Submitted to: American Society of Agricultural Engineers Meetings Papers
Publication Type: Other
Publication Acceptance Date: July 12, 1998
Publication Date: N/A
Technical Abstract: Evapotranspiration (ET) from well-watered alfalfa with no limitations to growth is one of two common reference ET (ETr) values used to scale ET from other crops to obtain crop coefficient values for irrigation scheduling. We tested Penman-Monteith (PM) and Kimberly Penman (1982) equation predictions of ETr against measured alfalfa ET under reference conditions. Alfalfa (Medicago sativa, var. Pioneer 5454) was seeded in Sept. 1995 and grown in 1996 and 1997 on Pullman clay loam (Torrertic Paleustoll) at Bushland, TX. The crop was well-watered with a lateral move sprinkler. Weighing lysimeters (3-m by 3-m surface area by 2.4-m deep) measured ET to 0.05 mm precision. Yield from 4 cuttings was 16.5 dry t/ha in 1996 and 16.4 t/ha in 1997. Crop water use averaged 1.01 m per year. Daily alfalfa ETr predicted using PM methods and half-hourly weather data compared well with our measurements (regression r**2 of 0.95, SE of 0.6 mm, and slope of 0.98). Use of daily weather data increased the SE to 0.8 mm (r**2 of 0.90, slope of 1.00) and introduced a positive offset of 0.5 mm. The Kimberly Penman (1982) equation used with daily weather data produced biased predictions (r**2 of 0.91, SE of 0.7 mm, intercept of 0.8 mm, and slope of 0.89). The ASCE Handbook 70 methods for predicting net radiation from solar irradiance worked well when applied to half-hourly data (r**2 of 0.98, SE of 0.6 MJ/m**2, and slope of 1.03). But these methods applied to daily data produced biased results (r**2 of 0.95, SE of 0.7 MJ/m**2, intercept of 1.5 MJ/m**2, and slope of 0.84). Use of the 1982 Kimberly net radiation equations with daily data produced slightly less biased results (r**2 of 0.97, SE of 0.5 MJ/m**2, intercept of 0.6 MJ/m**2, and slope of 0.86). Alfalfa ET was 1.15 times grass ET from a nearby separate experiment.