EVAPORATION MEASUREMENTS AND PREDICTIONS FROM SOILS UNDER CROP CANOPIES

Abstract

ABSTRACT Daily evaporation rates were measured with two sizes of lysimeters: (1) mini-lysimeters, 15 cm Inside Diameter (I.D.) and 20 cm long; and (3) micro-lysimeters 7.6 cm I.D. and 6 cm long. The field studies were conducted in the semi-arid climate of west central Nebraska and the sub-humid climate of eastern Nebraska. Silt loam, fine sand, and silty clay loam soils were used in the experiment. Crop roots were excluded from the lysimeters. The micro-lysimeters were refilled daily with soil in the field plot. The following day they measured evaporation. Since the micro-lysimeters most closely matched the field soil water, their measured evaporation rates provided validation for the mini-lysimeters, which were filled with undisturbed soil once each season. Soil water contents in the mini-lysimeters were matched to the surrounding field soil after major wetting events; however, the mini-lysimeters became wetter than the surrounding soil which was subject to water extraction by crops. For sprinkler irrigated com with full canopy cover of the soil, the evaporation rates from the mini-lysimeters averaged 0.2 mm/da more than the micro-lysimeters. Evaporation rates from the mini-lysimeters were compared with predictions from the combination of two energy based models. The Ritchie (1972) model calculated the net radiation at the soil surface under a crop canopy. The Idso (1979) model took this information plus air and soil surface temperatures to predict evaporation. There wasn't any on-site calibration of the model components. Idso's model would need further calibration to cover the range of soils in this study. The low values for net radiation at the soil surface under dense shading may lead to under prediction of evaporation from these models.