^áí^ A\y

X I'l CONSERVATION

IRRIGATIONS^

in HumjictÀreas

>

^/ \%' AGRICULTURE HANDBOOK NO. 107

Soil Conservation Service

U. S. Department of Agriculture
Irrigation Is an

Individual Matter

In humid areas, whether to irrigate is a question to be decided separately for each

farm. The farmer must consider his own needs and resources, and weigh costs and

benefits in relation lo conditions on his own farm. Soil conservationists, engineers,

county agents, and others who help farmers to plan their irrigation systems must take

these local conditions into account. Each farm, each system is an individual matter.

To make the right decisions and design an efficient system for a particular farm,

four things are needed:

(1) A soil and land-capability map of the farm. This map will tell what kind of soil

occurs in each part of each field. It will show the capability of each soil for cultivation

and irrigation, and its needs for conservation practices. In soil conservation districts

each farmer can get a copy of this map from the district or the Soil Conservation Service

office. Elsewhere, it will be necessary for someone skilled in soils to examine the farm

and prepare the soil map.

(2) A copy of the local Conservation Irrigation Guide. This gtiide is a compilation

of the best local information about soils and crops essential in planning the irrigation

system. It tells such things as the water-holding capacity and intake rate of each

soil, and the water-use rate of each crop. These factors vary so much from one place

to another that reliable information cannot be put in a handbook of this kind. The

local guides have been compiled by the Soil Conservation Service in cooperation with

State experiment stations. Agricultural Research Service, county agents, and others

who have such information. Soil Conservation Service offices have copies of these

guides.

(3) A conservation planfor thefarm. Successful irrigation must take into account

the capability and needs of the soil. For continued profits, irrigation must be fitted

into a basic soil conservation plan including a good soil management system. In soil

conservation districts, many farmers will already have such a plan; those who do not

can get help from the district in making them.

(4) Expert help in designing the system. Deciding the mechanical specifications

of a system to fit a particular farm is a highly complicated engineering problem. Once

the decision is made to proceed with an irrigation project, and the general features of

the system are determined to fit into the general conservation plan of the farm, the

planner needs the services of an expert in irrigation design. In soil conservation districts, engineers of the Soil Conservation Service are available for this work. Many

equipment dealers employ engineers, and other agricultural specialists are qualified to

help. But be sure to get expert help in designing the irrigation system.

This handbook cannot answer all the specific questions that arise in planning a

farm irrigation system. It cannot give the local information needed to answer many of

these questions. Rather, it presents basic information and principles of wide applica-'

tion, and seeks to guide the user through the analysis of an individual problem with the

use of the local irrigation guides and services mentioned above.

And finally, no matter how sound the planning or how good the design, an irrigation

system will not give good results unless it is operated correctly. To do this the Irrigator

must know the capabilities of his system and of his soils, and the water requirements of

his crops. With this knowledge he can apply the principles of conservation irrigation

to his own farm.
Contents

„ ... P'ee

Conservation irrigation farming 3

Need for irrigation 3

Benefits of irrigation 4

Costs of irrigation 4

Requirements for successful irrigation .... 5

Planning the irrigation system 6

Surveys for planning 5

Choosing method of irrigation 7

Design and operation of the system .... 8

Conservation irrigation guides 8

Moisture in the soil g

Available moisture 10

Intake rates JQ

Water movement in soils 10

Water use by crops H

Water-use rates H

Moisture-extraction pattern 11

Applying irrigation water 12

When to start irrigating 12

Irrigation efficiency 12

Depth to irrigate 12

Irrigation frequency 12

Amount to apply I3

Determining soií moisture 13

Water supplies 14

Amount of water needed 14

Sources of supply 15

Perennial streams 15

Reservoirs and lakes 16

Ground water 17

Pumps and power units 19

Pumps 19

Power units 20

Sprinkler irrigation 22

Types of systems 22

Kinds of equipment 22

Pumps 22

Supply lines 22

Main lines 22

Laterals 24

Sprinklers 24

Perforated pipe 25

Special equipment 26

Sprinkler irrigation—Continued Pap«

Design of system 26

System layout 26

Sprinkler selection and spacing 29

Maintenance of equipment 29

-Applying fertilizer in water 30

-Amount to apply 3Q

Method of apphcation 30

Frost control 31

Surface irrigation 32

Land preparation 32

Land smoothing 32

Rough grading 33

Land leveling 34

Water-distribution systems 36

Ditches 3^

Water-control structures 36

Ditch linings 37

Pipelines 3g

Applying fertihzer in water 3g

Furrow irrigation 3g

Where adapted 3g

Design of system 39

Distribution and control of water 41

Contour borders 42

Where adapted 42

Design of system 42

Distribution and control of water 43

Operation of system 44

Graded borders 44

Where adapted 44

Design of system 45

Distribution and control of water 47

Subirrigation 47

Where adapted 47

Distribution and control of water 48

Appendix 49

Minimum requirementsfor sprinkler design . 49

Average water-application rates from

sprinklers 51

Drops and checks for use in irrigation

ditches 52

For sale by the Superintendent of Documents, U. S. Government Printing Office,

Washington 25, D. C, January 1957

391260°—57-
Figure 1.—The humid area to which this handhook applies. Insets show typical rainfall patterns in comparison to

monthly moisture needs of important crops at representative stations for a normal and a dry year. The peculiar features of humid-area irrigation ¡rive way to problems of arid agriculture from east to west through the transition zone.
CONSERVATION IRRIGATION IN HUMID AREAS

By T. Jl. Quackenbiish, G. M. Renfro, K. H. Beauchamp, L. F. Lawhon,

and G. W. Eley, Irrigation Engineers, Soil Conservation Service

IRRIGATION IN HUMID AREAS differs from irrigation

in the arid West. Most important, production of

cultivated crops is possible in a humid climate

without irrigation; in an arid one it usually is not.

The line that divides humid from arid lands is not

a sharp one. It is a broad transition zone running

across Texas, (Oklahoma. Kansas, Nebraska, and

North and South Dakota (fig. 1). East of tliis zone

is the humid area for which this handbook is written,

lo the west different conditions demand or permit

different methods.

Within the transition zone itself, conditions change

gradually from humid on the east to arid on the

west. Irrigators in each locality will need to study

their own situation carefully, to compare it to typical

humid and arid conditions, and to judge which

methods of irrigation offer the best promise of success. In every case they will gain by relying to the

greatest possible extent on the results of local

research and experience.

East of this zone, the average annual rainfall

ranges from about 20 inches in western Minnesota to

60 inches in the Southeastern and Gulf States. In

most of this area rainfall usually is sufficient to produce good yields of most farm crops. But the rainfall does not always fall at the time it is most

needed (fig. 1). Frequent droughts in the growing

season reduce the yield and quality of crops.

Irrigation can overcome these deficiencies.

Conservation Irrigation

Farming

The margin of profit between farming with and

without irrigation is much smaller in humid than in

arid areas. The humid-area farmer, therefore, must

make sure he gets maximum returns from his investment to make irrigation profitable. This means

that he must not only have an efficient irrigation

system efficiently operated, but he must also use

good soil conservation and farm management

practices with it.

To use irrigation merely to rescue a crop already

suffering from drought wiîl do very httle in the long

run to increase crop production and profits. Good

conservation farming practices without irrigation

will usually pay off better than irrigation used as

"crop insurance" without the soil-management

practices needed with it.

Irrigation properly used in the humid area will

eliminate the drought hazard from farming; then

soil fertifity and aeration, not moisture, become the

major limiting factors in crop production.

The principles and methods of irrigation set forth

w this handbook are designed for typical humid conditions. They differ from arid-land irrigation in the

following important ways:

(1) Every system must provide for safe use or

disposal of surplus rainfall. This requires safeguards against soil erosion and provisions for

surface drainage n(jt ordinarily needed in arid areas.

(2) Seasonal water requirements are less than in

arid climates, but short-time peak-use requirements

may be as great.

(3) Farm crops generally do not root as deeply

in humid as in arid areas. This necessitates more

frequent but lighter irrigations during drought.

(4) \T inter rain and snow completely replenish

the soil moisture in most years so irrigation before

planting or after harvest is unnecessarv.

(5) The cost of land leveling often can be offset

largely by benefits from improved drainage, a

common need in humid areas.

(6) Humid-area soils generally have fine-textured

subsoils at 6 to 24 inches depth! This often limits

the extent of land levehng possible, and makes a

thorough knowledge of soil conditions especially

important to planning any grading or leveling.

Need for Irrigation

The average annual rainfall in humid areas would

be sufficient to meet the total moisture requirements

of most crops if properly distributed through the

growing season. This ideal rainfall pattern seldom

occurs, however, and many crops suffer even in

so-called "normal" rainfall years (fig. 1).

Moreover, runoff from high-intensity storms and

deep percolation to below the crop roots deprive

growing plants of a large part of the total rainfall.

The effect of short droughts on crops in the humid

area depends in part upon the kind of soil. A plant

growing in a sandy soil may show early signs of

drought, while the same plant in a silt loam may

survive a dry period without damage. These

differences affect the need for irrigation in each

locality.

In drv vears irrigation in humid areas is generally

profitable, but in some years it gives little advantage.

The farmer who is considering installing an irrigation

system should not be influenced too much by reports

of increased A ields obtained by his neighbors or experiment stations during dry years. It is the average increase in yield over a long period, including

both wet and dry years, that proves the true worth

of irrigation.

The expected results from irrigation must, of

course, be balanced against the cost of installing and

operating the system. These costs will vary widely

from farm to farm depending on such items as cost

of developing a water supply, type of soil, topog-
rapliy, method of irrigation selected, and the cost

and availability of labor.

Benefits of Irrigation

Irrigation helps to stabilize the farm economy.

During recent years the cost of farming has increased

so greatly that few farmers can afford even one crop

failure. The irrigator can make full use of seefl,

fertilizers, equipment, and labor every year with

more consistent, and usually larger, harvests.

Besides increasing per-acre yields, proper use of

irrigation can improve the quality of crops. By

maintaining adequate moisture in the soil durmg

the growing season, the farmer can produce higher

qualîty crops demanding higher market prices.

Time of harvest also can be more closely controlled.

In many highly competitive areas, the difference

between profit "and loss often hinges on the time

certain crops are put on the market. Irrigation

insures prompt germination and continuous plant

growth, making it possible to regulate planting

dates and time of maturity more closely.

With irrigation, fertilizers placed in the soil are

available at once to plants because moisture is

always present. Under these conditions larger

amounts can be used. Soluble fertilizers can be

apphed through irrigation water with a saving in

labor and equipment and with better control of

placement in the root zone.

Certain high-value cash crops can be protected

from frost damage with sprinkler irrigation.

Irrigation increases the survival of transplanted

crops by settling the soil into close contact with the

ro(jt system.

The efficient use of irrigation permits better

use of land in accordance with its capability. It

increases the capability of some soils for different

uses. Increased per-acre yields on the good land

permit devoting the more erodible land to grasses

and legumes, wood crops, or other uses. Irrigation

makes possible the successful seeding of needed

cover or soil-improving crops at the proper season

and helps in establishing vegetative cover on eroded

areas (fig. 2).

Costs of Irrigation

Irrigation costs may be divided into (1) fixed costs

and (2) operating and maintenance C(jsts. Fixed

IDA+ridiO

Figure 2.—Irrigation makes possible the growing of

perennial grasses or legumes to control erosion.

costs are all costs for which an initial outlay is

made or a capital investment is extended, including

taxes and insurance. Annual fixed charges include

depreciation and interest on the capital investment.

Annual operating and maintenance charges include

all recurring costs, such as fuel or power, labor for

water application and maintenance of the system,

and repair of equipment.

Developing the water supply is a fixed expense

for most irrigators. For those who can pump

directly from a perennial stream, this is a minor

expense. Where the source is a farm reservoir, the

initial investment may be $50 to .$200 per acre

irrigated. The cost of drilling a deep well to

obtain a specific flow of water, say 800 g. p. m., will

range from a few hundred dollars in one locality

to several thousand dollars in another.

For sprinkler irrigation, the greatest fixed cost

usually is for the equipment, including pump and

power unit, sprinkler heads, pipe and fittings, and

accessories. Most portable irrigation systems, when

adequately designed, cost from S80 to .?200 per acre

irrigated; a few cost more in unusual conditions.

Land-leveling costs for surface irrigation usually

range from S25 to $100 per acre.

Many factors affect the initial cost of an irrigation system. Some of the more important ones are:

(1) The nearness of the water supply to the

irrigated area affects the length and size of mainline pipe or ditch required. The closer the water

supply, the less it costs to get the water to the

irrigated area.

(2) Difference in elevation between the area to

be irrigated and the water supply determines the

static lift, which in turn affects the size of pump

and horsepower of the motor needed.

(3) The size of the area to be irrigated influences the per-acre costs of equipment and land leveling. Other factors being equal, the smaller the

area irrigated the greater is the cost per acre. This

is particularly true of areas of less than 5 or 10

acres.

(4) Control equipment that permits continuous

operation adds considerably to the cost of sprinkler

systems. These items usually include extra lateral

lines that can be moved while the system is operating,

and the valves and other fittings required at takeoff

points on the main line. The cost of this equipment

is usually more than offset by a saving in labor.

(5) The number of hours each day the system is

designed to operate affects the per-acre cost for

equipment. A sprinkler system designed to cover

an area in a certain number of days operating 10

hours daily costs nearly twice as much as one

designed to cover the same area in the same period

operating 20 hours daily.

Annual costs for power depend upon the type of

power unit used, the cost of fuel or electricity, and

the overall efficiency of the pumping plant. Except

for electricity, the power cost varies directly with

the horsepower delivered and the number of hours

operated during the season. Electric power rates

usuall) consist of a fixed or minimum charge, based
on the horsepower rating of the motor, and a diminishing schedule of rates for energy actually consumed.

The cost of electric power per hour of operation,

therefore, usually declines as the number of hours

of operation increases.

Labor costs vary widely. Operation of a sprinkler

system usually requires from 1/2 to 2 man-liours of

labor per acre per irrigation. Some of the factors

that influence labor costs are: Spacing of the lateral

lines; height, density, and planting pattern of the

irrigated crop; walking conditions in the irrigated

area; the amount of water applied at each irrigation

in relation to the total amount applied during the

season; and, of course, the design of the system

itself.

A well-planned sprinkler system with lateral lines

rotated to minimize the haul-back of pipe to the

starting position reduces labor costs. Buried main

lines with hydrant valves at each lateral position

eliminate the cost of laying and taking up main-line

pipe. Extra lateral lines that can be moved while

the system is operating help keep labor occupied.

The use of mechanized equipment for moving

lateral lines eliminates most of the labor required for

this operation, but adds to the initial cost of equipment.

Maintenance costs include lubrication and maintenance of the pump and power units, cleaning and

storing of pipe, replacement of worn coupler rings,

and repairs to sprinkler heads and valves. Annual

land smoothing is usually necessary where surface

irrigation is used.

As can be seen from the foregoing discussion, a

satisfactory cost estimate for irrigation on anv

individual farm can be made only after due consideration of all the factors involved.

Requirements for

Successful Irrigation

Some of the requirements for successful irrigation

are:

Land capability.—Irrigation should be confined to

soils that are capable ofproducing sustained high yields

of adapted crops when properly treated and managed.

The capacities and limitations of each soil for

storing moisture and plant nutrients, for withstanding

grading in land preparation, and other features

affecting irrigation design, must be taken into

account.

Adequate water supply.—A supply of water adequate to meet the needs of the crops being irrigated

must be available when needed.

Adequate labor.—The addition of irrigation to

other farm operations materially increases labor

requirements on most farms in the humid area.

Often one or more crops need to be irrigated at a

time when other crops must be planted, cultivated,

or harvested. Adequate labor must be available

when required for efficient operation of the system.

Adequate capital.—Irrigation is an expensive

practice compared with most farming operations.

This fact should be kept in mind when considering

Figure 3.—Benefits of irrigation are lost where unprotected soil erodes.

irrigation. The owner must be able to make an

initial capital investment adequate to install a welldesigned system planned to meet his individual

needs. He must have enough capital to operate and

maintain the system at least until the first year's

irrigated crop is marketed.

Erosion control.—High yields of irrigated crops

cannot be sustained on eroding land. For this

reason, irrigation should not be planned on land

subject to erosion until erosion-control measures

have been established (figs. 3 and 4). These measures may include terracing, contour cultivation,

stripcropping, and the use of cover crops. The

erosion-control practices and the irrigation system

need to be fitted together to provide ease of operation

and uniform distribution of water.

The hazards of erosion may be increased by

\L Uidl

Figure 4.—A complete conservation program is essential

to successful irrigation.
irrigation m the humid area. Land inider irrigation

is subject to the effect of natural rainfall as well as

irrigation water. A heavy rainstorm immediately

following irrigation may cause excessive runoff and

carry away valuable soil and plant nutrients.

If sprinkler irrigation is used, the water-ap])lieation rate must be no faster than the soil can absorb

the water. Excessive rates of application will cause

svu'face runoff.

The size of the drops also is important. Large drops

dislodge particles of soil that can easily be carried

awav by runoff water. On bare fields or new

seedings and row crops, rain and sprinkler irrigation

water may eom]>act and seal the soil surface. This

reduces the soil's abihty to absorb water and increases

the erosion hazard. Where crop cover or mulch is

present, the energy of falling water is dissipated and

little or no surface sealing or erosion results.

Some movement of soil particles occurs wherever

water flows over loose soil. This happens when

irrigation water is applied to furrows or borders

immediately after cultivation. By using small

streams of water on gentle slopes, erosion can be

kept to a minimum.

Adequate drainnge.—Effective drainage, both surface and internal, is essential to successfid irrigation

in humid areas. There is little point in irrigating a

crop during the early part of its growing season

only to have it damaged by poor drainage before it

reaches maturity.

Where the land is not naturally well drained,

artificial drainage must be ])rovided before the

irrigation system is installed. Most poorly drained

soils will benefit more from good drainage without

irrigation than they will from irrigation without

drainage.

Soil management.—A system of soil management

that maintains good tilth and soil fertility are

essential to successful irrigation. This is necessary

not only for maximum crop yields but also to insure

proper soil structure for irrigation water to soak in

and be held by the soil.

High fertility levels.—Maximum yields do not result from the apphcation of water alone, but from

aflequate air and moisture combined with adequate

plant nutrients. For this reason, a high level of

fertility must be maintained by the liberal use of

soil-improving crops, barnyard manures, and commercial fertilizers.

High plant population.—To get the most benefit

from irrigation the number of plants per acre needs

to be as great as can be produced without impairing

the growth of individual plants. It is usually profitable to increase the seeding rate or density of stand

of most crops when changing to irrigated farmin".

Only those crops and varieties best adapted to local

soils and climatic conditions should be used.

Control of plant diseases and insects.—Plant diseases are sometimes aggravated when irrigation w ater

is applied directly to the fohage by sprinkler irrigation. Disease-resistant varieties of crops should be

used where available. Repeated spraying or dusting

may be necessary to control insects. With sprinkler

irrigation, insecticides should be applied as soon as

practical after each irrigation.

Planning the Irrigation System

Irrigation, like any other farm operation, should

be undertaken only if it can be done successfully and

at a profit. The benefits must increase farm income

enough to cover the costs of purchasing, installing,

(operating, and maintaining the irrigation system

and leave a reasonable return on the owner's investment.

Moisture alone will not assure high yields of any

cro]), and successful irrigation involves more than

putting water on the land when rain fails. The

water must be applie«! to meet the growth requirements of the cro])s on the soils involved. Fertility

levels must be maintained and soil resources preserved for future production.

To avoid costh mistakes, the farmer must consider first whether irrigation is feasible on the farm.

This decision made, he needs to develop a sound plan

for using irrigation as a part of his total farm operation. Finally, he needs a correct design for the

system to meet the exact conditions on his farm.

Such an irrigation plan calls for careful surveys of

conditions on the farm; for reliable information on

the soils, crops, and adapted irrigation practices for

the area: and, usually, for technical services in designing the system. The resulting plan needs to be

coordinated with the conservation plan for the farm.

Surveysfor planning

Sound planning and design of an irrigation system

to fit all the foregoing variable factors requires

careful inventory of the conditions on the individual

farm. Such an inventory is needed to (1) determine

the feasibility of irrigating; (2) select the most

ada])table and efficient method of water application;

(3) provide a basis for the location and design of

field distribution systems, ditches, pipelines, structures, and pumping plants; and (4) provide information for the effective operation of the irrigation

system.

The surveys normally needed include:

Water-supply inventory.—The water-supply inventory consists of finding out what the possible

sources are and the rates and quantities available at

each. Variations in supply from one season to

another should be determined. If the supply declines during the growing season, it should be studied

by months, or shorter periods, in relation to crop

requirements.

Soil surveys.—Irrigation planning requires an accurate soil survey giving special attention to conditions that affect the application and retention of

water. The survey usually is plotted on an aerial

photograph and enlarged to the scale of the topographic maps used.

In addition to showing the kinds of soil and other

physical conditions that affect land capability, the

survey should get facts needed in designing the
irrigation system. It should indicate the general

condition of the topsoil, including structure, organicmatter content, and other factors which might influence rate of water intake.

The texture and permeability of each layer of the

subsoil should be known. This information indicates the freedom with which water and air can

move through the profile and the abihty of the soil

to retain moisture within the root zone of crops.

The depth of the soil profile and any possible

restriction to the penetration of plant roots should

be known. Where land leveling may be needed, the

depth to which the soil can safely be disturbed must

be known.

Kinds of soil must be accurately dra^vn on the

map. The location and extent of soils that differ

widely need to be considered in deciding how an

area might be subdivided, if necessarv, so that different methods of application and different amounts of

water can be used at each irrigation.

Any other soil conditions that might affect the

practice of irrigation should be shown, such as high

water table, restrictions to adequate drainage,

erosion hazards, plow soles or compacted areas, or

high salt content.

Topographic survey.—The kind of topographic

survevs needed depends upon the method of irrigation to be used and the irregularitv of the ground

surface. Detailed topographic survevs are timeconsuming and expensive: onlv enough information

need be obtained to permit sound and accurate

planning. Usually a careful inspection of an area

will reveal the nature and extent of surveys needed.

Any topographic survev begins mth a base map

showing the boundaries and dimensions of the fields

to be irrigated and the location of the water supply.

Such a map can be made by enlarging an aerial

photograph to the desired scale—usually 50 to 200

feet to the inch—and tracing from the enlargement.

For planning sprinkler irrigation, the topographic

information needed ordinarily includes only such

elevations as will: (1) Show direction of land slopes

for locating laterals and main fines; (2) show

changes in elevation along lateral-line settings so

the system can be designed to control variations in

sprinkler discharge; and (3) show maximum differences in elevation along the main line and between

the irrigated area and the water source. Where

slopes are gentle and fairly uniform, a few elevations

taken along the sides of the field, at control points

along possible main-fine locations, and at the water

source will be sufficient. On rolling land or wherever

lateral lines must follow contour rows, or on level

land where surface drainage is a problem, a more

detailed survey and a contour map are needed.

For surface or subirrigation methods not involving

land leveling, a less detailed survey will suffice.

The planner needs only those elevations that show

the natural ridges, depressions, and other features

that influence the location of contour border ridges,

subirrigation laterals, field supply ditches, and

drainage ditches.

For graded border, furrow, or other methods

requiring land leveling, a complete topographic

survey is required. Such a survey is usually made

by obtaining elevations in a grid at 100-foot intervals

parallel and at right angles to the proposed direction

of irrigation. Stakes set at each grid corner are

marked later and used as grade stakes as a guide

in leveling.

Where open ditches or permanent buried pipelines

are to be used to carry water from the source to the

field supply ditches, their location should be determined by a standard route-type survey. A profile

of each proposed centerline is needed. For computing

amount of excavation, cross sections need to be

surveyed at intervals not exceeding 100 feet along

the centerline.

Sources of power.—Sources of power available for

use with the system need to be determined and

compared. Where electric power is available, the

location of the nearest transformer, and features

such as phase, voltage, and horsepower limitations,

shovdd be noted. Particular attention should be

paid to the schedule of power rates and standby

charges.

Crop enterprises.—The crop enterprises should be

determined as closely as possible. The type, acreage,

and definite field boundaries for each crop should be

considered in relation to rotations planned. Cover

and soil-improving crops need to be taken into

account in estimating water requirements.

Farm operation schedules.—A clear picture of farm

operation schedules is needed to know how much

time can be devoted each day to irrigation without

neglecting other work. This information should

show if full-time irrigators are required during the

growing season or if incidental labor can be used at

different times of the day or night.

Choosing method of irrigation

The results of the foregoing survey will indicate

the method of irrigation best suited to the farm.

Three general types are used in the humid area:

(1) Sprinkler irrigation, (2) surface irrigation, and

(3) subirrigation. Three methods of surface irrigation are common; namely, furrows, contour borders,

and graded borders.

Each method is applicable to a particular set

of conditions; none can be used successfully under

all conditions. The principal features of the methods

adapted to humid conditions are as follows:

Sprinklers.—Water is sprayed through the air to

cover the landlike rainfall.

CROPS: All crops except rice.

SOILS AND TOPOGRAPHY: Any irrigable soil with

topography suitable for cultivation or for growing

perennial grasses and legumes.

IMPORTANT FEATURES: The method gives good

control of water appfication and generally good

efficiency; provides uniform wetting; can be used

on steep slopes; land leveling is not required unless

needed for drainage; initial costs are high; water

distribution is adversely affected by wind; power

required to operate sprinklers adds to cost.
Furroivs.—Water flows down furrows between the

crop rows.

CROPS: Row crops such as corn, cotton, soybeans,

truck crops, tree fruits, small fruits, and vineyards.

SOILS AND TOPOGRAPHY: All soils except coarsetextured ones that take water rapidly, on slopes

where furrow grades can be less than 0.5 foot per

100 feet.

IMPORTANT FEATURES: The method is adapted to

variable stream sizes: gives generally good efficiency;

provides good surface drainage; permits slight variations in furrow slope: portable and permanent

pipelines eliminate need for ditches; gated pipe and

siphons give positive control in distribution; land

leveling is required for good distribution of water;

irrigation water does not wash insecticides off foliage.

Contour borders (rice levees).—Contour ridges confine water on the land.

CROPS: Rice, pasture grasses, hay crops, small

grain, and row crops that can be flooded.

SOILS AND TOPOGRAPHY: Medium- to fine-textured

soils with smooth, reasonably uniform slopes of not

more than 1 foot per 100 feet.

IMPORTANT FEATURES: The method gives high irrigation efficiency and good surface drainage; large

streams of water are required; land preparation costs

are low; labor requirements are low; border ridges or

levees interfere with farming operations.

Graded borders.—Parallel ridges control the flow of

water down gentle slopes.

CROPS: Pasture grasses, hay crops, and small

grains.

SOILS AND TOPOGRAPHY: Medium- to fine-textured

soils on smooth slopes of not more than 2 feet per

100 feet.

IMPORTANT FEATURES: The method gives good irrigation efficiency and good surface drainage; medium

to large streams of water are required; land leveling

costs are generally high; labor requirements are low;

border ridges interfere with farming operations;

border pattern fits in wefl with furrow irrigation in

a crop rotation on slopes of less than 0.5 foot per

100 feet.

Subirrigation: A water table is kept at a controlled depth below the soil surface.

CROPS: All crops with shallow or medium root

depths, except rice.

SOILS AND TOPOGRAPHY: Very permeable soils

underlain with impermeable barrier or high water

table and with nearly level topography.

IMPORTANT FEATURES: Installation costs are low

except where the lines are used: labor requirements

are low; drainage is good; land leveling or smoothing

is usually required. The method is adapted to

limited areas.

Design and operation of the system

The irrigation system, regardless of type, must be

carefully designed to fit the soil, climate, topography,

and available labor on the individual farm where it

is to be used. In many years the difference between

a weU-designed and a poorly designed system will

mean the difference between "profit and loss.

8

Good design is not enough to insure maximimi

returns from an irrigation system. Operation in

accordance with the standards used in the design is

also essential. Many well-designed systems have

failed because they were not used properly. In order

to operate his system properly, each farmer must

know the intake rate, depth, and water-holding

capacity of his soil. He needs to be able to determine

the moisture content of his soil and must be willing

to irrigate when the soil needs water without waiting

for a rain that may not come.

Conservation irrigation guides

Irrigation design must be based on accurate informati<m about the soils of the field to be irrigated and

water-use requirements of the crops to be grown.

These vary so much from one locality to another

that local information must be used to be sure the

design will fit the conditions on the farm. Such

local information is available in most soil conservation districts and counties in the form of Conservation Irrigation Guides.

These guides are developed cooperatively by the

Soil Conservation Service, Agricultural Research

Service, Extension Service, and Experiment Stations.

They are prepared for each local area having similar

soils and nearly uniform water requirements for

crops. A group of soil, plant, and irrigation specialists acquainted with each area makes a complete

search of all available research information, analyzes

field studies, and reviews successful irrigation experiences to obtain the best local information about

soils and crops and their management under irrigation.

The Conservation Irrigation Guide tells the available moisture-holding capacity and intake rate of

each soil type. It gives peak water-use rates and

recommended depths to irrigate for different crops.

The guide recommends the adapted methods of

irrigation and gives the following types of information

for each situation: (1) Needed frequency of irrigation for different crops during season of peak wateruse rates, (2) probable field efficiencies of the irrigation systems, (.3) the amount of water that the system

should be designed to apply at each irrigation, and

(4) the estimated time required to apply this amount

of water. Figure 5 is an example of an irrigation

guide from Missouri.

Moisture in the Soil

To plan a conservation irrigation system for a

particular farm, and to operate the system to keep

the crops growing at an optimum rate, information

is needed on the capacity of the soil to take up and

retain water and the rooting habits and water requirements of the crop to be irrigated.

The soil may be considered as a reservoir for soil

moisture and plant nutrients. The roots of the plant

correspond to a pump with many inlets that penetrate deeper into the reservoir as the crop grows.

But the character of the plant limits the extent of
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root growth. Since water in the soil does not move

freelv to the plant roots, the supj)h witliin their

reach must be periodical!) replenished to kee)» the

plants growmg.

Available moisture

Different soils have different ca^iacities to store

water for plant use. After a soil is wet to saturation,

a part of the water drains to heyond the reach of

plant roots. When this free dramage stojjs and the

soil contains only the water it can hold in the ])ore

space against the force of gravity, the moisture

content is said to be at "field capacity." This point

is usually reached in from 4 to 48 hours after the

rain or irrigation, depending upon the rate of water

movement in the soil.

As plants use water from the root zone, the moisture content of the soil drops toward the ])oÍJit that

the roots no longer absorb water as fast as the leaves

transpire it; then wilting occurs. Crops suffer

permanent damage if the moisture level remains at

this "wilting point" for an appreciable length of time.

The moisture that plants can withdraw from a soil

between its field capacity and its wilting point is

known as "available moisture." A soil's capacity

to store available moisture is its "available moistureholding capacit^ ." Available moisture and moistureholding cajjacitv are measured, like ramfall, in the

equivalent of inches depth of water for the surface

area.

The available moisture-holding capacities of soils

vary primarilv with their textures. Coarse sands

will store the least available moisture in a certain

depth, and clays and mucks will store the most.

Available moisture-holding capacity is affected also

by other factors, such as organic-matter content,

salts, and compaction. In most soils these factors

vary with the different horizons of the profile, and

the moisture-holding capacities varv accordinglv.

Common ranges of available moisture-holding

capacities for soils of different textures are as follows:

inrfif'S It ufilrr ptT /i)i)l fij sail

Very coarse textures—very coarse sands 0.40-0.75

Coarse textures—coarse sands, fine sands, and

loamy sands 75-1.00

Moderately coarse textures—sandy loams and fine

sandy loams '. 1.00-1.50

Medium textures—very fine sandy loams, loams,

and silt loams 1.50-2.30

Moderately fine textures—clay loams, silty clay

loams, and sandy clay loams 1.75-2.50

Fine textures—sandy clays, silty clays, and clays. . . 1.60-2.50

Peats and mucks ' 2.00-3.00

Detailed information on available moisture-holding

capacities for particular soil types can be found in

local Conservation Irrigation Guides.

Intake rates

The intake rate is the rate at which water enters

the soil. It is dependent upon soil-surface conditions

and upon the rate at which the absorbed water can

pass through the successive soil layers and make room

for more water to be taken in. The soil layer with

the lowest transmission rate, whether at the surface

or in the subsoil, sets the limit on the intake rate.

The rate of water absorption is highest at the start

of water application; it declines as the soil becomes

wet and usually levels off at a near-constant rate set

bv the most restrictive soil layer through which the

water must pass.

Soil texture influences intake rate. Sands usually

absorb water most rapidly and fine-textured soils

most slowly. Soil conditions, such as structure and

organic-matter content, influenced by cultural practices and plant cover, also affect intake rates.

Effective intake rates vary also with the method

of water application, i. c, whether by sprinklers,

borders, or furrows.

Sprinklers.—Water application by sprinklers is

similar to rainfall. The intake rate of a soil is

modified bv the amount of cover on the land. Bare

fields or those with new seedings or in row crops take

in water more slowly than surfaces which have crop

cover or mulch.

Furrows.—When irrigation is by furrows, only a

part of the soil surface is flooded with water. The

amount absorbed is affected by the size of the

furrows and the distance between them. The movement of water is both downward and outward from

the furrow. This method gives still a different

intake rate than either sprinklers or borders.

Borders.—Under the border method of irrigation,

the surface of the ground is completely flooded.

The soil surface, therefore, has maximum opportunity

to take in water, and the rate is not the same as when

it is sprayed on with sprhaklers or run in furrows.

Intake rates for specific soil types and recommended methods of application can be found in local

Conservation Irrigation Guides.

Jf a ter movement in soils

Water movement in the soil is affected by the size

and arrangement of the pore spaces, which in turn

are influenced by the texture and structure of the

soil. Pore space is the percentage of volume not

occupied by soil particles—that which is occupied

by water or air. Coarse-textured gravelly and sandy

soils have the least total pore space, although the

individual spaces may be larger, and fine-textured

clay loams and clays have most pore space.

Soil structure breaks down with excessive tillage

or tillage when the soil is wet. A soil so treated is

apt to be puddled. Upon wetting and drying, the

surface soil bakes into a hard crust that reduces the

movement of air in the soil.

Differences in soil profiles may greatly affect the

movement of water in saturated soils. Regardless

of the intake rate or opportimity for water to enter

a soil, limiting factors below the surface, such as a

hardpan, claypan, or rock layer, a layer of sand, or

a heavy clay subsoil, may restrict the downward

movement of water. Therefore, in judging a soil

it is necessary to consider the entire profile to the

depth of root and water penetration.

10
The degree of saturation of a soil depends on the

rate at which water is apphed m comparison to the

rate at which it can move downward through the

prohle. If water is apphed no faster than the rate

of movement, the soil will be in a firm condition

at the end of the irrigation period. This is important when sprinklers are used, for it makes moving

the irrigation pipe easier. When water is applied

faster than it can soak downward, the top laver

remains saturated for some time after the irrigation.

This condition makes walking and moving of 7jipe

difficult and creates an erosion hazard should a rain

occur immediately after irrigation.

Water Use by Crops

A continuous supply of available moisture in the

soil is necessary for the best growth of crops. To

be able to rnamtam this favorable growing condition, the irrigator must know not only how much

water his soil will hold but also how fast his crops

use moisture and from what depth they withdraw it^

The irrigation system must be designed to supply

the right amount of water to meet these conditions

at the peak of the growmg season.

Water-use rates

The water required for optimum growth of a crop

includes both that transpired by the plants and that

evaporated from the surface of the adjacent soil.

This water requirement is conimonlv called "évapotranspiration" or "consumptive use," and is expressed in inches of water used per unit of time.

The water requirement or water-use rate of a crop

varies through the growing season with such things

as number of hours of daylight, amoimt of sunshine,

temperature, wmd speed', and humidity of the air.'

Generally, the water requirement is greatest when

days are long and sunny, temperature and wmd

movement are high, humidity is low, and the crop

is growing rapid Iv.

Different crops require different amoimts of moisture under the same weather and soil conditions.

For example, alfalfa uses more water than most

vegetable crops.

For designing an irrigation system, the average

daily water-use rate durmg the 6 to 10 days of most

rapid évapotranspiration of the season is sometimes

called the "peak use rate." This rate varies from

0.12 to 0.35 inch per day. Peak use rates to be used

in irrigation design are given in local Conservation

Irrigation Guides.

Moisture-extraction pattern

Plants absorb water mainly through the root hairs

and small roots. When conditions are favorable for

rapid growth, great numbers of root hairs are formed

daily. They move through the small pores in the

soil, absorbing water as they contact it. Adequate

moisture and air in the soil are favorable for good

root development and normal growth.

Figure 6.—Average moisture-extraction pattern of plants

growing in a soil without restriclive layers and with a

constant supply of available moisture.

Only a few plants develop roots and use soil moisture from depths greater than 5 to 6 feet. A good

man> annual plants ami some perennials have root

zones of 2 feet or less.

Each kind of plant has its own root-development

characteristics, which are mherent m the plant.

Most of the feeder roots are located in the upper

part of the root zone. In the humid area, a crop

usually starts the growing season with the soil at

field capacity as a result of sprmg and winter rains.

Therefore, when it becomes necessar\ to irrigate

Figure 7.—(Left), roots of oats are confined mainly to

the upper 10 inches of a sandy loam soil having a compacted "plowpan;" (right), where the plowpan is

broken by deep tillage the roots reach a depth of 24

inches. (Photos courtesy Connecticut Agricultural

Experiment Station.)

11
onlv the moisture which the plant has extracted

from the upper part of the root zone needs to be

replaced. The normal moisture-extraction pattern

of plants (fig. 6) is a good guide to needed irrigation

depths.

Root-growth and moisture-extraction patterns are

influenced by effective soil depths (fig. 7). Shallow

bedrock, hardpans, and elaypans restrict root development. Coarse sands and gravels ma) hold too

little moisture and plant nutrients for normal root

growth, or a high water table may stop it. All these

things affect the depth of soil that needs to be filled

with irrigation water.

Applying Irrigation Water

Irrigation should be planned to apply water before

the crop exhausts the available moisture in the

root zone. The system needs to be able to apply

the right amount of water to the entire field before

growth is retarded.

W hen to start irrlfiating

Practical considerations determine the moisture

level at which to start irrigation. A farmer naturall v

does not want to start when the soil does not seem

dry enough to warrant the trouble and cost of adding

more water. As the moisture level drops toward the

wilting point, crop growth is retarded and the time

left for covering the field before extreme drought

damage occurs becomes shorter. To install and

operate a system—particularly sprinklers—which

will irrigate a field under these conditions is correspondingly costly.

Research shows a marked reduction in crop yields

for nearly all crops if the moisture in the root zone

falls to less than 20 percent of the total available

water-holding capacity. On some soils some crops

are affected at the 35- to 40-percent level. The

critical level varies with the soil as well as with the

crop. Ihe cost in labor and equipment required to

maintain high moisture levels in the soil must be

carefully balanced against the effect on the yield and

quality of the crops involved.

Considering all these factors, it is usually most

practical to start irrigation when 50 to 60 percent of

the available moisture remains in the root zone. If

the irrigation system is properly designed, this

starting level allows enough time to cover the area

before yields are affected adversely.

Irrigation efficiency

Unfortunately, not all of the water applied during

an irrigation enters the soil profile and is held within

the root zone for plant use. This is true regardless of

the method of application used.

Unavoidable losses are caused by nonuniform distribution of water over the field, waste at the ends of

borders and furrows, percolation below root-zone

depth, and, with sprinkler irrigation, evaporation

from the spray and retention of water on the fohage.

Little can be done to reduce evaporation or retention

of moisture on the fohage. The other losses can be

12

held to a minimum by adequate planning, proper

design, and efficient operation.

"Field efficiency" of irrigation is the percent of the

total volume of water delivered to the field that is

stored in the root zone and ultimately consumed by

evaporation or transpiration. Other things being

equal, efficiency varies with the irrigation method

used; for a given method, it varies with the skill

exercised in the planning, layout, and operation of the

system; with climatic conditions; with the physical

properties of the soil; and with other factors. Assuming correct design and installation and reasonably

efficient operation, the ranges of efficiency that can

be expected from each method of irrigation are as

follows:

Percent

Sprinklers 65 to 75

Furrows 60 to 70

Contour borders 70 to 80

Graded borders 65 to 75

Subirrigation Up to 80

Depth to irrigate

Irrigation in the humid area ordinarily does not

require wetting the soil at each irrigation to the full

depth of the root zone. Because of the normal moisture extraction patterns described earlier (p. 11),

moisture content in the upper part of the root zone is

reduced to the critical level before the deep moisture

is exhausted. Each irrigation is planned to restore

the affected zone to field capacity. The depths of

needed irrigation for the crops to be grown are an

important factor in designing the system.

Recommended irrigation depths for different crops

are given in local Conservation Irrigation Guides.

Following are usual ranges for common crops on

deep, medium-textured, well-drained soils:

Inches

Corn 24-36

Cotton 24 - 36

Sorghum 20-30

Small grain 18-30

Soybeans 18-36

Alfalfa 36-42

Pasture 18-36

Potatoes 12 - 24

Tobacco ]^5 -24

Truck crops:

Shallow 9-12

Medium 12-24

Deep 24-30

Grapes 24-30

Fruit trees 36-60

Irrigation frequency

The frequency of irrigation needed varies according

to the water-use rate of the crop and the available

moisture-holding capacity of the soil in the root

zone. Most frequent irrigations are needed when

the crops are transpiring most rapidly. An irrigation system must be designed to meet the demands

upon it at this time.

The safe period between irrigations during the time

of most rapid évapotranspiration can be calculated

by dividing the amount of water to be replaced at

each irrigation by the peak use rate of the crop.

This gives the maximum number of days that can
be used to cover the irrigated area during the periods

of peak water use. It is also the minimum number

of days after starting irrigation before the crop will

need to be irrigated again if no rain falls in the

meantime. The irrigation svstem, therefore, needs

to be designed so that all the area can be irrigated in

this number of days.

Recommended irrigation frequencies for different

crops on different soils are shown in local Conservation Irrigation Guides.

Amount to apply

The amount of water applied at any particular

irrigation should be the amount required to bring the

moisture level in the soil up to field capacity plus the

amount required to overcome the unavoidable losses

discussed under Irrigation Efficiency. This amount

is usually expressed in inches of depth on the area

irrigated. It can be calculated by dividing the

amount of water to be replaced in the depth to be

irrigated by the estimated irrigation efficiency

expressed as a decimal.

For example, if the amount of water required to

refill the soil profile is 1.8 inches, and the irrigation

efficiency is 70 percent, the total depth of water to

be applied would be 1.8/0.70 or 2.6 inches.

In determining the amount of water to apply at

any time, the irrigator must first estimate or measure

the amount of available moisture in the soil to the

depth of irrigation. The difference between this

amount and the total available moisture-holding

capacity of the soil to this depth is the amount to be

replaced by irrigation.

Determining soil moisture

Moisture content of the soil can be either measured or estimated by one of the following methods:

Soil sampling.—Soil samples are taken from the

desired depths at several locations in each soil type,

weighed, dried in an oven, and then weighed again.

The difference in weight is the amount of moisture

in the soil, which can be converted to a percentage

or to inches of water remaining in the profile.

This method is the most accurate, but is not generally practical for farm use. Its accuracy depends

on the number of samples taken and the skill exercised in mixing and handling them. It requires the

use of facihties not ordinarily owned by farmers and

involves much time and labor. It is used primarily

in experimental work and is a standard against which

other methods of moisture determination are

compared.

Measuring instruments.—A variety of instruments

for measuring soil moisture are available commercially. These devices may not be as accurate as the

sampling and drving procedure, but they are being

improved. Their use can result in more efficient

water application which will more than pay for them

by increasing yields and saving water and labor.

The instruments now available are of two general

types: (I) tensiometers and (2) electrical conductivity-measuring devices. .

The tensiometer consists of a porous cup filled with

Figure 8.—Tensiometers for measuring soil moisture.

water and connected by a continuous water column

to a vacuum measuring device, either a gage or

manometer (fig. 8). The cup is placed in the soil at

the desired depth, and measurements are read above

ground on the vacuum indicator. These instruments

measure soil-moisture tension directly. A moisture

characteristic curve for each soil is needed to convert

moisture-tension measurements into available

moisture percentages.

At present, tensiometers do not satisfactorily

measure the entire range of available moisture in all

soil types. Thev are satisfactory for sandy soils

where tensions are low, but they are not so well

adapted to clay soils where tensions normally are

high. They are simple to operate and require a

minimimi of labor.

Electrical instruments (fig. 9) use the principle

Figure 9.—Electrical-conductance meters.

13
that a change in moisture content produces a change

in some electrical property of the soil or of an

instrument in the soil. Usually this ])roi)erty is

electrical conductivity.

Electrodes permanently mounted in conductivity

units—usualh blocks of plaster of ])aris. n\ Ion,

fiberglas. or gypsum—are buried at the depths to

be studied. Resistance or conductance meters

measure the changes in electrical resistance in the

blocks associated with changes in the moisture

content of the soil. Under usual conditions these

instruments can give good indications of moisture

content, if used according to manufacturers' instructions. For really accurate measurements, the blocks

need to be calibrated in the field.

Moisture accounting.—Calculated water-use data

can be used to maintain a daily "inventory" of the

moisture remaining in the soil. The "bookkeejjing"

starts when the moisture is known to be at or very

near field capacity. Each day the amount of water

used is subtracted from the total available moisture

in the profile. When the balance reaches a predetermined level, usually 50 to 60 percent of the total, time

to start irrigation is indicated.

The total available moisture-holding capacity of

the soil is taken from local Conservation Irrigation

Guides. Average dailv water use is computed from

climatologic-al data for monthlv periods. These

daily averages must be adjusted as temperature.

humitUty, sunshine, and wind velocity vary from

normal. The effective rainfall must also be measured as it occurs and added to the daily balance.

This procedure has the obvious advantage of

requiring no equipment or labor. Its accuracy

depends, first, on the accuracy of the computed daily

water-use rates and, second, on the irrigator's

judgment in adjusting these rates for variations in

the elimatological factors.

The "JeeV test.—The feel and appearance of soil

samples as taken in the field indicate the available

moisture content. The samples are compared with

the descrijitions in table 1 and the soil-moisture

content estimated by this guide. Obviously, this is

not an accurate method but with experience and

judgment the irrigator should be able to estimate the

moisture level within 10 to 15 percent of actual.

Water Supplies

Before investing in an irrigation system, a farmer

should assure himself that he has a water suppiv

that is (1) adequate to meet the requirements of the

acreage of crops to be irrigated, (2) of suitable quality, (3) dependable, (4) economically accessible, and

(5) legally available. These points should be carefully investigated, whether the water is to come from

a perennial stream, a reservoir or lake, or ground

water.

Amount of Water Needed

Whatever the source of water, it must be able to

deliver an irrigation stream large enough to cover

the irrigated area in the allotted interval. Or, converseh. the size of irrigated area should be limited

to what can be adequately irrigated by the available

irrigation stream.

The rate of dependable flow required depends on

the size of the area irrigated, the moisture requirements of the crops during periods of peak use, the

efficiency of the distribution system on the farm, and

the time allotted for one irrigation.

TABLE 1.—Guide for judging hoiv much moisture is available for crops

Feel or appearanee of soil

Soil moisture remaining

Coarse-textured

soils

Moderately coarsetextured soils

Medium-textured

soils

Moderately fine- and finetextured soils

0 percent Dry. loose sinple grained;

flows through fingers.

Appears to be dry; will

not form a ball with

pressure.'

Same as very light texture with 50 percent

or less moisture.

Tends lo stick together

sHghlly: sometimes

forms a very weak bail

under pressure.

Upon squeezing no free

water appears on soil

but wet outline of ball

is left on hand.

Dry. loose, flows

through fingers.

Appears to be dry; will

not form a ball.'

Tends to ball under

pressure but seldom

holds together.

Forms weak ball;

breaks easily; will

not slick.

Same as coarse-textured soils.

Powdery dry; sometimes slightly

crusted but easily

broken down into

powdery condition.

Soniew hat crumbly but

holds together from

pressure.

Forms a ball, somewhat plastic; will

sometimes slick

slightly with pressure.

Forms a ball; is very

pliable; slicks readily if relatively high

in clay.

Same as coarse-textured soils.

Hard, baked, cracked;

sometimes has loose

crumbs on surface.

Somewhat pliable; will

ball under pressure.'

Forms a ball; ribbons -

out between thumb

and forefinger.

Easily ribbons out between fingers; has

slick feeling.

Same as coarse-textured soils.

50 percent or less

50 to 75 percent

75 percent to field capacity (100 percent).

At field capacity (100

percent).

' Ball is formed by squeezing a handful of soil very firmly.

14
This required rate can be determined by the

formula,

^~ FH

Where:

Ç=the required flow in g. p. m.

¿=the irrigated area in acres

i)=the gross depth of apphcation in inches

F=the days required to complete one irrigation

H—Xht actual operating hours per day

Where losses occur in the water-distribution system, this required flow, (), must be increased to the

extent of these losses. There will be no losses in

pipelines, and only slight ones in lined canals, but

the losses m unlined canals mav be large.

Most farmers in humid areas prefer to irrigate

only in daylight. This requires a greater rate of

flow to irrigate a certain area than if the system operated 24 hours a àa.\.

For example, if a farmer wants to apply 2.4 inches

of water on a 40-acre field in 6 days, irrigating 12

hours a day, the required rate of flow would be computed as follows:

„ 453X40 acresX2.4 inches

^''"'^^"^P^^fff^l^'^^ffi,

6 days X12 hours

= 604 g. p. m.

Irrigating 24 hours per day, the required rate of

flow would be only 300 g. p. m.

Sources of Supply

Water for irrigation in humid areas nearly always

comes from a source deyeloped on the farm. There

are few organized group enterprises, such as are

common in the West. The indiyidual farmer generally must seek his irrigation water from perennial

streams, lakes or reservoirs, or wells on his own land.

Perennial streams

Farms adjoining streams can often use them as a

source of irrigation water. Since streams in the

humid areas depend ahnost entirely on rainfall,

their rates of flow are likely to yary during the year.

The supply of water is lowest during long dry periods

when irrigation needs are greatest. If the acreage

to be irrigated requires more water than the dryweather flow, the stream is not an adequate source

unless its water can be stored in a reservoir. The

smaller perennial streams and intermittent streams

are not dependable supplies except in conjunction

with storage structures.

Sometimes the farmer can increase the area irrigated by storing the overnight flow in an excavated

pit or reservoir. The reservoir should be large

enough to store the additional water needed to keep

the system operating the next day.

Unless the stream is large enough that the adequacy of the supply is obvious, the Irrigator needs

to determine the dependable dry-weather flow. He

can first check to see if the stream has been gaged by

the U. S. Geological Survey or some other Federal

or State agency. If not, it wiU be necessary to

measure the flow during a prolonged dry period.

Figure 10.—Rectangular weir.

Measuring water in streams.—Two common devices for measuring small streams of water are the

rectangular weir and the Parshall flume. The weir

is the simplest device and under standard conditions

is very accurate. The ParshaU flume is often more

convenient to use and is satisfactory for planning

irrigation. Detailed information on the construction

and use of these devices is available in U. S. Department of Agriculture Circular 843, Measuring Water

in Irrigation Channels With ParshaU Flumes and

Small Weirs.

The rectangular weir (fig. 10) commonly consists

of a bulkhead of timber, concrete, or metal across the

stream channel, having in its top edge a rectangular

opening through which the stream flows. This opening is called the weir notch; its bottom edge is the

crest; and the depth of water passing over the crest,

as measured at a definite point upstream from the

bulkhead, is the head.

The weir is installed so the notch is as near the

center of the stream as possible. The size of the

notch should be such as to cause the water to pond

above the weir and approach at a low velocity. The

water surface below the bulkhead must be low enough

that air moves freely below the stream as it leaves the

notch. A head of 0.1 foot over the weir is usually

enough to permit the stream to clear the downstream

edges of the crest and sides.

The depth of water, or head, as measured on a

scale above the crest, indicates the volume of water

passing over the weir. These amounts are read from

weir-discharge tables for different length notches

available in Circular 843.

The Parshall flume (fig. 11) is an excellent device

for measuring the flow in small streams. It can be

constructed of wood, sheet metal, or concrete, and

in a wide variety of sizes. The smaller sizes of metal

are portable.

The flume consists of a converging section, a

throat, and a diverging section. Two depth gages

on the side of the flume are set with the zero points

15
Figure 11.—I'arshall nicasuriiif; flximc.

even with the crest oí the flume. The depth of flow

is read from these gages and the corresponding rates

of flow are read from tables in Circular 843.

A ParshaU flume does not require as much ponding

as a weir for satisfactory results. Another advantage

is its ability to operate as a single-head device with a

minimum loss of head. When both a flume and weir

of equal crest length are operating under free flow

at the same discharge, the loss of head for the flume

is only about one-fourth as much as for the weir.

Qualitv of water.—The quality of water in streams

in the humid areas is generally satisfactory for irrigation. Some streams may be polluted with industrial wastes harmful to plants. If there is any question on this point, samples of the water should be

analyzed.

Water at the mouths of streams along the Atlantic

and Gulf coasts affected by tides often contains

enough harmful salts to make it unfit for irrigation.

Automatic tide gates can be installed in the smaller

streams to control the influx of tidewater. The gates

are opened to permit the flow of fresh water at low

tide and closed to keep out sea water at high tide.

Some farmers use tidal streams by irrigating only

during periods of low tide when fresh water is

running in them. In any event, the tidal streams

should be used for irrigation only after chemical

analyses have shown that their waters are safe.

Legal rights.—In most humid States the rights of

farmers to use water from streams are governed by

the riparian doctrine. Under the rules of common

law, a riparian owner is entitled to have the stream

=áerSbatg>aA

Figure 12.—Cross section of a typical farm irrigati

reservoir.

16

flow through or past his land undimimshed in quantity and unimpaired in quality except for domestic

use by other riparian owners. In many States the

courts have not determined whether irrigation is a

"domestic" use; in these States the farmer's right to

use water for irrigation is in doubt. Where such

doubt exists, the farmer should obtain legal counsel

before investing in an irrigation system.

Reservoirs and lakes

Storage reservoirs are increasing in popularity as

sources of irrigation water. Many farms have no

other dependable source.

Impounding structures.—Irrigation reservoirs usually are of the impounding type created by building

an earth dam across an intermittent or spring-fed

stream. Water is usually impounded from drainage

areas of 50 to 2,000 acres by dams 10 to 30 feet high;

the reservoirs hold 10 to 300 acre-feet of water. A

reservoir should be located as near as possible to the

irrigated field.

The acreage of crops irrigated from a farm reservoir is limited by the amount of water that will be

available during drought periods when irrigation is

needed. The required storage capacity of a reservoir

is determined from the maximum seasonal requirements of the crops to be irrigated, the effective rainfall expected in the growing season, the application

efficiency of the method of irrigation, the unavoidable

losses due to seepage and evaporation, and the

volume of dry-weather streamflow, if any. An allowance for anticipated sedimentation in the reservoir

should usually be made. Figure 12 shows a cross

section of a typical farm irrigation reservoir iUustrating the capacity needed to overcome losses as well

as that required to meet the seasonal requirements of

the crops.

The capacity of a reservoir is measured in acrefeet, which is the product of the surface area in

acres and the average depth in feet. For estimating

capacity of an impounding reservoir, four-tenths of

the maximum depth at the dam can be used as the

average depth. More precise results can be obtained

by taking cross sections of the reservoir with surveying instruments.

The watershed area above the reservoir must be

large enough that the storm runoff or reservoir

inflow is adequate to replenish the water supply as

needed. Major storm runoff usually occurs about

twice each year. The amount of runoff that can

be expected annually from a watershed depends

on many factors that vary widely over the humid

area. Engineering knowledge is needed to evaluate

these factors for a watershed and estimate the

expected annual runoff. It is often possible, however, to judge the adequacy of a contributing

watershed by studying nearby reservoirs with

similar areas.

Sometimes the water supplv from a small watershed can be augmented by off-channel storage in a

reservoir where water can be pumped or diverted

from a nearby stream during rainv seasons.

The cost of storing water in an impounding-type
-DITCH GRADE

GROUND LINE

Figure 13.—A battery of four shallow wells.

reservoir depends mostly on the topography of the

site but also on the character of foundation and

embankment material, the need for toe drainage,

and other factors. A study of a number of reservoirs

in the Southeast indicated that the cost of providing

storage" for 1 acre-foot of water ranged from |70

to $120.

The water in farm reservoirs usually is suitable

for irrigation. In some areas where tobacco is

grown, the runoff water may carry a crop disease

to the reservoir from where it may be spread to the

crop through the irrigation system.

The farmer's right to impound surface runoff for

irrigation ordinarily is not questioned if he owns the

entire watershed area. He may impound runoff

from land owned by others only if he does not damage

owners below him by depriving them of their

riparian rights to the water. In most humid

States a permit to impound water must be obtained

from a State or county agencv.

Excavated reservoirs.—Excavated reservoirs are an

important source of irrigation water in some localities. The storage basin is usually excavated in a

low-lying level area where the lateral movement

of water underground replenishes the supply. Thus,

there are two essential conditions for a dependable

excavated reservoir: (1) A high natural water table

under adjacent lands, and (2) a highly pervious

layer permitting the rapid lateral movement of

water within practical excavation depths, usually

12 to 20 feet.

The foregoing requirements and economic considerations limit the use of excavated reservoirs to

the irrigation of small areas, usually not exceeding

10 to 15 acres. The cost of these ranges from

about $200 each, up to $250 per acre-foot of storage

provided.

Natural lakes.—In Florida many natural lakes

are used for the irrigation of citrus crops and occasionally for vegetables. Lakes usually furnish a

391260°—57 .3

desirable quality of water. Their capacity often

restricts the acreage that can be irrigated. Many

of the same problems involved in using farm reservoirs also pertain to lakes.

Ground water

Many farms in humid areas do not touch perennial

streams and do not contain suitable sites for the

construction of storage reservoirs. For them, ground

water is the only prospective source of supply.

In many areas large quantities of water are

stored beneath the earth's surface in layers of

saturated sand and gravel. Not all this water can

be recovered, nor is it always near enough to the

surface for economic pumping. Where conditions

are favorable, however, ground water may be the

most dependable source of irrigation water.

Information on where and in what quantity

ground water is available can usually be obtained

from the State Geologist, the U. S. Geological

Survey, or from local well drillers.

Deep u-ells.—Deep wells are generally the most

dependable sources of supply. They are usually

drilled into a deep water-bearing stratum. The

type of pump required depends upon the depth to

the water level. Most deep wells require turbine

pumps, but where pressure causes the water to rise

to within a few feet of the surface a centrifugal

pump can be used.

The cost of a deep-well installation depends upon

the size and depth of hole and the type of pumping

equipment recpiired to obtain the desired rate of flow.

Cost estimates can best be obtained from rehable

well-drilling firms with experience in the locality.

Shallow uelts.—In some localities water can be

obtained near the surface from shallow waterbearing strata. Often these strata give low yields

and a battery of wells is needed to provide the

required flow of water (fig. 13).

There are several types of shallow wells; drilled

17
5

o

o

I

400 600

DISCHARGE (g.p.m.)

Figure 14.—Drawdown-discharge curves for two types of

aquifers.

and driven-point wells are most common. The

drilled well generally is more expensive but is better

suited for an irrigation system because it yields more

water.

Measuring well discharge.—The first question to

be answered about a well for irrigation is whether

it will dehver a dependable supply of water at a

rate fast enough to meet the needs of the area to be

irrigated (p. 14).

If the irrigation system is operated in daylight

only, the well can be made to serve a larger area by

pumping continuously and storing the overnight flow

in a small excavated reservoir.

A well should be thoroughly tested before additional monev is invested in an irrigation system.

The test consists of pumping the well at various

rates and measuring the "drawdown" in the well at

each discharge rate. By plotting the drawdown

against discharge a curve (fig. 14) is obtained which

shows the well's performance under several different

conditions. This information is of prime importance in (1) deciding the size of the area that can

be irrigated, (2) selecting a pump to fit the well,

and (3) selecting a power unit to drive the pump.

The drawdown in shallow open wells can often be

measured by lowering a tape into the well until it

touches the water. The distance the water level

drops from its position prior to starting the pump is

the extent of drawdown at any particular time.

For deeper wells, an electric drawdown gage

(fig. 15) can be used. The measuring line is lowered

between the pump column and well casing, and a

light or an ammeter shows when the electrode at

the end of the measuring line touches the water.

Another method of checking water levels is with

an air-hne and pressure gage installed with the

pump (fig. 15). A small air pump, such as a tire

pump or spray rig, is attached to the air line and the

pounds of pressure required to force the water out

of the submerged pipe multiplied by 2.31 tells the

number of feet the pipe is under water. This figure

subtracted from the length of the air line below

ground surface shows the water level.

18

A simple way to estimate the flow from a well is

to use a carpenter's square to measure the horizontal

and vertical proportions (dimensions X and Y of

fig. 16) of the arc of the stream from a horizontal

pi]ie flowing full. If the fall of water is sufficient to

obtain a vertical measurement of 12 inches, the

corresponding rate of discharge can be read directly

CLIP TO \

WELL CASING-^

45-VOLT

B BATTERY

AMMETER

BRUSH CONTACTREEL FOR INSULATED WIRESINGLE INSULATED LINE^

WEIGHTED WATERCONTACT POINT

MOTOR

PRESSURE GAGE

ATTACH AIR

PUMP HERE (0)

(X)

I I H^Xi

PUMP

COLUMN

-CASING

END OF AIR LINESHOULD TERMINATE

2 FEET ABOVE TOP

OF STRAINER

STRAINER

Figure 15.—Two types of drawdown gages: (Top),

electrical-contact gage; (bottom), air-line gage.

PUMP

BOWLS
Figure 16.—Flow from horizontal pipe. Dimensions to

be used in calculating discharge are: D, diameter of

pipe; X, horizontal distance; and 1', vertical distance.

from table 2. Or, the discharge can be calculated

from the following equation:

Q=2.83I>2X

Where

Ç= gallons per minute

JD= diameter of pipe in inches

X=horizontal distance in inches

y=vertical distance in inches

Where it is inconvenient to use this method, the

discharge from a well can be turned into a ditch and

measured with a rectangular weir or a Parsliall

flume as previously described.

Commercial flow meters usually give accurate

measurements, but these meters are expensive and

are usually not readily available in the humid States.

Quality of water.—Ground water in the humid

States is generally suitable for irrigation. Near the

Atlantic and Gulf coasts, however, salt has seeped

into many aquifers to the extent that the waters

are harmful to crops. If there is any question about

the quahty of well water, it should be tested before

use.

Legal rights.—The rights of a farmer to the use of

ground water for irrigation are not clearly defined

in most Eastern States. In many States the rule of

"reasonable use" apphes to ground water, and this

water may be used for irrigation oidy if there is no

adverse effect on other users. In some States and

in many local subdivisions, a permit is required

to develop ground-water supphes.

Pumps and Power Units

The pump and power units are a major part of the

cost of an irrigation system. The amount of money

that can profitably be spent for this equipment

depeiids on the value of the crops to be produced

and the number of hours the pump will operate each

season. If it is to be used only a short period each

year, an expensive unit involving high fixed costs

may not be economical. Cheaper pumps are

usually lower in efficiency and consequently require

more power but for short periods of operation this

may be more than offset by the lower first cost.

Specifications of pumps and power units are based

m part on the "head" or pounds of pressure which

must be overcome to move the water at the desired

rate. The elements of a pumping plant and of the

total head involved are diagramed in figure 17. To

convert head in feet to pressure in pounds per square

inch, multiply head by 2.3; to convert pressure to

head, multiply pressure by 0.433 (fig. 18).

Pumps

Centrifugal, turbine, propeller, and mixed-flow

pumps are commonly used for irrigation pumping.

Each type of pump is adapted to a certain set of

conditions under which it will give efficient service.

Centrifugal /Ju/n/>s.—Centrifugal pumps (fig. 19)

usually operate efficiently when pumping against

total heads of more than 12 feet. The suction hft

should not exceed 18 feet at sea level or 17 feet at

2,000 feet elevation. These pumps operate most

efficiently at all elevations if the suction hft is less

than 15 feet.

Centrifugal pumps are well adapted to most

sprinkler irrigation work since the total head requirements are usually high enough to produce good

efficiencies. For example, a sprinkler system operating at 50 pounds pressure would require a pump

to operate agamst a total head in excess of 115 feet.

Because of their low first cost, fight weight, long

hfe, ease of operation, and high efficiency, horizontal

centrifugal pumps are widely used for irrigation

pumping. They are well adapted to pmnping from

shallow wells, pits, reservoirs, streams, and ditches.

However, where the water level fluctuates widely

they cannot be used without raising and lowering

the pump.

Turbine pumps.—Deep-well turbine pumps (fig.

20) are adapted primarily for use in cased wells

where the water surface is too deep for a centrifugal

pump. They have been used successfully to pump

from 500 feet below ground. Turbine pumps are

as efficient as good horizontal centrifugal pumps.

TABLE 2.—Flowfrom horizontal pipesflowing full

[()=0.818 D'X when 1 = 12 inches]

Pipe diameter {D)

(inches)

Measured distance (A) in inches {Y=\2 inches)

12 14 15 18 20 22 24 26 28 30 32

2.. G. p. m.

39

157

354

627

980

1,413

C p. m.

46

183

414

731

1,145

1,650

G. p. m.

52

204

472

836

1,308

1,880

G. p. m.

59

235

530

940

1,470

2,120

G. p. m.

66

261

590

1,048

1,635

2,360

G. p. m.

T2

288

650

1,150

1,800

2,580

G. p. m,

78

314

708

1,252

1,960

2,820

G. p. m.

85

340

767

1,360

2,120

3,061

G. p. m.

91

366

825

1,460

2,285

3,300

G. p. in.

98

392

885

1,570

2,445

3,520

C. p. m.

104

418

945

1 670

4..

6...

8...

10.. 2 620

12.. ' 3 760

19
FRICTION HEAD

-SPRINKLER HEAD PRESSURE HEADFigure 17.—Elements of a pump and power unit on a sprinkler system and the corresponding elements of "total

head" used in calculatin" power requirements.

10 20

PRESSURE IN POUNDS PER SQUARE INCH

30 40 50 60 70

iiijiiiUi|iii'i|iiii|yiiiii|iiii|iiii|iiU^^ TMr iiiiiiiiil íii'iiniiíi _L iiiliifiihilli'iiiifii'iilili ii'iiiniili

80

JllL

90 100

iiniifiiili

0 10 20 30 40 50 60 70 80 90 100 110

HEAD I

Figure 18.—Scale for convert

They will give long and dependable service if properly installed and maintained. They are usually

more expensive than centrifugal pumps and are more

difficult to inspect and repair.

Propeller pumps.—Propeller pumps are useful for

pumping against heads of less than 10 feet. The

water is moved by the lift of the blades on the

impeller. These pumps will dehver large streams

of water under low heads. They are well adapted

to surface irrigation and to drainage. The first cost

is generally less than that for other types of pumps.

Propeller pumps deliver more water than centrifugal

pumps for a given size impeller and provide a more

nearly constant flow with changing pumping lift.

Mixed-flow pumps.—Mixed-flow pumps operate

efficiently at low heads of about 6 to 26 feet. The

impeller on a mixed-flow pump is a combination

structurally of the propeller and centrifugal types.

They pump best against heads intermediate between

those recommended for propeller and centrifugal

pumps, preferably 10 to 16 feet. They are best

adapted to deliver large streams of water.

Poicer units

The power required to pump water depends on

(1) the quantity of water, (2) the total head or

pressure against which it is pumped, and (3) the

efficiency of the pump.

Si:e oj power unit.—Table 3 shows the horsepower

required to pump different quantities of water

against total heads ranging from 10 to 240 feet.

20

120 130 140 150 160 170 180 190 200 210 220 230

N FEET

ing units of head and pressure.

This table is computed on the assumption that the

plant efficiency is 50 percent and it should be used

for preliminary estimates only.

Where the actual pump efficiency is known, the

horsepower requirement can be computed by the

following formula:

„ TT _g- P- m. X total head

^''"^,960 X efficience"

W here :

B. Hp.=brake horsepower required

g. p. m. =gallons per minute to be pumped

total head = static head + friction losses +

pressure head, in feet

efficiency = efficiency of pump expressed as a

decimal

If a water-cooled gasoline engine is used, multiply

the residt by 1.45; if an air-cooled engine, by 1.6; or

if a diesel engine, by 1.25.

Types of power units.—Many types of power units

can be used for operating pumps. An old automobile

engine belted to the pump may do the job at low

original cost, but operating cost is likely to be high

and service unreliable. Money is often wasted by

investing in old inefficient engines not suited to the

job.

It must be remembered that a farm tractor used to

furnish power will not be available for other farming

operations and may require modification of the cooling system. Farm tractors are not built for continuous operation such as is needed to power an irri-
Figure 19.—A centrifugal pump.

SD-537

gation pump. If a tractor is used, it should be large

enough so that it is not necessary to operate the engine at full throttle. Also, the motor should be

equipped with safety devices.

Where available at reasonable rates, electricity is

usually the most satisfactory source of power for

irrigation pumping. Electric motors offer high efficiency, rehability, compactness, and low cost of upkeep, which makes them especially desirable for

operating pumping plants.

Internal combustion engines are most widely used

where electric power is not available or where it is

too expensive. These include gasoline and diesel

engines. The former type may be adapted to burn

natural gas, kerosene, or distillates. Proper cooling

is very important when internal combustion engines

are used for irrigation pumping.

TABLE 3.—Power required to pump different quantities of water aiiainst different heads with pumping plant of

50-percent efficiency^

NC-32-1

Figure 20.—A deep-well turbine pump powered by an

electric motor.

Gasoline engines cost less initially than diesel engines and are better adapted to smaller loads and

shorter operating hours. Diesel engines are best for

heavy duty and generally give longer service. The

choice of an internal combustion engine for a given

job depends on the size of load, length of operating

periods, and the required life of the engine.

Irrigation pumping plants often operate for long

periods without attention. For this reason power

units should be equipped with safety devices to shut

them off when changes in operating conditions occur

that might cause damage. Such changes include

when: (1) Oil pressure drops, (2) coolant temperature becomes excessive, (3) pump loses its prime, or

(4) the discharge pressure head drops.

Discharge ïolal heail-

(G. p. m.) (C. f. s.)

25...

50...

100..

150..

200. .

250..

300..

350..

400..

450..

500..

600..

700..

800..

900..

1,000.

1,250.

1,500.

0.06

.11

.22

.33

.45

.56

.67

.78

.89

1.00

1.11

1.34

1.56

1.78

2.01

2.23

2.78

3.34

10

feet

20

feet

30

feet

40

feet

50

feet

Horse- Horse- Horse- Horse- Horseponer

Va

M

H

1

power

y2

1

2

2

poicer

2

2

3

poiter

1

2

3

4

potter

Vs

1

3

4

5

1 3 4 5 6

2 3 5 6 8

9 4 5 7 9

2 4 6 8 10

2 5 7 9 11

2 5 8 10 13

3 6 9 12 15

4 7 11 14 18

4 8 12 16 20

^ 9 14 18 23

5 10 15 20 25

6 13 19 25 32

8 15 23 30 38

60

feet

Horsepoiver

•M

2

3

5

6

8

9

11

12

14

15

18

21

24

27

30

38

45

70

feet

Horsepotfer

1

2

4

5

7

9

11

12

14

16

18

21

25

28

32

35

44

53

80

feet

90

feet

100

feel

Horse- Horsepower pouter

1 1

2 2

4 5

6 7

8 9

10 11

12 14

14 16

16 18

18 20

20 23

24 27

28 32

32 36

36 41

40 45

50 57

60 68

Horsepower

1

3

5

8

10

13

15

18

20

23

25

30

35

40

45

51

63

76

120

feel

poteer

2

3

6

9

12

15

18

21

24

27

30

36

42

48

55

60

76

90

140

feet

Horsepotter

2

4

7

11

14

18

21

25

28

32

35

42

50

57

64

71

88

106

160

feel

Horsepott'fr

2

4

8

12

16

20

24

28

32

36

40

48

57

65

73

80

101

121

IKI) 200 220

feel

Horsefeel feetHorse- H'trsepotter potter potter

•-> 3 3

5 5 6

9 10 11

14 15 17

18 20 22

23 25 28

27 30 33

32 35 39

36 40 44

41 45 50

45 50 55

55 61 67

64 71 78

73 81 89

82 91 100

91 101 111

114 126 139

136 151 166

210

feet

Hnrsepnnx'r

3

6

12

18

24

30

36

42

48

54

60

73

85

97

109

121

151

181

' Adapted from U. S. Dept. Agr. Farmers' Bui. 1857, Small Irrigation Pumping Plants.

21
Sprinkler Irrigation

In sprinkler irrigation the water is sprayed into

the air and allowed to fall on the land surfaee in a

uniform pattern at a rate less than the intake rate

of the soil.

The use of sprinklers for applying irrigation water

began in the United States about 1900. The introduction of light-weight aluminum pipe and quick

couplers after World War II reduced the labor cost

of moving portable sprinkler equipment. More economical and dependable power plants also lielped

make sprinkler irrigation more acceptable to farmers. Consequently, this method of irrigation has

spread rapidly during recent ) ears.

Some advantages of the sprinkler method are:

(1) It is adapted to most soils that can be irrigated and is particularly adapted to sandy soils.

(2) It can be used for all major farm crops, except

rice.

(3) It is adapted to both level land and steep

slopes; necessary erosion-control measures on steep

land can be used with sprinkler irrigation.

(4) It requires little or no land leveling if surface

drainage is adequate. This point is of particular

importance on shallow soils where leveling would

expose large areas of infertile subsoil.

(5) Very light applications for new seedings and

small plants can be made efficiently.

(6) Farming operations are not hindered by field

ditches and dikes.

(7) Small streams of water can be more efficiently

used.

(8) Soluble fertilizer can be applied and controlled

economically and efficiently.

(9) Frost can be controlled on certain fruit and

truck crops.

(10) Most of the equipment can be moved from

field to field.

Some limitations are:

(1) It usually requires the highest initial investment of any system, except where extensive land

leveling is necessary for surface or subirrigation.

(2) Wind can cause uneven water distribution.

(.3) Power requirements are high because of the

high pressures required to operate sprinklers.

(4) Water must be relatively free of sand and

debris.

(5) Water cannot be applied efficiently at low

rates under hot, windy conditions.

Types of Systems

There are three general types of sprinkler irrigation

systems:

(1) Permanent.—Permanent systems are those

having the pipes permanently located. Usually the

pipes are buried and do not interfere with farmwork. Installation costs are much higher, but labor

and maintenance costs are less than for semipermanent or portable systems. Permanent systems using

movable sprinkler heads and quick-coupled riser

22

pipes are used primarily in nurseries, orchards, citrus

groves requiring overtree sprinklers, and for other

high-value crops.

(2) Semipermanent.—Semipermanent systems

usually have the main lines buried and the laterals

])ortable. The water supply is from a fixed point.

This type of system is used where the area to be

sprinkled is permanently located. Installation costs

are somewhat less than for permanent systems but

labor and maintenance costs are more.

(3) Portable.—Portable systems have both portable main lines and laterals. Installation costs are

less than for permanent and semipermanent systems

since both mains and laterals can be used in more

than one location. Labor and maintenance costs are

higher. These svstems are designed to be moved

around the farm from field to field or even from farm

to farm. The portable system with a fixed water

supply is especially adapted to the irrigation of crops

in a rotation with nonirrigated crops. If water is

available at several locations on a farm, the system

can be operated with a portable pump. This generally reduces the initial cost and provides for flexible operation in the irrigation of one or more crops

in a rotation.

Kinds of Equipment

The component parts of all sprinkler systems are

generally similar (fig. 21). Pumps provide the

needed pressure. Supply, main, and lateral pipelines convey the water to the sprinklers. Special

equipment can and often should be added to improve

operations and reduce labor requirements.

Pumps

Sprinkler systems require fairly high-pressure

pumps, such as the turbine and horizontal centrifugal types. Turbine pumps are used to pump from

deep ground water. Centrifugal pumps are used

with surface water supplies and shallow wells. For

further information on size and kind of pumps refer

to the section on Pumps.

Supply lines

Supply lines convey the water from the pump to

the main line on the field to be irrigated. Permanent lines can be used where it is not necessary to

move during the irrigation season; otherwise portable lines are more economical. Permanent lines

should be buried deep enough not to interfere with

farming operations. A way to drain the line to prevent damage from freezing must be provided. Steel,

iron, asbestos cement, plastic pipe, and reinforced

concrete with special joints are suitable materials for

buried lines. Light-weight aluminum lines are best

for portable supply fines.

Main lines

Main lines convey the water from the supply line

to the lateral lines. Buried permanent mains make

farming operations easier and require less maintenance than portable lines. Portable mains are more
GIANT SPRINKLER HEAD

y-y^^^ LATERAL

D PLU&

COUPLER

INTERMEDIATE PRESSURE SPRINKLER

:^'2-FL0W CONTROL VALVE

HIGH RISER

TAKE OFF VALVE

CONSISTING OF

VALVE TEE COUPLER

AND VALVE OPENING

ELBOW

PIPE FOR FILLING CONTAINER

PIPE FOR EMPTYING CONTAINER

CONTAINER FOR

FERTILIZER

SOLUTION

Figure 21.—The parts of a sprinkler irrigation system.

23
ÀHkiiiKiéiTyyy l

Figure 22.—A side-roll lateral.

economical, especially when used to irrigate several

fields; however, the^ require more labor. The same

materials are suitable lor mains as lor siippl) lines.

La tero Is

Lateral lines convey the Mater from the main to

the sprinklers. Quick-coupled aluminiun pipe is the

best for most portable laterals. Buried permanent

laterals are sometimes used for orchards, nurseries,

and small gardens.

Special types of sjjrinkler laterals have been developed to save labor in moving the lines. These special types are almost always more costly than the

conventional quick-coupled ])ortable lateral.

Side-roll laterals.—Some laterals are moved by a

hand-propelled or power-driven mechanism located

at about the center of the line (fig. 22). Each line

is mounted on wheels with the pipe serving as an

axle. It is connected to the main line bv a length

of flexible hose. This type of lateral is best adapted

for use with close-growing crops on rectangular,

smooth, well-drained fields.

Pull-type laterals.—Another type of lateral is

mounted on wheels or skids and pulled lengthwise

with a tractor or truck (fig. 23). Each lateral is

towed in a curved route to where the pijje is in line

with the new lateral position; it is then pulled back

and joined to the main line. Pull-t>pe laterals are

usually used with forage crops grown on medium to

sandy soils that remain firm when wet. Some eroi»

damage may result when the lines are pulled across

muddy fields.

Self-propelled laterah.—A single lateral moimted

on wheels that are driven by hydraulic cvlinders

rotates around the field with one end pivoted at the

center. Water is fed into the system at the pivot

from a well located there or from a supply piped to

that point. The size of sprinkler nozzles increases

outward from the pivot so that the amount of water

discharged is varied to compensate for the increasing area covered by each sprinkler. This type of

system is adapted to square, flat fields. Special provision must be made to irrigate the corners which

fall outside the range of the last sprinkler.

24

Sprinklers

Sprinklers spray the water onto the land through

nozzles on the sprinkler heads which may be of either

revolving or stationary type. Revolving-head (fig.

24) sprinklers are most widely used for farm crops.

The single-nozzle head usually has a small nozzle

and the sprinklers are close-spaced along the lateral

line. The two-nozzle head has one nozzle, called a

range nozzle, set to throw water to the outer part

of the wetted circle. The other, called the spreader

nozzle, is smaller and applies water to the area near

the sprinkler. Sprinklers of this kind can be used

to apply water at average rates of 0.2 to 1.0 inch or

more per hour.

Revolving-head sprinklers operate satisfactorily

through a wide range of application rates and operating ])ressures. They are classified as follows:

Low-pressure sprinklers.—Sprinklers that operate

at 5 to 15 pounds pressure per square inch are designed with special thrust springs or reaction-type

arms to operate at low pressures. Thev will apply

water at relatively high rates over small wetted

diameters. Only a fair distribution pattern can be

expected. Because of the low operating pressure,

the waterdrops are fairly large and may cause puddling of bare soil.

Low-pressure sprinklers are adapted to small

acreages where gravity pressure can be utilized.

Their use is confined to soils where intake rates are

more than 0.5 inch per hour.

Moderate-pressure sprinklers.—Sprinklers that operate at 15 to 30 pounds pressure per square inch

are usually designed with a single oscillating nozzle

or a long-arm double nozzle. They are capable of a

fairly wide range of apphcation rates at recommended

spacings over moderate wetted diameters. The

distribution pattern is reasonably good at the upper

pressure limits. The waterdrops can be broken up

fairly well with proper jjressure.

Moderate-pressure sprinklers are adapted primarily to imdertree sprinkling in orchards. They

can be used for small acreages of field crops with

pressures of 20 to 30 pounds per square inch.

Intermediate-pressure sprinklers.—Sprinklers that

operate at 30 to 60 pounds pressure per square inch

are commonly designed with two nozzles that discharge water to an iimer and an outer circle. This

type of sprinkler has a wide range of application

Figure 23.—A pull-type lateral.

W N'yuul2
f-^'^W^-^v

Figure 24.—A re^ olving-liead sprinkler.

rates at recommended spacings. Wetted diameters

are 70 to 130 feet. Waterdrops can be well broken

up with proper combinations of operating pressure

and nozzle size. Under these conditions, distribution patterns are usually very good.

Intermediate-pressure sprinklers are probably the

most popidar because they are adapted to a wide

variety of soils and crops and can be used successfullv for overtree systems in orchards.

High-pressure sprinklers.—High-pressure sprinklers

are similar to intermediate-pressure sprinklers except

that they are designed to operate at pressures of 60

to 100 poimds per square inch. Application rates at

recommended spacings exceed 0.3 inch per hour, with

rates above 0.5 inch being more common. The

wetted diameter ranges from 120 to 230 feet. Distribution patterns are good except when wind velocity is high; then maximum spacings need to be reduced as wind increases.

Sprinklers of this type are primarily adapted to

truck crops, field crops, and pasture, particularly on

finer-textured soils. They provide fast coverage

with limited equipment.

Giant or hydraulic-type sprinklers.—Giant sprinklers

are designed to discharge a large high-velocity stream

of 100 to 600 g. p. m. iiato the air at pressures of more

than 80 to 120 pounds per square inch. Air resistance breaks up the stream into fine drops. Application rates at recommended spacings usually exceed

0.5 inch per hour. Wetted diameters range from 200

to 400 feet. The distribution pattern is acceptable

when the air is calm. Waterdrops are well broken up

when design pressures are maintaiiaed. Some washing of the soil may occur with clean-tilled crops as

the sprinkler is starting or stopping.

Giant sprinklers are adapted to the irrigation of

pastures, tall-growing crops, such as corn, where the

moving of pipe is difficult, and on isolated areas that

require complicated lateral settings when smaller

sprinklers are used. They are often used because

they can cover large areas rapidly and labor requirements for moving are less than for smaller sprinklers.

391260°—57 4

Cost of operation is usually higher due to the high

operatmg pressure required. They should be used

only on soils that take water rapidly.

Undertree sprinklers.—Some sprinklers are designed

to keep the nozzle streains below the tree foliage and

fruit in orchards. Stream trajectories are lowered by

changing the angle of the nozzles. Wetted diameters

are limited by the flat water stream. Where coverage

is confined to a single tree mterspace, application

rates are generalh above 0.3 inch per hour. Where

laterals can be moved more than one interspace each

setthag, the range of water application is increased.

Moisture-distribution patterns are fairly good.

"When laterals are moved more than one tree interspace, shifting of the lateral to give a diamond pattern

of sprinkler settings heljjs to offset the effect of interference by the trees with the distribution of the sprav.

Placing the sprinkler Imes in alternate tree rows on

succeeding irrigations will even the overall distribution.

Undertree sprinklers are adapted to all orchards

but are recommended especially for soft fruits subject to damage by water contact and where wind

distorts overtree sprinkler patterns (fig. 25).

Overtree sprinklers.—Regular moderate- to intermediate-pressure sprinklers can be mounted on risers

high enough for the stream to clear the trees at least

20 feet from the sprinkler. Moisture distribution is

normally good, but heavy foliage tends to concentrate water around the edge of the tree where it

drops from the foliage. These sprinklers are adapted

to use with all fruits not subject to damage from

water contact, such as citrus and apples.

Perfora ted pipe

A perforated pipe sprinkler system (fig. 26) sprays

the water out of small holes in the upper part of the

lateral pipe. Usually several rows of perforations

are used so that the jets of water cover a rectangular

pattern on both sides of the pipe with fair uniformity.

Perforated pipe is normally operated at pressures of

CAL-1374

Figure 2.5.—Undertree sprinklers used in an orchard.

25
CAL 6').i7

Figure 26.—Perforatt-d pipe being used lo irrigate

pasture.

15 to 25 pounds per square inch. This method is

primarily adapted to land of gentle and uniform

slope, its use is generally limited to soils having

intake rates of 1 inch or more per hour. Wind

seriously affects the distribution pattern. Since the

pipe lies on the groimd. tall crops interfere with the

water distribution.

Some perforated pipe s^ stems have only one row

of holes and distribute the water by oscillating the

pipe through an arc of about 180° with a waterpowered motor. The pipe is supported 3 to 8 feet

above the ground on posts fitted with roller sockets

so it can turn with little friction. These oscillating

sprinklers are most commonly used for permanent

systems in small nurseries and vegetable gardens.

Special equipment

Special equipment can be added to the sprinkler

system to improve performance, reduce labor requirements, and facilitate operations.

Valves are used to control pressure in the lateral

lines. They are needed in systems where main-line

pressures vary significantly at lateral takeoff points.

Takeoff valves can be used where the main and

lateral join, or straight-line valves can be located in

either the main or lateral line. In multiple-lateral

systems, these valves permit moving individual

laterals without shutting down the entire system.

This saves labor, especially when the lateral is some

distance from the pump.

Flow-control valves are used to regulate the pressure and discharge of individual sprinklers where

topography causes unequal pressures along the

laterals. They are seldom needed on level fields or

smooth slopes. Automatic backflow-control valves

protect equipment where water is pumped to hi^h

elevations.

Pressure gages tell the operator whether the required pressure h being supplied to the system. A

gage on the discharge side of the pump checks on

pump operation. A gage on the lateral just off the

main fine helps in regulating pressures in the lateral.

This is needed especially on rolhng land where the

required lateral-line [iressure varies with different

settings. A pitot gage is used for checking pressures

at the individual nozzles.

Debris screens or strainers are usually needed to

keep the system free of trash that might plug the

s|)rinkler nozzles if the water comes from a stream,

ditch, lake, or reservoir. Details of design will varyaccording to the size and amount of debris carried

in the water.

Desilting basins may be required to trap sand and

suspended silt in water from streams and open

ditches. Desilting basins and debris screens can

often be combined.

Uooster pumps may reduce operating costs when

used to provide adequate pressure for small areas

that lie considerably above the main irrigated area.

This permits the main part of the system to operate

at lower pressures than would be required if the

entire system were served by one pump.

Design of System

Designing a sprinkler system to fit soils, crops,

and labor conditions of an individual farm and to

deliver water in the amounts and rates required by

the farm irrigation plan is an engineering job. The

parts of the system must be fitted together to operate

efficiently and economically. Therefore, it is generally best to purchase the entire system from one

dealer who will follow adequate design criteria and

guarantee the operating efficiency of the system.

The American Society of Agricultural Engineers

and the Sprinkler Irrigation Association have developed and published a list of "Minimum Requirements for the Design, Installation and Performance

of Sprinkler Irrigation Equipment" (Appendix).

These standards should guide the planning of a

system to fit the general requirements of the farm

irrigation plan.

System layout

The layout of the system must be fitted to the

conditions on each farm discussed in the section on

Surveys for Planning. For a single field of regular

shape, the layout may be simple. When the system

is to be used on several fields or on odd-shaped

areas, developing the plan may be a full-scale engineering problem requiring aíternate layouts and

careful analysis of economical pipe sizes.

Some of the most important factors to consider in

system layout are the following:

Soils.—The soil survey used in making the general

larm irrigation plan is an important guide in deciding specifications of the system.

More detailed examinations are needed to find

significant differences in water-intake rates, available

moisture-holding capacities, and effective soil depths.

When a field has soil areas differing significantly in

these respects, it may be wise to subdivide it on the

basis of the amount to be appüed at each irrigation.

If the intake rate varies for the different soUs, the

system should be designed to apply water so as not

to exceed the slowest rate.

26
lopography.—Topographic conditions must be

investigated to determine the general land slope and

the maximum differences in elevation. This is

needed for (1) locating laterals and main lines, (2)

controUing variations in sprinkler discharge due to

pressure changes, and (3) computing pump and

motor requirements.

Generally, main lines should nm up and down the

slope and laterals across the slope to minimize

variations in pressure at the sprinkler heads. Figure

27 illustrates alternative layouts to fit variations in

topography.

Crops.—The types of crops to be grown on the

irrigated area must be considered. Orchards and

vineyards generally limit sprinkler system lavout,

sprmkler spacing, and types of sprinkler heads.

Different crops, and the use of cover crops and

mulch in the rotation, may affect peak water-use

requirements and water application rates.

Shape of field.—Irregular-shaped fields generally

•i-o —o—o—oFigure 27.—Some typical layouts of sprinkler systems showing effects of topography: (A) Layout on moderate,

uniform slopes with water supply in center; (B) layout illustrating use of odd number of laterals to provide required

number of operating sprinklers; (C) layout with gravity pressure where pressure gain approximates friction loss

and permits running lateral downhill; (D) layout illustrating area where laterals have to be laid downslope to

avoid wide pressure variation caused by running laterals upslope; (E) layout with two main lines «""dges to avoid

running lateral uphill; and (F) layout with two main lines on the sides of the area to avoid running laterals uphill.

27
(21 PUMP

r SPACING FROM EDGE OF STRIP

u

UNEVEN MEADOW STRIP- A/g

°'^'^m'-:yC'?0^à

ivSv :; -; UNEVEN MEADOW STRIP- NO ^RR\o,a.f\Q^-~¿i-.--'¿''}I

[ ]PUMP

Fifiure 28.—Examples of system layout with striperopping: (Top), where both meadow and row crops are to

be irrigated at the saine time; (bottom), where only the

row erops are to be irrigated.

increase the initial cost of the system and make

operation more difficult. The system layout should

permit the use of as near the same nuniher of sprinklers at each lateral setting as possible. Therefore,

fields of uniform M'idth and length best fit the pattern

of sprinkler layout. Sometimes the boimdaries of

irregular-shaped fields can be changed to fit this

rectangular pattern. The layout must be such that

the number of settings for each lateral required to

cover the field does not exceed the nmnber of settings

per day times the maximum number of days between

irrigations in the peak use period.

Conservation practices.—The sprinkler system layout and the conservation practices on the same field

need to be coordinated. On a contour stripcropped

field the laterals should follow the stri])s as nearly

as possible, and spacing of laterals and strips need to

be adjusted to one another (fig. 28). When both

crops of a stripcropping pattern are to be irrigated

at the same time, the w idth of strips should be equal

to the lateral spacing. By jilacing the laterals on

the meadow stri])s, damage to small row crops is

avoided, and it is not necessary to walk on muddy

ground to move the lines.

If the meadow strip is not to be irrigated with the

row crop the w idth of row-crop strip should be some

multiple of the lateral spacing. Even so, some overlapping of the sprinkler spray onto the meadow strip

will be necessary to insure uniform water distribution

over the strip being irrigated. If the row-crop strip

is of uniform width the lateral can be laid the full

length between two rows. Crossing of rows with the

]}ipe may damage some crops, such as strawberries.

Cro]} stri])s need to follow gentle curves so the

laterals can bend enough at the connections to follow

the rows or strip boundaries. Generally, 20-foot

lengths of pipe arc best for contour stripcropped

fields. Leveling or smoothing the ground before

JaA ing out the system may make possible more imiform width of strijis and long gentle curves.

Layout and operation will be easier on terraced

land if ])arallel terraces can be used. On fields with

nonjiarallel terraces, laterals may have to cross

terraces. Land leveling or smoothing prior to constructing the terraces hel])s to straighten out their

alinement and may make possible the use of parallel

terraces.

The system must be designed to properly irrigate

the different crops in the rotation.

If a ter supply.—The location of the water supply

affects the design and layout of the system. A water

su])plv near the center of the area to be irrigated

permits shorter mains and a cheaper system than if

the source is near the edge or outside. Not only

more but larger pipes are required for main and

sup])ly lines if water must be pumped an extra

distance.

Labor supply.—Farm operation schedules and

availability of labor are important factors of design.

Minimum labor requirements usually call for the

greatest initial investment for equipment. If no

extra labor is available on the farm for irrigation,

the system is usually designed for either one or two

sets per 24-hour day; sprinklers are moved in the

morning or evening, or both, as a routine farm chore.

This type of operation requires more equipment than

if the system is designed for more frequent moves and

more hours of operation per day. Irrigation schedides and the moving of eqiii^jinent should be planned

for least interference with other farming activities,

such as cidtivation, spraying, harvesting, or rotation

grazing schedules. Extra equipment is sometimes

required to fit irrigation into the overall farming

operation.

Criteria for system layout.—The following points

are a guide to the layout of mains, submains, and

laterals:

(1) Provide for the simultaneous operation of the

number of sprinklers which will discharge the

required amount of water.

(2) The niunber of settings for each operating

lateral must not exceed the number of settings per

day times the maximum ntimber of days allowed for

completing the irrigation during the peak use period.

(3) Select lateral lengths and pipe sizes so presstire along the laterals will not vary more than 30

percent, and preferably not more than 20 percent.

(4) Run main lines up and down the predominant

land slopes.

(5) Rim laterals across slope as nearly on the

28
contour as possible to minimize pressure variation.

(6) Use pipe of one, or not more tlian two, sizes

in laterals, especially in systems with more than one

lateral.

(7) If there is a choice of water sources, the one

nearest the center of the design area will result in

the most economical combination of pipe sizes.

(8) Split-line operation with 2, 4, or 6 laterals

operating at the same time is desirable for large areas.

(9) Plan the layout to rotate multiple laterals so

as to avoid long hauls back to the starting ])osition.

(10) Consider areas of significantly different soils

in the same field.

(11) Rough, rolling, or broken topography often

calls for major modifications in design.

Sprinkler selection and spacing

Sprinklers must be selected to fit the soil and

crop requirements previously discussed. They must

not apply water at an average rate faster than the

intake rate of the soil. Manufacturers Hst sprinkler

nozzle sizes, pressures, and application rates for

different spacings for the sprinklers they produce.

The planner of a system can select from this information one of several combinations which meet

the limiting intake rate of the soil (Appendix).

The discharge rate of a sprinkler system can be

adjusted through a wide range by varying sprinkler

type, nozzle size, and operating pressure. In deciding the best spacing, it is, therefore, possible to

consider other factors, such as imiformitv of application, wind velocity and direction, riser height, crops,

farming operations, and labor schedules.

Each sprinkler apphes water in a circular area.

The diameter of the wetted area is governed by the

nozzle size and the water pressure. The water does

not fall evenly over the area, but more reaches the

ground near the sprinkler than at the outer edge of

the circle. Variations in pressure affect both the

30 20 10 0 10 20 30

0.6

0.4

0.2

30 20 10 0 10 20 30

20 10 0 10 20

DISTANCE FROM SPRINKLER (FEET)

Figure 29.—Patterns of water application by a rotatingtype sprinkler operating at different pressures: (Top),

pressure too low; (center), pressure satisfactory; (bottom), pressure too high.

rate of discharge and the pattern of distribution

(fig. 29).

Therefore, for uniform application, the sprinklers

need to be spaced so their wetted areas overlap

(fig. 30). The distance between sprinkler heads on

y

SPRINKLER-

.40

.40

.55

.47

,40

.55

.45

\42'

.55

.47 '

/

.40/

\ /

.40\

.47 \

.50

.40

.40

.40

,40

40

-SPRINKLER

\ /

SOFT^.x'

/^ \

\

-40 FT,-

Fiffiire 30.—Comhined application of water from four

overlapping sprinklers spaced 40 x 50 feet; average

application rate 0.4.3 inch per hour.

the laterals should be 0.3 to 0.5 the diameter of the

wetted circle; the distance between lateral positions,

or the distance the lateral is moved, should not

exceed 0.65 the wetted diameter. This lateral .spacmg should be reduced to about 0.3 the wetted diameter in areas where wind velocities frequentlv are

more than 10 miles per hour. For each spacing a

certain average pressure must be maintained at the

sprinkler to discharge water at a particular rate.

Each system is designed for a certain sprinkler

size, spacing, and operating pressure; it must be

operated as designed if it is to give satisfactory

results. Changing the spacing or number of sprinklers, adding or substituting pipe, or operating at a

different pressure will throw the svstem out of

balance so that correct application of water is not

possible.

Maintenance of Equipment

Though relatively simple, the maintenance of

sprinkler equipment is highly important. Lack of

care can cause a delav in irrigation at a crucial

time that ma^' result in reduced vield or even failure

of the crop. All equipment should be inspected

periodically; worn or damaged parts should be

replaced at once.

Pumps and power units are most likely to suffer

from neglect. The manufactmer's recommendations

should be followed closely in operating and maintaining these units.

Care in handling and hauling portable pipe,

sprinklers, and risers is especially important. Improper handling of this equipment often causes more

damage than wear in use.

Most maintenance work can be done in the winter,

in rainy seasons, or at other times when the system

29
is not in use. A few off-season maintenance jobs

are:

(1) Take sprinkler heads apart and inspect them

for excessive wear.

(2) Replace enlarged nozzles. Some suppliers

offer this service to their customers and charge

onh for repair parts.

(3) Clean portable irrigation jiipe and replace

worn gaskets.

(4) Store pipe above the ground, preferably

indoors.

(5) Give special attention to pumps, motors, and

internal combustion engines and to such safety

devices as low-pressure and high-temperature cutoff

switches. A qualified mechanic should inspect these,

and repairs and replacements should be made

promptly.

Applying Fertilizer in Water

With a sprinkler irrigation system many fertilizers

can be applied in the irrigation water to almost any

crop at anv time during the growing season. This

method is quick, economical, easy, and effective,

and requires little extra equipment. The uniformity

of fertilizer application is the same as that of water

distribution.

The major fertilizer materials differ in the way in

which they can be applied because of differences in

their chemical activity in the soil.

Most nitrogen fertilizers dissolve readilv in water

and can be ap])lied through sprinklers provided

they do not contain free ammonia.

Potash also can be applied through the sprinkler

system, but there is little to recommend this method

unless unexpected potash deficiencies occur during

the growing period.

Phosphorous fertilizers are only partly soluble in

water. Therefore, this material usually is best

applied dry before or at the time of planting. Ammonium phosphate is water-soluble and can be

applied through the irrigation system, but the phosphates are likely to become fixed by combination

with calcium in the soil and not move down through

the profile like nitrogen and potash.

TABLE 4.—Amount of fertilizer to use for each setting

of sprinkler line i

Lerifilh

of line

Distance

moved each

Benin t;

N litrient ai>p ied per acre -

20 pounds 40 pounHa 60 pounds 100 pounds

Fed Pounds Fournis Pounds Pounds 40 6 12 18 30 330 60

80

9 18 27 45

12 24 36 60

40 12 24 36 60 660 60 18 36 54 90 80 24 48 72 120 40

60

18 36 54 90 990 27 54 81 135 [ 80 36 72 108 180

1 From Mich. Agr. Ext. Bui. 324, Fertiliziiif; Through

-itr^tKin 'Walter- ^ °

Other elements, such as boron, magnesium, manganese, and copper, should be applied in dry form.

Small amounts can be applied through the irrigation

system if deficiencies appear during the growing

season.

Amount to apply

The amount of anv fertilizer to be run through the

svstem at each setting of the sprinkler line can be

figured from table 4. To find the amount of bulk

fertilizer required for a certain rate of application,

divide the pounds to be applied at each setting by the

percentage of the nutrient in the fertilizer. For

example, if an application of 40 pounds per acre of

nitrogen is recommended, the length of the line is

660 feet, and the distance moved is 60 feet, 36 pounds

of nitrogen needs to be applied at each setting. If

the fertilizer material is ammonium nitrate containing 33 percent nitrogen, divide 36 by 0.33 to get

a])proximately 110 pounds, the amount of bulk

fertilizer needed at each setting.

The approximate percentages of elemental nutrients in fertilizers commonlv applied through irrigation water are as follows:

Nitrogen fertilizers: Percmt

Ammonium nitrate 33. ()

Ainmoniuin sulfate 20. 5

Sodium nitrate 16. 0

Cal-nitro 20. 5

Calcium cyanamide 22. 0

Potash fertilizers:

Potassium chloride 55. 0

Potassium sulfate , 50. 0

Ammonium sulfate is the least soluble of the commonly used solid forms of nitrogen fertilizer. About

280 pounds can be dissolved in 50 gallons of water.

About 108 pounds of potassium chloride can be

dissolved in 50 gallons of water.

Method of application

The method of adding the fertilizer to the water

varies with the type of pump.

Systems using centrifugal pump.—With a centrifugal pump, the fertilizer solution can be drawn into

the suction side of the pump (fig. 31). To do this,

the suction pipe is tapped near the pump, and a

CENTRIFUGAL

PUMP

'^DISCHARGE PIPE

PIPE FOR FILLING

— CONTAINER

U LINE FOR EMPTYING

y CONTAINER

SUCTION PIPE

Irrigation Water

30

CONTAINER FOR FERTILIZER SOLUTION

(55 GAL. OIL DRUM MAY BE USED)

Figure 31.—Equipment for distrihuting fertilizer

through a centrifugal pump.
TURBINE PUMP UNIT

_SIDISCHARGE PIPE

ï

ïï SH

PIPE FOR

FILLING

CONTAINER

TO SPRINKLERS

-!v/Avw///<>\\4 SMALL MOTOR DRIVEN

GEAR OR PADDLE PUMP

:"jj-:'"^1 ^ PIPE FOR EMPTYING CONTAINER

CONTAINER FOR FERTILIZER SOLUTION

Figure 32.—Equipment for pumping fertilizer solution

into discharge pipe of pump or into sprinkler line.

short piece of pipe is threaded into the opening. A

valve is attached to the end of this pipe to regulate

the flow of the solution. Another piece of pipe or

garden hose is attached to the outlet end of the valve

with the other end extending to near the bottom of

the fertilizer solution container. A similar installation can be made at the discharge side of the pump

for filling the container wth water to dissolve the

fertilizer.

Systems using turbine pumps.—A small pump can

be used to inject the solution into the irrigation

pump discharge pipe (fig. 32). A rotarv, gear, or

piston pump, capable of developing pressure higher

than that deUvered by the irrigation pump, is

needed. The small pump of a spray rig is adequate.

Another method is to extend a % or /ó-inch copper

tube down along the outside of the column of the

pump and tap it into the suction pipe below the pump

bowls. Some Irrigators simply pour the fertilizer

or fertilizer solution into the well between the waterdeliverv column and well casing.

Injecting fertilizer into sprinkler line.—The fertilizer solution can be injected at almost any location

on the main line or at the intake of a sprinkler line

with a small pmnp connected as illustrated in figure

32.

Dealers have equipment for introducing fertilizer

into the pipeline between the pump and sprinklers.

'TURBINE PUMPUNIT

DISCHARGE PIPE

TO SPRINKLERS

ASPIRATOR UNIT

TUBE TO ASPIRATOR

FOR EMPTYING

CONTAINER

SHUTOFF

VALVE

^CONTAINER FOR FERTILIZER SOLUTION

Figure 33.—Equipment using aspirator unit to draw

fertilizer solution into pipeline.

These applicators use a pressure difference somewhere in the line, usually between couplers, aroxmd

elbows, or at a reducer, to force the fertiUzer solution

into the pipe. A large pressure tank fitted with two

valves holds the fertilizer. High-pressure hoses

extend from the valves to fittings on the pipe (fig.

Insoluble-type fertilizers must be finelv ground to

avoid clogging the nozzles. If nozzles are one-eighth

inch or larger in diameter they arc not likely to stop

"P- . . The fertilizer should be applied toward the end of

the setting to avoid loss through leaching. Allow

10 to 20 minutes for applying the fertilizer, then run

the system for another 10 to 15 minutes to wash off

the plants and clean out the pipelines, pump, and

valves.

Frost Control

Sprinkler irrigation systems can be used to protect

crops from frost. Since the ordinary sprinkler svstem covers only a small area with anv one setting of

the line, it is usually necessary to add lateral lines

and sprinklers for adequate frosit control. The

entire field needs to be covered with a fine mist during the time of freezing temperatures.

The wetted areas of adjoining sprmklers should

just meet or overlap slightly. A triangular spacing

covers the largest area with the fewest sprmklers.

Application rates of about 0.10 to 0.15 inch per

hour are adequate. Very small single-nozzle sprinklers are best. Two-nozzle sprinklers can be used by

plugging the spreader nozzle. Satisfactory nozzle

size depends upon the spacing, as follows: 60 x 60-

foot triangular spacing, 'á- or 51>-inch nozzles; 80 x

80-foot spacing. Ym- or 'i.-inch nozzles; and 90 x 90-

foot spacing, J4-inch nozzles. The latter is practical

onh if the operating pressure is high enough to spray

the water ov^er the entire area between sprinklers.

Studies at Michigan State University indicated that

small single-nozzle sprinklers should complete a

circle in 20 seconds or less to control frost.

The frost-control system should be turned on

before air temperature at plant level gets down to

32° F. Most growers believe that applying water

until the ice is melted off is best. The field becomes

a mass of ice, but the temperature stays above freezing point of the plant fluids as long as the water is

being applied. Damage usually occurs if the water

is turned off too soon after the air temperature rises

above 32°. To be safe, the sprinklers should run

until all the ice has melted off the plants.

An automatic alarm is needed to warn the Irrigator at night when to turn on the system. An

electric thermoswitch set at plant level in the field

with wires to a loud bell in the house will serve this

purjjose. The alarm should be set to sound when the

temperature reaches 34°. The system shoidd be

laid out and tested well in advance of the time when

it mav be needed.

Frost control with sprinklers has been used successfully on low-growing cro]is such as tomatoes,

cuciunliers, peppers, beans, cranberries, and straw31
berries. If the temperature drojis lar below freezing, the ice that accumulates on trees may be heavy

enougli to break the branches. Ice is likely to break

down such crops as sweet corn, celery, pole beans, and

tall flowers. This method of frost control is not

practical for tall, thin plants.

Surface Irrigation

Surface irrigation involves transjiorting and applying water directly on the soil surface. It usually

requires some land preparation to shape the surlace

to control the flow of water. This type of irrigation,

therefore, is most applicable to nearly level land whenlittle grading is needed, or to deep soils where considerable earth can be moved without seriously reducing productivity.

An efficient water-distribution system of canals,

ditches, or pipelines is essential to successful surface

irrigation. It must also provide for control of erosion and for adequate drainage.

The methods of surface irrigation best adapted to

humid conditions are (1) furrows, (2) contour borders,

and (3) graded borders.

Land Preparation

All surface methods of irrigation require a smooth

land surface for imiform distribution of water. Good

land preparation to obtain this smoothness is the

first step in installing any surface irrigation system.

This job well done gives the following benefits:

(1) Efficient control of water residting in uniform

distribution; (2) improved surface drainage, which

is especially important in humid areas; (3) less soil

erosion and fertility loss because of better control

of water: and (4) irrigation of larger acreage with a

limited water suppK due to more efficient use of

water.

Besides land clearing, three types of land preparation are used in humid areas. These, in order of

amount of work required, are land smoothing, rough

grading, and land leveling. The choice of one over

another is governed by the character of the land, the

method of irrigation, and the cn^ps involved in each

case.

The depth of topsoil that can be disturbed without

reducing productivity often limits the extent of

leveling that is practical. This is especially true in

humid areas where many subsoils are fine textured.

Topography also often limits the degree of preparation that is feasible. Where the surface is so irregular

that fine leveling would be prohibitive in cost, other

crops and methods of irrigation requiring less complete land preparation should be considered.

The method used to apply water often dictates

the degree of land preparation required. Some

methods require a fine degree of leveling; others require only the removal of minor surface irregularities.

The value of the crop to be irrigated often limits

the amount of money that can profitably be spent

on land preparation and thereby influences the degree of leveling done.

32

Land smoothing

Land smoothing, sometimes called "planing," is

the removal of minor irregularities in the surface

without altering the general topographic pattern.

These high and low spots are usually so slight that

they are not easily noted by the naked eye. Cuts

and fills are correspondingly light.

All irrigable soils can be smoothed, but the practice is useful primarily on nearly level land or land

with slight and regular slopes. Land smoothing is

also the final operation in land leveling. In one

way or another it may be used with any type of irrigation system.

Land smoothing is generally done before laying out

a contour border irrigation system. It straightens

the contour ridges, makes farming and operation of

the system easier, and results in more uniform application of water.

Smoothing can also be used to good advantage wth

subirrigation. It permits the artificial water table to

be controlled at a more nearly uniform depth below

the land surface, resulting in more even crop growth

over the field.

Where sprinkler irrigation is to be used on nearly

level, poorly drained land, smoothing may be the

most practical means of providing adequate suiface

drainage.

Surface irregularities are often caused by tillage

implements. These can best be removed by periodic

land smoothing.

Land smoothing is done with landplanes, levelers,

and floats. A landplane has an adjustable blade

about the middle of a frame mounted on wheels

(fig. 34). Landplanes are made in lengths up to 80

feet and with blade widths up to 15 feet. This great

length makes it possible to do a smoother job with

this machine than other equipment but large tractors

are needed to pull it.

The automatic leveler is smaller than the landplane. It also has a movable blade at about the

middle of the frame that automatically loads dirt

from the high places and dumps it in the low places.

Levelers are available in a variety of sizes, some of

which can be pulled by farm tractors (fig. 35). They

do a good job of smoothing.

The common wooden float or drag is built in many

different designs. Being shorter than the other types

of smoothing equipment, these do not leave the land

surface nearly so smooth.

All types of smoothing equipment operate on the

same principle; i. e., the blade removes the high spots

and pushes the soil into low areas ahead. Usually

three or more passes of the equipment in cross directions are needed, the last pass being made in the

direction of irrigation.

Land smoothing is usually contracted for either

by the acre or by the hour. The cost depends on the

type of equipment used and the number of "passes"

made over the land. Costs usually range from $1.50

to $2.00 per acre for each pass of the landplane.
Figure 34.—Smoothing land with a 60-foot landplane.

NEV-696

Rough grading

Rough grading is the removal of knolls, mounds, or

ridges, and the filling of pockets, gullies, and other

low areas in a field. These irregularities are much

greater than those described under land smoothing

and are clearly visible. Cuts are relatively heavy,

often amounting to 2 feet or more. Rough grading

is usually followed by land smoothing or land

leveling.

Since knolls or mounds normally occupy only a

small percentage of a field suitable for irrigation,

the productive topsoil is not disturbed over a wide

area. Therefore, this tvpe of land preparation can

be used on most irrigable soils and under all topographic conditions where surface irrigation is

adapted.

It is good practice to excavate the high spots to

about 6 inches below the finished ground surface

and to fill the holes with topsoil from adjacent

areas. Allowance needs to be made for settling of

the fills and swelling of cuts.

If the soil removed from the high spots can be

placed in nearby low areas or spread over adjacent

areas—which it usually can—a track-type tractor

and dozer blade can be used most economically

(fig. .36). For long hauls, a rubber-tired carrier-type

scraper pulled by a large tractor is more economical.

Rough grading is usually contracted for by the

equipment-operating hour, since the yardage of

excavation is difficult to estimate without unwarranted time-consuming field surveys.

Figure 3.5.—Automatic leveler pulled by farm tractor.

33
LA 61750

Figure 36.—Rough grading with a bulldozer.

Land leveling

Land leveling is the reshaping of the land surface

to a planned grade. As practiced in humid areas,

this usually consists of leveling the surface of each

field to one or a series of planes (fig. 37). These

planes usually have some slope both in the direction

of irrigation and at right angles to this direction.

Land leveling normally is essential to successful

and efficient irrigation by either the graded border

or graded furrow method. The plane or planes

must have slopes suitable to the method of irrigation,

as follows:

Graded borders—0.10 to 2.00 feet per 100 feet

length of border; 0.00 to 0.10 foot cross slope per

border width.

Graded furrows—O.OS to 0.50 foot per 100 feet

length of furrow; 0.00 to 2.00 feet per 100 feet cross

slope. Cross slope must be reduced where fin-rows

are less than 6 inches deep.

AZ-5330

Figure .37.—Land leveling with a wheel-type tractor

and a carryall scraper.

Land leveling is limited to soils that are suitable

for irrigation by border or furrow methods and that

are deep enough that the needed cuts and fills can

be made without permanently reducing productivity.

Yields may be lower immediately after some landleveling jobs, but most soils can be restored to their

original productivity in 2 or 3 years by the use of

good conservation and soil-management practices.

Where damage is likely to be more permanent, land

leveling is not practical.

Topography often limits the extent of land levehng

that is feasible. In humid areas it is generally

considered too costly to move more than 600 cubic

yards of earth per acre. Leveling is usually limited

to land which can be graded economically to slopes

not exceeding 2 feet per 100 feet. Slopes of less than

1 foot per 100 feet are preferable.

A topographic survey is needed to make the landleveling plan. Such a survey is usually made by

3 4 5 6 7 8 9 10 II 12

Figure 38.—Topographic map made by grid method, showing elevations at 100-foot spacings and contour lines at

0.5-foot intervals.

34
the grid method, in which stakes are set and elevations recorded at intervals of 100 feet or less (fig.

38). The stakes are later marked and used as grade

stakes in earth moving.

The finished plan can take the form of a landgrading map showing the depth of cut or fill at eacli

stake in a 100-foot grid pattern (fig. 39). Such a

map shows the equipment operator at a glance

where the material is to be excavated and where it

is to be spread.

Making a land-leveling plan is a complex job

requiring the services of an experienced engineer.

The slopes of the resulting plane must fall within the

limits practical for the type of irrigation and still

conform as nearly as possible to average slopes

before grading if costs are to be held to a minimum.

The "cut" yardage must be in near balance with the

"fill" yardage Avith due allowances for compaction

and shrinkage.

Differences in slope patterns within a field often

require that it be divided into parcels to be leveled

separately (fig. 40) if the amount of earth moved is

to be kept to a minimum.

Ifthe natural slope is so steep as to require material

reduction, the least amount of grading wiU be needed

if each parcel is held to the minimum length that

can be irrigated satisfactorily (parcels A and B in

fig. 40). Abrupt changes in steepness (parcels B

and C and parcels F and G) usually require that

each portion be leveled separately. Sharp changes

in the direction of maximum slope (parcels D and E),

causing borders or furrows to run in different directions, require separate treatment of the parcels.

Where the natural slope is too steep to permit

leveling to a plane, or where the soil profile is too

shallow to permit deep cuts, the land can be leveled

in narrow strips across the main slope. These strips

are called "benches" and the practice "bench leveling." Since this method leaves a sharp break in

slope between the benches, a ridge is needed along

the lower edge of each bench to hold the water and

prevent erosion (fig. 41).

The rubber-tired, carrier-type scraper is used on

most land-leveling jobs. This implement, with one

operator, excavates, loads, hauls, and spreads the

earth. These scrapers are pulled bv large tractors.

Capacities range from 5 to 23 cubic yards. Thev

are especially well adapted to jobs where cuts are

heavy or hauls long.

A smaller implement, recently developed, is the

elevating scraper. It is also a rubber-tired, carrier

scraper that performs all operations with one operator.

It is available in 5-, 8-, and 10-cubic-yard sizes and

can be pulled by a medium to large farm tractor.

It is fast and well suited to most land-leveling jobs.

The rotary scraper is used to a lesser extent in land

leveling. It is much smaller and lighter than the

carrier scrapers and can be powered bv a farm-size

tractor. It is especially suitable for light work and

is used primarily by farmers who wish to do their own

leveling rather than employ a contractor.

After the grading job has been completed and

checked bv the engineer, there still will be small

irregularities in the surface that must be removed.

This is best done with a landplane or automatic

leveler, as described under Land Smoothing.

QUANTITIES; 5,868 CUBIC YARD CUT

3,523 CUBIC YARD FILL

Figure 39 -Leveling-plan map based on topographic map shown in figure 38. Amount of cut or fill is shown on

ealh^ade stake left at points'^where elevations were taken in survey. The shaded area is to be cut, the unshaded

^ IS to be filled.

35
HIGHEST POINT

LOWEST POINT/

Figure 40.—Division of a field for land leveling to fit

slope patterns. Arrows indicate direction of irrigation.

The cost per acre of leveling a field depends on a

number of factors; the principal ones are the volume

of excavation required and the length of haul. This

volume ordinarily varies between 200 and 600 c^ibic

vards per acre. The cost of excavation usually is 12

to 18 cents per cubic yard of earth moved.

A newly leveled field should not be planted immediately to a perennial crop, such as alfalfa. When

irrigated, new fills may settle, and the soil in cuts

may swell, causing irregularities in the surface.

t4^-'{f¿

^il.B-l8oy

Figure 41.—Bench leveling with a motor grader.

For the first year or two after leveling, a field should

be planted to annual crops so these irregularities

can be removed by smoothing.

Maintenance is'required to keep the surlace ot a

leveled field smooth. This can be done by going

over a field at least once with a landplane, leveler, or

float each time a seedbed is prepared. Plowing must

be done to avoid dead furrows and backfurrows.

Two-way plows are widely used for this purpose.

Water-Distribution Systems

Ditches or pipelines transport irrigation water

from the source of supply to the area to be irrigated.

They need to be carefully planned to deUver the

required amount of water without excessive losses

and without soil erosion.

Ditches

Earth ditches are usually the cheapest way to

transport irrigation water on the farm. In some

cases, they may serve for both irrigation and drainage.

Ditches are best adapted to clay or loam soils.

Excessive seepage does not occur in these soils and

(htchbanks are more stable than in sands or sandy

loams.

For efficient irrigation, the Avater surface in the

ditch must be 0.5 to 1.0 foot higher than the adjacent

ground to be irrigated (fig. 42). To build the

ditchbanks to this height usually requires more

dirt than should be excavated from the ditch channel.

Where land leveling is done, this additional fill

material can be supplied as part of the leveUng job.

It is important that ditches be large enough to

carry the amount of water needed. Ditches that are

too small increase labor requirements and result in

low irrigation efficiencies. Table 5 gives the ditch

sizes needed for different irrigation streams.

When it is planned to irrigate directly out of a

ditch with turnouts or siphon tubes, the ditch should

be nearly level (less than 0.10 foot fall per 100 feet).

In a level ditch^ water can be backed up for a maximum distance, requiring a minimum of check dams

and labor to control the irrigation flow.

Water-control structures

Ditches should be of gentle grade to avoid highvelocity flows which would cause soil erosion. If

irrigation ditches must be constructed on steep

slopes, drop structures are necessary to control the

velocity of the water (fig. 43).

Other water-control structures, such as checks and

chutes, may be needed to control irrigation water

and save labor. Checks can be built of concrete

Figure 42.—Cross section of a typical unlined irrigation

ditch.

36
(fig. 44) in the ditch, or can be built of wood or metal

and placed in the ditch. By placing stoplogs or

flashboards in the structures, the flow of water can

be partially or wholly directed into another ditch,

thereby controlhng the water-surface elevation so

the proper irrigation stream is delivered to the

furrows or border strips.

Drawings of several types of drops and checks are

in the Appendix.

In temporary ditches (those that have to be

rebuilt each year), water is usually controlled with

portable devices such as canvas, plastic, or metal

checks and dams (fig. 45). These structures can be

moved for use in different places and can be stored

out of the way during the nonirrigation season.

Ditch linings

Earth ditches can be lined with impervious material to prevent excessive seepage and the growth of

weeds in the channel.

One of the main problems in the use of earth

ditches is the control of weeds. Weeds in a ditch

obstruct the flow of water. If allowed to go to seed,

TABLE 5.—Sizes of ditches required for different

irrigation streams.

GRADE O.OS FOOT PER lOn FEET

Stream size

(Cubic feet per second}

0.5.

1.0.

2.0.

4.0.

8.0.

Size of ditch 1

r

Base width 1 Water depth

Gallons per

minnie Inchra Inches

225 12 8

450 18 9

900 24 11

1.800 36 13

3,600 48 17

GRADE 0.1 FOOT PER 100 FEET

0.5 225 12 6

1.0 450 15 9

2.0 900 18 10

4.0 1,800 24 13

8.0 3,600 36 16

GRADE 0.2 FOOT PER 100 FEET

0.5.

I.O.

2.0.

4.0.

8.0.

225 12

450 15

900 18

1.800 18

3,600 24

5

5

9

12

15

GRADE 0..5 FOOT PER 100 FEET

0.5 225

450

900

1.800

3.600

12

12

18

18

24

4

1.0 6

2.0. 7

4.0. 10

8.0. 13

NEB-1808

Figure 43.—A series of concrete chute drops in an irrigation ditch down a steep slope.

they may be spread over the farm in the irrigation

water.

Concrete.—Concrete is the most widely used ditchlining material (fig. 46). It is permanent and fairly

cheap, and is readily available in most localities. It

will prevent weed growth in channels.

Three methods of fining ditches with concrete are

generally used: (1) Hand placement with or without

forms, (2) mechanical placement with slip-form, and

(3) application under air pressure. AH three

methods are satisfactory if the following precautions

are observed: (1) Proper preparation of subgrade;

(2) use of proper kind and proportion of cement,

gravel, sand, and water; (3) proper mixing, placing,

finishing, and curing of the concrete; and (4) proper

maintenance after ditch construction.

Soil cement offers possibilities for use as a ditchlining material where sand and gravel are not

available but where subgrade soils or those nearby

are sandy. Soil cement is made of portland cement

' Side slopes 1.5 to 1.

M;i(-mo7

Figure 44.—A concrete check with flashboards in place.

37
WYO 700

Figure 45.—A canvas check used to turn walor in a

ditch.

and natural soil. Careful control of the mixture is

necessary. The amount of cement to be used depends on the type of material with which it is mixed:

normally about 12 percent cement by volume will

give satisfactory results.

Asphalt.—Asphaltic concrete and buried asphalt

membranes reduce seepage and weed growth, but

they are not widely used for farm irrigation ditches

because special equipment not readily available to

the farmer is required for their installation.

Prefabricated asplialt membranes are manufactured in rolls similar to tarpaper and can be laid

easily by hand. This material is subject to damage

by livestock and weather. Covering it with a layer

of earth prolongs its life but increases the growth of

weeds and the difficulty of cleaning ditches.

Bf/n7/flèrics. —Butyl-coated fabrics of fiberglas,

cotton, ravon, and nylon have been tested as ditchlining materials. Butyl reinforced with nylon has

given best results. This material is highly resistant

to weathering, but is damaged by the trampling of

livestock.

Plastics.—Plastic films have shown promise where

not subject to damage by livestock.

Earth and bentonite.—Seepage can sometimes be

reduced by lining the ditch with one or more layers of

impervious silt or clay or with bentonite. In the

humid areas, however, this procedure is only a temporary expedient since it does not help in the control

of weeds, and earth linings are easily damaged in

ditch maintenance.

NEB-1806

Figure 46.—An irrigation ditch lined with concrete.

Pipelines

Pipelines offer many advantages over open ditches

for transporting irrigation water. They can be

buried so they do not interfere with farming operations, and they require httle maintenance. Their

primary disadvantage is the high initial cost. Reductions in water losses and labor requirements, however, usually more than offset this additional cost.

Low-pressure pipelines (heads less than 25 feet or

pressures less than 10 pounds per square inch) are

used primarily with surface irrigation methods.

Where the lines are permanent (usually buried), they

are often made of concrete or plastics. Portable lowpressure pipelines are usually made of light-weight

metal, treated canvas, flexible plastic, or rubber.

High-pressure pipelines are used when operating

pressures in excess of about 10 pounds per square

inch are required, as with sprinkler systems.

Permanent high-pressure lines are usually buried

and are made of steel, asbestos-concrete, or reinforced concrete with special joints. Portable highpressure lines are usually made of light-weight metal

with pressure-seal couplings.

The use of pipelines to carry irrigation water is

increasing rapidly in the older irrigated areas. In

the humid areas where ditch maintenance costs are

high, the use of pipelines is recommended wherever

possible.

Applying fertilizer in water

Fertilizer can be appUed through a surface irrigation system by placing it in the ditch or pipeline

before the water is turned into the furrows or border

strips. The fertilizer must first be dissolved and

then thoroughly and uniformly mixed with the water.

The dissolved fertihzer can be sprayed into the

irrigation stream through nozzles spaced at intervals

across the supply ditch a short distance from the

area to be fertihzed.

Another method is to place a wooden structure

consisting of a V-shaped divider and baffles in the

supply ditch. A container, such as an oil drum in

which the fertihzer has been dissolved, is mounted on

the structure so the solution can be discharged into

the water at the divider. The turbulence of the

water in the structure niLxes the fertilizer.

Furrow Irrigation

In furrow irrigation, water is applied by running

small streams in furrows between the crop rows.

The water soaks into the soil and spreads out into the

crop root zone between furrows.

W here adapted

Furrow irrigation can be used with all field crops

planted in rows, truck crops, tree fruits, small fruits,

and vineyards. The land to be irrigated should have

productive soil deep enough to permit needed land

leveling without exposing harmful amounts of subsoil. The water-intake rate should be at least 0.2

inch per hour and not more than 4 inches per hour.

38
Loams and day loams are best adapted for

furrow irrigation because their available moistureholding capacities are high and their intake rates are

right. Very sandy soils are not well adapted because their high intake rates make short rows

necessary.

Topography should be smooth and uniform. Irregular topography increases leveling costs and labor

requirements.

The land must be leveled so that water can travel

the entire length of the row without ponding. This

means that all high and low spots must be removed

and the land given enough slope to let the water flow

down the furrows. The leveled grade must be maintained from year to year.

Water should be available at the rate of at least

10 g. p. m. for each acre to be irrigated.

Some advantages of the furrow method are:

(1) Where land leveling is not excessive, initial

investment is low since the furrows made in cultivating row crops can be used to convev water.

(2) Uniform water distribution and high efficiency can be obtained with proper design and

operation.

(3) The water does not wet foliage and wash off

insecticides.

(4) The required land leveling provides for excellent surface drainage.

(5) Small streams of water can be used efficiently.

Design of system

After land preparation is completed, the system

can be designed to fit the soil and topographic features of the field. Furrow spacing, furrow grade,

cross slope, furrow streams, and length of run

should conform to the local Conservation Irrigation

Guide. The specifications in this section will serve

as general guides if more specific local information

is not available.

Furrow spacing.—Furrows can be spaced to fit

the crops grown and the standard machines used

for planting and cultivating. Crops like corn and

potatoes have furrows between all rows. Bedded

crops, such as lettuce, carrots, and onions, may have

pairs of rows between furrows, ^'ide-spaced crops,

like tomatoes, fruits, and berries generally require

more than one furrow between crop rows.

Furrows are spaced close enough that water will

spread from their sides into the ridge and root zone

of the crop before too much moves down below the

root zone. The way water moves through different

soils affects practical spacings of furrows (fig. 47).

Care must be taken during tillage to keep furrows

deep enough to carry the irrigation stream. A furrow 6 inches deep is required to carry a furrow stream

of 40 to 50 g. p. m.

Furrow grade.—In humid areas, furrow grades

should not exceed 0.3 foot per 100 feet. Steeper

grades may permit erosion from high intensity

storms. Furrow grades up to 0.5 foot per 100 feet

may be safe if runs are short enough to prevent

accumulation of enough water to cause erosion.

DEPTH

FEET

Figure 47.—Typical patterns of moi'íture penetration

by the same amount of water from furrows in different

soils.

A minimum furrow grade of 0.05 foot per 100 feet

is needed to assure surface drainage.

Cross slope.—The slope of the land at right angles

to the direction of irrigation, often called the cross

slope, should not exceed 2.0 feet per 100 feet.

Where furrows are less than 6 inches deep, the cross

slope should be less than 2 percent, and it mav be

necessary to use shorter runs to jirevent accumulation of water and overtopping of the furrows.

Furrow streams.—At the beginning of an irrigation the largest stream of water that will not cause

erosion is used in each furrow. This stream, which

is allowed to flow until the water reaches the ends

of the furrows, is called the "initial" stream. Its

purpose is to wet the entire length of each furrow

as quickly as possible. Reducing the difference in

time that water infiltrates at the upper and lower

ends of the furrow gives more uniform distribution

of water and improves the efficiency of irrigation.

The size of the initial furrow stream is determined,

for the most part, by the furrow grade and cross

section. Following are usual maximum nonerosive

furrow streams for different furrow grades:

Gallons

per

Furrow grade, percent minute

0.05 66

.10 66

.15 66

.20 50

.25 40

.30 33

.40 25

.50 20

After the initial stream has reached the lower end

of a furrow, the stream is reduced or "cut back" to

one that will just keep the furrow wet throughout

its length with a minimum of waste at the end.

This cutback stream flows until the required amount

of water has been apphed.

39
TABLE 6.—Intake rate for total area irrigated by furrows

Furrow intake rale (^'ulloiiH per minute

per 100 feet)

0.1

.2

.3

.4

.5

.6

.7

.8

.9

1.

2

3^

4.

5.

6.

7.

8.

9.

10,

Furrow Spacing

18 indien 20 inellen 22 inehcB 21 incheH 27 inches 30 inches 36 inches 48 inches

Inrftes per liu-hrs per Inrltps fuT inrtws prr fnrhps pr-r Inches per Inches per Inches per

hour lititir hiiur hour hour hour hour hour

0.06 0. 06 0. 05 0. 05 0.04 0. 04 0.03 0.02

.13 .12 . 11 .10 .09 .08 .06 .05

.19 . 17 .16 .14 . 13 . 12 .10 .07

.26 .23 .21 .19 .17 .15 .13 .10

.32 .29 .26 .24 .21 .19 .16 .12

.39 .35 .32 .29 .26 .23 .19 .14

.45 .41 .37 .34 .30 .27 .23 .17

.52 .47 .42 .39 ..34 .31 .26 .19

.58 .52 .48 .44 .39 .35 .29 .22

.65 .58 .53 .48 .43 ..39 .32 .24

1.29 1.16 1.06 .97 .86 .78 .65 .48

1.94 1.74 1..58 1. 45 1.29 1. 16 .97 .73

2.58 2. 33 2.12 1.94 1.72 1.55 1.29 .97

3.23 2.91 2.64 2.42 2.15 1.94 1.62 1.21

3.88 3.49 3.17 2.91 2.59 2.33 1.94 1.45

4. .52 4.07 3.70 3.39 3.01 2.72 2.26 1.70

5.17 4.66 4.23 3.88 3.45 3.10 2.58 1.94

5.82 5.24 4.76 4.37 3.88 3.49 2.91 2.18

6.47 5.82 5.29 4. 85 4.31 3.88 3.24 2.43

To compute the proper size of cutback streams,

the average intake rate of the soil must be known in

gallons per minute per 100 feet of furrow. Actual

rates should be determined for each field by methods

set forth in Agriculture Handbook No. 82, Methods

for Evaluating Irrigation Systems, or taken from the

local Conservation Irrigation Guide. The usual

ranges of intake rates for different textured soils

follow :

Galliftis per

minute per

100 feel of

Fine textures: Jurrmr

Dense clays 0 to 1

Silty clays to clays 0.5 to 2

Clay loams to silt loams 1 to 2

Medium textures:

Silt loams to loams 1 to 3

Moderately coarse textures:

Fine sandy loams to sandy loams 1.5 to 10

Coarse textures:

Loamy fine sands to loamy sands 5 to 15

The proper size of the cutback stream is this

intake rate times the length of run in hundreds of

feet. For example. 800-foot furrows on a silt loam

with an intake rate of 2 g. p. m. per 100 feet would

take a cutback stream of 16 g. p. m.

To determine how long to allow this stream to run,

the amount of water to be applied in inches is divided

by the intake rate in inches per hour. This intake

rate is affected by the spacing of the furrows, and

the furrow-intake rate must be converted to total

intake for the irrigated area (table 6). In the

example, if the planned application were 4 inches

and the spacing of the furrows 36 inches, the cutback stream would need to run ^^ or 6.1 hours.

It may not be necessary to cut back the initial

furrow streams to get high irrigation efficiencies on

some soils. This would be true where intake rate

is high when water is first applied and verv low

later. An example is a fine-textured soil that tends

to crack upon drying.

40

If at least three-fourths of the water to be applied

is absorbed during the first fourth of the computed

irrigation time, the use of cutback streams is questionable. As this ratio increases, the practice has

even less value. In such cases a stream in excess of

the average intake rate of the soil but less than the

maximum allowable nonerosive stream is satisfactory. After this stream wets the lower end of

the furrow, the Irrigator cuts off the water and turns

it into other furrows. Such a procedure results in a

saving of labor w ith little or no sacrifice of efficiency.

Length of run.—The optimum length of run is

usually the longest furrow that can be safely and

efficiently irrigated. If the run is too long, water

soaks in too deep at the head of the furrow bv the

time the stream reaches the lower end. This means

that the upper end of the furrow is overirrigated

and the lower end may even be underirrigated.

If the run is too short, extra cross ditches are

required, and labor requirements will increase.

The length of run best for a particular field is

affected by the erosion and drainage hazards, the

kinds of soil, the slope of furrow, and the size

of stream that the furrow will carrv.

The intake rate of the soil affects the length of row

that can be irrigated efficiently. Runs must be

shorter on a porous sandy soil, for example, than on

a tight clay soil on the same slope. Also, the same

furrow s should not cross two widely different kinds

of soil, as from a sandy loam to a clay loam. Rather,

the field should be divided so each type of soil can

be irrigated separately.

If the furrows are too long on soils having poor

internal drainage, runoff from rainfall may collect

at the lower ends and damage the crop. The danger

of erosion from rainfall mav also limit the safe

length of run. The size of the furrow also affects

the length of run by limiting the size of furrow

stream that can be used.
The length of furrow which can be effectively

irrigated may be as short as 150 feet on soils with

intake rates in excess of 2 inches per hour, or as

much as 1,000 feet on soils with low intake rates

and free drainage. A good guide is to make runs

of a length that the maximum nonerosive stream

reaches the end of the furrow and needs to be cut

back in about one-fourth apphcation time. The

safe length varies for different furrow grades and

amounts of water applied on different soils (table 7).

TABLE 7.--Safe lengths of run for different furrow grades and amounts of water applied on different soils '

Furrow gra le (perceiu)

Fine-textureil noils Mediuin-texliireil sttils Moderately coar8e-texlured

soils

Coarse-textured soils

2-inch

irrigation

4-inch

irriK^ilion

2-inch

irrigation

4-inch

irrigation

2.inch

irrigation

4-inch

irrigation

2-inch

irrigation

4-inch

irrigation

0 05

Fe<-I

800

800

800

1, 000

1, 000

950

800

550

Feet

800

800

800

1,000

1,000

1,000

900

550

Feel

800

800

800

920

800

720

620

550

Feel

800

800

800

1,000

1,000

1, 000

880

620

Feel

660

660

660

560

500

450

380

340

Feel

920

920

920

800

700

640

540

480

Feet

300

300

300

260

225

205

175

150

Feel

425

10 425

15 425

20 360

25 320

30 290

40 250

50 220

' Assuming maxiinuni iioiierosivt' stream will be used in furrows of adequate eross section; smaller furrows will require smaller

streams and shorter runs. \^ here drainage is a problem, the length of run shotjld not exceed that shown in the local Drainage or

Irrigation Guide, whichever is shorter.

Distribution and control of water

Control devices are required to meter the proper

amount of water from the field ditch or pipeline into

each furrow. The most common devices are siphons

for field ditches and gates, valves, or sleeves for

pipeUnes. *

Each system needs enough of these devices to

take the entire flow of the pipeline or ditch. A few

extras make moves easier. The following are

common methods of distribution and control of

water in furrow irrigation.

Siphon tubes.—Siphon tubes (fig. 48) are widely

used because they are easy to set and to move from

one location to another. Thev are made of hght

metal, rubber, or plastic. Some metal siphons have

gates at the lower ends for controlling the flow.

Two or more small tubes can be used in each furrow

if the stream is to be cut back after the initial

wetting. The stream can be reduced by removing

one or more of the tubes, or by using a smaller one.

The rate of discharge is affected by the size of the

tubes and the variations in the operating heads;

i.e., the difference in elevation of the water surface

in the supply ditch and at the outlet of the tube

(table 8).

Gated pipe.—Gated pipe is made of light-weight

metal with small gates spaced to match the furrows.

The pipe comes in sections easy for one man to

handle and fitted with simple watertight connections.

The gates can be adjusted to control the flow

of water into the individual furrows (fig. 49). The

advantages of gated pipe are: (1) There is no loss

TABLE 8.—Discharge from siphons operating under

different heads '

nea i in inches—

of siphon

(inches) 1 2 3 4 5

C. p. m.

2

5.5

8

10

13

17

21

25

28

33

36

6 9

V2

G.p. m. G.p. m.

1.3

3

4

6

8

10

13

15

17

19

21

G. p. m,

1.6

4

5

7

10

13

16

18

21

24

27

G. p. m.

1.8

5

7

8

12

15

18

22

25

29

32

G.p. m.

2.1

6

9

11

15

19

24

28

32

36

40

G. p. m.

2.7

% 7

1 ....

11

11/, 13

lîi

1 %

18

23

lU 28

iVa 33

VÁ 38

1% 44

2

ARK-619J3-D

Figure 48.—Siphon tubes distribute water to furrows.

1 Head is the dilTerencc in height of water in supply ditch and at discharge

end of tube.

41
Figure 49.—Gated pipe delivers water directly into the

furrows.

of water between the field ditch and tlie furrow;

(2) it is not necessary to construct head ditches

across the crop rows in the field; and (3) the pipe

can be uncoupled and laid parallel to the ro\NS or

removed from the fields during cultivation, or

periods when irrigation is not needed.

Distribution hose.—A vinyl-coated glass, cloth,

butvl, plastic, or canvas hose can be used to distribute

water to the furrows. Kacii section is fitted with

outlet tubes spaced to match the furrow. The hose

usually comes in 100-foot lengths that can be

rolled up and carried from field to field. Surface

hose has the same advantages as gated surface ])ipe.

except that it is much more susceptible to deterioration by weathering and use.

Buried pipelines and risers.—A distribution system

consisting of buried pipelines with risers ending in

distribution pots can be used to convey water to

the furrows. This system is generally used in

orchards and vineyards. It usually requires the

highest initial investment, but has the advantage

of greatly reducing labor and maintenance costs.

Contour Borders

Contour border irrigation is the application of

water to a gently sloping field that has been divided

into strips boimded by low earth ridges or levees on

the contour. The ridges confine the water to each

strip until the required irrigation is completed. For

most farm crops, when the desired amount has

soaked into the soil the excess is drained off and used

to irrigate the next border strip below. In irrigating

rice, water is held on the entire field for long periods

of time to control weeds.

W here adapted

For successful contour border irrigation, soils

should be medium- to fine-textured. They should

be able to store at least 1.5 inches of available moisture per foot of depth or at least 3 inches in the root

zone of the crop. The intake rate should not exceed

0.5 inch per hour. For rice culture, the soil must

have a restricting layer just below the root zone with a

permeability rate of not more than 0.01 inch per hour.

Topography should be smooth and reasonably uniform, with a maximum slope of not more than 1 percent. Where slopes in small parts of a field exceed

1 percent, land leveling or grading is required.

A water supply of at least 10 g. p. m. per acre

irrigated is needed. The irrigation stream must be

large enough to permit rapid flooding of each border

strip. A minimum of 225 g. p. m. (0.5 c. f. s.) per

acre in the largest strip is recommended. Larger

irrigation streams utilize labor more efficiently by

permitting one man to irrigate a larger acreage in

a day.

Advantages of the contour border method are: (1)

Efficient and uniform distribution of irrigation water

is easily obtained, (2) maximum utilization can be

made of rainfall, (3) adequate drainage facilities are

easily provided, (4) operation of the system is simple

and easy, (5) initial installation costs are low, and (6)

labor and other operational costs are relatively low.

Limitations on the use of contour borders are: (1)

They are restricted to the soil and topographic conditions described; (2) land smoothing is required; (3)

relatively large irrigation streams are usually required; (4) young crops may be damaged on soils

that crust after being wet; and (5) maintenance of

border ridges, ditches, and structures is a problem.

In Arkansas and Louisiana where these systems

are used extensively, initial installation costs range

from $15 to $30 per acre, depending upon the land

smoothing required. These costs do not include the

development of the water supply. After installation,

one man can irrigate as much as 300 acres.

It is desirable to smooth the surface of the area

to be irrigated by two or more passes with a landplane, the number of passes depending on the relative smoothness of the existing surface. This

smoothing of the surface tends to permit better

alinement of the borders, a more uniform application

of water, and naore effective drainage.

Design of system

A contour border irrigation system in the humid

areas should adhere closelv to the natural drainage

pattern to provide for the removal of excess rainfall

(fig. 50). The border ridges follow the contour with

one end of each ridge ending at the natural drainageway, if one exists. W- or V-shaped drainage ditches,

which also convey irrigation water, are usually

located in the natural depressions (sec. B-B, fig.

50).

Border strips.—Where the contour border method

is used with row crops, cultivation is usually completed before the border ridges are built. Crop

rows, therefore, must be carefully laid out so that

when the ridges are built later each row will drain

toward the upper side of a ridge and will contain

no pockets or high spots.

The desirable vertical interval between border

ridges is 0.20 foot. This interval can be increased

to Ó.40 foot where topography is such that a smaller

vertical interval would result in a horizontal interval

of less than 40 feet.

To provide uniform penetration of water and rapid

42
CONCRETE PIPE CULVERTS

SEE SECTION B-B

C. M. PIPE TURNOUT

SEE SECTION C-C WELL OR OTHER SOURCE OF SUPPLY^^

i DRAINAGE CHANNEL' FOR BORROW ONLY

CONTOUR LEVEE SECTION A-A _ . EOUAL TO LEVEE HEIGHT'

* IO'MINIMUM

I.O'MINIMUM-DEEPER FOR

PIPES OVER 12" DIAMETER

0.5 MINIMUM

CONCRETE PIPE CULVERT

LENGTH 8-0". DIAMETER

DETERMINED BY DRAINAGE

AREA. MINIMUM DIAMETER-12"

1.0'MINIMUM DOUBLE DRAINAGEWAY SECTION B-B

PIPE TURNOUT WITH GATE

SUPPLY DITCH SECTION C-C

Fifnire 50.—A typical layout of a contour border system.

removal of the excess, border strips should not ex- Where the ridges are temporary, as for rice or row

ceed 600 feet in length. The area of individual crops, their side slopes should be no steeper than 2

strips should not exceed 2 acres for each cubic foot to 1, horizontal to vertical. In permanent pastures

per second (450 g. p. m.) of water available, and 1 the ridges should have side slopes no steeper than 4

acre per cubic foot per second is better. to 1 to permit the use of mowing machinery and

Border ridges.—The settled height of the ridges to resist trampHng by livestock,

must equal the sum of the vertical interval between Distribution and control of iiater

them, the depth of water applied, and the freeboard , , . ,• , , ^ r^ r i i i

needed. The freeboard should be at least 0.25 foot. A dramage ditch, at least 0.50 loot deep and with

An additional 0.30 foot must be provided for settling. side slopes no steeper than 2 t:^ 1, is needed along

Thus, the unsettled ridge height might be computed the upper side of each ridge. The ridges are usually

as follows' constructed with a grader, using the dirt removed

Feet from the ditch (sec. A-A, fig. 50).

Vertical interval between ridges 0.20 Where water enters a drainageway from both

Depth of water to be applied (3 inches) 25 sides, it consists of two V-shaped ditches with a

Freeboard 25 levee between them. The levee should be wide

Allowance for seitling ^ enough to serve as a roa(lwa>. ])roviding easy access

Minimum unsettled height 1.00 to all parts of the field. \\ here the water enters

43
Í6«'

y/^.

%

>v

—-::^ró^£^

Figure 51.—A corrugated metal pipe turnout for contour border irrigation.

from one side, one ditrh and a levee are needed.

The drainage ditches must be 0.50 foot deeper than

the ditches along the upper sides of the border

ridges, or a minimum of 1 foot.

The center levee must be as high as the border

ridges. Side slopes of levees should be no steeper

than 2 to 1. and in pastures they should be at least

4 to I.

In order to hold water on the border strips, the

ridges must cross the drainageways; drainage is

provided by a pipe culvert in the drainage ditch

under each border ridge (fig. 51). Usually two 4-

foot joints of concrete pipe or an 8-foot section of

metal pipe is used. The diameter of the pipe is

determined by the drainage area above. The pipe

should be at least 12 inches in diameter and large

enough to remove 2 inches of water in 24 hours.

Culverts are plugged at their upper end to hold

irrigation water on the border strip until the desired

amount has soaked into the soil. For irrigating

rice, where water is to be held on the field for several

davs, spillways are installed in the contour ridges to

convey the water from one stri]) to the next and to

control its deitth.

Supply ditches along the upper side of the area

to be irrigated convey the water to the highest

border strip in each series. Each ditch consists of

two parallel levees built of soil from a borrow area

between. Since the system operates by gravity,

water is conveyed above ground surface and a head

of at least 0.50 foot, and preferably 1 foot, ntust

be provided at the turnouts. Tops of the levees,

therefore, must be at least 2 feet above the highest

part of the irrigated area. Side slopes usually

should be not steeper than I/2 to 1 (sec. C-C, fig. 50).

Water enters the upper border strips through pipe

turnouts. An 8-foot section of corrugated metal

pipe with an inexpensive light steel gate on the

upper end is easy to install and is satisfactory.

The diameter of the pipe depends upon the planned

rate of flow to the borders and the head available to

obtain that flow.

Operation of system

Irrigation with a contour border system begins

with the culverts in the drainageway plugged. The

turnout gate in the supply ditch is opened, and water

enters the upper border strip of a series. When

water reaches the desired depth on this strip the

first culvert in the drainageway is partially opened,

allowing water to flow into the next lower strip; or,

in rice irrigation, the water may be controlled automatically by spillways in the contour ridges. When

water reaches the desired depth on each strip, it is

turned onto the next one in the same way.

After water has been on the upper strip long

enough to permit infiltration of the desired amount,

the plug in the culvert controlling that strip is

removed, and the excess drains off quickly. This

process is repeated downslope to complete the irrigation. With careful operation only the excess water

on the lowest strip of each series need be drained off

as waste.

The time of the first irrigation of row crops is

especially important, for flooding may damage young

plants. Most row crops should not be irrigated by

the contour method until the plants are at least 45

days old.

Graded Borders

A graded or parallel border irrigation system consists of parallel strips of land separated by low earth

ridges on a field that has been leveled to a desired

grade (fig. 52). The strips are level crosswise between borders and normally run down the main

slope. Where the land has been leveled in benches,

the strips run across the main slope.

A large stream of water is turned in at the head

of a border strip and moves downslope as a sheet

confined by the ridges or borders, soaking into the

soil as it goes. As soon as the desired amount of

water is applied—usually before the sheet has

reached the lower end—the water is cut off. When

the receding water reaches the lower end the entire

strip has been irrigated.

W here adapted

The graded border method is adapted to most

soils where depth and topography permit the required land leveling at a reasonable cost and without

permanent reduction in soil productivity. Slopes

ordinarily should not exceed 2 feet per 100 feet.

This method is adapted to a wide range of soil

textures, excepting only those having extremely high

or extremely low intake rates. Soils with too high

intake rates require very large irrigation streams and

very short border strips to avoid excessive water

losses by deep percolation at the upper ends of the

strips. On the other hand, soils with extremely low

intake rates require correspondingly small irrigation

streams; with these it is difficult to get adequate

coverage and to avoid excessive runoff from the

lower ends of the strips.

The graded border method is especially suited to

the irrigation of close-growing crops not damaged by

44
temporary flooding. It is used primarily for irrigating îiay crops, pasture grasses, and small grains.

The graded border method has a number of advantages: (1) The initial investment is relatively low,

since the border ridges can be constructed cheaply

with ordinary farm implements; (2) with a good

system one man can irrigate a larger acreage than

with some other irrigation methods, and labor costs

are correspondingly low; (3) cost of pumping, if any,

is low since the system does not operate under

pressure; (4) uniform distribution and high water-use

efficiencies are possible with proper design, layout,

and operation; (5) relatively large irrigation streams

can be used efïicientlv; (6) operation of the system

is simple and easy: and (7) properly graded border

strips provide for excellent surface drainage if adequate outlet facilities are provided.

On the other hand, this method has the following

limitations: (1) It can be used only on certain soils

and slopes, as previously discussed; (2) a more precise

land-leveling job is required than for other surface

methods; (3) relatively large irrigation streams are

needed, often in excess of 2 c. f. s. or 900 g. p. m.:

(4) light applications, less than 2 inches in depth,

are difficult to make efficiently; (5) young crops

may be damaged or extra tillage required on soils

that bake or crust after being wet; and (6) the border

ridges must be maintained and they tend to interfere

with mechanized farming.

Design of system

Precise land leveling is essential to successful

irrigation by the graded border method. The final

slope shoulâ not exceed 1 foot per 100 feet in the

direction of irrigation for hay crops and small grains.

Slopes of up to 2 feet are permissible for irrigation

of permanent pastures. The graded slopes should

be uniform throughout the length of the border

strips, although some variation can be tolerated to

avoid excessive leveling costs. In such cases, the

steepest slope in any border strip should be no

great<'r than iVz times the flattest slope, or the difference between slopes should be no greater than 0.50

foot per 100 feet, whichever is less. The grade

should increase or decrease consistently without

und (dation.

\V here the crop to be irrigated is to be rotated with

row crops irrigated by the furrow method, the slopes

in the direction of irrigation should conform to the

requirements of the furrow system.

The border strips should be level crosswise; in no

event should the cross slope exceed 0.10 foot in the

width of one strip. Greater cross slopes cause the

water to concentrate along one side of the strip

giving uneven irrigation.

Size of border strips.—The dimensions of border

strips depend upon the size of the irrigation stream

available, the intake rate of the soil, the slope of the

land, and the amount of water to be applied.

This information should be obtained locally by

field surveys and from the local Conservation Irrigation Guide. The table of dimensions of border strips

and size of irrigation streams (table 9) includes

common recommendations from local guides covering a wide range of conditions. These general

recommendations should be used only where more

specific local information is not available.

The width, or distance between border ridges, is

governed largely by the slope of the land and the

size of the irrigation stream that is available and can

be safely turned into the strip. Where possible,

the selected border-strip width should be a multiple

of the width of the least flexible farm implement to

be used in the field.

The length of a border strip is limited by the size

of the irrigation stream that will flow without

causing erosion. For example, with a slope of 0.50

Figure 52.—Irrigation with graded borders.

45
TABLE 9. Sizes of border strips and irrigalion streams for different slopes and amounts of water applied

on different soiis

Wl.llh of

IIODIIT

strip

Fine-lextured soil, intake rule 0.4 in./tir.

Meiliijn -textureil soil, intake

in./lir.

rate 1.0 Moderately coarse-textured soil, intake

rate 2.0 in./hr.

Slope f>f border elrip 2-iiirli irrif-'iilioii -t-iiirli ¡rri^'<ilion 2-inch irrigation 4-inch irrigation 2-inch irrigation 4-inch irrigation

(perceiu)

Irrigation

stream

Length >

of border

strip

Irrigation

Hi rea ill

Length 1

of ln>nler

strip

; ..,./

1,.320

1,320

1, .320

1,320

1,320

1.320

1. 320

1.320

Irrigation

stream

Length '

of border

strip

Irrit,'iilif>n

Htream

Length 1

of.horder

strip

Irrigalion

stream

Length 1

of border

strip

Irri.

gation

stream

Length 1

of border

strip

0.25 50

50

45

40

40

35

30

30

C./. s.

4.2

3.9

3.2

2.6

2. 4

1.8

1.3

1.0

Fi'ft

1,320

1,320

1,320

1,320

1,300

1. 1.50

1,050

890

C. /. s.

2.3

2. 2

lis

1.4

1.4

1. 1

.9

.9

C. f. ».

8.5

6.6

4.5

2.9

2.4

1.8

1.3

1.2

1,0.30

860

720

580

500

450

400

340

C./. s.

5.7

5.4

4.4

3.0

2.4

1.8

1.3

1.0

1, 320

1,320

1,320

1.000

930

840

760

6.50

C./. 5.

8.4

6.7

4.5

3.0

2.4

1.8

1.3

1.0

Fnt

530

450

370

300

260

230

210

180

C. /. s.

8.5

6.6

4.5

3.0

2.4

1.8

1.3

1.0

Fcm

960

.35 800

50 670

.75 550

1.00 460

] 25 410

1.50 370

2.00 320

1 Lengths of hortler «tripa are for perniiinent [>.

lengthe or 1,000 feel.

(iture. W here alf;ilfa. annual ha> cropti, or umall frrain» are irrigated, border etripH should not exceed these

foot per 100 feet, a flow of 0.10 c. f. s. per foot of

border-strip width may be used safely. Witli a slope

of 2 feet per 100 feet, the safe size of stream is about

0.035 c. f. s. per foot of width. Thus, the larger

streams that can be used safely on the flatter slopes

permit longer border strips to be used. On finetextured soils with slopes of less than 0.2 foot per

100 feet, the length of strips is limited by surface

drainage requirements.

The area of the border strip should not be greater

than the maximum irrigation stream will cover

uniformly without excessive losses in the time allotted

for the irrigation set.

Irrigation streams.—The size of the irrigation

stream depends on the texture and intake rate of the

soil, the size of the border strip, and the depth of

application required. Coarse-textured soils with high

intake rates require large streams to spread the water

over the entire strip ra])idly and avoid excessive

losses due to deep percolation. Fine-textured soils

with low intake rates require smaller ones to avoid

excessive losses to runoff from the low"er ends of the

strips. For the same reason, larger streams are

Figure 53.—Determining intake rate of a soil with

cylinder infillronieter.

required for large strips than for small ones, and for

shallow applications than for deep ones.

It is important that intake rates of the soil be

determined in the field. Soil Conservation Service

technicians obtain this information with cylinder

infiltrometers (fig. 5.3). Knowing the intake rate

and the slope of a field, an experienced technician

can calculate the border-strip dimensions and size of

irrigation stream that will give most uniform distribution and greatest efficiency.

Border ridges.—Border ridges need to be high

enough to control the flow. They must be higher

for the flatter slopes than for the steeper ones. The

base of the ridge should be broad so that farming

operations can be carried on over it. The sides of

the ridge should be no steeper than 2 to 1, horizontal

to vertical. Thus, on a steep slope where a ridge of

minimum settled height of 4 inches is needed, the

base should be at least 24 inches wide. On flatter

lands where an 8-inch ridge is needed, the minimum

base width should be about 48 inches.

Two implements commonly used to build border

ridges are the border disk (fig. 54) and border drag.

A variety of other implements can be used. Some,

like the border disk, leave shallow ditches along both

CAL-6974

Figure 54.—Building a border ridge with border disk.

46
MON Í-12S

Figure 55.—A permanent turnout in bank of supply

ditch.

sides of the border ridges. These ditches need to be

filled bv blading or smoothing between the ridges;

otherwise, the water flows down the sides of the strip

in these channels and does not wet the soil evenly.

Distribution and control of ivater

Water is brought to the heads of the border strips

in head ditches, sometimes called "field laterals." or

in pipelines. Earth ditches are most economical on

gentle slopes; they are satisfactory on medium- to

fine-textured soils with moderate to low intake rates.

On coarse soils with high intake rates, water losses

during conveyance are excessive in unlined ditches.

Pipelines are more satisfactory under such conditions.

Where head ditches are used, water flows into the

border strips through open turnouts, pipe turnouts,

or siphons over the bank of the ditch. Turnouts are

usually permanent structures and are used where

the border ridges are to be maintained permanently

(fig. 55).

In loose sandy soils or where large irrigation

streams are desirable, siphons that will discharge uj)

to 2 c. f. s. each are advantageous. One or more

siphons can be used to deliver the desired rate of flow

to a single border strip. Such large siphons require

the use of a priming pump, but they are easy to

handle and positive in action. The investment in

enough siphons to irrigate a large field is relatively

small compared to the cost of adequate permanent

turnouts. The use of siphons leaves the head ditch

free of obstructions and permits ditch maintenance

by machine instead of hand labor.

A system of buried pipelines with an outlet valve

in each border strip is most satisfactory from the

standpomt of farm operations. Pipelines do not have

to be laid out on grade. Water losses in conveyance

are negligible. No land is taken from cultivation,

and there is no problem of weed control.

Subirrigation

Subirrigation is a metho<l of applying water beneath the ground surface. It is usually done by

creating an artificial water table and maintaining it

at some predetermined depth, usually 12 to 30 inches,

below the ground surface. Moisture reaches the

plant roots through capillary movement upward.

Where Adapted

Several conditions are requisite to successful subirrigation. These are:

(1) An adequate supply of water relatively free

of salts must be available throughout the growing

season.

(2) The area to be irrigated must be nearly level

and comparatively smooth. Land leveling or

smoothing is usually required for best results.

(3) There must be a permeable layer, such as

organic soil or sand, immediately below the surface

to permit rapid movement of water both laterally

and vertically.

(4) Beneath the permeable layer must be a barrier

against excessive losses through deep percolation.

This barrier may be a relatively impervious layer in

the substratum or a permanent high natural water

table on which an artificial water table can be built.

(5) The distribution system must permit the

water table to be raised to a uniform depth below

the ground surface of the entire area.

(6) An adequate outlet for drainage of the irrigated area must be available.

This method of irrigation has several major advantages in areas where it is adapted: (1) Subirrigation can be used effectively on some soils that are

difficult and expensive to irrigate by other methods,

(2) installation costs are low except where water is

introduced through tile, (3) labor requirements are

low, (4) the method is adaptable to a large variety

of crops and does not interfere with tillage practices,

(5) high efficiency in water use is possible, and (6)

the same system can provide for both irrigation and

drainage.

Subirrigation also has rather severe limitations.

The principal one is the unusual combination of

natural conditions for successful subirrigation, discussed in the following paragraph. Only water of

good quality can be used and soils may become saline

unless water apphcation is carefully controlled.

High fertility may be difficult to maintain.

There are few places in the humid States where

all these conditions exist, and they are of limited

area. Hence, the opportunity for subirrigation is

limited. Among the better-known areas where subirrigation is practiced are the Everglades of south

Florida and the Elatwoods of the Florida Coastal

Plain. There are smaller scattered areas of organic

soils in the Great Lakes States of Michigan, Indiana,

Minnesota, Wisconsin, and Ohio. In these States

the practice is known as "controlled drainage,"

since the water table is controlled by a system of

47
FLA-D 25. 1

Figure 56.—A subirrigation system using open ditches for water distribution.

structures in drainage canals and additional water

normally is not needed for irrigation.

Distribution and Control of Water

Water may be introduced into the soil from open

ditches (fig. 56), mole drains, or tile drains. The

former method is most widely used since it is relatively inexpensive and is adaptable to all soils that

can be subirrigated.

Since the main or supply ditches are used for

drainage also, their depths and cross-sectional areas

are often determined bv the volume of water that

must be removed from the area. These ditches

usually contain check structures equipped with

flashboards to control the water level. The structures are sjjaced at vertical intervals not exceeding

6 inches. The flashboards are removed to permit

drainage when irrigation is not required.

Feeder ditches, drains, or tile lines are spaced to

keep the depth of the water table within limits

prescribed for the site.

Mole drains can be used successfully onlv in

organic soils, as in the Florida Everglades.

Tile is sometimes used in the organic soils of

Michigan, Indiana, Minnesota, Wisconsin, and

Ohio, and in the sands of the FJatwoods area of

Florida. The principal deterrent to the use of tile

is the high cost of installation which mav amount to

several Imndred dollars per acre. This cost is

justifiable only for very high-value crops.

48
Appendix

Minimum Requirements for the Design, Installation, and Performance of Sprinkler

Irrigation Equipment '

These minimum requirements pertain to the

design, installation, and performance of sprinkler

irrigation equipment and include dealer-purchaser

responsibilities. The design and performance requirements are concerned particularly with those factors

that are directly related to land, crops, and farm

operations. The dealer-purchaser responsibilities

recognize successful operation of a sprinkler system

as depending on both buyer and seller.

Part I. Design and Performance

1. Application Rate. A portable sprinkler irrigation

system, when properly designed and operated,

shall meet the following conditions with respect

to water application:

a. Water shall be applied at a rate that does

not cause runoff during the normal operating period nor cause it to stand on the

surface of the ground after the sprinkler

line is shut off.

b. Determination of the pro]ier rate of application shall be the responsibility of the person

designing the svstem. Bare-ground infiltration rates for different types of local

soils may be obtained from responsible

agricultural technicians. In the absence

of such technical adyice, the designer may

estimate the proper application rate on the

basis of past experience with similar soil

types.

2. System Capacity.

a. For regularly irrigated areas, the system

shall haye the capacity to meet the peak

moisture demands of each and all crops

irrigated within the area for which it is

designed. Sufficient time must be allowed

for moying laterals and for permitting

cultural practices on the land. The cajjacity

must also allow for reasonable water losses

during application periods.

b. For supplemental irrigation and/or special

uses, the system shall haye the capacity

to apply a stated amount of water to the

design area in a specified net operating

period.

3. Depth of Water Application. In the design of the

system, total depth of application (equivalent

rainfall) per irrigation shall be goyerned by the

capacity of the soil to store moisture and the

depth of the principal root zone of the crop to

be irrigated. Information on both of these

' This recommendation was sponsored by the Siibcommiltee on Sprinkler Irrigation, Soil and Water Division

American Society of Agricultural Engineers, and received

ihe official endorsement of the Sprinkler Irrigation Association at its annual convention at Biloxi, Miss., Nov. 1951. it

was approved Feb. 1953 as an official ASAE recommendation.

factors may be obtained from agricultural

technicians or may be estimated by the designer

on the basis of his past experience with similar

soil types and crops.

4. Uniform (? ater Application. Since uniformit\ of

ap|)lication is affected by both pressure in the

line and spacing of sprinklers, recommendations

for desirable operating pressures and spacings

for different types of s|)rinklers and nozzle sizes

shall be obtained from the sprinkler manufacturer.

Differences in pressures at the sprinklers shall

be ke]>t to a minimum to assure reasonably

uniform distribution of water over the entire

design area. A common rule, which should be

adhered to as closely as practicable, is to limit

pressure differences along a sprinkler lateral to

20 percent of the higher pressure.

Excessive pressure differences in the main or

supply line result in widely varying pressures

at the head of the laterals. Often these excessive

variati<ms cannot be controlled by pipe size

alone. Then the only practical alternative is to

design for adequate pressure at the lateral

takeoff, where pressure in the main will be

lowest, and instruct the operator how to regulate

pressures into the other laterals by adjusting

the takeoff-valve openings.

5. Crop Damage. Water shall be applied in a wav

that will not damage plants or fruit.

Part II. Dealer-Purchaser Responsibilities

A. Dealer Responsibility

1. Proper Design. ^^ hen the system is planned bv

a dealer or his representative, the dealer shall

assume full responsibility for the proper design

of the svstem he proposes to furnish. Design

requirements to fit the system to conditions of

soil topography, water supply, and crop enterprise shall be ascertained by the dealer either

directiv or bv obtaining such information from

recognized reliable sources.

When design requirements are furnished in

writing bv the purchaser, the dealer's responsibility shall be limited to design of the system to

meet the stated conditions.

When plans and specifications arc furnished

in writing bv the purchaser, the dealer's responsibility shall be limited to supplying equipment that will satisf> the requirements of the

specifications furnished.

When the purchaser buys the system piecemeal, he absolves all dealers of responsibility

for the performance of the system as a unit.

2. Proper Installation. The dealer or his re]iresentative shall assume full responsibility for the

proper installation of the system.

Pumps and power units shall be set on a firm

base, and care shall be taken to keep the pump

and the motor or engine in proper a linement.

Wiring and starting eqtdpment for electrically operated plants shall comjily with ajiproved

standards. Electric motors shall be provided

with overload and low-voltage protection.

49
Internal-combustion engines shall be provided

with protective devices. Thermostats shall be

snpjdied that stop the engine when water or

oil temperatures exceed the safety point.

T\ here conditions are such that a failure of the

water supply might cause the jnnnp to lose its

prime, the pumping plant shall be protected

by a device that stops the engine. These devices mav be dispensed with where conditions

are such that there is little probability of walersup]ily failure or when the pumping plant is

constantlv attended.

3. Operating Instructions. The dealer or dealers

furnishing equipment 7-cquired for a complete

s¡irinkler system shall furnish the ])urchaser.

in writing, such instructions, performance charts,

and layout drawings as arc required to insure

pro])er operation, in accord with design conditions and normal expected life for the type of

equipment furnished.

4. Performance ïf arrantv. When a dealer or associated group of dealers assumes res])onsibility

for designing and installing a sprinkler irrigation svsteni, a warranty shall be furnished stating specially the performance expected for

watcr-a])plication rate, capacity, rate of coverage for a S])ecific design area serving sj)ecific

crops, and crop acreages as stated by the purchaser and mutuallv understood to be the basis

for the design.

\^ arrantv shall be based on trial of the system

during operation under the range of o])erating

conditions imposetl on the system. The warranty shall not be expected to cover any conditions that are beyond reasonable control of

the dealer either in design or in installation.

Values used for infiltration rate, peak-use rate

of moisture by crojts, or capacitv of soils to

retain water for plants cannot be expected to be

accurate for every condition of soil. Evidence

that the dealer has made reasonable efforts to

obtain values from reliable sources shall be

sufficient reason to absolve him from responsibility if such values do not represent local

conditions.

^ hen a dealer or dealers assume responsibiUty for the installation of a system in accord

with specifications supplied by the purchaser,

a warranty shall be furnished stating the performance expected as to friction loss in the system, pump and engine motor characteristics,

and other pertinent data pertaining to the

specifications.

.5. Equipment Warranty. The dealer or dealers

assuming responsibiüty for the installation of

the system shall furnish warranties coverin<i

the quality of material and workmanship of

each piece of equipment furnished in accord

with the original manufacturer's guarantee,

and shall provide for replacement of defective

parts shown to have failed because of poorquality materials or poor workmanshij).

6. Maintenance and Repair Service. Dealers selling

SO

sprinkler irrigation systems in a territory shall

maintain an inventory of replaceable parts and

required equijtment repair service. The extent

of this available service shall be such that users

in the tcrritorv will be assured of reasonable

service that will avoid crop loss due to shutdown of a system for replacements or repairs.

I}. Purchaser Responsihility

1. Operation in Accord with Instructions. The purchaser and user of a s[>rinkler irrigation system

shall assume resj)onsibility for failure of the

system to perform properly if, after receiving

all data furnished by the dealer, he fails to opcrate the system in accord with all conditions

assumed in the design of the sy stem To obtain

full life of all equipment, the user shall observe

the stated limits of operating conditions set

forth by the manufacturer.

2. Care and Maintenance Recommendations. The

purchaser shall follow the dealer's recommendations for care and maintenance of the equipment.

This applies to periods of use as well as nonuse

of the equi])ment.

Part III. Definitions

Design Area. The specific land area which the

su]i)>lier or designer and the purchaser mutuallv understand is to be irrigated by the sprinkler system.

Sprinkler Irrigation System. All equipment required to apply water to the design area, from the

source of water supplying the system to the revolving sprinkler, nozzles, or perforated pipe.

If ^yatcr is already available to the design area,

the system includes only the equipment required to

develop the necessary pressure and apply the water

to the area.

If both water and pressure are available, as in the

case of an existing pressure line, the system includes

only the equipment required to take the water under

pressure from the supply une and apply it to the

design area.

Sprinkler Lateral. A line of portable pipe or

tubing with sjirinklers. nozzles, or perforations along

the hnc. A lateral may be one of several operated

from a common main supply line, or it may be a

single unit supplied directly from the water source.

Application Rate. The equivalent rainfall rate

expressed in inches of water depth per hour (acreinches jjer acre per hour). For system with rotating

sprinklers, the rate is computed on the basis of the

spacing of lateral settings, the spacing of the sprinklers along the lateral, and the average discharge of

the sprniklers. For perforated pipe systems, the

application rate is computed from lateral spacings,

length of lateral, and average flow into the lateral.

Infiltration Rate. The rate at which soil will take

in water during the irrigation period, expressed in

mches of water depth per hour.

Peak-Moisture Demand. The maximum demand

that occurs during periods of maximum temperature

and crop groMth. This peak demand for moisture

on the part of a crop results from transpiration by

plants and direct evaporation from the soil.
Average Water-Applicalion Rates (Inches Per Hour) from Sprinklers

Spacing

(feet)

20X20.

20X30.

20X40. .

20X50. .

20X60. .

(Tiillnns per miniilc from each sprinkler—

25X25.

30X30.

30X40.

30X50.

30X60.

40X40.

40X50.

40X60.

40X80.

50X50.

50X60.

50X70.

50X80.

60X60.

60X70.

60 X80.

70X70.

70X80.

70X90.

80X80. .

80X90. .

80 X 100.

100X100

120X132

0.24

.16

.12

. 101

.08|

0.48

..32

.24

.19

.16

. LS .31

0.72

48

36

29

24

46

I 0.97

.64

4«

.39

..32

.62

.11

1.21

.80

.60

.48

.40

.77

.54

.40

.32

.27

.30

. 24

.20

.15

. 19

. 16

.14

. 12

. 13

. 11

.10

1.45

.96

.72

.58

.48

.93

. 64

.18

..38

..32

.23

.19

. 17

.14

. 16

. 1 1

. 12

1.70

1.13

.85

.68

..56

1.09

.19

.16

.14

.14

. 12

1.94

1.29

.97

.77

.64

1.24 1.40

.97

.72

.58

.48

.54

.43

.36

.27

.35

.29

.25

.22

.24

.21

.18

1.61

1.21

.97

.81

1.55

1.07

.81

.64

..54

14 16 17

1.77

1.33

1.06

.88

1.70

.66

.53

.44

.33

.43

.35

.30

.27

..30

.25

.22

.22

.19

.141

.12]

.11

1.93

1.45

1.16

.97

1.86

2. 10

1.57

1.26

1.05

2.02

1.39

1.05

.84

.70

1.70

1.36

1.13

1.50

1.13

.90

.75

.84

.68

.56

. 1.2

1.81

1.45

1.21

1.61

1.21

.97

1.93

1.55

1.29

1.72

1.29

1.03

.86

2.06

1.64

1.37

1.83

1.37

1.09

.91

.96 1.02

. 78i . 82

. 61 . 68

. 48| . 51

.54

.45

.39

.34

. 35i .38

. 30 . .12

.26 .28

.58

.481

.411

JÎ6|

. 1.0|

.31!

. 30

.66

.55

.47

.41

,13 .16

37 .39

32 . 34

.28

.24

.22

.21

.19

. 17

.30

.261

.23'

.20'

.18

.33

.29

.26

20 I 25 I 30 35 40 1 50 60

2. 18

1.74

1.45

1.93

1.45

1. 16

.97

1.09

.87

. 72

.51

1.84

1.53

2.04

1. 53

1.22

1.02

1. 14

.92

.77

.57

.73

.61

..52

. 16

18 . 51

11 . 13

36 . 38

..36

.31

.28

.37

.33

.29

.27

.24

.22

.29

.26

.23

1.94

1.61 2.02

2.15

1.61

1.29

1.07

1.20

.97

.81

.61

.77

.61.

.55

.48

.39

.34

.31

2.02

1.61

1.34

1.51

1.21

1.01

.76

1.93

1.61

1.46

1.21

.91

1.16

.97

.83

.73

2.25

1.87

2.11

1.70

1.41

1.06

1.36

1. 13

.96

.85

67 .81

57 .69

50 .61

49 ..59

43 ..52

38 .46

38 .45

33 .40

30 .36

.91

.81

.71

.69

.60

.,54

.53

.47

.42

. 16 ,18| . 19 .31,

1.94

1.61

1.21

1.55

1.29

1. 10

.97

1.08

.92

.81

, ,

90 100 110 I 120 200 300

For other «lischarfies and/or epacings. use

the fullow¡ii;í formula;

R=(96.3)(g. p. m.)

Si XS„

where R ^application rate, inches per hour

g. p. in = discharge from sprinkler, galhiiis

per minute

Si = ,spacing of sprinklers on lateral

Sm = spacing oflateral on main

2.01 ,

i..í2;i. 82 2. 12

1.61

1.38

1.21

1. 35

1. 15

1. 00

.39 .48

,11 .12 , 18

1.93

1.61

1. 16

1.93

1.70

2.21

1.94

1.62 1.88 2. 16

1.38 1.61 1.84

1.21 I.42I1.62

1. 18 1.38 1.58

1.011.2111.39

.92il.08 1.23

.58

30 .37

1.06

.94

.85

.65

2. 18

2.08

1.82 2. 02 2.22

1.78

1..56

1..39

1.21(1.36

1.07|l.21

. 97 1. 09

.77 .87

1.9712. 16 , ,

1.73 1.91 2.08

1.54 1.70 1.85

1.51

1. 34

1.21

.97

1.66

1. 48

1.33

1.81

1.61

1. 45

1.07|1. 16

.61 .67

42

94

73 1.22:1.84

' From "Sprinkler Irrigation," Sprinkler Irrigation Association, 1955.

Ul
Dro|)s and Checks for Use in Irrigation Ditches

-2'-0"MIN.-

U, S. GOVERNMENT PRINTING OFFICE: 1957