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Agroclimatology

Advection Influences on Evapotranspiration of Alfalfa in a Semiarid Climate

USDA–Agricultural Research Service Conservation and Production Research Laboratory, P.O. Drawer 10, Bushland, TX 79012

^* Corresponding author (jtolk{at}cprl.ars.usda.gov)

Received for publication February 1, 2006. Advective enhancement of crop evapotranspiration (ET) occurs when drier, hotter air is transported into the crop by wind, and can be an important factor in the water balance of irrigated crops in a semiarid climate. Thirteen days of moderate to extremely high ET rates of irrigated alfalfa (Medicago sativa L.) were evaluated using energy balance and atmospheric coupling models to examine the magnitude of ET enhancement due to advection. Alfalfa ET was measured using precise, monolithic weighing lysimeters. The average ET of the selected days was 11.3 mm d^–1, with ET exceeding 15 mm d^–1 on 3 d, with mean 24-h vapor pressure deficit (VPD) of 2.1 kPa and 2-m wind speed of 4.4 m s^–1. Evapotranspiration due to available energy (net radiation + soil heat flux) was fairly stable at an average of 6.6 mm d^–1 whereas advected atmospheric deficits and sensible heat flux (H) added as much as 10.5 mm d^–1 to ET, with H providing an average of 42% of the energy used in ET. Overnight ET losses due to continued H flux gains and VPD resulted in ET losses as large as 3.0 mm. Advective enhancement of ET plays a significant role in the water balance of the semiarid region of the southern High Plains.

Abbreviations: AE, available energy (R_n + G) • DOY, day of year • ET, evapotranspiration • ET_eq, equilibrium evapotranspiration • ET_imp, imposed evapotranspiration • ET_imp-night, nighttime imposed evapotranspiration • LAI, leaf area index • VPD, vapor pressure deficit

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