Depth of Water to Apply

Seasonal water demand and peak daily use vary considerably from crop to crop and from one field to the next. Deciding when to irrigate and how much water to apply are the two most difficult decisions to make in managing irrigation systems. Many different methods have been developed to help answer these two questions. The following discussion will explore some of the factors influencing these decisions and present one method for determining how much water to apply and the irrigation interval.

The amount of water used by plants depends on five factors:

1. The length of the growing season

2. The amount of daylight per day

3. The daily temperature

4. The speed and direction of the wind

5. The crop's stage of growth

For any given plant, the daily rate of use will increase until the plant reaches maturity, and then it will decrease, Figure 19.1. The peak water use rate occurs at the height of the growing season. Table 19.1 shows the peak use for a number of crops with both short and long growing seasons, and the root zone depth from which each crop extracts most of its moisture.

Two characteristics of soil must be considered in determining how much water should be applied: (1) the rate at which soil can absorb (store) water, or the infiltration rate; and (2) the total amount of water that can be stored. The infiltration rate is determined by the soil texture and the total amount of water that can be stored is determined by the soil depth. Loams and clays can hold more water than sands, as shown in Table 19.2. Also note that the type and the depth of subsoil make a difference in the amount of water that can be stored per irrigation. For example, soils having more compact subsoil can store a greater amount of water than the same soils having a uniform depth.

Peak use

Plant maturity

T"5

Peak use

T"5

Growing season (days)

FIGURE 19.1. Typical daily water demand.

Growing season (days)

Table 19.1. Root zone depth and peak water use rate.

Length of growing season

Root zone depth of

Table 19.1. Root zone depth and peak water use rate.

Length of growing season

Root zone depth of

principle moisture

180-210 days

210-250 days

Crop

extraction (in)

Peak use (in/day)

Peak use (in/day)

Alfalfa

48

0.29

0.32

Beans

24

0.20

0.20

Corn

36

0.30

0.35

Cotton

36

0.28

Grain sorghum

30

0.20

0.20

Melons

30

0.20

0.22

Other truck

24

0.20

0.22

Pasture

24

0.28

0.30

Peas

36

0.19

Potatoes

24

0.38

0.20

Small grain

30

0.20

0.22

Sugar beets

36

0.28

0.30

Tomatoes

48

0.20

0.22

Vineyards

48

0.22

0.25

The rooting depth of the crop also influences the amount of water that should be stored. Any water that infiltrates into the soil to a depth below the root zone is lost to the plant. These factors were taken into consideration during the development of Table 19.1 and Table 19.2.

To use these tables first consult Table 19.1 for a particular crop and growing season length and obtain values for (1) the root zone depth of principal moisture extraction and (2) the peak daily water use. Then Table 19.2 can be used to determine the number of inches of water to store per irrigation. Note that you must know the soil type or profile on which the irrigation is to take place and the root zone depth of principal moisture extraction of the crop.

Problem: What is the root zone depth, peak daily use, and net amount of water to store per irrigation for a crop of corn grown on a location where the season is greater than 210 days on a silt loam soil over compacted subsoil?

Table 19.2. Net amount of water to store per irrigation.

Net amount of water to store (in) for various root zone depths

Table 19.2. Net amount of water to store per irrigation.

Net amount of water to store (in) for various root zone depths

Soil profile

24 in

30 in

36 in

48 in

Coarse sandy soil, uniform to 6 ft

0.85

1.10

1.30

1.75

Coarse sandy soil over compacted subsoil

1.50

1.75

2.00

2.50

Fine sandy loam uniform to 6 ft

1.75

2.20

2.60

3.00

Fine sandy loam over compacted subsoil

2.00

2.40

2.80

3.25

Silt loam uniform to 6 ft

2.25

2.75

3.00

4.00

Silt loam over compacted subsoil

2.50

3.00

3.25

4.25

Heavy clay or clay loam soil

2.00

2.40

2.85

3.85

Solution: Using Table 19.1 and Table 19.2:

Length of growing season

Root zone depth of principle moisture 180-210 days 210-250 days

Crop extraction (in) Peak use (in/day) Peak use (in/day)

Root zone depth of principle moisture 180-210 days 210-250 days

Crop extraction (in) Peak use (in/day) Peak use (in/day)

Alfalfa

48

0.29

0.32

Beans

24

0.20

0.20

Corn

36

0.30

0.35

Cotton

36

0.28

Grain sorghum

30

0.20

0.20

Melons

30

0.20

0.22

Other truck

24

0.20

0.22

Pasture

24

0.28

0.30

Peas

36

0.19

Potatoes

24

0.38

0.20

Small grain

30

0.20

0.22

Sugar beets

36

0.28

0.30

Tomatoes

48

0.20

0.22

Vineyards

48

0.22

0.25

Root zone depth = 36 in. Peak daily use = 0.35 in/day.

These figures show that the net amount of water to store per irrigation is 3.25 inches, Table 19.3.

Next determine the amount of water to apply. The amount of water to apply must be greater than the amount to store because irrigations systems do not operate at 100% efficiency. Some of the water will evaporate, some runs off, and some percolates below the crop root zone. The term application efficiency is used to describe these losses. It is defined as the ratio of the depth of water stored to the depth of water applied, expressed as a percent. The application efficiency of a well-designed irrigation system will be between 60 and 80%. Application efficiency can

Table 19.3. Net amount of water to store for sample problem.

Net amount of water to store (in) for various root zone depths

Table 19.3. Net amount of water to store for sample problem.

Net amount of water to store (in) for various root zone depths

Soil profile

24 in

30 in

36 in

48 in

Coarse sandy soil, uniform to 6 ft

0.85

1.10

1.30

1.75

Coarse sandy soil over compacted subsoil

1.50

1.75

2.00

2.50

Fine sandy loam uniform to 6 ft

1.75

2.20

2.60

3.00

Fine sandy loam over compacted subsoil

2.00

2.40

2.80

3.25

Silt loam uniform to 6 ft

2.25

2.75

3.00

4.00

Silt loam over compacted subsoil

2.50

3.00

3.25

4.25

Heavy clay or clay loam soil

2.00

2.40

2.85

3.85

be used to determine the amount of water to apply in the following manner:

Depth of water stored (in)

Depth of water to apply =-

Application efficiency or:

where DWA = Depth of water to apply (in); DWS = Depth of water to store (in); AE = Application efficiency (as a decimal).

Problem: How much water should be applied for the corn crop in the previous problem if the application efficiency is 70%?

Solution:

DWS 3.25 in

AE 0.70

It is also necessary to determine an irrigation interval, which is the number of days it takes the crop to use up the water stored in the soil. The irrigation interval is determined by dividing the amount of water stored in the soil by the plants daily use. For peak use the irrigation interval is:

where IRI = Irrigation interval (days); DWS = Depth of water to store (in); PDU = Peak water use (in/day).

Problem: What is the irrigation interval for the corn crop in the previous problem? Solution:

DWS 3.25 in

For this corn crop, if 3.25 inches of water is stored in the soil per irrigation, at the time of peak use it must be irrigated every 9.3 days.

If there is rainfall during the irrigation period, the irrigation interval should be adjusted accordingly. For example, if there were 1.25 inches of rain on the corn crop in the previous problem, then we would divide the amount of rain (in) by the water needs of the crop (in/day) and extend the interval the corresponding number of days. For the peak demand of the corn crop, 1.25 ^ 0.35 = 3.6 days. Instead of irrigating again in 9.3 days, the irrigation interval could be extended to 12.9 or 13 days.

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