Extreme Evapotranspiration by Irrigated Alfalfa: A Consequence of the 1976 Midwestern Drought

The rate of evapotranspiration by irrigated alfalfa at Mead, NE (41°09′N; 96°30′W, elevation 354 m) reached record levels in 1976. Evapotranspiration was measured with precision weighing lysimeters in a field 1.9 ha in size. Evapotranspiration ranged during the growing season from 4.75 to 14.22 mm day−1 and exceeded 10 mm day−1 on one-third of the days studied. On each day of study the ratio of latent heat flux density (LE) to the sum of the net radiation and soil heat flux densities (Rn + S) was such that LE/(Rn + S) > 1, indicating the occurrence of significant sensible heat advection. On clear days during mid-summer the net radiation provides energy sufficient for evaporation of no more than 7 mm day−1. Sensible heat advection provided the remaining energy consumed in evapotranspiration. The unusually strong sensible heat advection likely was due to the generally dry condition of surrounding regions during the drought of 1976.