When and How Much?

U.S. Department of AgricultureNatural Resources Conservation Service

California Department of Water ResourcesWater Conservation Office

Irrigating withFurrow, Border, and Sprinkler Systems

University of CaliforniaCooperative Extension

The U.S. Department of Agriculture (USDA) prohibits discrimination in its programs on the basis of race, color, national origin, sex,religion, age, disability, political beliefs, and marital or familial status. (Not all prohibited bases apply to all programs.) Persons withdisabilities who require alternative means for communication of program information (Braille, large print, audio tape, etc.) shouldcontact USDA’s TARGET Center at (202) 720-2600 (voice and TDD). To file a complaint, write the Secretary of Agriculture, U.S.Department of Agriculture, Washington, DC 20250, or call 1-800-245-6340 (voice) or (202) 720-1127 TDD). USDA is an equalemployment opportunity employer.

Revised August 1997

Rooting Depth for Irrigation

Crop FeetAlfalfa 3.0-6.0Almond, apple, apricot 4.0-5.0Asparagus 3.0-6.0Barley 2.0-3.0Beans 2.0-4.0Citrus 3.0-5.0Corn 2.0-4.0Cotton 4.0-5.0Grape 3.0-4.0Ladino Clover 2.0-3.0Lettuce 1.0-2.0Melons 3.0-4.0Oats 2.0-3.0Olives 4.0-5.0Onions 1.0-3.0Pasture (perennial) 2.0-4.0Peach, pear, plum 4.0-5.0Safflower 3.0-5.0Sorghum 3.0-4.0Squash 3.0-4.0Sugar beets 4.0-6.0Tomato 3.0-4.0Walnut 5.0-7.0Wheat 2.0-3.0

Rooting DepthThe depth of soil from which a crop can extract water is theeffective crop rooting depth. This depth will vary with thestage of crop growth. Rooting depth may be restricted to adepth considerably less than the normal root depth by soillimitations (i.e., hardpan, high water table, and so forth).Irrigation design rooting depths for several mature irrigatedcrops, grown on deep, permeable well-drained soil, are givenbelow.

TABLE 2

21

Irrigation...When and How Much?Conservation irrigation water management means controllingor regulating irrigation water applications in a way that willsatisfy crop water requirements without wasting water, erodingsoil, or degrading water quality. It involves applying water inaccordance with specific crop needs in amounts that can beretained in the soil for crop use and at rates and times that areconsistent with soil intake characteristics and erosion hazard.

SoilThe amount of water a soil can hold, and that is available forplant use, is determined by the soil’s physical properties. Soiltexture and depth are the most influential factors in determiningthe Available Waterholding Capacity (AWC) of a soil. Manysoils have varying textures at different depths. Table 1 givesthe general range of AWC per foot of depth for several soiltextures.

TABLE 1 Available Waterholding Capacity forVarious Soil Textures

Available Moisture Range Average (in./ft.) (in./ft.)

Very coarse to coarse-textured sand 0.5-1.25 0.90

Moderately coarse-textured sandy loams 1.25-1.75 1.50 and fine sandy loams

Medium texture—very fine sandy loams 1.50-2.30 1.90 to silty clay loams

Fine and very fine texture—silty clay to clay 1.60-2.50 2.10

Peats and mucks 2.00-3.00 2.50

Soil Texture

When To IrrigateMost surface- or spring-irrigatedcrops are normally irrigated whenabout 50 percent of the availablemoisture has been removed.Exceptions would be specialtycrops needing certain conditionsfor product quality.

One method of estimating the amount of water depleted fromthe soil is the “feel” method, using Table 3. The depletionlevel estimates can be made by taking soil samples at variouslocations in the field with an auger, soil tube, or shovel atapproximately 1-foot increments for the entire crop rootzone. The total amount of moisture depleted at each locationis the sum of the 1-foot increments for the crop rootingdepth. This is also the new amount to be replaced by irriga-tion in order to refill the soil profile to its capacity. Addi-tional allowance must be made for deep percolation, runofflosses, salinity leaching, and application efficiency.

Example 2. Estimating When To IrrigateAssume the following:• The same soil and crop as in Example 1.• An allowable moisture depletion of 60 percent of the total

available in the 4-foot root zone.• Allowable depletion level = 0.60 x 5.0 inches = 3.0 inches

This means that irrigation should begin when about3 inches of moisture have been lost by this calculation.

4

Total Available MoistureThe total amount of water available for plant use in the rootzone is the sum of the AWC per foot for the various soil typeswithin the effective rooting depth. See the example below.

Example 1. Calculation of Total Available Moisture in the Root ZoneAssume the crop is beans with a 4-foot rooting depth (Table 2)and is grown in a soil with the following characteristics:

Depth(in.)

Texture AWC (from Table 1)

8”

12”

Sandy loam

Silty clay loam

Coarse sand

CropRootingDepth

(in./ft.)

28”

^

4’

^ ^^

^

^^

^

1.9

1.5

0.9

Depth(in.)

Layerthickness

(ft.)

AWCper foot

(in./ft.)

Availablemoisture

(in.)

0-8 812

x 1.5 1.0=

2812

20-48 x 0.9 2.1=

Total available moisture = 5.0

1212

8-20 x 1.9 1.9=

3

Application EfficiencyApplication efficiency is ameasure of irrigationperformance. It involves thetotal amount of water applied,the uniformity of thedistribution, losses (such asrunoff, evaporation, and deeppercolation) and the net amountof water needed to meet plantgrowth requirements andleaching. Common applicationefficiencies for various types ofirrigation systems, under good-to-excellent management, arelisted below in Table 4.

Profile of Wetted Soil

Water Surface

Root Zone

Run

off

End

of F

ield

Wetted Depth

(in excess of leaching requirement)Deep Percolation

TABLE 4 Irrigation Application Efficiencies Under Good-to-Excellent Management

Approximate Range of Application Efficiencies(in percentage)

System

Furrow 70-85

Basin 75-90

Sprinklers 70-85

Graded Border 70-85

Drip (Trickle) 80-90

6

TAB

LE 3

Guid

e fo

r Es

timat

ing

Avai

labl

e So

il M

oist

ure

Depl

eted

by

the

“Fee

l” M

etho

dM

oist

ure

Defic

ienc

y(in

./ft.)

SO

IL T

EX

TU

RE

CLA

SS

IFIC

A TIO

NM

oist

ure

Defic

ienc

y(in

./ft.)

Coa

rse

(loam

y sa

nd)

San

dy(lo

amy

sand

)M

ediu

m(lo

am)

Fin

e(c

lay

loam

)

(fie

ld c

apac

ity)

(fie

ld c

apac

ity)

(fie

ld c

apac

ity)

(fie

ld c

apac

ity)

App

ears

ver

y da

rk, l

eave

sw

et o

utlin

e on

han

d, m

akes

ash

ort r

ibbo

n

App

ears

ver

y da

rk, l

eave

s w

etou

tline

on

hand

, will

rib

bon

abou

t1

inch

App

ears

ver

y da

rk, l

eave

sle

aves

slig

ht m

oist

ure

onha

nd w

hen

sque

ezed

, will

ribbo

n ab

out 2

inch

esA

ppea

rs m

oist

, mak

es a

wea

k ba

ll

App

ears

slig

htly

moi

st, s

ticks

toge

ther

slig

htly

Dry

, loo

se, f

low

s th

roug

hfin

gers

(w

iltin

g p

oin

t)

Qui

te d

ark

colo

r, m

akes

aha

rd b

all

Fair

ly d

ark

colo

r, m

akes

ago

od b

all

Slig

htly

dar

k co

lor,

mak

es a

wea

k ba

ll

Ligh

tly c

olor

ed b

y m

oist

ure,

will

not

bal

l

Ver

y sl

ight

col

or d

ue to

moi

stur

e (w

iltin

g p

oin

t)

Dar

k co

lor,

form

s a

plas

tic b

all,

slic

ks w

hen

rubb

ed

Qui

te d

ark,

form

s a

hard

bal

l

Fairl

y da

rk, f

orm

s a

good

bal

l

Slig

htly

dar

k, fo

rms

a w

eak

ball

Ligh

tly c

olor

ed, s

mal

l clo

dscr

umbl

e fa

irly

easi

ly

Slig

ht c

olor

due

to m

oist

ure,

smal

l clo

ds a

re h

ard

(wilt

ing

po

int)

Dar

k co

lor,

will

slic

k, r

ibbo

nsea

sily

Qui

te d

ark,

will

mak

e th

ick

ribb

on, m

ay s

lick

whe

n ru

bbed

Fairl

y da

rk, m

akes

a g

ood

ball

Will

bal

l, sm

all c

lods

will

flat

ten

out r

athe

r th

an c

rum

ble

Slig

htly

dar

k, c

lods

cru

mbl

e

Som

e da

rkne

ss d

ue to

unav

aila

ble

moi

stur

e, c

lods

are

hard

and

cra

cked

(wilt

ing

po

int)

.0 .2 .4 .6 .8 1.0

1.2

1.4

2.0

1.8

1.6

.0 .2 .4 .6 .8 1.0

1.2

1.4

2.0

1.8

1.6

5

Procedure for Calculating Depth of Water Applied

Step One

Step Two

Step Three

Area irrigated (acres)Furrows Borders/Basins Sprinklers

Area =(Number of

furrows)X (Length

in feet)(Spacingin feet)

X

(43,560)

Area =(Lateral length

in feet)X

(Movein feet)

(43,560)

(A) Area per border =

X(Lengthin feet)

(Widthin feet)

(43,560)

(B) Total area acres =

X(Numbero f

borders)

(Areaper

border)

Flow(in gallons per minute (GPM))

Siphons Pumps Sprinklers

(A) See Table 7 for GPM per siphon

(B) Total flow =

(A) See Table 6 for GPMper sprinkler

Number ofsiphons[ [X GPM persiphon[ [ Number ofsprinklers[ [X GPM persprinkler[ [

(B) Total flow =

Depth of Water Applied(inches)

Use total flow from Step Two with Table 8 and your set time to get acre inches of water applied.

Depth applied (inches) =(Acre inches of water applied)

(Area irrigated from Step One)

TABLE 5

7 8

Center Pivot

Area per center pivot

( .0314)X (P)X

(43,560)

2

Where: P = Percent of full circle

Laterallength in

feet[ [

If a flow meter is notavailable, see Tables 9,10, and 11 for GPM flowfrom pump with freedischarge.

9

How Much To ApplyThe gross amount of water to be applied is the net amountrequired to refill the crop root zone (to meet plant require-ments), divided by the application efficiency.

ManagementSo far, the discussion has assumed flexibility in the schedulingof irrigation and the total amount applied. An irrigation isscheduled when the amount of soil moisture depleted from theroot zone approaches the allowable depletion level. Theamount of water applied is based on replacing the depleted soilmoisture. This involves decisions for when to irrigate and howmuch to apply in order to optimize moisture conditions for cropproduction and minimize irrigation costs.

Occasionally, water deliveries are on a fixed schedule whendeliveries are made every 10 to 15 days. Under these condi-tions, it is not necessary to decide when to irrigate, but onlyhow much to apply. The amount applied then should be basedonly on the soil moisture depleted since the last irrigation.

The relationship between the total amount delivered,delivery flow rate, set time, gross application, and the area to beirrigated is as follows:

Where: D = Cross application depth (inches)A = Area irrigated (acres)Q = Flow rate (cublic feet per second)T = Set time (hours)

Gross application required =Depleted moisture to be replaced Application efficiency (decimal)

Total amount delivered = D x A = Q x T

This equation can be used to calculate gross application depthapplied, the area to be irrigated, the flow rate required, and theset or delivery time.

When the flow rate, time of set, and area irrigated are known,the gross application depth (D) can be calculated by:

When the flow rate, the set time and the gross applicationdepth are known, then the area that can be irrigated (A) can becalculated by:

When the gross application depth, the set time, and the areairrigated are known, then the flow rate (Q) needed can be calcu-lated from:

When the gross application depth, the flow rate, and the areato be irrigated are known, then the set time (T) can be calculatedfrom:

D =Q x T

A

A =Q x T

D

Q =D x A

T

T =D x A

Q

10

11

NozzleSize(in.)

Sprinkler Nozzle Discharge (in GPM)Nozzle Pressure (#/sq. in.) 1

TABLE 6

3/32

7/64

1/8

9/64

5/32

11/64

3/16

13/64

7/32

1/4

9/32

5/16

11/32

3/8

13/32

7/16

15/32

1/2

17/32

9/16

5/8

30

1.4

1.9

2.4

3.2

3.9

4.7

5.5

6.5

7.6

9.9

12.4

15.2

25

1.3

1.7

2.2

2.9

3.5

4.3

5.0

5.9

6.8

35

1.5

2.0

2.6

3.4

4.2

5.1

6.0

7.1

8.3

10.7

13.4

16.5

19.7

22.8

27.2

40

1.6

2.2

2.8

3.6

4.5

5.4

6.4

7.6

8.9

11.4

14.3

17.7

21.1

24.4

29.1

33.9

38.9

43.6

45

1.7

2.3

3.0

3.8

4.7

5.7

6.8

8.1

9.4

12.1

15.2

18.9

22.4

26.0

30.9

35.9

41.1

46.0

51.6

57.5

70.0

50

1.8

2.4

3.2

4.0

5.0

6.0

7.2

8.5

9.9

12.8

16.1

20.0

23.6

27.6

32.7

37.8

43.3

48.4

54.0

60.6

73.6

55

1.9

2.6

3.3

4.2

5.2

6.3

7.5

8.9

10.3

13.5

17.0

21.0

24.8

29.2

34.3

39.7

45.4

50.7

56.4

63.6

77.2

60

2.0

2.7

3.5

4.4

5.5

6.6

7.8

9.2

10.7

14.1

17.9

22.0

25.9

30.6

35.9

41.5

47.4

53.0

58.8

66.5

80.8

65

3.6

4.6

5.7

6.8

8.1

9.5

11.2

14.8

18.7

23.0

27.0

32.0

37.4

43.3

49.4

55.3

61.2

69.4

84.4

70

5.9

7.1

8.4

9.8

11.6

15.4

19.5

23.9

28.1

33.3

38.9

45.1

51.4

57.2

63.5

72.2

87.8

75

8.7

10.2

12.0

16.0

20.3

24.8

29.2

34.5

40.3

46.8

53.3

59.6

65.8

74.9

91.0

1 Approximate diameter of coverage. For sprinkler with two different nozzles, the wetteddiameter is coverage of the larger nozzle.

60

70

80

90

100

110

125

145

205

185

165

WettedDiameter

(ft.)

Diameter of Siphon(inches)

Water surface in furrow

Head

Water surface

Field ditch

Siphon Tube Discharge (in GPM)

.7

1.3

3

5

8

10

13

21

32

46

86

1.0

1.6

4

6

10

13

16

27

40

57

106

1.2

1.8

5

7

12

15

18

32

48

65

122

1.5

2.1

6

9

15

19

24

41

54

82

153

1.7

2.7

7

11

18

23

28

50

65

100

200

3/8

1/2

3/4

1

1-1/4

1-3/8

1-1/2

2

2-1/2

3

4

9” head6” head4” head3” head2” head*

* Head is difference in height of water in supply ditch and center of discharge endof tube or water surface to water surface if tube outlet is submerged.

TABLE 7

Number of furrows =Q - GPM stream

q - GPM of siphon tubes

12

TA

BLE

8Co

nver

sion

Tab

le—

Gallo

ns P

er M

inut

e (G

PM

) an

dCu

bic

Feet

Per

Sec

ond

(CF

S) t

o Ac

re In

ches

of W

ater

App

lied

GP

M 45 90 135

180

225

270

315

360

405

450

495

540

585

630

675

720

765

810

855

900

990

1080

1170

1260

1350

18 1.8

3.6

5.4

7.2

9.0

10.8

12.6

14.4

16.2

18.0

19.8

21.6

23.4

25.2

27.0

28.8

30.6

32.4

34.2

36.0

39.6

43.2

46.8

50.4

54.0

16 1.6

3.2

4.8

6.4

8.0

9.6

11.2

12.8

14.4

16.0

17.6

19.2

20.8

22.4

24.0

25.6

27.2

28.8

30.4

32.0

35.2

38.4

41.6

44.8

48.0

14 1.4

2.8

4.2

5.6

7.0

8.4

9.8

11.2

12.6

14.0

15.4

16.8

18.2

19.6

21.0

22.4

23.8

25.2

26.6

28.0

30.8

33.6

36.4

39.2

42.0

12 1.2

2.4

3.6

4.8

6.0

7.2

8.4

9.6

10.8

12.0

13.2

14.4

15.6

16.8

18.0

19.2

20.4

21.6

22.8

24.0

26.4

28.8

31.2

33.2

36.0

11 1.1

2.2

3.3

4.4

5.5

6.6

7.7

8.8

9.9

11.0

12.1

13.2

14.3

15.4

16.5

17.6

18.7

19.8

20.9

22.0

24.2

26.4

28.6

30.8

33.0

10 1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

16.0

17.0

18.0

19.0

20.0

22.0

24.0

26.0

28.0

30.0

9 .9 1.8

2.7

3.6

4.5

5.4

6.3

7.2

8.1

9.0

9.9

10.8

11.7

12.6

13.5

14.4

15.3

16.2

17.1

18.0

19.8

21.6

23.4

26.2

27.0

CF

S .1 .2 .3 .4 .5 .6 .7 .8 .9 1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.2

2.4

2.6

2.8

3.0

6 .6 1.2

1.8

2.4

3.0

3.6

4.2

4.8

5.4

6.0

6.6

7.2

7.8

8.4

9.0

9.6

10.2

10.8

11.4

12.0

13.2

14.4

15.6

16.8

18.0

7 .7 1.4

2.1

2.8

3.5

4.2

4.9

5.6

6.3

7.0

7.7

8.4

9.1

9.8

10.5

11.2

11.9

12.6

13.3

14.0

15.4

16.8

18.2

19.6

21.0

8 .8 1.6

2.4

3.2

4.0

4.8

5.6

6.4

7.2

8.0

8.8

9.6

10.4

11.2

12.0

13.0

13.6

14.4

15.2

16.0

17.6

19.2

20.8

22.4

24.0

48 4.8

9.6

14.4

19.2

24.0

28.8

33.6

38.4

43.2

48.0

52.8

57.6

62.4

67.2

72.0

76.8

81.6

86.4

91.2

96.0

105.

6

115.

2

124.

8

134.

4

144.

0

36 3.6

7.2

10.8

14.4

18.0

21.6

25.2

28.8

32.4

36.0

39.6

43.2

46.8

50.4

54.0

57.6

61.2

64.8

68.4

72.0

79.2

86.4

93.6

100.

4

108.

0

24 2.4

4.8

7.2

9.6

12.0

14.4

16.8

19.2

21.6

24.0

26.4

28.8

31.2

33.6

36.0

38.4

40.8

43.2

45.6

48.0

52.8

57.6

62.4

67.2

72.0

20 2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

22.0

24.0

26.0

28.0

30.0

32.0

34.0

36.0

38.0

40.0

44.0

48.0

52.0

56.0

60.0

Set T

ime—

Hour

s

13

2

4

6

8

10

12

14

16

18

20

2

4

6

8

10

12

14

16

18

20

2

4

6

8

10

12

14

16

18

20

X

D Y

9339

6471

5177

4394

3851

3445

3126

2865

2647

2461

16-inch6977

4829

3859

3271

2864

2559

2319

2123

1959

1819

14-inch

4969

3435

2742

2322

2031

1812

1641

1500

1383

1283

12-inch3312

2288

1826

1545

1350

1204

1089

996

917

850

10-inch2005

1386

1106

937

819

731

662

605

558

517

8-inch

1044

724

580

493

433

387

352

323

299

278

6-inch419

294

238

204

1181

164

150

139

130

122

4-inchY

(in.)

When X=12 inchesD=Inside diameter of pipe

TABLE 9

13011

9060

7285

6213

5472

4919

4485

4132

3836

3584

16-inch9652

6721

5403

4608

4058

3648

3325

3063

2844

2656

14-inch

6819

4750

3820

3259

2871

2582

2355

2170

2015

1883

12-inch4508

3144

2532

2163

1908

1718

1569

1447

1346

1259

10-inch2713

1897

1532

1313

1161

1049

960

888

829

777

8-inch

1421

999

811

698

621

563

519

482

452

426

6-inch601

423

345

298

265

242

223

208

196

185

4-inchY

(in.)

When X=18 inchesD=Inside diameter of pipe

TABLE 10

16226

11345

9159

7843

6936

6261

5731

5301

4942

4635

16-inch11983

8384

6773

5803

5135

4638

4249

3932

3668

3443

14-inch

8434

5907

4778

4099

3632

3284

3012

2791

2607

2450

12-inch5569

3909

3168

2723

2418

2191

2041

1870

1750

1648

10-inch3373

2375

1931

1665

1483

1348

1242

1157

1086

1026

8-inch

1815

1281

1044

903

806

735

679

634

597

565

6-inch806

565

458

393

348

315

290

269

251

236

4-inchY

(in.)

When X=24 inchesD=Inside diameter of pipe

TABLE 11

14