Irrigation Notes

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California State University, Fresno, California 93740-0018

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September 1988

Water Sources for Irrigation

Kenneth H. Solomon and David Zoldoske

One of the first questions any grower must answer when considering an irrigation system is: What will be the water source? Growers can't do much irrigating without an adequate supply of good quality water.

There are actually four water supply questions: How much will you need? What sources can you use for supply? How much water can that source provide? What is the quality of the water from your planned source?

WATER QUANTITY

The amount of water used during the growing season depends on crop and climate. The type of plant, stage of growth, and length of growing sea-son are significant factors. Weather variables such as temperature, wind, relative humidity, and sunlight during the growing season determine the potential demand for water from a crop. Of course, rainfall may supply a portion of the crop's water needs.

Note that not all rainfall can be used by the crop. Short duration, high intensity rain deposits water faster than the soil can absorb it, causing some of the rainfall to run off. "Effective rainfall" is the amount of rain that soaks into the ground, available for later plant use.

During the growing season, the irrigation system must be able to apply the crop's water needs not met by expected effective rainfall. Local agricultural agencies or the weather service can provide the best estimates for seasonal crop water needs and effective rainfall.

In addition to seasonal water needs, consider the daily water use rate during the time when the crop requires the most water. This is called the "peak-use" rate of water demand. Your irrigation system should be capable of supplying enough water to the crop to meet requirements during the peak-use period, particularly if there is no effective rain during that period. Local agricultural agencies can assist you with an estimate of the peak-use rate for various crops.

Water can be measured in inches (of rainfall or irrigation), acre-inches (one inch of water applied to one acre), and gallons per minute (GPM). For example, a 100 acre orchard with a peak-use rate of 0.25 inches per day requires 100 acres x 0.25 inches = 25 acre-inches per day. If you plan to operate the irrigation system a maximum of 20 hours per day (it's wise to leave some slack for down time, maintenance, etc.), this is a water rate of 25 acre-inches ÷ 20 hours = 1.25 acre inches per hour. Since one acre-inch per hour equals roughly 453 gallons per minute, 1.25 acre-inches per hour x 453 = 566 GPM.

One more adjustment needs to be made: no irrigation system can apply water with 100 percent efficiency. Efficiencies depend on the type of irrigation system, your intended management practices, and conditions specific to your field. Consult your farm advisor or a competent irrigation designer regarding a reasonable efficiency figure to use in your planning. The CIT Irrigation Note "Irrigation Systems and Water Application Efficiencies" (CATI Publication No. 880104) lists ranges of attainable efficiencies for different types of irrigation systems.

The water flow rate determined in the previous step needs to be divided by the efficiency (in decimal form) to compute the total water flow requirement. Assuming an 80 percent efficiency for this example, 566 GPM ÷ 0.80 = 708 GPM. This is the flow rate that must be available from your water source if the irrigation system is to keep up with the peak-use water requirements of the crop.

SOURCES OF WATER

Now that you know how much water is needed, you should consider likely sources of water. Investigate your water rights to any potential water source. Don't plan on irrigating from any source unless you are sure you have a legal right to draw the amount of water you'll need. If you plan to drill a well, check with the state water agency about necessary permits.

Common water sources include: rivers and streams; lakes, ponds and reservoirs; and wells. Rivers and streams may appear to be convenient sources, but flows may vary throughout the year, perhaps failing when you need them most. Check the stream flow history. If the reliable flow is too low to meet your water needs, some form of on-farm water storage will be required.

Lakes, ponds or reservoirs are good sources of water, if their storage capacity plus the water flowing into them, is adequate. In some areas, wells may provide the water you need. Usually deep wells are the most dependable. Talk to local well drillers and local water agencies to obtain an estimate of how deep you'll have to drill, how much water you can expect, and what the cost will be. These same people can also advise you on the proper well design (casing, screen, gravel pack, etc.) for your area.

You should also examine the location of your field relative to possible water sources. Consider both the distance and elevation differential between field and water source. There will be a cost to convey water from a distant source to the field, and there will be an annual operating cost to pump water from a lower elevation to a higher one. Try to estimate the total cost of pumping and delivering water from each source to your field before making a choice. For example, it may be cheaper to drill a well at the field to be irrigated than to pump water from a distant river or lake.

The quantity of water available from a water source is often difficult to estimate. It is probably best to obtain professional advice. For example, the U.S. Geological Survey or a local government agency may have measured the flow from a river or stream, including during a prolonged dry period. You should base your plans on the reliable stream flow, not just the current flow. Lake or reservoir volume may also be difficult to estimate. Wells should be tested after drilling to determine the actual water delivery rate. Well tests may be arranged through your local well driller or well supply dealer.

WATER QUALITY

Consider not only the amount of water available, but its quality. Water quality may vary during the season, depending on the source of contamination. If possible, analyze water samples throughout the year. Salts, industrial wastes, and organic acids and stains (from decaying plant or animal waste material) are potentially harmful contaminants. Soil and water testing laboratories can analyze your water (and soil) samples, and offer advice as to the best management practices to follow in your circumstances.

Other foreign matter in the water such as sand, silt or algae may affect your irrigation system. Such materials can clog a micro-irrigation system while sand can reduce the life of pumps, fittings and sprinklers. Water treatment may be necessary to properly deal with these problems.


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