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Title: BOWEN RATIO COMPARISONS TO LYSIMETER EVAPOTRANSPIRATION IN SEMI-ARID CLIMATES

Author
item Prueger, John
item Hatfield, Jerry
item Aase, J
item Pikul Jr, Joseph

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: In semi-arid climates, irrigation is required for reliable crop production. Currently there is a lack of timely crop-water-use information necessary for efficient allocation of scarce water resources. Precision weighing lysimeters provide accurate measurements of daily water use but are expensive, few in number, require high maintenance, and are non-mobile. A study was conducted in Montana on lentils to determine how much water the crop used and how two different techniques for measuring water use compared with each other. The one method was a lysimeter which measures the water use by the lentil crop directly by weighing the amount of water in a column of soil. The other method measures water use by indirectly determining the effect of the crop on the atmosphere above the crop. These two methods were compared throughout the growing season for two years. It was found that the indirect measurement method provided an accurate measurement of water used both on a hourly and daily basis. The advantage of the indirect method is that it can be used in any field and is considerably less expensive than lysimeter methods. This type of information will provide decision-making aids to extension agents and farms to better manage scarce water resources needed for semi-arid agriculture.

Technical Abstract: Water use in agriculture by different cropping systems is of primary interest in determining crop water use efficiency of different tillage practices that will lead to reduced crop production risk. Lysimeters are considered the standard for evapotranspiration measurements; however, these units are often nonreplicated and are few in number at any given location. Our objective was to determine if a simple Bowen ratio system with nonexchanging psychrometers could provide accurate measurements of evapotranspiration from lentil in a semi-arid climate. The study, conducted in 1993 and 1994 on two adjacent 180 x 180 m fields with lysimeters (1.68 x 1.68 m x 1.83 deep) in the center of each field, was on a Williams loam (fine-loamy, mixed Typic Argiboroll) soil near Sidney, MT. A Bowen ratio system comprised of two nonexchanging psychrometers and anemometers at 0.25 and 1.25 m above the plant canopy surface was placed in the lentil field along with a net radiometer and soil heat flux plate. Precipitation during the growing season from planting to swathing varied from 367 mm in 1993 to 227 mm in 1994. In 1993, the Bowen ratio system underestimated lysimeter evapotranspiration because, even though the lysimeter was drained, it was often near saturation due to above normal precipitation. Agreement was closer in 1994 when there was no excess water in the lysimeter. Half-hourly evapotranspiration fluxes from the lysimeter and Bowen ratio agreed to within 10% throughout the season. Bowen ratio systems with nonexchanging psychrometers can provide satisfactory estimates of daily evapotranspiration and can be used to estimate evapotranspiration in semi-arid climates.