Alfalfa reference et measurement and prediction

摘要

Alfalfa evapotranspiration (ET) is one of two common reference ET (ET_R) values. The other is grass ET. We tested Penman-Monteith (PM) and the 1982 Kimberly Penman (KP) equation predictions of ET_R against measured alfalfa ET under reference conditions. Alfalfa (Medicago saliva, var. Pioneer 5454) was grown in 1996 through 1999 on Pullman clay loam (Torrertic Paleustoll) at Bushland, TX. The crop was well-watered with a lateral move sprinkler. Monolithic weighing lysimeters (3-m by 3-m by 2.4-m deep) measured ET every half hour to 0.05 mm precision. Yields were 16.5, 16.4, 20.6, and 15.2 dry Mg ha~(-1) in 1996 through 1999. Crop water use averaged 1.01 mperyearin 1996 and 1997,andwas 1.16 m in 1998. Daily ET in this windy, semi-arid environment occasionally exceeded 14 mm. Daily alfalfa ET_R predicted using PM methods and half-hourly weather data compared well with our measurements (regression r~2 of 0.94, SE of 0.6 mm, slope of 0.94, and intercept of 0.2 mm). Use of daily weather data increasedthe SE to 0.8 mm (r~2 of 0.90, slope of 0.98) and introduced a positive offset of 0.7 mm. The KP equation used with daily weather data produced more biased predictions (r~2 of 0.91, SE of 0.7 mm, intercept of 0.9 mm, and slope of 0.88). The ASCE Manual 70 methods for predicting net radiation from solar irradiance worked well when applied to half-hourly data (r~ of 0.97, 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 0.94, SE of 0.8 MJ/m~2, intercept of 1.5 MJ/m~2, and slope of 0.85). Use of the KP net radiation equations with daily data produced slightly less biased results (r~2 of 0.97, SE of 0.6 MJ/m~2, intercept of 0.7 MJ/m~2, and slope of 0.87). Alfalfa ET was 1.15 times grass ET.