Surface energy balance and surface coefficients of a corn field during non-growing periods

Efficient use of water in agroecosystems requires accurate quantification of evapotranspiration (ET) during growing and non-growing (dormant) periods. Prediction of ET during the growing season has been well researched, but relatively little information exists on evaporative losses during non-growing periods. The objective of this research was to evaluate ET in relation to available energy, precipitation, and reference ET for a corn field during the non-growing period (October 15 through April 30). Three consecutive non-growing periods were studied: 2004-2005 (season I), 2005-2006 (season II), and 2006-2007 (season III). Bowen ratio energy balance system-measured ET was approximately 50% of available energy during average and wet seasons (seasons I and III) and 41% of available energy during a dry season (season II). Cumulative ET ranged from 133 to 167 mm and exceeded precipitation by 21% during the dry season. Mean surface (crop) coefficients were 0.63 and 0.48 for grass and alfalfa-reference crops, respectively. Regression equations were developed that yielded reasonable estimates of ET for the average and wet seasons. Considering non-growing period evaporative losses in water budget calculations would enable water regulatory agencies to better account for water use over the entire year rather than only for the growing season.