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Giant scales weigh water; scientists check moisture use and loss by crops.

Giant Scales Weigh Water

Scientists check moisture use and loss by crops.

To a rabbit scampering across it, it would look like any ordinary wheat field in the Great Plains. The innocent rabbit would never know that it had just had its weight recorded by federal scientists.

Hidden in the middle of the field are four massive ground-level soil tanks poised on underground scales. Called weighing lysimeters, these 50-ton instruments are definitely not designed to weigh rabbits, although they can distinguish weight changes as slight as a pound.

Instead, the USDA Agricultural Research Service uses them to monitor the weight losses caused when water either evaporates from the soil or plant leaves or is transpired by plants. The rate of evapotranspiration is critical in an area this dry--so dry that, according to Bobby A. Stewart, not a month goes by, on average, that more water wouldn't evaporate from a standing water surface than is gained from rainfall. Stewart is director of the ARS Conservation and Production Research Laboratory in Bushland, Texas.

The Bushland lab has four weighing lysimeters, each in the middle of an 11-acre field. Each lysimeter is really "a box of soil on a scale," says Arland D. Schneider, an ARS engineer.

Each box is 10 feet by 10 feet by 7.8 feet deep. The support staff at the lab enclosed the 780-cubic-foot blocks of soil in steel tanks and hired contractors to lift them onto the scales. The crane operator didn't believe Schneider when he told him each would weigh 100,000 pounds--until he started lifting one.

Visitors are amazed when Schneider opens a trap door and invites them to climb down 12 feet beneath the field with him. In a room deliberately painted white and brightly lit to make it "user-friendly," observers can readily see that the soil tank plots sit on scales.

Schneider explains that the goal of the Bushland research team is not a lysimeter in every wheat field. "The whole purpose of this continual monitoring is to see if we can improve evapotranspiration predictions from weather data, rather than having to measure changes as they take place," Schneider says.

Since evapotranspiration is a measure of a crop's water use, the ability to predict its rate is necessary for farmers in deciding when to turn irrigation pumps on and for researchers using computers to simulate crop growth.

Schneider is a member of a research team that includes agricultural engineer Terry A. Howell, soil scientists Jean L. Steiner and Steve R. Evett, and biologist Judy A. Tolk.

The team has grown wheat, corn, and grain sorghum on the lysimeters in the past 3 years. "On the average, the seasonal water use was no greater than expected," Schneider says. "But I was surprised by the tremendous variability in water use from day to day."

There was also more loss of water to drainage than expected, he says, as he describes the 16 tubes that drain water from each lysimeter.

In cooperation with Sam J. Smith, an ARS water quality researcher in Durant, Oklahoma, the researchers analyze the waste water for fertilizer leaching. They have found some leaching in the loam soil typical of several million irrigated acres in the Bushland-Amarillo area of the west Texas High Plains.

Schneider explains that every day at 5 minutes after midnight, the computer at the lab's headquarters a mile away phones underground computers in the wheat field to collect data. The data is ready to be processed in the morning when technicians arrive at work.

Schneider points out some of the key weather instruments located at each site: an anemometer to measure windspeed and wet and dry bulb thermometers for calculating relative humidity. "There are also a number of sensors to measure how much sunlight is reflected upward by plants and how much is getting through to heat the soil," he says. Other instruments measure soil water content, soil temperature, and the transfer of heat in and out of the soil.

"We also have a fully equipped weather station located nearby that collects data such as windspeed and direction, temperature, and evaporation," he says.

"When we moved each 780 cubic feet of soil, we moved it as one block, intact," Schneider says, "rather than excavating it layer by layer and filling it in reverse. The actual process involved using hydraulic jacks to force the temporarily bottomless lysimeter tank into the ground. Once it reached ground level, workers placed steel pipes through the soil below the tank to form a temporary bottom. A pair of cranes lifted the tank and its 50-ton load out of the hole and then a permanent bottom was welded in place.

"This method lets us avoid errors caused by disturbing the soil," he says. "Actually, we didn't have a choice because this soil has a natural caliche (calcium-carbonate) layer 3 to 5 feet deep that can't be reconstructed anymore than a rock could be smashed and reassembled.

"We've also placed the plots in the middle of large fields so that there are 125 yards of crop between the lysimeter plants and the field edges in any direction," he says. "It's vital to place the lysimeter far enough into a field so that plants on its surface respond exactly like those growing in the middle of a farmer's field."

A large mechanical sprinkler irrigator can water any one of the four fields or two fields simultaneously, to compare irrigated and nonirrigated farming. For example, Steiner has tested sorghum in high and low levels of winter wheat residue, produced by growing the wheat with and without irrigation. Steiner plans to do more research to compare the effects of crop residue left on the surface by different methods of tillage.

Evett is using the weighing lysimeters to monitor evaporation from bare soil. "With our evaporation potential higher than our rainfall potential, we have to depend on rainwater stored in soil. Great Plains farmers usually leave dryland fields bare or fallow for 11 months between crops to give the soil a chance to store moisture," he says. Rain is anathema to scientists who want to control the amount of water an outdoor plant gets. The solution is a moving shelter. Rainfall activates a specially designed building, causing it to automatically move over and close its doors around the 48 small lysimeters at the site.

Each of the lysimeters is 30 by 40 inches by 7.8 feet deep, containing a 65 cubic feet block of undisturbed soil.

Twenty minutes after the rain stops, the doors open and the rain shelter retreats, allowing normal conditions again.

Twenty-four of the small lysimeters contain soil cores collected at Garden City, Kansas--250 miles north of Amarillo. It took a caravan of five 18-wheelers to haul the soil, Schneider says. "This silty loam soil is what you find in a large area around Garden City," Schneider says.

Twelve other lysimeters are filled with a fine sandy loam soil from Big Spring, Texas, which is 250 miles south of Bushland. The final 12 lysimeters have the loam collected at the Bushland lab.

"You can walk just 40 feet here and pass the three major soil types you'd normally have to take a 500-mile-long trek from Texas to Kansas to see," Schneider says. "This gives us a rare opportunity to accurately extrapolate our results to a wider region. We can also do drought experiments on three different soils under the same climate conditions."

These lysimeters are weighed periodically by being lifted slightly by a crane supported by the rain shelter building. The crane is suspended within the building to cut wind-caused vibrations during weighing.

"We're just getting started with these lysimeters. Last winter and spring, we grew winter wheat. This summer we're studying four varieties of grain sorghum to see how they are affected by late-season drought in these soils," he says.

The rain shelter and lysimeters provide a unique combination. They are considered among the best of this type of facility in the world. ARS also conducts soil and water research with lysimeters at Fresno, California, Coshocton, Ohio, and Temple, Texas.

PHOTO : Biologist Judy Tolk checks leaf temperatures with an infrared thermometer at the rain-sheltered lysimeter site. (K-4426-1)

PHOTO : Arland Schneider climbs from the underground lysimeter utility room while soil scientist Steve Evett conducts bare soil evaporation experiments. (K4425-1)

PHOTO : Twelve feet beneath the field, engineer Arland Schneider checks equipment used to weigh one of the large lysimeters. (K-4425-14)

PHOTO : Under the movable rain shelter, biologist Judy Tolk uses a hoist to lift and weigh one of the small lysimeters. (K-4429-1)

Bobby A. Stewart, Arland D. Schneider, and Jean L. Steiner are at the USDA-ARS Conservation and Production Research Laboratory, P.O. Drawer 10, Bushland, TX 79012. Phone (806) 356-5732.
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Author:Comis, Don
Publication:Agricultural Research
Date:Jan 1, 1992
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