Water use in agriculture by different cropping systems is of 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 (ET) measurements; however,
these units are often not replicated 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 ET from lentil (Lens culinaris Medikus) in a
semiarid climate. The study was conducted in 1993 and 1994 on two
adjacent 180- by 180-m fields with weighing lysimeters (1.68 by 1.68
by 1.83 m) located in the center of each field, 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 was 367 mm in 1993 and 227 mm in 1994. In 1993, soil
water content of the lysimeter was greater than the field after large
precipitation events around Day of Year (DOY) 210, even though
the lysimeter was drained. After this time, the lysimeter ET exceeded
that measured by the Bowen-ratio system. Agreement was closer in
1994, when precipitation was near normal and there was no excess
soil water in the lysimeter. Cumulative ET totals from the lysimeter
were reflective of the seasonal precipitation patterns. Differences
between the lysimeter and Bowen-ratio occurred when there was
excess precipitation and inadequate drainage from the lysimeter. Half-hourly
ET fluxes from lysimeter and Bowen-ratio values agreed to
within 10% throughout the season. Bowen-ratio systems with nonexchanging
psychrometers can provide satisfactory estimates of daily
and seasonal ET and can be used to estimate ET in semiarid climates
