Nighttime evapotranspiration from alfalfa and cotton in a semiarid climate

Nighttime evapotranspiration (ETN) has typically been neglected in estimating water loss from land surfaces. Our objective was to quantify the contribution of ETN 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 ETN 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, ETN was as much as 12% of ET24 with single nighttime losses approaching 2 mm. Model calculations showed that virtually all ETN was the result of imposed atmospheric conditions, primarily vapor pressure deficit. However, ETN 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.