Seasonal Need

Seasonal water demand is the amount of water (in inches) that a crop must have during one growing season for maximum production. The seasonal water demand will vary from season to season, and for each crop and region. Table 19.4 contains some typical values for three regions of the United States.

Knowledge of the seasonal water demand for a crop in a given area is useful for determining the contribution of irrigation to the cost of production and the total amount of water that will be needed. When the cost of water is known (usually expressed as dollars per acre-feet), as well as the seasonal demand of the crop, the normal rainfall during the growing season, and the number of acres, it is possible

Table 19.4. Typical seasonal water demand for some crops (in).

Western region Central region Eastern region

Table 19.4. Typical seasonal water demand for some crops (in).

Western region Central region Eastern region

Alfalfa

36.0

36.0

33.0

Corn

23.0

25.0

21.0

Cotton

31.0

20.0

19.0

Grain sorghum

20.0

22.0

21.0

Oranges

33.0

Hay

31.0

36.0

Sugar beets

36.0

29.0

Tomatoes

19.0

14.0

Source: Planning for an Irrigation System, American Association for Vocational Instructional Materials (AAVIM), Athens, Georgia.

Source: Planning for an Irrigation System, American Association for Vocational Instructional Materials (AAVIM), Athens, Georgia.

to estimate the total amount of water that will be needed and what it will cost. To determine total seasonal use, the efficiency of the irrigation system also must be considered. Typical efficiency is 60-80%.

Problem: How much water (ac-ft/yr) is needed to supply 120 acres of cotton in the central region if the normal rainfall during the growing season is 5.0 inches, and the efficiency of the irrigation system is 70%?

Solution: Using units cancellation:

1800 ac-ft

Problem: What is the total water cost if the price is $25.00 per acre-foot? Solution: Using units cancellation:

yr ac-ft yr yr

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