Methods for Evaluating

IRRIGATION

SYSTEMS

LIBRARY

MAY 1 1956

Agriculture Handbook No. 82

Soil Conservation Service

UNITED STATES DEPARTMENT OF AGRICULTURE
Contents Page

Furrow irrigation 4

líquipnient needed for furrow test 4

Procedure 4

Analysis of results from irrigation (rial 6

Sample analysis 9

Border irrigation 11

Equipment neetled for test 11

Procedure 11

Sample analysis 13

Problems encountered in border irrigation 15

Sprinkler irrigation 17

General ])eri'ormance requirejuents 17

Method 17

Equi])ment needed for field tests 17

Procedure 17

Sample analysis 21

Washington, D. C. Issued April 1956

Fui- sale by the Supi-Tintrndent of Documents, U. S. Goverunieut Piiutiiig Ciffiee

W;ishiugton 25, D. C. - Price 20 cents
Methods for Evaluating

IRRIGATION SYSTEMS

By Wayne D. Criddle, Sterling Davis, Claude H. Pair, and Dell G. Shockley, Irrigation Engineers '

ItJANY IRRIGATION SYSTEMS, both sur-

-LTJ- lace and sprinkler, are poorly adapted to the

soiJs and topography. Intake rates and waterholding capacities of the soils often were not

known before a field was laid out for irrio-ation

Frequently, little effort was made to learn how

much water could be run in each furrow or border

strip without causing soil erosion. The length of

irrigation run needed for proper distribution of

moisture m the root zone of the crop seldom was

determined. Often, sprinkler irrigation systems

failed to apply water in accordance with soil

characteristics and crop needs. Improper operation of well-designed irrigation systems also has

wasted water, damaged hmd, and reduced production and net income.

Excessiye irrigation wastes not only water. It

also leaches water-soluble nutrients "beyond the

plants' reach. Too-heayy irrigation on higher

land often causes waterlogging of rich, lower

lying lands. To correct this usually requires

installing a costly system of drains as well as

reclamation. On a field being irrigated, by surface or grayity methods, ineflîcient" use of irrigation water usually shows up in poor yields at the

upper and lower parts. This is because the upper

part lost nutrients by erosion and leachmg and

was kept too wet for good growth, while the

lower part receiyed too little water.

How to apply irrigation water to crops without

eroding soil is a problem. Improyement in its

fertility and structure usually enables soil to take

in water faster. As the intake rate increases,

larger streams of water must be deliyered to the

furrows and borders to get uniform irrigation.

Even though erodibility may be decreased by

good soil management,'the efl'ect of the larger

streams may more than offset any such gain.

Irrigation engineers. Soil Conseryation Service

technicians, and other professional people working

in irrigation should find the evaluation methods

described here useful in helping farmers attain

greater efficiency in irrigation. These evaluations

will also lead to better design criteria for local

soils, crops, and climatic conditions.

Furrow- and border-evaluation trials by the Soil

Conservation Service showed the need to control

irrigation streams, especially where water is

' This handbook is a consolidation and revision of three

provisional handbooks on irrigation methods that were

prepared while the authors were employees of the Soil

Conservation Service.

delivered through large, open ditches in which

the fiow may fluctuate. Such fluctuations can be

controlled rather simply. The method sugo-ested

here uses relatively large overflow-type controls

and submerged orifice-type controls of smaU

capacity.

T-rE^^'^'V'^^^ o^er the overflow type varies as

tl'' , where H is the head of water on the weir.

Thus, a relatively small increase in depth of

water causes an appreciable increase in amount.

The flow through a submerged-orifice type of

control, however, varies as ID. Increasing the

depth of water in front of the orifice from 4 to 9

inches would increase the flow in the ratio of

1.5 to 1, or 50 percent (9'^40. Such an increase m depth over the weir-tvpe control would

increase the flow in the ratio of 3.37 to 1 or 237

percent (9'='-^4'''-).

In furrow-irrigation trials, an overflow structure

may be used in the field lateral, with undershottype controls to divert water from the lateral to

the equalizing ditch and to regulate the flow from

the equalizing ditch to each "furrow (fig. 1). A

second oyerflow-ty])e control may be used in an

overflow ditch that leads back to"the field lateral,

or to additional furrows, to handle sur])lus water

from the equalizing ditch. Such an arrangement

is satisfactorjr for stabilizing furrow streams when

the water supply fluctuates^

In liorder-irrigation trials, a relatively long

overflow-type structure is recommended for the

field lateral and an undershot type of control

to the border strip (fig. 1).

7 tBorder ridge

Undei5hol gates -^

(Can use siphons or ony

ondee control)

M'f^""''^' '" ' '""""'""""'"""""""""

^

L

Tesi urrows <

¡iS^

k

- Spiles

Teat 1

Figure 1.—Suggested layojit for conducling irrigation

field trials.
Furrow Irrigation

lliose are sometimes failed corrugations

Furrows are used for nearly all row crops that

are irrigated by surface methods. Close-growing

crops—such as small grains, hay, and pasture—

on slopes and on soil that ])akes or crusts badly

after ])eing wet may also be irrigated with small

furrows.

or rills.

In furrow irrigation, water is delivered to a head

ditch or pipeline along the upper edge of the field.

It is then diverted intd furrows running down or

across the slope. Furrows should ])e long enough

to permit economical handling of farm equipment

between head ditches but not too long for safe

irrigation. Runs should be as long as good soil

conservation will allow. This will keep the turning of farm machinery and settings of furrow

streams to a minimum.

Since most erosion occurs when land is planted

to furrow-irrigated crops, evaluations of furrowirrigation systems and practices are extremely

important, especially where slopes are rather

steep. Erodibility of the soil, size of the stream,

steepness of the slope, and shape of the furrow

are factors involved. Increasing either slope or

stream size tends to increase erosion. Decreasing

stream size and slope and using wide, shallow

furrows tend to decrease erosion.

Increasing the size of stream in a bare, V-type

furrow on the steeper grades does not materially

increase the rate at which the water enters the soil.

A furrow stream of 1 gallon per minute will put

about as much water into the soil per foot of

furrow as will a stream 10 times as large. This

generally is not the case on the gentler slopes, nor

where the furrows are broad or grass covered. By

using smaller streams, however, the Irrigator

usually can save both water and soil, with but

little more time spent in irrigating if lengths

of run are correct.

Equipment Needed for Furrow Test

In general, the equipment for tests of an

irrigation system is simple. The items below are

needed for furrow-irrigation tests:

1. Engineer's level and rod.

2. Chain or tape.

3. Stakes.

4. Stopwatch and regular watch or clock.

5. Shovel.

6. Spiles, siphons, gated pipe, or some other

positive means for controlling discharge of water

into furrows.

7. Wide-mouthed gallon jar or can, if the stream

is to be measured volumetrically, or some other

accurate measuring device such as free-flow or

submerged orifices, or small Parshall flumes.

8. Forms for recording evaluation data.

The first five items are available in any irrigation locality. The Made-mouthed jar can be

Fiííiire 2,—Measuring flow in furrow

orifice plates.

bought at a confectionery store for a few cents,

or a can or bucket may be used instead. 'Spiles

may be bought, or built of lath, sheet metal, or

lumber. Orifice plates (fig. 2) and small Parshall

flumes (fig. 3) are not available commercially but

can be built by local sheet-metal shops.

Procedure

The procedure for gathering data to evaluate

a furrow-irrigation system is to divert difl'erent

sized streams into several furrows and check the

rate at which the stream fronts advance down

them. Each stream is measured at the head of

Figure 3.—lAIeasuring flow through a small Parshall

flume.
Figure 4.—Measiirin«; water voliimetrically into a furrow with use of bucket and stopwatch. Gated surface

pipe is used in controlling the flow of water to the

furrows.

the furrow and at one or more points down field

to determine how mucii water enters the soiL

Before tm-ning water on:

1. Choose several uniform furrows for testing.

2. Set stakes at 50- or 100-foot stations down

the field.

3. Run levels on each station to determine

average slope and variation in slope.

4. Set spiles, orifice plates, or other controls

at heads of the furrows. If the streams are to

be measured volumetrically, dig a hole at the

lower end of each spile hirge enough for the

container to catch the flow (fig. 4).

5. Select outflow-measuring points down the

test furrows and install measuring devices.

Figure 5.—Soil Conservation Service technicians estimating how much additional soil moisture can be

stored in the root zone of a potato field.

Figure 6.—Erosion being caused by furrow siream too

large for slope and soil condition.

6. Estimate how much water can be stored in

the crop-root zone (fig. 5). It is assumed that

this test will be run at the time the crop is to be

irrigated, or, if land is not cropped, when the

soil moisture is relatively low.

After turning water on:

1. Set constant-flow streams—a different size

for each furrow. Analysis will be easier if the

spread in stream sizes is rather large. The

largest stream should cause erosion (fig. 6) or

be too large for the furrow to carry (fig- 7). The

smallest sliould be too small to advance to the

end of the furrow, regardless of how long it is

allowed to run. The size of the medium stream

may be estimated from the formula Q=10/S,

where Q is the stream size in gallons per minute

and S is the slope of the furrow in percent.

Sometimes, several guard furrows must be set

on either side of the test furrows to prevent undue

lateral seepage losses.

2. Record the time when water starts to flow

into each furrow and when it reaches the stations.

3. Measure streams periochcally at the intake

to the furrows and record results.

4. Inspect each furrow for erosion or overtopping and estimate the maximum allowable stream.

Flowing water nearly always causes some erosion,

so cloudiness in the water for the first 5 minutes

after a stream passes a point may be permissible.

Obvious movement of soil particles and vertical

cutting or undercutting along the furrow banks

after the initial wetting would be serious erosion.

This indicates the neecl for using a smaller stream.

5. Periodically measure flows in tlie furi'ows

at the outflow-measuring points. Continue tliese

measurements until flows become practically

constant. Tliis may not happen on tlie finete.xtured soils for several liours.

(3. After the irrigation, cut a trench across and

at right angles to tlie furrows at several places to

disclose the wetting pattern.

336916°—55-
Fisiire 7.—Furrow stream exceeds the carrying capacity

of the furrow.

7. Check the adequacy of the iri'igatioii 24

hours or more after tlie water is turned off. In

the fine-textured soils, 2 or 3 days may be rec|uired

for all the free moisture to be distributed.

8. Determine what adjustments, if any, are

needed for safe and efficient irrigation.

Analysis of Results From

Irrigation Trial

The most important factors are: (a) water

needed to refill the soil-moisture reservoir, (b)

intake rates, (c) time required to refill tiie soilmoisture reservoir, (d) maximum furrow spacing,

(e) maximum allowable time to get water through

the furrows, (f) maximum allowal)le furrow stream,

and (g) maximum allowable length of run.

W^oter needed to refill

soil-moisture reservoir

Before irrigating, subtract the amount of

moisture in the root zone from the field capacity

to find the amount of water needed to refill the

soil-moisture reservoir (fig. 8). Laboratory detei-minations of soil-moisture conditions just

before and after irrigation are desirable, of course.

But if they cannot be made readily, the soilmoisture conditions can be judged with a reasonaljle degree of accuracy by the "feel" method

(table 1).

Intake rates of the soil

The rate at which the soil absorbs water usually

decreases rather rapidl}^ for a time after the start

of an irrigation. After several hours, however,

it usually becomes nearly constant. When the

intake rate during a normal irrigation is plotted

on log paper on the vertical axis and time on

the horizontal axis, the resultant curve has a

general sliape indicated b}' the formula I=KT'^

Where:

/=Intake rate of the soil.

T=Time that water is on the surface of the

soil.

/v = Intake-rate intercept at unit time.

iî,=Slope of the line (vertical scaled distance

divided b}' horizontal scaled distance).

In furrow irrigation, only part of the land surface is in contact with the water, so the equivalent

field-intake rate ^^ will vary with both the rate

'" Field intake rate is defined as the rate at which water is

absorbed in acre-inches per acre per hour.

\^BLE 1.—Guide fur juilijiiKj how much moisture is apailable for crops

Feel or appearance of soil

Soil moisture remaining

Very light texture Light te.xture Medium texture Heavy and very heavy texture

0 percent50 percent or less.

50 to 75 percent75 percent to field capacity (100 percent).

At field cai)acity (100

percent).

Dry, loose single

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 to slick together

slightly, sometimes

forms a ^ ery weak

ball 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.i

Tends to ball under

pressure but seldom holds together.

Forms weak ball,

breaks easily, will

not slick.

Same as very

te.xture.

light

Powdery dry, sometimes slightly crusted

but easily broken

down into powdery

condition.

Somewhat crumbly but i

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 very light texture.

Hard, baked, cracked;

sometimes has loose

crumbs on surface.

Somewhat pliable,

will ball under pressure.'

Forms a ball, ribbons

out between thumb

and forefinger.

l']asily ribbons out

between fingers, has

slick feeling.

Same as very

texture.

light

1 Ball is formed by squeezing a handful of soil very firmly.
1 2 1 3 1 4 5 1 6

7 8 9

10

Soil texture

Available

moisture

capacity

Soil moisture

twiore irrigation

Depth

((eet) Very

tieavy

Heavy Medium Ligtil

Very

light

Moisture

tjeficiency

Incfu^ ;vrc.«i l„cl:a /íiLÍa.^

0-1 X 2.00 20 0.40 1.60

1-2 X 2.00 40 0.80 1.20

2-3 X 1.25 50 0.62 0.63

34 X L25 50 0.62 0.63

4-5

5-6

Total 1 6.50 2.44 4.06

Available moisture capacity in inches of water per foot (iepth of soil under average c

heavy, 2.20; medium. 2.00; light, 1.25; very light, 0.75.

nditions: very neavy. 2.00;

Figure 8.—Form for figuring the amount of water

needed to refill the soil to its moisture-holding

capacity.

at which water is absorbed from the furrow and

the furrow spacing;. Since the amount of water

entering and leaving a furrow or a section of it is

usually measured in gallons per minute, the furrow-intake rate is commonly computed in gallons

per minute per 100 feet of furrow. Dividing the

intake rate in gallons per minute per 100 feet of

furrow by the furrow spacing in feet gives the

approximate field-intake rate in inches per hour.

Figure 9 ma}' be used in making this computation.

Time required to refill the

soil-moisture reservoir

The area under the curve—from the formula

I^KT"—divided by 60 is equal to the depth of

water {B) absorbed by the soil when the curve

is plotted from the formula I=KT'^ with tinie

(T) expressed in minutes and intake rate (/) in

inches per hour. The area under the curve as

determined bv integration is:

600=

;í + 1

r"+i and r=

r60P(n + l)7'+i

A layer of slowly permeable soil within or near

the root zone may cause a temporary perched

water table and wide liorizontal movement of the

water. In contrast, an extremely permeable

undcrlayer may curtail horizontal movement.

Therefore, the soil profile and pattern made by

the percolating waters should l)o known before

furrow spacing is recommended.

Maximum alloivable time for getting

water through the furrows

If the intalie rate is uniform throughout tiie

length of the furrow, absorption of water by the

soil will be uniform along each segment for equal

periods of time. Water, however, normally advances much more slowly down a furrow wlien it is

dry than it flows from tlie furrow after it is wet

and the water is turned oft". Ordinarilj^, the water

is in the upper end of the furrow longer than at

points farther down.

For nearly uniform iri'igation of a field, the

stream fronts should reach the lower ends of the

furrows within one-fourth of the total time

needed to refill the soil in the root zone. Thus,

the "opportunity time" for the soil to absorb

water will be about 25 percent greater at the upper than at tlie lower end of the field. But the

water-intake rate of the soil decreases with time,

frequently inversely proportional to tlie square

root of the elapsed time. So, the amount of

water absorbed during this extra time will be less

than 2.5 percent of tiie total. If the square-root

relationship is correct, about 12 percent more will

be absorbed at the upper than at the lower end.

The average deep-percolation loss for the full

lengtli of the furrow would, therefore, be in the

magnitude of 5 percent (fig. 10).

Maximum allowable furrotv stream

Hardest to determine, because of the lack of

accurate information, is the maximum allowable

Maximum furroiv spacing

Furrow spacing in row crops is usually fitted to

the type of crop or equipment. Spacing can be

varied in some crops, as, for example, in orchards

or close-growing crops irrigated with corrugations.

In medium-textured soils that are homogeneous

throughout the root zone of the crop, the depth

and width of the wetted bulb of soil beneath the

furrow are frequently about the same. Maximum furrow spacing in such soils should not exceed the depth of root zone of the mature crop.

When the plants are small, with undeveloped

root systems, much of the irrigation water may

percolate down below tlieir reach if enough is

applied to get adequate lateral penetration with

the maximum furrow spacing. Higher application efficiencies should be possible wlien plants

are more nearlv mature.

Generally, the soil profile is not homogeneous.

20 Î

35

5-40

• 45

50

Fi"ure 9.—Nomograph for converting intake rate m

gallons per minute per 100 feet of furrow lo acreFriches per acre per hour for various furrow spacmgs.
Ground surface^

' j-r''-!'!'y!^^

Water added to the soil when stream first

reaches lower end ..^ '^''^

d2

5]_|

— — — — — Hottom ofroot zone^

^oTi bVdTerpei^olation

Figure 10.—Profilo along furrows showing water penetration when intake rate is inversely proportional to square root

of elapsed time.

Assume

T'=Timc required to refill root zone

T','4=Time required for furrow stream to reaeh lower end of furrow

i/i, fl;, (¡3, fti, and 0/5 = Depths of water ahsorbed in equal time increments T/4

With intake varying inversely with the square root of lime

d,= (V2-l)rf„ d3=(^/ï-^/2)d„ di=(2~^'3)flu and d,= (^JS-2)d,

Then

Dz,=<ii(l+V2-l+V3-V2+2-V^)= 2di-Absorbed at lower end

Dc;=íí,(l+V2-l+ V3-V2+ 2-V3+ V5-2)=V5rfi-Absorbed at upper end

Average deep pereolation—(V-5^2)-^

Average depth absorbed = - ^ =(2+ yó) ~

To find the percentage of water absorbed that will be lost to deep percolation:

^"«^^'^-^^T 100(.5-2) 23.6 ., 1 = -,- = r^í5V ='5-'í percent

(2 + VB)| (2+V5) 1-2-Í6

furrow stream. Tliis is largely a matter of judgment, and any two people working independently

may arrive at somewhat different conclusions.

Until a quick, reliable method is devised to measure the erosion caused by streams of different

sizes, and the amount of erosion permissible on

the various soils is déterminer], decision on the

maximum allowable stream will have to rest on

judgment.

Unfortunately, surface irrigation cannot be

practiced without some movement of soil material.

There may be damaging erosion at the top

of a sloping irrigated field, where the stream is

largest, but practically no soil movement at the

bottom. Removal of soil from the top and deposition farther down may be as serious as if soil were

removed from the field. Site conditions affect

the seriousness of soil erosion. C'ertainly, removal of an inch fi'om a field having many

feet of good soil is less a threat to permanent

agriculture than removal of an inch of soil from a

field underlain at 15 inches by rock. Accordingly,

serious erosion needs to be defined locally.

Erosion may be no problem on nearly flat

grades. The limit on the size of furrow stream

may be the carrying capacity of the furrow.

Because the intake rate decreases with time and

some ponding occurs in the furrow, it usually is

well to "cut back" the furrow stream after it

has reached or approached the end of the furrow.

Unless this is done, loss of water by surface waste

is likely to be heavy. A cutback may not be

necessary where grades are nearly flat and furrows

have adequate storage capacity.

The maximum stream, as previously determined, need not always be used for good irrigation. A smaller stream wfll be satisfactory if it

will reach the lower end of a field within the

"one-fourth time" criterion.

Maximum alloiiable length of run

The maximum allowable length of run is the

longest distance in which the maximum allowable

furrow stream can eft'ect nearly uniform distribution of water in the soil. This can be decided

after plotting the rate at which the different size

streams advance and determining the maximum

size sti-eam and maximum time that can be allowed for it to get down the furrow. Somewhat

higlier efficiencies might be obtained if the

fields are shorter than the maximum allowable.

8
ilevation

1 1 2 1 3 4

station Clocl^

time

(24-hr,)

Elopsed

time

Clock

time

54-t.r)

Elapsed Clock

time time

(24-hr.)

Elopsed

time

Clock

time

(24-hr.)

Eiopsed

time

0 +00

Feet

100.0 8:oo

Minutes

8:02

Minutes

start 8:05

Minutes

start 8 :io

Minutes

start stort

1 fOO 98.2 9:3 0 90 824 22 8:i8 13 8:20 10

2 too 96. 1 13: 33 333 9:31 89 8:47 42 8:37 27

3 +00 93.9 ii:i7 195 9:35 90 9:07 57

4+00 92.0 15:25 443 10:43 158 9.42 92

5 +00 90. i i6:35 513 12:04 239 10:33 143

6+00 68.4 i3:38 333 1 1:32 202

7 +00 86. i i5:42 457 12:36 266

8 +00 83.9 13:47 337

1 +50 ii:o7 187

2+40 16:20 500

Estimated maximum ollowabi« furrow stream 5.0 g.p.m.

Figure 11.—Rate-of-advance dala.

But, if the fields are only slightly longer than the

maximum allowable, a lower irrigation efficiency

might be preferable to cutting the run in two.

Sample Analysis

The information needed and methods of analysis can best be illustrated by a sample evaluation

of a cornfield. Rows are 42 inches apart. The

soil is medium-textured and uniform througliout

the root-zone depth of 48 inches. The slope in

Furrow No. 1 Furrow No.2

inllo« stotlon 0+00 Outflow Stotion Inflow station 0 + 00 Outflow station 1+00

Clock

time

Sec /

gal.

Flow

g.p.m.

Clock

time

Sec./

gol.

Flow

g.p.m.

Clock

time

Sec./

gol.

Flow

g.p.m

Clock

time

Sec./

gol.

Flow

g.p.m.

8:00 30 2.0 8:02 15 4.0 8:27 100.0 0.50

8:50 31 .6 1.90

8:30 29.8 2.0 a: 35 14.9 4.0 9:20 24.6 2.44

10:00 ?l.4 2.80

9.05 30.0 2.0 9:07 15.0 4.0 11:10 20.0 3.00

12:30 19.2 3. 1 2

10:30 30.1 2.0 10 35 15.2 3.9 14:00 18.2 3.30

16: 00 17.7 3,40

12:00 30.0 2.0 12:03 15.0 4.0

13:30 30.0 2.0 133 1 15.0 4.0

16:30 30.0 2.0 16:35 15.0 4.0

1 Furrow No 3 Furro w No 4

Inflow Station 0+00 Outflow stotion Inflow St otion 0 + 00 Outflow stotlon 6+00

Clock

lime

Sec./

gal.

Flow

g.p.m.

Clock

time

Sec./

gal.

Flow

g.p.m.

Clock

time

Sec./

gol.

Flow

g.p.m.

Clock

time

Sec. /

gol.

Flow

g.p.m.

e:05 10 6.0 8:10 7.5 8.0 1 1.40 300.0 0.20

12.00 42 9 1.40

e:40 9.8 6.1 8:45 75 8.0 12 40 26.1 2.30

14: 10 18.8 3.20

9:10 10.0 6.0 9:1 5 7.5 8.0 I6:i5 14 4 4.16

10:3a 10.0 6.0 10:40 7.5 e 0

12:05 10.1 5.9 12:07 7.5 8.0

13:33 10.0 6.0 13:35 7.5 8:0

16:38 10.0 6.0 16:40 75 8.0

Figure 13.—Rale-of-advaiice curve for determining

maximum length of furrows.

the direction of irrigation is 2 percent.

Furrow stream rate-of-advanee and flow measurements are recorded on figures 11 and 12,

and plotted on figure 13.

Water-storage capacity

The soil was examined by 1-foot increments to a

4-foot depth just before the irrigation trial, and

it was found to be of uniform texture througliout.

Its capacity for holding usable water was estimated

at about 2.0 inches per foot of depth, or 8.0

inches in the root zone. The depth of water

needed to refill the root zone was estimated to be

3.7,5 inches.

Water-intake rate of the soil

The water-intake rate of the soil was determined

by measuring the flow into and out from the

furrow. Wlien the average intake-opportunity

time was 14 minutes, water was being absorbed

at the rate of 3.40 g. p. m. per 100 feet of furrow.

With the furrow spacing 42 inches, this is eciuivalent to a field-intake rate of 0.94 inch per hour.

After 478 minutes, the intake rate was 0.60

g. p. m. per 100 feet of furrow, or a field rate of

0.17 incli per hour. Ta})le 2 gives complete results;

values are plotted on figure 14. The variation of

field-intake rate witli time is given by the formula

7=3.58 T-"-'.

Time required to refill the

soil-tnoistiire reservoir

Since the formula for the intake-rate curve is

known, the time required to put the 3.75 inches

into tiie soil can be computed with tlie formula:

^rmDjii+m^+i

Figure 12.—Furrow-stream measurements.

From figure 14 K is seen to be 3.58 and 7i=

Thus
y_r(60X3.75) (-0.5+1.0)"[-».5+i

L 3.58 J

r225X0.57 „^_ . ,

= 16.5 hours

Maximum furrow spacing

Since a row crop is on the sample field, furrow

spacing is fixed. With the deep, medium-textured

soil found here, however, the 42-inch furrow

spacing should give reasonably good lateral

wetting by the time the proper amount of water

is put into the soil. Excavation across the

furrow indicated this to be the case.

Maximum allowable time to get

water through tJte furroiv

Since 16 hours would be required for the soil

to absorb the necessary 3.75 inches of water, the

furrow stream has to reach the lower end of tlie

field in 4 hours to meet the criterion for good

irrigation. Thus, the water would need to be on

the upper end of the field 20 hours—4 hours for

the furrow streams to reach the lower end plus

16 hours needed to store 3.75 inches of water at

the lower end.

TABLE 2.—Aleasuremerü offurrow-intake rate

FURROW NO. 2

Elapsed time—•

Clock

Inflow Outflow

Less

in

furrow

Intake

per

100 ft.

time

(24-

hr.)

StationStation Aver0 +00 1 + 001 age

Min- Min- Minutes utes utes G.p.m. G.p.m. G.p.m. G.p.m.

8:02 Start

22

25

0

3 14

4. 0

8:24

8:27 0. 60 3. 40 .3. 40

8:50 48 26 37 i^„.. 1. 90 2. 10 2. 10

i):20 78 56 67 1. .56 1. 56

10:00 118 96 107 1. 20 1. 20

11:10 118 166 177 3. 00 1. 00 1. 00

12:30 268 246 257 3. 12 .88 . 88

14:00 358 336 347 3. 30 . 70 . 70

10:00 478 456 467 4. 0 3. 40 . 60 . 60

FURROW NO. 4

8:10 Start

202

210

8.0

Con11:32 0

11:40 8 109 0. 20 7.80 1. 30

12:00 230 28 129 .stant 1. 40 6. 60 1. 10

12:40 270 68 169 Flow 2.30 5. 70 . 95

14:10 360 158 259 3. 20 4. 80 . 80

16:15 485 283 384 8.0 4. 16 3. 84 . 64

20

^

k

:: s

Ö 7

il

8"

i ^

iz

o

ii

^ —^ ---- — - -^ - h— — ^ - -J --

Furrow intoke rote

\(l,p.m./IOO'

D Furrow #2

o Furrow#4 _ _

^^ . 1=13,0 T

-U.b

^ ^

Eaulvalent field ^^^ 10.0

\

^

In ta ke rote

nches per hour

^-v^42 furrow spocina

-^ u

;

^

n = -5.= -D.5 1

|5.0 _ _.

s 7

EEtEtOz 1

-^— ----^^

--1 -

" Spocinfl (inches)

" ^ -^ ,

ç .4

S .3

ô

• -2

o

1

- .

^

^

\

•N

- ^ ^-

1 For furrow No. 4 station 6 + 00.

I Z 315676910 2 3 456769100 200 300 1000

Elapsed time in minutes

Figure 14.—Intake-rate curves for furrow irrigation.

Maximum allowable furrow stream

Furrow streams of 2, 4, 6, and 8 g. p. m. were

tried. The largest eroded more soil than could be

allowed. The 6-g. p. m. stream washed some

loose soil during the initial wetting and continued

to move a little soil as the stream was running.

This was evidenced by some cloudiness of the

water and a small amount of undercutting of the

furrow bank. The 4-g. p. m. stream caused little

soil movement, even with the initial flush. It was

decided that 5 g. p. m. were the safe maximum

furrow flow for these conditions.

Because of the decrease in the intake rate with

time and ponding in the furrow, the stream should

be "cut back" after it has reached or approached

the end of the furrow. In the example, the furrow

stream could be reduced to about 3.5 g. p. m.

shortly after it reached the lower end of the furrow,

with further cuts from time to time as desired.

Maximum allowable length of run

Figure 13 shows the maximum allowable length

of run. The length of run is indicated by the

point where the line for the maximum allowable

time to get the water through the furrows intersects the plotted curve of the maximum allowable furrow stream. In this case it is 420 feet.

Conclusions

On this sample field, the 800-fcot-long furrows

cannot be irrigated safely without excessive deeppercolation and/or soil' losses. The maximum

nonerosive furrow stream is 5 g. p. m., which will

advance only 420 feet in ji the tiine needed to

refill the soil-moisture reservoir. An intermediate

head ditch should be built so the field can be

irrigated in two 400-foot runs. Also, if 5-g. p. m.

streams are used to get the water through the

furrows, they should be "cut back" to about 3.5

g. p. m. shortly after they reach the lower ends to

prevent excessive losses from surface runofl".

10
Border Irrigation

Border irrigation can be used for all closegrowmg crops, some row crops, and orchards

where topography and soils are suitable. It

permits efficient, rapid, and relatively- easy irrigation if the borders are properly ' constructed.

Labor requirements are low. Relatively laro-e

streams of water are required. The layouts mu'st

have nearly flat, uniform grades and'good land

preparation. Border strips must be level

transversely.

Although border irrigation is suitable for most

soil types, it may not be desirable on fine-textured

soils where surface baking occurs unless moisture

to get the crops established can be applied some

other way. Sprinklers or corrugations between

the border ridges are sometimes used. Often,

natural precipitation will suffice to start the crop!

This section describes a technique to gather

information for evaluating an existing borderirrigation system and determining the proper size

streams of water.

Equipment Needed for Test

Equipment needed to conduct field tests on

border-irrigation systems includes:

1. Engineer's level and rod.

2. Chain or tape.

3. Stakes, ax, and crayon.

4. Regular watch or clock.

5. Shovel and soil auger.

6. Weirs, submerged orifices, Parshall flumes,

calibrated siphons, or other devices for measuring

water to and from border strips.

7. Forms for recording data.

Procedure

The first step in evaluating border irrigation

IS to determine the "basic intake rate" - and

water-holding capacity of the soil in the root

zone. Then release a stream of predetermined

size into a border strip, and time both its advance

down the strip and how long the water covers

each part. Compute the depth of water applied

and determine the uniformity with which it is

absorbed. Repeat this process one or more times

if the stream is found to be not of proper size.

Before releasing water into border strips:^

1. Choose a border strip into which a constantsize stream can be directed, and which will permit

-In this handbook, the "basic intake rate" is the rate

at which n-ater will enter the soil after a period of several

hours, when the change in rate becomes very slow.

^ It is assumed that the trial run will be made at the

time the crop is in need of irrigation. If the test is run

when the soil is wet, the intake rate of the soil may be

much slower and the rate of advance of the water much

faster than when the soil is in need of irrigation.

measurement of the waste water if runoiï occurs *

2. Measure the width of the border strip

3 Set stakes at 50- or 100-foot stations down

the border.

4. Run levels on each station to determine the

average slope and variation in slope.

5. Set weirs or other measuring devices at the

upper end of the border strip, and at the lower

end it surface runoff is expected.

6. Determine, by excavation if necessary, the

depth oi the root zone of the crop on the field.

7. Estimate the amount of water needed to fill

the root zone.

8. Measure or estimate the basic intake rate

oí the soil. One method frequently used is to

drive a cylinder about 15 inches long and 9 to

12 inches in diameter into the soil to a depth of

4 to 6 inches, add water, and determine the rate

at which it is absorbed during various periods

9. Determine from figures 12 and 13 the approximate size of stream to use.

_ Turn water on: Make sure that the stream

size does not fluctuate. Do not use a stream

large enough to cause erosion or overtop the

border ridges. If the strip is wide, more than

one opening from the head ditch to the border

strip may be required to avoid erosion near the

turnout.

After turning water on:

1. Record the time water starts to flow into

the border strip and the time the front of the sheet

of water reaches each station. If the front is

an irregular line across the border strip, use the

average of the times that the different parts

reach each station.

2. Record the time when the waste stream, if

there is any, starts, and measure the flow periodically until it ceases.

3. Record the time when water is turned off at

the head of the field and the time when the sheet

of water recedes past each station. This requires

good judgment. On slopes above 0.5 percent, a

large part of the water in the border strip when

the supply is shut oft" may move down slope in a

fairly uniform manner. "On these fields, record

recession time at each station when the water has

disappeared from the area above it. If the recession line across the Ijorder strip is irregular,

record the time when there is about as much

cleared area below as there is water-covered area

above the station. On slopes below 0.5 percent,

a smaher proportion of the water moves down the

strip. Some may be trapped in small depressions

and may not be absorbed for some time after

surrounding areas are clear. Since the important

thing is to determine when the intake opportunity

' On soils having a fairly rapid intake rate, border strips

laid out on a flat slope may have little, if any, surface

runoff. On the other hand, fairly uniform distribution of

the water down the border on steeper slopes with tight

soil may be impractical without some surface waste.

11
TABLE 3.—Suggested maxiyntnn safe irrigation

streams in cubic feet per second per foot of border

strip width '

Slope Maximum

stream

(Q) '

Slope

iS)

Maximum

stream

(Q) '

Cubic feet Cubic feet

Percent per aecond Percent per second

0. 3 0, 150 1. 0 0. 0(iO

. 4 . 120 1. 5 . 043

. 5 . 100 2.0 . 035

. 6 . 080 2. 5 . 030

. 7 .077 3. 0 . 02G

. 8 .070 4.0 .021

. 9 . 064 6.0 . 018

^ For border strips without sod protection. Larger

streams mav he used with sod cover.

- Q = 0.0(iS-"-\

is essentially gone, the recession time usually may

be recorded for a station when 80 to 90 percent

of the area between it and the next station upstream has no water on the surface.

4. Check the adeqtiacy of the irrigation at a

number of places with an auger or soil tube 24

hours or more after the water is turned off. Two

or more days may be reqtiired for the free moisture

to be distributed in the fine-te.xtured soils.

5. Plot the "rate of advance" and the "recession curves" of the sheet of water.

6. Determine the average depth of water

absorbed by the soil and imiformity of distribution.

7. If analj'sis of data indicates that some adÍÍ

.07

.06

Í °'

D .03 T3

'5

S .02

o

«

E .01

o .009

° .008

o .007

Ï .006

« .005

O .004 O

J: .003

o

o

.- .002

o

E

o

a>

5 001

" X5009

= .0006

=, .0007

.0006

.0005

.0004

.0003

^ á 1 i 1 1 1 1 ^...f% Note For slopes other than

0.5 percent, refer to figure

16 for correction factors

\

^ ?

-^

^''Of ~^ c ^ ~<ie--"•'',

/^^~~- .^

~-\i r- i^rS-^ .^

---¿ -^ 0" \.^

^^.7

/If ^'

--._ :::

^^

^.

-^ '\

'-^.^ / /-^ •-...^ "• -, '"'...^ ^ "--> r - "^ -^^ ""-. '-^..^Z ., -...^

~~-, -.H 05;^

^^ ^^

\^ ---, /" -,, --,

--., t- /--- ~--, -'--

-.,

^"•.,^

--,_

A\^"-^ ~~

^"^-- ^ -^

^ 4íí^.

-..^

k.,^

\ ■■--,^-., --

::::

U/- <[« - -^ ^ ~..^ "^-^'^ .- -^^ •-^^ h^

M

'^"""^P/o , --.^ ~--,^

^-^r " --^^ ---,_

/ Indicates approximate

'— range of intake rotes for

various soil textures ~"---.

^^ ^

0

Fig

12

I 2 3 4 5 6 7

Depth of wafer required to till root zone of crop (inches)

lire 15.—Curves for eslimaling unit-border streams

on slopes of 0.5 percent.

.6 .7 8 .9 1.0

Irrigation slope--percent

3.0 40 5.0

Figttre 16.—Factors for use in adjusting unit streams

from figure 15 for slopes other than 0.5 percent.

justments are desirable, make them, and repeat

the entire process.

As with furrow irrigation, determining the

maximum allowable stream is largely a matter of

judgment. Erosion may be significant, however,

on slopes of 0.3 percent or more which have poor

vegetative protection, if streams larger than those

indicated in table 3 are used.

Where a dense sod has been established on stable

soils, border streams up to twice the size indicated in table 3 have been used safely.

On slopes less than 0.3 percent, the maximum

stream usually will be governed by the height of

border ridges. On such slopes, with cover crops,

streams of 0.15 c. f. s. per foot of border strip

width may be expected to have flow depths of

6 to 8 inches. Streams of 0.2 c. f. s. per foot of

])order-strip width may be more than 8 inches deep.

Border ridges Avith settled heights more than 8

inches are usually diffictdt to build and maintain,

so the use of streams larger than about 0.12 to

0.15 c. f. s. per foot of border-strip are generally

inadvisable.

The intake rate of the soil, slope, width and

length of the border strip, depth of application,

height of the border ridges, and erosion are all

considered in determining the size of border

stream to use. Figures 15 and 16 show relationships of these factors, based on empirical data

from many sites.

Stream sizes are shown as "unit streams" (the

stream required for each 100 feet of border strip

1 foot wide). Sizes of border streams can be

determined by multiplying the unit stream by the

product of the border-strip width in feet times the

length in hundreds of feet (border-strip area in

hundreds of square feet). The unit streams shown

in figure 15 are for a 0.5-perccnt slope. For other

slopes, multiply these unit streams by the slope

factors in figure 16.

Intake rates used in figure 15 are basic rates.

They are not the average rates for the irrigation

period, nor are they necessarily the rate at the

time an irrigation is completed. Often the required

amount of water may enter the soil before the

basic intake rate is reached.

It must l)e remembered, however, that although

texture may give an indication of the intake rate,

other factors may cause the actual rate to differ

greatly from one estimated on the basis of texture

alone. The design of any border-irrigation sys-
tern should be based only on measured intake

rates under existing field conditions.

Sample Analysis

The easiest way to explain the evaluation process and the reasons for gathering the information

is to describe a complete field trial and analyze

the results. A loam soil witli a basic intake rate

of 1.0 inch per hour, and irrigated as the farmer

was accustomed to doing, is used. The border

strip has a 0.5-percent slope, is 20 feet wide and

1,000 feet long. The crop is wheat. Four inches

of water need to be put into the root zone, which

is about 4 feet deep.

Determining basic intake rate

Rings (fig. 17) were used as described on page 11

to measure the water-hitake rate. Table 4 shows

the data. The variation of intake rate with time

is plotted in figures 18 and 19. Studies to date

indicate that the intake rate as measured by rings

usually is comparable to the actual intake rate

under border irrigation.

The intake-rate curves show that the final, or

basic, rate is about 1.0 inch per hour, but the accumulated intake curves show that the first 4

inches of water would percolate into tlie dry soil

in 40 minutes. Therefore, deep percolation will

occur if water is held on the land longer than about

Figure IT.—Intake rings in operation. The rate at

which water enter.s the soil at various times and the

time required to store a given amount of water in the

soil are observed. The final (Ijasic) intake rate and the

depth of water to be applied are used as a basis for

determining the size of the border-irrigation stream

needed.

40 minutes. Some deep percolation may occur if

the soil in the root zone is to be fully wetted

througliout the length of run but it can be held

to a minimum by properly balancing the size of

stream, area of border strip, and time of application.

TABLE 4.—Eing-intake ¡lata

Observed

time 24-

hr. clock

Elapsed timeDistance to watersurface from

reference point

Intake during

periofl

Accumulated intake

during test

8:00__

8:10..

8:20__

8:30..

8:45..

9:00 .

9:30..

10:00.

11:00.

13:00.

15:00.

17:00.

Since

last

reading

Minutes

10

10

10

15

15

30

30

60

120

120

120

Since beginning

of test

ñlinuíes

10

20

30

45

60

90

120

180

300

420

540

Before

filling

After

filling Depth

Average

rate

per hour

I Average

Depth ' rate

por hour

Inehes

Fill

Inches

0. 0

Inches Inches Inches

1. 72

2. 74

3. 49

4. 39

5. 19

0. 59

7. 74

9. 04

12. 04

15. 04

17. 24

Inches

1. 72

1. 72 10. 3

10. 3

2 74

1. 02 0. 1

8. 2

3 49

. 75 4. 5

7. 0

4 39 . 0

. 90 3. 0

5. 9

80

. 80 3. 2

5. 2

2 20

1. 40 2.8

4. 4

3 35 .0

1. 15 2. 3

3. 9

1 90

1. 90 1. 9

3. 2

4 90 . 0

3. 00 1. 5

2. 5

2 40

2. 40 1.2

2.2

4 GO

2. 20 1.1

1.9

13
\ \ \ y

12

10

B

6

4

2

—1—^/—i—1—

f/ : i

' "^Z 1 ' i

i --/ i- (— \

- 1 -1 + -j T

/ : n"^""-'"Wi„.

( 1 '1 """ PONuJl, (l„./h,'

\ \ \ ; i '

0 100 200 300 400 500 600

Elopsed time—minutes

Fiíínre 18.—Ring-intake curve.

Rate of advance of iiater

The water's rato of advance clown the bordcT

strip is extremely important. The water was

run onto the land for 230 minutes hi the first

trial (table 5 and ñg. 20), but it took 300 minutes

for it to reach the lower end of the field. Feveolation into the soil was not uniform. Much

more water was received by the upper than the

lower end of the border strip.

Recession of sheet of water

How long the water stands on a border strip

after the stream is turned off usually is significant. See table 5 and figure 20 for the recession

data. Note that the water was turned ofi' 70

minutes before the front of the water sheet reached

the end of the border strip.

Analysis of data

Inasmuch as only 40 minutes were needed to

fill the root zone to field capacity, much water

was lost by deep percolation in the 1,000-foot run.

The shaded area above the "advance curve" in

figure 20 represents the time water must bo on the

field at any point to refill the root zone. The area

between the shaded area and the "recession curve"

50 ~ ~[~ _

r _

40

^

Intoke INNrate-

_-^inch«s per hour

<M III ^^

r

a-

_ 9 — -hH — " - ^y

"--» At V V '

-M p^-

^r Q,

» 3 J *.

~°-

o

-^ -'

, -oAccumu oled

- inches

^k ~-D.^

-p ' — — '— — '- ^ ~.- • ^^-

'

o o ooooooo

Elopsed lime-minutes

Fifiiire 19.—lîing-inlake curves.

shows the time during which deep percolation

takes place.

An average depth of 8.3 inches of water (fig. 21)

was applied to the field during the 230 minutes,

giving an irrigation efficiency of 4.0/8.3X100, or

only 48 percent. There was no surface runoff, so

52 percent was lost as deep percolation.

The unit stream (QJ, used by the farmer on

400

300

3 200 -

100

200

100

i i ^1 i

^^^„iSrr^L^il^^.^^-^..;., I i

: : ^^>-'<^*~' ! i Second:trial

0 2 4 6 8 10

Distance down border strip (hundred feet)

Figure 20.—Advance and recession curves from borderirrigation field trial.

14
TABLE 5.—Data from horder-i)ii(jation field trial

FIRST TRIAL

Station Elevation

Time water is on the land Measurement of water retained

Advance of water

sheet

Recession of

water sheet

Inflow station

0 + 00 OutHow station

tune I . ^ ,

(24-hr.) I t""^

Clock

time

(24-hr.)

Elapsed

time '

Clock

time C

(24-hr.)

f. s. Time C. f. s. or

g. |). ni.

0_

1.

2^

3_

4.

5.

6_

7_

9_.

10_

0

1

2

3

4

5

6

6-^60.

Feet

8:00

8:12

8:27

8:47

9:10

9:36

10:10

10:45

11:28

12:14

13:00

M iniitrs

0

12

27

47

70

96

130

165

208

254

300

11:50

11:56

12:07

12:18

12:33

12:49

13:07

13:20

13:32

13:42

13:48

Minutes

230

236

247

258

273

289

307

320

332

342

348

8:00

8:05

9:00

11 :30

11:50

On No outflow

1.00

1. 00 ' 1. 00 I

Off I

SECOND TRIAL

14:00

14:12

14:27

14:47

15:10

15:36

16:10

16:30

0

12

27

47

70

96

130

150

15 30

15 32 1

15:41

15:57

16 12

16 31

16 50

17 10

90

92

101

117

132

151

170

190

14:00

14:10

14:30

15:00

15:30

On I Xo o\ilflow

1.00 I

1.00

1. 00

Off

1 After water is turned on at head of field.

this field, was the total stream (Q) divided by the

width of the strip (W) in feet and the length of

the field (L) in hundreds of feet or:

^''=îA=20^0=°-°05^-^-^-

Faults of this irrigation s^^stem could be corrected in either of two ways. First, the stream

could be increased so that it would reach the

lower end of the field more quickly than before.

It then would be turned off sooner. vSccondly,

the border strip could be shortened. Additional

water was not available, so it was decided to

shorten the length of run.

The unit stream needed to replace 4 inches of

water in the root zone in a soil with an intake rate

of 1.0 inch per hour is given in figure 15 as 0.0075

c. f. s.; or, for a 20-foot strip, 0.15 c. f. s. for each

100-foot length. If 1.0 cubic foot per second is

the maximum stream available, the maximum

length of run should be about 1.0/0.15X100 or

660 feet.

Data from the second trial, using the shorter

length of run, are also given in table 5 and plotted

in figure 20. They indicate fairly efficient api)lication, with the water well distributed throughout

the border strip.

Figure 21 shows that the use of a unit stream of

0.0075 c. f. s. for 90 minutes gives a total application of al)Out 5 inches. With only 4 inches being

stored in the root zone, the application efficiency

was 4.0/5.0X100 or 80 percent.

Problems Encountered in

Border Irrigation

These figures on stream-area relationships are

for use onlv as a guide, but they may be used to

obtain tentative answers to some proljlems in

border irrigation. Following are three examples

of the use of this information:

A. Knoivn:

1. Length of field: 1,320 feet.

2. Basic intake rate: 0.25 in./hr.

15
1^- 10 100—=1^

- .09 90 —

- oe 80 —

70^^ ~ — 07 60^^

^

— 06 SO -^^

^05 «140-^^

^.04 O30 ^

-^ -»Íp- ^~.oi §20^=.

O 15 -^

*: T"— 02 Q. § O 9 - — OIS \ ^ e --

"D ^ 0 7-=^ i

— 010

« 6 -^

Ë 5^1

^ 009

- ooe - h- K 4^=

- — — 007 .1 =r 5-^s

- 006

C 5^ Ü —- J- 001 O ^^^ ^"^ =

o ~ O -

° ^-— 004 \ ~zï^

.^ _ËL, ^ 003

Cl 1 hour ^=-

\ 50-^

"o "• „ 40 -=

O ^'^ .002

O J- a. Ú ^10~^ 5 o =.

o —

— 0013

|üO^1-

V — I

o ' 0010

i ^^

Ê —

— 0009

— 0008 -|io^IL

:^0007

— 0006 EXAMPLE- CL =

" ^^ OÛOi Unit streom = 0.02c.f.s. CL

° s —El

-^ 3T. = Depth of application required:3,9in. ^

=> 'M ^ 0004

Therefore- *) -^

j

^ 0003

^ 0002

=-,00015

Application lime - 27 mm.

t J —

iz —

h

— 0001

B. Known:

1. Size of available stream is 1.0 c. f. s.

2. Desired width of border strip: 24 feet.

3. Basic intake rate: 1.5 in./hr.

4. Slope: 0.5 percent.

5. Root zone of crop will hold 4 inches of

water.

Desired:

1. Length of border strip recommended.

2. Time required to apply water.

Ansiver:

1. The recommended unit stream is 0.011

c. f. s. (fig. 15).

Therefore, the length of border=

Figure 21.—Chart for determining time required to

apply various depths of water vith different size of

unit streams.

3. Slope: 0.5 percent.

4. Root zone of crop will hold 5 inches of

water.

5. Desired border-strip width: 40 feet.

De-sired:

1. Size of stream to be used.

2. Length of time required to irrigate each

strip.

An.sicer:

1. The unit stream (fig. 15) is 0.00133 c. f. s.

Therefore, the stream for the strip is

0.0013X13.2X40 = 0.08 c. f. s.

2. A unit stream of 0.00133 c. f. s. will apply

5 inches depth of water in about 8.3 hours

(fig. 21). Assuming an application efficiencv of 70 percent, the total time is

8.3/OJO, or about 12 hours.

1 cf.

24X0,011 X100= 378 ft.

A unit stream of 0.011 c. f. s. will apply 4

inches depth of water in 48 minutes (fig. 21).

Assuming an application efficiency of 70

percent, the stream would have to be on

the field for about 70 minutes.

C. Known:

1. Length of field: 440 feet.

2. Basic intake rate: 2.5 inches per hour.

3. Slope: 1 percent.

4. Width of border desired: 20 feet.

5. Depth of application required: 2.0 inches.

De-sired:

1. Desirable stream to use.

2. Time required to apply water.

An-siver:

1. The recommended unit stream for a slope

of 0.5 percent is 0.034 c. f. s. (fig. 15).

Multiplying this by the slope factor

(fig. 16) shows the unit stream recommended for a 1.0-percent slope to be

0.0292 c. f. s. (0.034X0.86). The stream

required per foot of width for a 440-foot

run is 0.0292X4.4, or 0.128 c. f. s. With

a slope of 1.0 percent, however, the maximum allowable stream is 0.060 c. f. s.

per foot of border width (table 3). Therefore, the field should be irrigated in two

runs, using 1.28 c. f. s. border streams

(2.2X20X0.0292=1.28).

A unit stream of 0.0292 c. f. s. will apply

2 inches depth of water in 9.5 minutes (fig.

21). Assuming an application efficiency of

65 percent, the water needs to be run on

the border strip for only about 15 minutes.

16
Sprinkler Irrigation

Sprinkler irrigation is used for almost all crops

and on nearly all soils. It may be the only way

that crops can be irrigated satisfactorily on soil's

having a very high intake rate, steep slopes,

shallow soils, or irregular topography.

This section suggests a method to gather data

on sprinkler equipment and field operating conditions, and to analyze estal)lished systems. It

should be helpful in working out needed adjustments in design, installation, and operation. It

should lead also to developing better sprinkler

design criteria for local soils, crops, and climate.

General Performance Requirements

Seven main factors should be checked in any

revolving-head sprinkler system to determine

adequacy of design and operation, and what

adjustments in layout or use may be needed.

1. Application rate: Water should not be applied faster than the soil will absorb it. Yet, it

should be applied fast enough to prevent excessive

evaporation losses.

2. Depth of application: The amount of water

applied during an irrigation should not be greater

at the point of lightest application than can be

held by the soil within the root zone of the crop.

Greater amounts should be applied only when

leaching to remove harmful salts is necessary.

3. System capacity: The equipment should be

able to replenish the soil moisture at a rate at

least equal to the peak rate of use by the crop.

4. Uniformity of application: Water should be

applied as uniformly as practical over the field.

The point of lightest application usually should

have received at least 80 percent as much water

as the average for the field. Uniformity of

application is affected by differences in the discharges of individual sprinklers along a lateral and

on different laterals. It also is affected by the

uniformity of spray distribution within the effective area of individual sprinl-ders.

5. Wafer losses: The greatest water loss in a

well-designed and operated sprinkler system

occurs through wind drift and evaporation l:)etween

the sprinkler nozzle and the ground. Drop sizes

and application rates affect these losses. For

efficient water use, such losses should not be more

than 10 to 15 percent of the flow through the

system.

6. Economical pipe sizes: Distribution-pipe sizes

should be such that there is an economic balance

between pipe cost and power cost.

7. Crop damaye: Water must be applied in a

way that will not damage the crop physically.

Method

The following procedure is suggested for use in

checking a sprinkler-irrigation system:

1. Determine the water-distribution pattern

in the efl'ective area between two sprinklers on a

lateral.

2. Determine the amount of water needed to

refill the soil in the root zone.

3. Make an inventory of the sprinkler-svstem

parts and determine the operating procedure.

4. Field check the operating characteristics of

the system.

5. Analyze the data ol)tained.

6. Make recommendations for revision of the

s.ystem or changes in operating procedures, if

necessar3^

Equipment Needed for Field Tests

The following items are generally required for

the field evaluation of a sprinkler system:

1. Pressure gage (0 to 100 pounds) with pitottube attachment to measure pressure at the

sprinkler nozzles.

2. Soil auger and shovel.

3. Stopwatch and regular watch or clock.

4. Calibrated container to measure the discharge from individual sprinklers (1- to 5-gallon

capacity, depeiuling on the size of sprinkler).

5. Two pieces of rubber garden hose about 4

feet long.

6. About 50 spray gage cans (quart oilcans, or

other cans tall enough to catch the maximum

application).

7. Graduate to measure the water cauglit in tlie

spray gage cans (500 cc. capacity graduated to 10

cc).

8. Chain or tape (100 ft.).

9. Forms for recording data.

Procedure

The order to be followed in evaluating sprinklersystem performance can be varied, but the following step-by-step procedure is suggested:

Refill capacity of soils

Examine the soil in the root zone before tlie

spriiddcr test, so that the amount of soil moisture

to be added can be estimated.' Determine by

excavation if necessary the depth of the root zone

of the crop. If the crop is not mature, estimate

tlie depth of penetration bj^ the roots of the

mature plants.

Inventory and determine

operating procedure

Obtain and record data on sprinkler-system

design and operation. Use a sheet similar to

figure 29 as a checklist and to record the data.

5 It is assumed tliat trial run will be made at the time

the crop is in need of irrigation. If the test is made when

the soil is wet, the amount of water that would normally

be required for an irrigation will need to be estimated.

17
Most of the information can bo obtained from tlie

farmer and bj' field observations of tlie equipment.

Water-distributioii pa ttern

Set up s])rinkler lateral ready to operate, and

set out tlie spray cans in symmetrical pattern

across the lateral between two sprinklers. Tlie

cans should be about .5 feet apart where the

sprinkler spacing is less than 30 feet, and about

10 feet apart where sprinklers are .'30 feet or more

apart (fig. 22). The area selected must be far

enough from end of the sprinkler lateral to obtain

the normal overlapping of the wetted circles from

adjacent sprinklers. Set cans level and an inch

or two into the ground so they will not be overturned (fig. 23). On sodded areas, it may be

desirable to support the cans with sliort stakes.

In tall-growing crops, they should be supported

above the vegetation.

Prevent the sprinklers on each side of the spraygaging area from turning when the water is first

turned on so the initial jets from the nozzles will

not fall into the measuring cans. After normal operating pressure has been built up in the system,

measure and record the pressure and discharge

of the sprinklers on each side of the spray-gaging

area and the first and last sprinkler on the lateral.

Volumetric measurements of discharge mav be

40'

Previous loferai

Posilion "A"' "JZ

Fiíínre 22.—!Spray gage can layout.

Figure 23.—^Method of setting spray gage cans.

made by placing a hose over the nozzle and

directing the water into a calibrated container

(fig. 24). Gallon containers are adequate for

discharges up to 10 g. p. m. Use larger ones on

sprinklers of greater capacity.

Release the sprinklers after the measurement,

recording the time of release. Run the system

until a sulistantial depth of water (usually % inch

or more) has been caught in the spray gage cans

midway between successive lateral positions, but

not long enough to cause overflow from anj^ can.

Normally, the longer runs provide the more nearly

accurate data. For convenience in computations,

cut oft' the test run on the hour.

Stop the sprinklers so that no more spray can

fall into the cans. Record the time of stopping

and again measure pressure and discharge of the

sprinkler on each side of gaging area. Then turn

oft' the water, measure tlie catch in spray gage

cans, and record the measurements. Volumetric

measurements with a graduate are faster and more

nearly accurate than direct depth measurements

with a ruler. Quart-size oilcans hold almost

200 cc. per inch of depth. Readings to the nearest

10 cc, then are equivalent to 0.05-inch accuracy.

System operating cJiaracteristics

With the sprinkler system in operation, measure

lateral and main-line pressures at various points.

Measure the pressures in the lateral pipelines at

18
Figure 24.—JMethods for measuring sprinkler discharge: (Left), method for measuring small flows. (Right), method

for measuring larger flows. Both nozzles have to be measured to get total discharge.

fii'st sprinkler from the main-line outlet, the high

point in the lateral line, and at the end sprinkler.

When feasible, measure the main-line pressure

at the pump, at the highest point and at the point

farthest from the pump. If a pitot-tube attachment is used, pressures are measured at the

nozzles of the sprinklers (fig. 25). Otherwise,

pressure gages should be connected into pipelines

before water is turned on. Operating costs are

lowest and application of water is most nearly

uniform when pressures differ little between the

various points.

Observe the rate at which water enters the soil,

especially in areas around the fastest filling

spray gage cans. Water applied at any point at

one revolution of the sprinkler should have disappeared before water again is applied to that

point. There should be no movement of water

over the surface, and more than the slightest

ponding is generally unsatisfactory. Intake observations usually should be made after the

sprinklers have been operating for several hours.

The area wet during the preceding lateral setting

often has a lower intake rate than the dry area.

Surface movement of water on either area is

indicative of too high an application rate.

Determine the adequacy of irrigation by

checking, with a soil auger, the depth of water

penetration on an area from which the sprinlder

lateral had been moved long enough for the water

to distribute itself in the soil.

Check for crop damage caused by sprinkling.

Such damage maj^ be caused by drop impact,

unfavorable jet trajectory, or inadequate riser

height.

Note wind direction and estimated oi- measured

velocities, temperature, and humidity.

Record all observations.

Analysis of data

Overall adequacy of the system and needed

adjustments in design or operation can be determined by a careful analysis of the field data.

1. ATinimum inferml between irrigations: Divide

the depth of water in inches to be applied at each

irrigation by the peak consumptive-use rate of

the crop. When peak rates are not known, these

average values may be used in estimating minimum intervals:

Hot climate 0..30 to O.o5 inch per day

Moderate climate. _ .25 to .W inch per day

Cool climate .20 to .25 nicli per da}^

Local data should be used when available.

2. Pattern efficiency: Determine this from the

S])ray-gage-can records. Tliese recoixls, liowever,

sliow the depths of water caugiit during only one

lateral setting and do not take into account the

overlapping that would occur from adjacent lateral settings. An additional run must be made or

the records adjusted to sliow tleptlis that would be

caught during a com])lete irrigation. The avoa

covered by tlie "south group" of gages (fig. 22)

will receive water from the lateral in both i)Ositions "B" and "C." Water distribution to the

"south group" of gages from position "C" can l)e

assumecl to be the same as was measured in the

"north grou])." Thus, records for the "north

group" can be superimposed on tliose for the

"south group" to obtain the total-depth-of19
Figure 25.—Using pressure gage with pilot tube to measure pressure. (Left), tip of pitot tube must be centered in

water jet, and must be held parallel with center of jet. (Right), hold about one-eighth-inch from nozzle and

twist Into position, observe gage for maximum reading, which will occur when pitot tube is properly positioned.

application values for that area. "North g-roup"

gage records must be kept in their proper position

relative to lateral position "C," so that all water

caught in gages having the same number can be

added togetlier for an adjusted catch.

Divide the sum of the adjusted depths in the

"south group" by the manlier of gages in tliat

group to get the average depth of catch.

Inspect the adjusted gage records for minimum

depth of catch. Since the value at any one

gaging site may be affected by experimental

errors, it is best to use as the minimum the

average for the 2.5 percent of tlie gages liaving the

least adjusted water depths.

Divide the minimum depth of catch by the

average depth of catch for the area to determine

the pattern efficiency.

3. Water lasses: Subtract the average depth of

water reaching the ground, as measured in the

spray gage cans, from the average depth of application as calculated from the discharge of the

sprinklers on each side of the gaging area. Convert the sprinkler discharge to average depth as

follows :

Inches Gallons per minute x 0(i.3 x hours operated

depth S])rinkler spacing (ft.) x lateral sjjacing (ft.)

4. Application eßiciency for the (ja(jed area:

Divide the minimum depth of catch by the average

depth discharged from the sprinlcler nozzles adjacent to the gaging area.

5. Ap])Hcation efficiency for the lateral: Divide

the minimum depth of catch by the computed

depth for the average discharge of all the sprinklers

on the lateral. Since sprinkler discharge is a function of pressure, differences of over 20 percent in

pressures along the lateral generally result in

unsatisfactory lateral efficiencies.

When the discharge of each sprinlder on the

lateral is not measured, the average is considered

to be the discharge of the first sprinkler minus

three-fourths of the difference between first and

last sprinklers.

6. S]/sfem efficiency: If lateral pressures can be

maintained about the same for all settings, lateral

efficiency and system efficiency can be considered

identical. If lateral pressures vary at dift'erent

settings, however, additional evaluation trials may

need to be made to determine overall system

efficiency.

7. Time required for each lateral setting: Divide

the inches depth of water required to fill the root

zone by the average depth caught per hour in the

area oi^ minimum application

8. Number of lateral moves per day: Divide 24

hours by the required hours of operation plus the

time required to move the lateral. If system is

not operated on a 24-hour day basis, use total

operating and moving time available per day.

9. Number of days to cover field: Divide total

number of lateral moves needed to cover the field

20
by the product of the number of lateral moves per

day and the number of laterals

10. Sprinkler-system capacity : Multiply the maximum number of sprinklers in operation by the

average sprinkler discharge in gallons per minute.

11. Pump pressure head: Multiply the pressure

in pounds per square inch, measured at the pump,

by 2.31 and add the difference in elevation in feet

between the pump and the water level on intake

side of pump.

Conclusions and recommendations

The calculations and operation inventory give a

basis for conclusions as to the adequacy of design

and effectiveness of operation and recommendations for improvements.

Sample Analysis

A sprinlder system in a moderately cool climatic

area is evaluated to illustrate the application of

the procedure and methods of analyzing the field

data.

This system irrigates a rectangular 20-acre field

of alfalfa which has a peak-use rate of 0.25 inch

per day. Water is pumped from a well midway

along the north side. Two sprinkler laterals

operate from the buried main pipeline, which is

along the centerline of the field (fig. 26).

Both laterals have the same size pipe and

sprinlder nozzles. Lateral Xo. 1, at the 21st pipeline outlet from the pump, was selected for evaluation.

Refill capacity of soil

The soil is a coarse-textured sandy loam from

0 to 36 inches and a light-textured fine sand}" loam

from 37 to 72 inches. The average total available

water-holding capacity of the 6-foot profile was

6.0 inches. Before irrigation, the profile contained

3.1 inches of available moisture, so 2.9 inches had

to be added by irrigation.

-40'-

h -Spfoy-gogB area Wind diractio

-Lottrol No I-300'of 2"diam8l8r loiarol

4"aiamoler buried mom pipolmslJOO'

H h.^ I-I-H I.J-1 I-LH h^H l-Lf (J-, HL-J ^J-, (.J-I HJ^ 1-I-H HI-I I-^-I I-^ h^H 1-^ h

•

Outlet volvflt

Lotsrol No2-3O0'of 2 diameter loterol0.03 0.00 0.00 0.00-^Total catch in 4 hrs. (In,)

0 0 0 0-•—Sproy-goge can

.00 ,00-^Averag« catch per hour

Sprinkler No.7^

Sprinkler No.8

.01

.09

0

.02

.36

0

.09

.88

0

.22

1.13

0

,28

1.36

0

,34

.00

,08

0

,02

,36

0

,09

,64

0

,16

,08

0

,02

,40

0

.10

,60

0

.20

, le I

0

,29

05)

01

43

I I

92

D

23

19

,30

Sprinkler No,6

Sprinkler No,5

Figure 26.—Typical layout for 20 acres of sprinklerirrigated alfalfa.

Figure 27.—Spray gage can measnremenls lateral No. 1

at 21st main-line outlet.

Water-disfrihiition pattern

The water-distril:)ution pattern for the lateral

between the 6th and 7th s])rinklers was determined,

using spray gage cans. The sprinkler lateral was

run 4 hours for this test. Figure 27 shows location

and amount of water caught in each spray can,

and the computed average depth per hour.

Inventory and operating procedure

Items 1 to .5, figure 28, give data on the sprinklers, pipeline, pump, and water source, gathered in

field observations. Item 6 tells how the farmer

operates his sprinkler system

System operating characteristics

Pressure and discharge measurements taken

at sprinklers 1, 6, 7, and 8 on lateral No. 1 are

recorded in figure 29. The aver-age of tiie two

discharge measurements on each sprinlder is

9.5, 8.9, 8.7, 8.6 g. p. m., respectively.

Observation sliowed tlie soil was taking the

watei' satisfactorily, with no water movement

over the surface or ponding. The maximum rate

of application was found to be 0.34 inch per hour

(fig. 27).

21
Adequacy of irrigation was checked on an area

from which the sprinl-cler lateral had been moved

24 hours earlier. The root zone was filled to

capacity.

There were no signs of crop damage caused

by sprinkling.

Aualysis of data

1. Aliinmum interval between irrigations: The

peak consumptive-use rate of alfalfa in this area

is 0.25 incli per day. The moisture to bo replaced

is 2.9 inciies. Therefore, the minimum interval

between irrigations will be

2.9

0.2,T ai.G (use 11 days).

2. Pattern (fficieneij: Figure 30 shows the water-

(Hstribution pattern, determined from tlie spray

gage measurements. Since tlie gages are located

in a uniform pattern, the average depth of application over the gaging area ecjuals tlie sum of the

depths (7.33 inches) divided by the number of

gage cans (24), or 0.30,5 inch.

The minimum depth, or the average of the

catch in 6 cans, is 0.250 inch.

0.22 + 0.25 + 0.2.-) + 0.2.3 + 0.25 + 0.2S

G

=0.250

Pattern efficiency is (0.250-^-0.305) - (100 = 82

percent, which intlicates good sprinlder performance. It is necessary to apply an average of 3.5

inciies (2.9^-0.82 = 3.5) to supply at least 2.9

inches of water to all parts of the field.

3. Water losses: The average discharge of sprinklers 6 and 7 was 8.8 g. p. m., ecjuivalent to a

deptli of application of 0.354 inch per hour for

a 40 -X 60-foot spacing. Since the average depth

of application in the spray gages was 0.305 inch,

the loss between the nozzles and the gages was

0.049 inch, or nearl}^ 14 percent. Probably only

a part of this loss is evaporation and wind drift,

since some splash may have occuri'ed and the

average catch in the gage cans may have been

slightly lower than the average depth applied

over the ground surface. In any case, a loss of

14 percent is not unreasonably high.

4. Application efficiency for the gaged area is

the minimum depth of application (0.250 inch)

divided by the average depth of application

measured at the nozzles (0.354 inch) X 100, or

71 percent. This is satisfactory.

5. Application efficiencij for the lateral: Tlie

application efficiency of lateral No. 1 is the minimum depth of application (0.250 inch) divided

by the depth of application (computed for the

average sprinkler discliarge on the lateral) times

100. Tlie average sprinkler discharge is 9.5 —

% (9.5-S.ß)=S.S g. p. m.

As the average discharge for all sprinklers on the

lateral is the same as the measured discharge of the

two sprinklers ad.i'acent to gaging area, the application cffii'iency for the lateral is the same as the

efficiency computed for the gaging area.

Farm .T«v» n-rham

Field No. 1 Ac r e s .

1. SreINKLER INFCRMAT ION

.Locat ion,

20

narihnn ,'^fïï)

. Crop_ AT fA1 ffl

Date Aiign.qt. 1 j IQ'^I

3.

4.

5.

6.

Mode 1

-S6-

Make Rainhtrd

Nozzle size '^/l^^-fr 1 /RWRated discharge

SHÎINKLE3Î LATE3?ALS: Number of laterals. 2.

Sprinkler spacing /iQ ft. Distance moved

Lateral No. 1 3QQ f t . 2 in. diameter and

Lateral No. 2 300 ft . 2 in. diameter and

Lateral No. .3 f t . ' in. diameter and

JiQ- Number of nozzles^_i__

p. m. at jtC? lbs, pressure

Number of sprinklers per lateral.^

An ft. Riser height. Jt.

ft.

ft.

ft.

MAIN-LINE Pires:

Sizel^3flû_ft

Number of outlet valves

Jl in. diameter ______ ft.

2Z. spacing of valves

.in. diameter ft.

.in. diameter

in. diameter

__in. diameter

.60 ft.

^in. diameter

PUMP: Make Tnyftr.qoll-Ranri Mcxlel No. 2-CRVNL

Capacity l'?Q g. p. m. at 1^0 ^ft. head.

. r. p

Source

Hor s e powe r 7—1/2

WATCR INFCRMATICN

Delivery schedule

OHKATIONS: Number of lateral moves to cover field

Well

_ Ser ia1 No.

Type power

011i96íi8l

Electric

Elevation pump to water level

Quantity 2QQ

JiQ_ ft.

p. m.

f:r>nf.^ minng

M

Number of moves per day. .Number of laterals operating

Hours sprinklers operated each day 11—1/2 Hours to move lateral^

Maximum number of sprinklers operating 16 Minimum number of sprinklers operating l6

REMARKS: F1rat. yf=iar in alfalfa. T-a^f. ^ yo^tra rnw PT'npqFigure 28.—Sprinkler-system design and operation.

22
Time sprinklers started . ICit'^n a. m.

Duration of test run It- hours.

Y/ind direction HE Estimated velocity ¿

Temperature QD-Qc: degrees. Humidity T.nw

Time sprinklers stopped ?»3ri

.m. p. h.

Main-line pressure at pump l|t; p. s. i. At high point in field_U2—p. s.i.

At end of main line . )i9 p. s. i.

Water-application rate observât ions. Rat.a ^^af.i^fAc+.ory—M^^-jmum 0.^). In./hr.

Spi inkier pressure and discharge measurements

Sprinkler Sprinkler Sprinkler Sprinkler Spr inkier

No 1 No. A No. 7 — No . R No

O u Ü o Ü

OJ OJ OJ 0) (3J • —1 M • H tn • '-i !A -H t/l —^ t/1

c '-^ 1 c —i 1 c 1—( 1 C 1—1 1 r -H 1

tu •1-1 •H 1 •H H 1 •H •H 1 •H H 1 •H 1 E 'J- U, CJ '4H 1- aj <*. U 0) <^-. U 1) 4- U 0)

•H 0) . OJ Oí) . D . 0.' M . OJ • OJ M , o; . 0) no , <¡> . eu bi .

H u zr o c 11 E 1- cr 0 C U fc U O" o c u F i- cr O c u. F l_ n* 0 C u g

D fi tJ -H QJ D 1/1 +J -H ro D i/l J-J -H Cî 3 ¡fi -M -H 03 tn I-' -1 0]

<A \ CU X . en \ aj x; • t/l ^^ 03 . tA \ « X . t/1 \ c« s: •

00 . OJ ^ o a t/i . 0) -t-" Ü a t/1 tu -i-J o a t/) tu 4-1 u U t/1 , OJ -i-i u CL,

4) ¡A E C t/1 aj M E C t/1 <U tfl E C c/) 0) t/1 E C t/1 IIJ t/1 £ C m

U ^ •H 0 •H . u 02 H 0 H . u £i •H O -^ . U J2 •H o •H . u n •H O •H

CU -H H Ü Q ùi CL, ^ H u Q M PL, -H H o Q M DH -H H u O w PH ^H H (J a M

Latíïral No.J,

10:20 U2 6.3 9.5 37 6.7 9.0 36 6.9 8.7 35 7.0 8.6

2»35 U2 6.Î4 9.U 37 6.8 8.8 36 6.9 8.7 35 7.0 8.6

Lateral No. _2

lîOO hk 37

.

Capacity of calibrated container 1,n gallons

Figure 29.—Field-evaluation dala.

23
' ' '~y '0.34' ' ' 0.32' ' ' 'ö'

Sprinkle

0,20

.02

Sproy-gaqe cons*

0.22

09

Cafchper hour ^'

from south can^.

0.1 I

.22 Colch per hour

from north con"

,33

Adjusted colch y

per hour g Q4

.28

°32

0.00

.34

0 23

.02

0.23

.09

0.09

.16

0,03

.22

0.01

.30

'O.2B' 'V)~' '

.00

0.30

.02

0.23

10

O.ll

.20

0.02

.29

0.00

.31

31

Lotero I^

.28

0.24 0

.01

.25

0.23

0

. 1 1

.34

0.12 5

.23

.35

0.03 2

.30

.33

0.00 2

;- '

^ Lateral10

-40'

.32

—(y.

H

Fitíiire 30.—Watcr-tlistributioii pattern on sample area.

On longer laterals, the averag'e sprinkler discharge is usually greater than the ihscharge of

the two s])rniklers near the end of the lateral.

Here application efficiency of the lateral would

be lower than for the gaging area.

6. System efficiency: Since the main line in this

example is equipped with takeout valves, operating pressures can be maintained approximately

equal for all lateral positions. The overall system

efficiency, tlierefore, is considered equal to the

efficiency at lateral No. 1.

It should be noted that with split-line operation,

as in this example, operating pressures may be

lowest when the laterals are o])])osite each other

in the center of the field. This will always be

true when a uniform size main is used and laid

on a uniform grade. When this condition is

found, the evaluation trial should be made at a

lateral setting near the center of the field. Due

to topographic conditions, the minimum operating

pressure in this system was at the far end of the

main line.

7. Time required for each lateral setting:

The time required to apply the net 2.9 inches of

water to fill the soil profile to field capacity will

be 11K bours (2.9-^0.25).

8. Number nf lateral moves per day {24 hours):

With llK hours' operating time and Vi hour to

move the lateral, there can be two moves per day.

9. Number of days to cover field: Forty-four

lateral settings are needed to cover the field.

Each of the 2" laterals is moved twice a day thus

making 4 sets per day. The number of daj^s

required will be 44^-4=11.

10. Sprinkler system capacity: Average sprinkler discharge for lateral No. 1 was 8.8 gallons per

minute. The discharge of spriidvlers on lateral

No. 2 was not measured, but the manufacturer's

rated discharge for the average pressure as computed from pressure measurements in figure

29, was 9.0 g. p. m. Svstem capacity must be:

(8.8X8) + (9.0X8) = 142'.4 g. p. m.

11. Pump pressure head: (45X2..31)-f40=144

feet.

Conclusions and recommendations

This S3'stem is well designed, with equipment

capable of meeting crop requirements in peakwater-use periods. Application rate is satisfactory for the soils. The water-distribution pattern

is reasonably uniform. Lateral and main pipelines are adecpiately designed for efficient operation

costs. The capacity of the pump is adequate.

Operation of the system is satisfactory, but

the ecpiipment will just meet the peak-use requirements. The peak-use frequency is 11 days

and the field can be irrigated in 11 daj^s. This

leaves no safety factor for breakdowns or other

interruptions during the peak-use period. In the

off-peak periods, however, when the rate of water

used by the crops is less, the S3^stem need not be

operated continuously.

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