Evapotranspiration (ET) by irrigated alfalfa in eastern Nebraska, as determined with precision weighing lysimeters during 1967, varied qualitatively and quantitatively with season. ET in early spring may exceed the net radiation. Low temperature, however, induces a strong canopy resistance and ET can then be significantly lower than atmospheric conditions suggest possible, Maximum ET measured during this season was approx. 6.5 mm day#x2212;1
Strong local and/or regional advection may prevail in late spring and ET rates may be as high as 12 mm day-1. ET may exceed Rn by 80% during this season which is often characterized by very low humidity when winds stem from SW through N.
Mid-summer is characterized by hot and humid conditions under predominantly S and SE winds. ET exceeds Rn only little during this season. Maximum ET rate is about 7.5 mm day-1.
Nocturnal ET is common during the alfalfa growing season. Strong temperature inversions account for a downward delivery of sensible heat sufficient to explain this effect. As much as 1 mm per night can be transpired by alfalfa.
The Energy Balance Bowen ratio method estimated hourly ET more closely than did an aerodynamic method uncorrected for atmospheric stability conditions. Neither method gave estimates more reliable than ±0.151y rain-1.
The Penman method underestimates ET under potential evaporation conditions by as much as 30%, particularly when advection is strong. The van Bavel method for estimating potential evaporation is very sensitive to the value assigned the roughness parameter. With Z0 = 2 cm this method estimates potential evaporation of 8 to 28% more than with Z0 = 1 cm. The van Bavel method is also sensitive to windiness, underestimating ET on calm days and overestimating on windy days.