Biological Systems Engineering


Date of this Version



Published in Journal of Irrigation and Drainage Engineering 135: 3 (June 1, 2009), pp. 313–322; doi: 10.1061/(ASCE)IR.1943-4774.0000001 Copyright © 2009 ASCE. Used by permission.


Effective water resources planning, allocation, management, and use in agroecosystems require accurate quantification of actual evapotranspiration (ETc) during growing and nongrowing (dormant) periods. Prediction of ETc for a variety of vegetation surfaces during the growing season has been researched extensively, but relatively little information exists on evaporative losses during nongrowing periods for different surfaces. The objectives of this research were to evaluate ETc in relation to available energy, precipitation, and grass and alfalfa-reference ET (ETo and ETr) for a maize (Zea mays L.) field and to analyze the dynamics of surface coefficients (Kc) during the nongrowing period (October 15–April 30). The evaporative losses were measured using a Bowen ratio energy balance system (BREBS) on an hourly basis and averaged over 24 h for three consecutive nongrowing periods: 2004–2005 (Season I), 2005–2006 (Season II), and 2006–2007 (Season III). BREBS-measured ETc was approximately 50% of available energy (Rn – G; Rn is net radiation and G is soil heat flux density) during normal and wet seasons (Seasons I and III) and 41% of available energy during a dry season (Season II). Cumulative ETc ranged from 133 mm in Season II to 167 mm in Season III and exceeded precipitation by 21% during the dry season. The ratio of ETc to precipitation was 0.85 in Season I, 1.21 in Season II, and 0.41 in Season III. ETc was approximately 50% of ETo and 36% of ETr in both Seasons I and III, whereas in Season II, ETc was 32% of ETo and 23% of ETr. Overall, measured ETc during the dormant season was generally most strongly correlated with radiation terms, particularly Rn, albedo, incoming shortwave radiation, and outgoing longwave radiation. Average surface coefficients over the three seasons were 0.44 and 0.33 for grass and alfalfa-reference surfaces, respectively. Using geometric mean Kc values to calculate ETc using a Kc ETref approach over the entire nongrowing season yielded adequate predictions with overall root mean square deviations of 0.64 and 0.67 mm day–1 for ETo and ETr, respectively. Estimates of ETc using a dual crop coefficient approach were good on a seasonal basis, but performed less well on a daily basis. Regression equations that were developed (accounting for serial autocorrelation in the ETc and ETref time series) yielded good estimates of ETc. Considering nongrowing period evaporative losses in water budget calculations would enable water regulatory agencies to better account for water use in hydrologic balance calculations over the entire year rather than only for the growing season and to better assess the progression and availability of water resources for the next growing season.