when and how much? · tion in order to refill the soil profile to its capacity. addi-tional...
Post on 23-Oct-2020
1 Views
Preview:
TRANSCRIPT
-
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
top related