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II. Water and the Hydrologic Cycle

II. Water and the Hydrologic Cycle

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WATER AND ITS RELATION TO SOILS AND CROPS



5



the 0-H chemical bond, is a factor of prime importance in determining

the physical properties of water.

The high heat of vaporization, a property of water that is of great

significance in relation to the hydrologic cycle, is a manifestation of the

high degree of hydrogen bonding of water. Such bonds, which have to be

broken in transforming water from the liquid to vapor state, also account

for the fact that this transformation takes place at a temperature 260” C.

above that of another simple molecule, methane, which has nearly the

same molecular weight but is free of hydrogen bonding between its

molecules.

Hydrogen bonding and the tetrahedral distribution of electron pairs

around the oxygen atom also serve to explain the unusual increase in

volume that occurs when water freezes. The open nature of the spatial

arrangement of the water molecules arising from the bonding between

the water molecules gives ice a lower specific gravity than water. The ice

structure, upon melting, partially collapses, with water molecules OCCUPYing the “open spaces” in the ice structure. The facts that ice is less dense

than water and that water has maximum density at a temperature slightly

above the freezing point are both properties of great significance in the

role of water in the thermal and hydrologic phenomena of the earth and

its atmosphere.

Hydrogen bonding is also responsible for the viscous nature of water

and for the rapid decrease in this property as temperature increases. The

intermolecular hydrogen bonds are disrupted by heat. Other important

consequences of hydrogen bonding are the properties of adhesion,

cohesion, and surface tension, properties that largely determine the

retention and movement of water through porous media, such as soil

and plant tissues.

A final illustration of the unique properties arising from water’s

molecular structure is the solvent action that is so intimately related to

the role of water in biological systems. Water acts as a solvent for

organic and some inorganic compounds by the mechanism of hydrogen

bonding. In the case of saltlike compounds, water acts as a solvent by

means of charge interaction as a consequence of the separation of charge

between the hydrogen and oxygen atoms in the water molecule.

In addition to the physical phenomena discussed above, stemming

largely from the unique ability of the water molecules to associate through

hydrogen bonding, the molecular structure of water has profound effects

on its chemical properties. These properties depend on breaking the

strong hydrogen-to-oxygen bond, resulting in the formation of the positive

hydrogen ion and negatively charged hydroxyl ion. Through this mechanism, water becomes an active participant in chemical reactions and, as



6



M. B. RUSSELL AND L. W. HURLBUT



such, is involved in most of the important chemical processes occurring in

nature.

Throughout the remaining sections of this article, water is considered

in terms of its more macroscopic and familiar properties and in its

behavior in soils and plants. The reader is asked to remember, however,

that the observed behavior of this truly unique compound is, in the

final analysis, traceable back to the structure and electronic configuration

of the water molecule itself.



B. THEAGRICULTURAL

WATERSUPPLY

M. B. Russell and L. W. Hurlbut

University of Illinois, Urbana, Illinois, and University of Nebraska, Lincoln, Nebraska



Water may be considered as the lifeblood of the earth. Its mobility,

energy transformations, and physical and chemical behavior impinge on

every facet of organic life. We live in and are part of the unending flux

of water known as the hydrologic cycle. This complex series of interconnected flows and phase changes is shown in part in the schematic diagram

in Fig. 1.

The water that is agriculturally useful during any one year is an

extremely small part of the world's total water supply. Including ground

water to a depth of 12,500 feet, total supply is estimated to be about

165 trillion acre-feet. Roughly 93 per cent of this amount is found in the

oceans and seas, and 7 per cent in fresh-water forms. The latter consists

primarily of ground water (about 5 per cent), and polar ice and glaciers

(about 2 per cent). The total amount of water in lakes, rivers, and soil

moisture is about 1 per cent of the total fresh-water supply, or only about

0.08 per cent of the world's total water supply. A summary of estimated

quantities of water in the several parts of the earth's hydrosphere is shown

in Table I. Interchange of water is continuous, at varying speeds, among

the several parts of the hydrosphere. In some instances the transit time is

of the order of thousands of years, as in the case of deep ground-water

movement or the cyclic movement of water through the polar ice caps

and glaciers. Short-term cycles of only a few hours are also common, as in

the case of the return of water to the atmosphere by evaporation from the

wet soil surface immediately following a rain. The part of the hydrologic

cycle of greatest general agricultural concern is the annual precipitation

cycle. Each year about 89 billion acre-feet of water fall on the land

surfaces of the world. This amounts to 7%times the moisture content of the

earth's atmosphere, and 13%times the estimated amount of water stored

in the soil. Roughly four-fifths of annual precipitation returns directly to



WATER AND ITS RELATION TO SOILS AND CROPS



7



8



M. B. RUSSELL AND L. W. HURLBUT



TABLE I

Estimated and Relative Quantities of Water in the Earth's Hydrospherea

Acre-feet

Total water

Total fresh water

Ground water to 12,500-ft. depth

Lakes and streams

Atmosphere

Soil moisture

Plants and animals

Annual precipitation

Annual runoff



165,000 X

11,000 x

8,200 X

118 x

12 x

6.5 x

0.9 x

89 x

17 x



Ratio to annual precipitation



lo9

109



lo9

109

109

109

109

109

109



1850

124

92

1.3

0.14

0.07

0.01

1 .o

0.2



Adapted from Ackerman and Lijf (1959).



the atmosphere, as evapotranspiration, with the remaining one-fifth

accounted for in stream flow. Except for the relatively small amounts of

water used from the ground-water reserves, whose cycle of depletion and

recharge is much longer, practically all agricultural water use is identified

with the annual precipitation cycle and involves the use of relatively

short-term, low-capacity storage media.

The water resources of continental United States are tabulated in

summary in Table 11. These data indicate that average annual precipitaTABLE I1

Water Resources of Continental United States"

Annual precipitation

4.75 X 109 acre-feet

Annual runoff

1.3 X 100 acre-feet

Estimated total usable ground water 47.5 X 109 acre-feet

Soil moisture

0.6 X lo9 acre-feet

Lake storage

13.0 X lo9 acre-feet

Average annual precipitation

30 inches

Average annual runoff

8 inches

Average soil moisture storage

3.7 inches

a



Adapted from Ackerman and LSf (1959).



tion is about 30 inches and average annual runoff is about 8 inches. Usable

ground-water reserves are estimated to be equal to ten years of precipitation, and the total storage in lakes is 3%times the yearly precipitation.

The average amount of available water stored in the soil for the area of

the United States, however, is only about 3%inches of water.

If the water supplies discussed in the preceding paragraphs were

uniformly distributed over the United States, and if seasonal distribution



WATER AND ITS RELATION TO SOILS AND CROPS



9



of the precipitation were matclied to crop needs, there woulcl be fcw

areas of agricultural water shortage in this country. Neither of the two

foregoing conditions exist, however, with the result that many areas are

characterized by a marked imbalance between available water and

agricultural needs. Geographic distribution of precipitation and runoff

is shown in Figs. 2 and 3. Figure 4 shows the manner in which agricultural

water use, as measured by potential evaporation, varies throughout the

United States. The preceding figures indicated that, on the average, the

eastern part of the United States and parts of the Pacific Northwest are

regions of water surplus. The area west of the 95th meridian is, except for

some of the mountain areas, a region of moisture deficiency if potential

evaporation is taken as an index of agricultural water need. Even in the

regions of average annual water surplus, water deficiencies are common

in specific localities, because of (1) failure of seasonal distribution of

rainfall to match seasonal water needs, ( 2 ) deviations of annual rainfall

from average values, ( 3 ) excessive runoff resulting from high intensity of

precipitation, steep topography, or low infiltration rate, as with frozen soil,

and (4) low soil-moisture storage capacity for supplying crop needs

between rains.

Current rainfall and soil moisture constitute the “working water

supply” for crop production. Because of its agricultural significance, water

storage by the soil is of great importance, even though it averages only

about 12 per cent of annual rainfall and 0.01 per cent of the world’s freshwater supply. Even so, the soil plays an important role in the hydrologic

cycle. As a water storage medium it reduces runoff peaks, supplies moisture for growing plants, and retains a significant portion of precipitation in

a manner permitting its early evaporation back to the atmosphere.

The water storage capacity of soil is a function of its depth and

physical composition, The volume fraction of voids multiplied by the

soil depth is a measure of the gross water storage capacity of a unit area

of soil. In many soils the volume fraction of voids varies with depth,

making necessary an integration over each of the soil horizons to obtain

the total profile storage capacity.

In well-drained soils, and in dry regions where the subsoil is perennially dry, not all of the soil pores remain filled with water. Therefore

the effective storage capacity of a soil is determined by the volume

fraction of pores that remain water-filled after water essentially ceases to

move downward. The volume fraction of water retained under such conditions is affected by soil texture, ranging from 0.08, for sands, to 0.30,

for clays. For soils of intermediate textures such as loams and silt loams,

0.25 is a good approximation of the gross field water storage capacity.

Using this figure, we find that 3 feet of a silt loam soil will store 9 inches



10

M. B. RUSSELL AND L. W. IIURLBUT



FIG.2. Average annual precipitation for United States (Evans and Lemon, 1957).



WATER AND ITS RELATION TO SOILS AND CROPS



11



Y



wo



FIG.3. Average annual runoff for United States (Langbein and Wells, 1955).



12

M. B. RUSSELL AND L. W. HURLBUT



m



.‘*



a



30-36



81



36-42



I 1



42-40

over 4 0



inches

19



I



m



Y



G



FIG. 4. Average annual potential evapotranspiration for United States (Thornthwaite, 1948).



13

of water. However, not all of this water is available to plants. The volume

fraction iinavailable to plants is also a function of soil texture, increasing

from about 0.04, for sands, to 0.18, for clays, with 0.10 being a good approximation for soils of medium texture. As shown in Fig. 5, about 60

per cent of the effective storage capacity of well-drained soils may be

considered available to plants. Factors affecting the retention of water by

soils, the laws governing its movement, and its availability to plants are

discussed in later sections of this review.

WATER AND ITS RELATION TO SOILS AND CROPS



FIG.5. The effect of soil texture on water retention ( U . S . Dept. Agr. Yearbook

Agr. 1955, p. 120).



In localities where rainfall and soil-moisture storage are inadequate

to meet crop needs, other components of the hydrologic cycle must be

drawn on to correct the deficiency. Surface water from streams and

lakes and ground water are the sources that can be used. It can be seen

from Tables I and I1 that each of these sources of water is much larger

than the annual rainfall, and each has an order of magnitude larger than

the soil moisture supply. However, as with annual precipitation, surface

and ground-water supplies, as shown in Figs. 3 and 6, are not uniformly

distributed and, in fact, are largely concentrated in those areas where

current rainfall and soil storage are most adequate. Thus, in the humid

region east of the 95th meridian, all streams of any size are permanent,

and annual runoff exceeds 10 inches in most areas. Even there, surplus

stream flow undergoes a pronounced seasonal variation. Except in Florida

and the southeastern coastal plains, half or more of the annual runoff

occurs in three months of the year. Since the period of peak stream flow



GROUND-WATER



AREAS IN THE UNITED STATES

i

I

A



Y



!+



P



UNCONSOLIDATED AND

SEMICONSOLIDATED AQUIFERS

CONSOLIDATED- ROCK AOUIFERS



\.



BOTH CONSOLIDATED AND UNCONSOLIDATEDROCK AQUIFERS

NOT KNOWN TO BE UNDERLAIN BY AQUIFERS THAT WILL

GENERALLY YIELD AS MUCH AS 50g.p.m. TO WELLS



FIG. 6. Ground-water areas in the United States (Thomas, 1955).



WATER AND ITS RELATION TO SOILS AM) CROPS



15



is in the late winter or early spring, it does not coincide with the period of

maximum agricultural need. Therefore, to achieve maximum use, runoff

must be impounded for periods of about six months.

Average annual runoff is much less in the Great Plains region than

in the more humid Eastern States, and seasonal concentration is more

pronounced-50 to 70 per cent in a three-month period over most of the

region. Flow in major streams in this region is stabilized to some degree

by runoff from the bordering mountains to the west.

Runoff patterns in the western third of the United States reflect the

mountainous nature of much of this area. Rainfall and soil storage are

generally insufficient for intensive agricultural production. Irrigation,

based on impounded mountain streams and on ground-water supplies, is

widely practiced throughout this region. Runoff from the lowland areas

is slight except in the Puget Sound area.

As might be expected, the ground-water supplies depicted in Fig.

6 reflect in a general way the precipitation and evapotranspiration patterns

and the geologic structures of the country. Three major types of groundwater areas are shown in Fig. 6: (1) the channels and associated alluvial

deposits along water courses, ( 2 ) loose sands and gravels in glacial drift

and outwash, and ( 3 ) such consolidated rocks as limestones, basalt, and

sandstones. In 1950, ground-water withdrawals accounted for about 20

per cent of all the water withdrawn for municipal, rural, industrial, and

irrigation use. The last accounts for more than 60 per cent of all ground

water used in the United States. Nearly all of rural use, 25 per cent of

municipal use, 7 per cent of industrial use, and 25 per cent of irrigation

use are supplied from ground-water sources. The usefulness of a groundwater source is determined by capacity, depth and recharge rate of the

aquifer, and, in some instances, the chemical quality of the water. Large

quantities of ground water are held in clays and fine-textured materials of

such low permeability that the discharge rate from wells is too low for

practical utilization. It is estimated that 80 per cent of all water obtained

from wells in the United States comes from unconsolidated sand and

gravels, 5 per cent from limestone, 3 per cent from sandstone, and 2 per

cent from basalt. For ground-water aquifers to serve as a continuing

source of water, recharge rate must equal withdrawal rate. In some areas

in the southwestern United States the underground water reserves are

being steadily depleted (Fig. 7) and remedial measures are being employed to increase the recharge rate (Mitchelson and Muckel, 1937;

Muckel and Schiff, 1955).

The chemical nature, or quality, of surface and ground-water supplies

has an important bearing on their agricultural usefulness. Water quality

for irrigation is determined by total concentration of soluble salts, con-



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