“You don’t put money into an irrigation system without thinking hard about it,” said Guy Fipps during a discussion at the Irrigation and Trade Conference in McAllen. Fipps says growers should consider:
-- Irrigation technologies—drip, furrow or center pivot—probable irrigation strategy—full irrigation; supplemental irrigating during short, dry periods; or deficit irrigation, supplying less water than the crop needs.
-- Farmers should also consider whether they will supply fertilizer and chemicals through the system.
-- Another important consideration, especially in the Rio Grande Valley, is if water supply is adequate.
“Generally we want our system to meet peak crop water demands,” said Fipps, Extension agricultural engineer in College Station. “And different crops need different amounts of water: from .16 inches per acre per day for citrus to .32 inches for corn. As an example, forage, which takes .25 inches per day, would require about 6,800 gallons per acre per day.
In evaluating available technology, level of control is important. Farmers want to put out the precise amount for specific crop needs. “One of the basic strategies is to fill up the root zone, let the crop use it until it is depleted, then fill it up again,” said Fipps.
Accurate calculation includes the soil’s water holding capacity, the depth of the root zone and the minimum moisture level that should be maintained. Much depends on whether the soil is clay, loam or sand.
Farmers should take a water quality sample. In the Rio Grande Valley, growers are concerned with salinity of river water since high salt content may damage foliage with spray irrigation or accumulation in the soil. Groundwater users should determine salinity, sodium and boron levels.
System efficiency is important. How much water would is lost in the air? How uniform is water distribution?
Purchase price of each system will be a major factor. The size of a farmer’s field may influence price, along with the labor requirements of each system.
A farmer must look at the cost of pressurizing water in each system. Because it takes a lot of energy for impact sprinklers, a low pressure system may be more economically feasible in some cases.
Drip systems allow frequent irrigation with small amounts of water and maintain optimal soil moisture. Of the many drip products, the most popular is drip tape, which is reasonably priced. Purchase costs can be high, especially when using plastic mulch. After installation, the labor cost is low.
The keys to success in drip irrigation include good filtration, control of clogging, soil moisture management, and planting a high value crop.
Furrow irrigation waters an entire field quickly. When water first contacts the soil in a furrow, the infiltration rate is high. As the water continues to run, the infiltration rate diminishes. If water is shut off and allowed to infiltrate, the surface soil particles consolidate forming a partial seal that reduces intake rate when water is again applied.
This means that the water moves down the furrow and does not infiltrate the soil.
By using surge flow irrigation farmers increase efficiency by 10 percent to 30 percent. Surge flow works by intermittently applying water to alternating sides of a field.
The keys to success include having a large water stream per row (25 or more gallons per minute for each row), using plastic or aluminum gated pipe, and using the surge flow method. Purchase costs are low, operating skill is low and efficiency ranges from poor to good.
Fipps recommends center pivot or linear move for large acreage with sprinkler irrigation. Both use the same type of sprinklers and have similar designs.
The cost to install center pivot is high – about $325 per acre for larger fields and $500 to $1000 for smaller fields. Linear move machines cost 50 percent more than the same length pivot, with cost per acre depending on field length. Benefits of either system can include reduced labor, improved water distribution and management convenience.
If choosing center pivot it is important that the system is properly designed. Choosing a water applicator with low pressure requirements, such as the LEPA system, minimizes pumping costs and water evaporation loss.
“You have to do your homework,” Fipps said. A farmer must collect data on his field as well as data on the basic irrigation technologies. When he has narrowed these down to one type system, he should get a site-specific design and a detailed cost estimate from several dealers.