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Cut Water Losses
They say a penny saved is a penny earned, but does that same axiom apply to water losses during irrigation?
Both university and industry experts warn that focusing on just one aspect of a sprinkler system (such as cutting losses to evaporation) could lead to losses elsewhere. Danny Rogers at Kansas State University explains that applied water is lost to the air, off of foliage, by running off the field, and to deep percolation of moisture in the field.
“The two components of air loss are droplet evaporation and water drift,” Rogers explains. “Droplet evaporation is the water that evaporates while in flight from the nozzle and before it reaches the crop canopy or the soil surface. Drift losses are water droplets that move off the field or onto a non-targeted area of the field, usually by wind.”
Recent research has shown that losses to the air are small compared with other potential water losses, provided a pivot has properly designed and operated nozzles. In fact, one study showed that as long as droplet sizes were within normal droplet size range, direct evaporation of water droplets was less than 1% of the discharged water.
Cut Leaf Losses
Foliage losses, meanwhile, refer to water that is lost as it sits on plant surface. Interception is water that is caught and held on the plant material surfaces and eventually evaporates into the atmosphere.
To reduce losses in the canopy, you need to employ nozzles that either produce a smaller wetting area (application area) or are placed closer to the canopy. Certainly, positioning nozzles deep into the canopy will also help limit crop interception.
“Canopy evaporation continues to decrease and can be eliminated with application systems like LEPA (low energy precision application) and MDI (micro-irrigation drip lines), which deliver irrigation water directly to the ground,” Rogers points out. “However, reducing canopy evaporation should not be at the expense of creating water runoff.
“Once the water reaches the ground, it can be lost in several ways,” he explains. “If water application rates are higher than the soil intake rate, water can either be held in surface storage or it can start to move along the soil surface and become runoff.”
Water that moves within the field also reduces the efficiency of the application, either because the soil receiving the runoff is over-watered or the excess water is lost to evaporation or deep percolation.
Deep percolation loss, of course, involves water that enters the soil profile that is in excess of the available water storage capacity of the root zone. The best way to manage deep percolation loss, Rogers explains, is through the adoption of an irrigation scheduling method, such as climatic-based (ET) irrigation scheduling or soil-based irrigation scheduling.
Balance the Way Water Is Applied
Rogers cautions, however, that rather than fixing one problem, producers must consider the balance of the overall system. A trend in recent years, he says, has been to use low-pressure nozzles to help reduce operating costs.
On the other hand, lower-pressure nozzles often increase the average application rate, increasing the potential for runoff.
In one Texas trial (which used a LEPA system on a clay loam soil), the intake rate of the soil was less than the sprinkler application rate, causing over half of the applied water to be lost through runoff. As a result, corn yield was reduced by nearly 25%, which proves that solving one problem without balancing the whole system can, potentially, make things worse.