significance to farming operations. Major strides have
been made in recent years in the agricultural utiliza-
tion of these poorly drained lands of the state. In
fact, it is safe to predict an even greater expansion in
these areas as their potentialities are better under-
stood. The present need is for more emphasis on the
dynamic soil-water-plant entity in our cooperative and
organized research program. It is the purpose of this
paper to call attention to some of the soil-water-plant
relationships which are embraced by the problem of
water table control on agricultural lands.
The soil-water system without the plant should
perhaps receive first consideration, because the plant
is supplied with water as a result of the retention of
water by the soil and the movement of water through
the soil. The upper limit of water retention is com-
plete saturation which occurs below the water table,
and the lower limit is the air-dry state. A unit volume
of water can be removed from the soil with increasing
difficulty from saturation to the air-dry condition.
Thus, soil water has a certain energy of retention, and
it moves in the soil over energy gradients. The rate
of soil water movement is directly related to the energy
gradient or driving force and to a conductivity factor.
It is fortunate from the standpoint of water conser-
vation that the conductivity factor operates to retard
soil water movement even though the driving force
may be high. At complete saturation water conduc-
tivity or hydraulic conductivity is at a maximum de-
pendent upon the size distribution of the interstitial
spaces in the soil. For the unsaturated case the term
capillary conductivity is used. This latter property of
the soil decreases as the water films in the interstices
become smaller with the drying of the soil. The low
value of the capillary conductivity at the air-dry state
represents one of the soil moisture conserving features
of the conductivity factor. Another characteristic
of the conductivity factor is its relatively low value
even at moisture contents in the optimum range for
plant growth. This results in the surface drying of
well-drained soils and the relatively slow drainage to
greater depths. Both of these phenomena help to


*Assistant Soil Physicist, Agricultural Experiment Station,
University of Florida.


bring about the miracle of water storage in soils in
the energy range for plant use.
Now consider the soil in cross section or in profile
with a water table present. Following a rainfall or
irrigation the moisture content of the soil increases
from the surface downward to the water table. There
is a point, however, above the water table at which the
water content begins to increase very rapidly as the
water table is approached. This zone immediately
above the water table is called the capillary fringe and
varies in depth with the nature of the soil interstitial
spaces. The smaller the interstices the deeper the
capillary fringe. This can be seen from the well-
known equation for the rise of a liquid in a capillary
tube

2T
h =-2- cos 0
rdg
where h is the height of rise, T is the surface tension
of the liquid, r is the radius of the capillary tube, d is
the density of the liquid, g is the gravitational constant
and 0 is the angle of contact of the liquid with the
sides of the capillary.
When the capillary fringe extends to the soil sur-
face evaporation of water occurs at a maximum rate.
In fact, water may evaporate from some soils at a faster
rate than from a free water surface. As the water
table is lowered the capillary fringe loses contact with
the soil surface, and it is then possible for the surface
soil to dry more rapidly than the conductivity of the
soil can permit a resupply of water to the evaporating
surface. Further water loss by evaporation is then
greatly reduced since the water must move through
the dry surface layer in the vapor state. Clearly, these
facts have implications on the conservation of ground
water. The least loss of water by evaporation and
transpiration takes place where the water table is low
enough to keep the capillary fringe well below the
soil surface and the soil is free of vegetation.
In view of the above principles the question of dust
mulching as a water conservation practice may have
real meaning. If the rate of surface drying can be has-
tened by breaking the soil surface then the quantity of
water lost should be materially reduced. It is impor-
tant to note that the dust mulch must be created be-
fore the soil surface becomes dry. With some soils
this is not practicable. The maximum value of the
dust mulch can be realized when the water table is


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near enough to the soil surface to make a significant
contribution of water to the evaporating surface yet
far enough below the soil surface so that the surface
is not readily rewetted by upward movement of water.
In addition, the drying conditions affect the relative
value of a dust mulch. In certain cases weather con-
ditions favoring a slow evaporation of moisture may
actually cause a greater water loss than conditions
more favorable to rapid drying.
The retention, movement, and loss of soil water in
relation to a water table has increased import when
the plant is added to the complex soil-water system.
Most plants grow best in a soil-water-air system in
which the air volume is at least 10 per cent. Thus,
plant roots grow into the capillary fringe but usually
stop short of the water table because of inadequate
aeration. The capillary fringe, therefore, can be an
important source of water for the plant if the water
table is stabilized. An uncontrolled water table some-
times fluctuates in a manner which limits the plant
root system. A rapid drop in the water table during
a short drought period brings about a water deficit
for the plant with a shallow root system. Drainage to
prevent the occurrence of high water tables of long
duration will insure a greater water supply to plants
because of the possibility for deeper rooting. It is
important to note that whether the water supply to
plants is increased with drainage depends upon the
depth of looting and the amount of water-table
control possible. In some instances the water supply
may be increased by raising the water table so that
the plant roots are in the capillary fringe. Where the
soils are very porous an irrigation system may be
based on a controlled rapid fluctuation of the water
table. However, drainage conditions and efficiency of
water use may dictate a stable water table combined
With surface irrigation. Ideally, the water table should
be controlled at a depth which will allow the surface
soil to dry, the plant root system to extend to a maxi-
mum depth, and the plant to obtain water from the
capillary fringe.
The plant growing in the soil is an additional
source of soil-water loss. The extensive plant root
system can remove water from soil depths not generally
affected by surface evaporation. However, since many
row crops do not completely cover the soil some of


the fundamental principles of water loss from a bare
soil still hold; that is, a dry soil surface greatly reduces
water loss. Therefore, good management practice
would dictate letting the soil surface remain dry as
much as possible. Important implications are apparent
in relation to the recent attempts which have been
made to translate evapotranspiration measurements
into irrigation timing. Most evapotranspiration meas-
urements are made under conditions which are sup-
posed to give a maximum water loss. These conditions
may or may not exist with a crop in the field, and even
if they are made to exist the resultant high irrigation
requirement may be far from economical.
In connection with scheduling irrigation from esti-
mated water-use rates some knowledge of the water
storage capacity of the soil is needed. This informa-
tion is also necessary in the design of irrigation sys-
tems. Data of this nature are not easy to obtain for
several reasons. First, the presence of a capillary fringe
in the root zone provides for an unknown quantity
of water to be supplied from outside the root zone.
Secondly, on well-drained soils the amount of water
available to the plant will depend upon the soil pro-
file characteristics, the depth of wetting, and the
nature of the drying conditions following a rainfall or
irrigation. Also, it is necessary to make almost all
storage measurements directly in the field. Even if
a good water storage estimate is obtained one is still
confronted with variable water use rates which depend
upon stage of growth, density and depth of vegeta-
tion, extent of soil shading and perhaps with the phy-
siological conditioning of the plant by previous soil
moisture treatment. An efficient irrigation control
based on soil moisture estimates rather than measure-
ments can be realized only as our present knowledge is
used effectively and as new information is added to
that which is available.
These are some of the more important soil-water-
plant relationships which are fundamental to the
scientific control of the water table on agricultural
lands. Water-table control will approach a more ideal
state as research supplies the answers to many questions
relating to soil-water retention, movement and losses
by drainage, evaporation and transpiration. The im-
port of the problem is apparent from the fact that
water-table control is a matter of life and death to the
plant-and to man!


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