Submitted to: ASA-CSSA-SSSA Annual Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 10/1/2007
Publication Date: 11/1/2007
Citation: Howell, T.A., Tolk, J.A., Evett, S.R., Copeland, K.S. 2007. Hourly and daily evapotranspiration of alfalfa under regional advection [abstract]. ASA-CSSA-SSSA Annual Meeting, November 4-8, 2007, New Orleans, Louisiana. 2007 CDROM.
Technical Abstract: Regional advection often affects the evapotranspiration rates of irrigated crops in the Southern High Plains. In 1998, during a 10-day period (13-22 June) of unusually strong advection, high evapotranspiration (ET) rates for unstressed, irrigated alfalfa (Medicago sativa) were measured with two precision weighing lysimeters at Bushland, Texas. Each lysimeter field was 4.2 ha, and each lysimeter was 3 m by 3 m by 2.3 m deep containing a Pullman clay loam (fine, mixed, superactive thermic Torrertic Paleustoll) monolith located in the center of the fields. Weather parameters (net radiation, net short-wave irradiance, 2-m wind speed, and 2-m air temperature and relative humidity) were measured at each lysimeter along with soil heat flux (at 50 mm depth) and soil temperatures at 10 and 40 mm depths to calorimetrically correct the heat flux measured by the four plates to the surface. Similar weather parameters (incident solar irradiance, 2-m wind speed, 1.5-m air temperature and relative humidity) were measured in an adjacent weather station over an irrigated grass (Festuca arundinacea) surface. A standardized Penman-Monteith (PM) equation was used to compute daily and hourly alfalfa ET rates for the approximately 0.55 to 0.62-m tall alfalfa. Daily mean standardized PM ET errors ranged from less than 2 to 3.9% for the 10 days while the totalized hourly ET error was -1.6%. The computed hourly ET rates were predicted slightly better (r**2 increased from 0.97 to 0.98 and the standard error decreased from 0.086 to 0.074 mm h**-1) using the individual weather data measured at the lysimeter compared with ET computed using the weather station data for both lysimeters. The standardized PM equation predicted ET accurately even under strong regional advection; but on average the mean 10-day ET wasn't significantly more accurate as estimated by the PM equation compared with a Penman equation designed for unstressed alfalfa.