Ernest L. Clawson and Sean Hribal

Irrigation is an important part of many crop production systems in northeast Louisiana. The amount of irrigation a crop requires is affected by the rate at which that crop uses water. Crop water use is known as the evapotranspiration (ET) of the crop and includes both water lost by the soil and water used by the plants.
 
Predicting crop ET is difficult because it is affected by crop species and by crop growth stage. In addition, it is influenced by weather conditions, which suggest ET values measured in a given year are likely to differ from the values for the same crop in a different year.
 
To overcome this challenge, irrigation researchers often focus on the relationship between crop ET at a given growth stage and contemporaneous weather conditions. ET measurements are expressed as a percentage of a reference ET, calculated from weather parameters such as solar radiation and wind speed. Once they are determined, these percentages (known as crop coefficients) can be used to estimate ET values in future years. This is done by calculating reference ET from a nearby weather station, which accounts for current weather influences, and multiplying it by the crop coefficient.
 
One of the best methods for measuring ET over short intervals is the weighing lysimeter. This instrument consists of a container filled with soil and resting on a scale with its upper surface at ground level. The container allows water losses only through the soil or through the crop planted in the lysimeter. Changes in the container mass, therefore, represent the actual ET of the crop planted in it.
 
Paired weighing lysimeters were installed at the Northeast Research Station, St. Joseph, La., in 2005. Each includes an inner tank filled with soil and resting on four load cells within an outer tank. The load cells record the inner tank’s mass. A data logger records the mass once per second and stores the data as five-minute averages. Differences in inner tank mass from midnight to midnight represent the daily ET of the crop.
 
The design of the lysimeters includes a sufficient rooting depth for field crops (approximately 4.5 feet), and both tanks are braced for structural stability. The inner tank is aligned within the outer tank so the two do not touch at any point. A flexible rubberized fabric seals the gap between the two tanks at the soil surface to keep out rainwater. Unwanted water can be pumped from the outer tank through an access pipe. Accumulated drainage water from the inner tank can be pumped through a standpipe connected to a perforated pipe drain at the bottom of the tank. The inner tank is one crop row wide.
 
A calibration performed in spring 2007 demonstrated that the lysimeters have linear responses to changes in mass. In addition, each change in mass was measured to within the equivalent of a 0.008-inch depth of water – a very small percentage of the total ET expected for a 24-hour period. Thus, the precision of the lysimeters is sufficient to measure daily crop ET.
 
In practice, an irrigated area is maintained around the lysimeters to ensure they are in an environment similar to large-scale irrigated agriculture. This helps to promote representative measurements of the crop ET. Because tractor tires and tillage implements cannot pass over them, many operations on the lysimeters are performed by hand. Growth stages and other important crop parameters are measured regularly. The inner tanks are pumped to avoid excessive accumulations of water in them. A weather station under construction nearby will record the data needed to calculate reference ETs. Like the lysimeters, the weather station will be surrounded by a large irrigated area.
 
Future research will focus on developing crop coefficients for irrigated cotton and other crops.