Water use in agriculture by different cropping systems is of interest
in determining crop water use efficiency of different tillage practice
s that will lead to reduced crop production risk. Lysimeters are consi
dered the standard for evapotranspiration (ET) measurements; however,
these units are often not replicated and are few in number at any give
n location. Our objective was to determine if a simple Bowen-ratio sys
tem with nonexchanging psychrometers could provide accurate measuremen
ts of ET from lentil (Lens culinaris Medikus) in a semiarid climate. T
he study was conducted in 1993 and 1994 on two adjacent 180-by 180-m f
ields 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 Arg
iboroll) soil near Sidney, MT. A Bowen-ratio system comprised of two n
onexchanging psychrometers and anemometers at 0.25 and 1.25 m above th
e 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 199
3. In 1993, soil water content of the lysimeter was greater than the f
ield after large precipitation events around Day of Year (DOY) 210, ev
en though the lysimeter was drained. After this time, the lysimeter ET
exceeded that measured by the Bowen-ratio system. Agreement was close
r in 1994, when precipitation was near normal and there was no excess
soil water in the lysimeter. Cumulative ET totals from the lysimeter w
ere reflective of the seasonal precipitation patterns. Differences bet
ween the lysimeter and Bowen-ratio occurred when there was excess prec
ipitation and inadequate drainage from the lysimeter. Half-hourly ET f
luxes from lysimeter and Bowen-ratio values agreed to within 10% throu
ghout the season. Bowen-ratio systems with nonexchanging psychrometers
can provide satisfactory estimates of daily and seasonal ET and can b
e used to estimate ET in semiarid climates.