Meeting Crop Water Demand

Wasted water is money down the drain; ensure that applied water is being used by the crop.

• By Amanda Huber •

calvin trostle, university of georgia, irrigation

Calvin Trostle, of the UGA C.M.
Stripling Irrigation Research Park, talks
about different moisture monitoring devices.

With top-soil moisture registering as very short in six peanut states in mid-May, producers are hoping the drought trend breaks in favor of timely rains. Rarely though is irrigation not needed at some point during the production season.

“Weather conditions from year to year are variable, can be difficult to plan for and have a large impact on crop growth, development and yield,” says Wes Porter, University of Georgia Extension irrigation specialist. “So we must find ways to adapt to changing conditions and manage our crop to these varying conditions.”

Many options are available to aid in determining when and how much to irrigate peanuts. Porter says when we talk about irrigation scheduling, we are really talking about determining how much water is needed and when to apply it to the field to meet crop demands.

“The purpose is to increase profitability and/or crop quality by increasing the efficiency of using water and energy or by increasing crop productivity,” he says. “We want to make sure that what we put out is being used by the crop and not being lost to runoff or evapotranspiration.”

The Price Of Wasted Water

University of Georgia Ag economists put the average cost of irrigation at $7.50 per acre inch of water applied. “Using this average, for 1,000 acres of irrigated land at 10 inches of irrigation, that’s $75,000,” Porter says. It’s obvious why every drop needs to be used by the crop and not wasted.

To that end, Porter says the current water-use curves for most crops are based on outdated historical evapotranspiration data and are in need of updating. He says they are testing a new water-use curve for peanut that reduces the currently published curve by four to five inches. A new water-use curve for cotton was published in the 2018 cotton production guide, but more testing in needed before the information on peanut is released.

Checkbook Overdrawn

2017 UGA irrigation trial resultsOf the methods used to schedule irrigation, based on a USDA National Agricultural Statistic Service Survey, visible stress, feel of soil and calendar schedule top the list.

“If you are waiting on true visible stress, that’s lost yield potential on almost every crop we grow,” Porter says. “We need to use a better method.”

The UGA Checkbook is another most commonly used methods for irrigation scheduling. However, Porter says caution should be exercised when implementing the checkbook because it was developed based on historical averages and is not an exact fit for years that are either wetter or drier than normal as it will over- and under-predict water need in those years respectively.

“The total estimated water requirement based on the UGA Checkbook for peanuts is 23 inches,” he says. “It was developed based on historical evapotranspiration estimations over a number of years and is a very conservative method, meaning it will typically ‘over-water’ when compared to the actual crop requirement.”

Porter says irrigation research over the past three years suggests that the UGA Checkbook over-irrigates peanuts causing both yield losses compared to other scheduling methods and costing much more from unnecessary irrigation events. Research is still being performed to determine exactly how much it should be reduced.

Better Methods Available

Online scheduling tools available are the USDA’s IrrigatorPro and University of Florida’s PeanutFARM. Both methods use local data to estimate peanut maturity through growing degree day models. By tracking rainfall and evapotranspiration, the tool will offer an estimated irrigation requirement.

Porter says more advanced irrigation scheduling methods include soil and plant sensors.

“Two of these sensors, for example, are capacitance and tensiometic soil moisture sensors. Meter and Watermark are two commonly known sensors of each type. A plant sensor includes SmartField’s SmartCrop canopy temperature sensor.”

A favorable feature of the sensors is the wide variety of options and the ability to be integrated into a system for the producer.
The UGA Easypan has proven time and again that it is a reliable method for estimating field evapotranspiration. It is also simple and inexpensive to build.

Porter says that any method chosen should be based on the producer’s comfort level with technology and irrigation management.

2017 Testing

EasyPan method of measuring ET

The EasyPan is a proven method of
measuring field evapotranspiration.

Because of abundant rainfall in 2017, Porter found no significant differences in yield between the irrigation scheduling treatments. However, there were differences between the amount of irrigation that was applied and in the Irrigation Water Use Efficiency (IWUE).

“Since there was only irrigation applied to establish a stand and to activate herbicides, it had the highest IWUE. This was a high yield for a dryland crop,” he says.

The two treatments with the most irrigation applied, even with the abundance of rain, and the lowest IWUE were the UGA Checkbook and the 50 percent UGA Checkbook treatments. “Both of these treatments show that there was no additional benefit for the irrigation above what was applied.”

Porter says the UGA Smart Sensor Array or WaterMark probe system had not only the highest IWUE but also had the highest yield. IrrigatorPro, UGA EasyPan and PeanutFARM, all had respectable yields and decent IWUE.

Reducing Water Footprint

Even without planning to, peanut producers have become more water efficient simply through the seed they plant. Newer peanut varieties have increased yields without increasing water consumption, which makes them more efficient at water use in terms of production. Knowing when and how much to irrigate through an irrigation scheduling method increases that efficiency and profit potential on the farm.