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Agroclimatology

Nighttime Evapotranspiration from Alfalfa and Cotton in a Semiarid Climate

USDA-ARS, Conservation and Production Research Lab., P.O. Drawer 10, Bushland, TX 79012

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

Received for publication September 30, 2005. Nighttime evapotranspiration (ET_N) has typically been neglected in estimating water loss from land surfaces. Our objective was to quantify the contribution of ET_N to daily (24-h) ET (ET₂₄) of irrigated and dryland cotton (Gossypium hirsutum L.) and irrigated alfalfa (Medicago sativa L.) grown in a semiarid climate. The results were then examined using a Penman–Monteith ET model which separates control of ET into its radiation (equilibrium) and atmospheric demand (imposed) components. Nighttime ET was measured at Bushland, TX using weighing lysimeters containing monolithic soil cores of Pullman clay loam (fine, mixed, superactive thermic Torrertic Paleustoll) for alfalfa in 1998 and cotton in 2001. Measured ratios of ET_N to ET₂₄ ranged from an average of 3% for a dryland cotton crop to 7.2% for irrigated alfalfa over a season. In the largest events, ET_N was as much as 12% of ET₂₄ with single nighttime losses approaching 2 mm. Model calculations showed that virtually all ET_N was the result of imposed atmospheric conditions, primarily vapor pressure deficit. However, ET_N was also related to sensible heat transfer to the canopy. Nighttime ET can be an important part of total ET of irrigated crops in a semiarid environment.

Abbreviations: DOY, day of year • ET, evapotranspiration • ET_eq, equilibrium ET • ET_imp, imposed ET • ET_N, nighttime ET • ET₂₄, daily (24-h) ET • G, soil heat flux • H, sensible heat flux • IRT, infrared thermometer • LAI, leaf area index • E, latent heat flux • (R_n + G), net radiation + soil heat flux • , decoupling factor • r_a, aerodynamic resistance • r_s, surface resistance • VPD, vapor pressure deficit

This article has been cited by other articles:

				J. A. Tolk, S. R. Evett, and T. A. Howell Advection Influences on Evapotranspiration of Alfalfa in a Semiarid Climate Agron. J., October 31, 2006; 98(6): 1646 - 1654. [Abstract] [Full Text] [PDF]