Agricultural Irrigation Systems

The summer drought has many farmers considering an irrigation system for the first time. There are many common terms and requirements for irrigation systems that may be helpful for those considering a purchase in the near future make wise choices.

Many crops and forages require 1 inch of water per week for optimum yield production. Some crops require quite a bit more - cotton requires up to 1.25 inches per week or more. Corn can require more than 2 inches per week. The irrigation system should be designed to provide the crop’s water needs.

No irrigation system is 100% efficient. Drip systems are typically 95% efficient - that is, 95% of the water pumped reaches the crop. Center pivot systems can be from 70% up to 90% efficient. Traveling gun systems are generally 70% efficient, which means that up to 30% of the water (or more) pumped in a traveling gun system is lost to evaporation.

If we were to pump 1 inch of water through a traveling gun system (and some center pivot systems) during a hot, dry, windy day the crop would actually receive 0.7 inches of water - 70% of the water pumped. So to apply 1 inch of water to the crop we may need to actually pump 1 / 0.7 = 1.43 inches of water. Quite a loss! Watering at night (explained later) helps lessen this loss. The total amount pumped is called the "gross" amount irrigated. The amount of water that reaches the plant after evaporative and other losses is called the "net" amount of water irrigated.

To apply 1 inch of water to one acre (also called 1 acre-inch) requires 27,154 gallons of water. We can use this figure, the amount of time the irrigation system will operate, and the depth of water we plan to apply to determine the flow rate needed for the system.

Irrigation firms in South Carolina typically design drip systems to provide one week’s water need in 84 to 120 hours per week. Traveling gun systems are designed to operate 90+ hours per week to apply 1.5 to 2 inches (gross) of water. Center pivot systems are designed to apply 1 inch of water (gross) every 3 days operating 24 hours per day. The table below shows a range of acreages and irrigation operating times - and the flow rates needed to apply the water in the time specified.

Acres Irrigated	"Net" Inches Applied each week	Hours the System is Operated	Typical Drip System Flow Rate (95% eff)	Typical Center Pivot Flow Rate (80% eff)	Typical Traveling Gun Flow Rate (70% eff)
10	1	120	40 gpm	47 gpm	54 gpm
10	1	90	53 gpm	63 gpm	72 gpm
25	1	120	99 gpm	118 gpm	135 gpm
25	1	90	132 gpm	157 gpm	180 gpm
50	1	120	198 gpm	236 gpm	269 gpm
50	1	90	265 gpm	314 gpm	359 gpm
100	1	120	397 gpm	471 gpm	539 gpm
100	1	90	529 gpm	628 gpm	718 gpm

These flow rates are based on typical irrigation system efficiencies as noted in the chart. The required flow rate for a particular field may vary depending of field shape and irrigation systems coverage.

Night watering has one major benefit - lower evaporative losses. Sprinklers and traveling gun systems that are 70% efficient during the heat of the day may be 80% to 85% efficient at night. This is due to three factors related to evaporation. The first is a higher relative humidity - the night air is generally more humid, which slows evaporation. The second is a lower wind speed - winds are usually slower and lighter during the night, which also means less evaporation. The third factor is solar radiation and heat, which also plays a part in evaporative losses. Increasing the efficiency of your irrigation system by 10% or more is certainly a good reason to consider night irrigation. (Note that drip systems do not spray water in the air and thus are generally as efficient during the day as at night)

Night watering has long been thought to increase disease problems for various crops and plants. The culprit is not night watering, but rather the "leaf wetness period." The leaves of the crop are wet for a period of time every night, beginning with dew fall and ending when the morning sun dries the dew from the leaves. If irrigation can be started after the dew falls - and ended so that the leaves dry at the normal time in the morning - then there should be no increase in disease problems. If the leaf wetness period is extended by the irrigation system any spores that may fall on the leaves have an enhanced chance to germinate due to the presence of moisture on the leaves.

If the irrigation system must be operated for longer than this "wetness period" each night consider turning the system off so that the leave may dry normally in the morning, then turning the system back on until 11:00 a.m. or so to finish the irrigation. This is not always practical for an agricultural irrigation system - especially during times of drought - but if disease problems persist this management practice will help control them.

If you are considering an irrigation system there is a list of irrigation suppliers in South Carolina on the web at http://www.clemson.edu/irrig under the "Agricultural Irrigation Equipment" heading. Irrigation system costs depend on system type, amount of water applied, water supply, elevation, and many other factors. A good rule of thumb to start with is $1,000.00 per acre for the irrigation equipment. This figure can be as low as $500.00 per acre for large (100+ acres) center pivot or drip systems - and as high as $2500.00 per acre for a small, 5 acre traveling gun system.

One final note - it is generally not economically feasible to irrigate pastures or tall fescue hay fields. Coastal Bermuda hayfields may be profitably irrigated in some instances with good management. The economic viability of other forage crops and row crops depends on the management system used by the grower - and on the commodity prices, of course!