Theory and Application of a Residual Energy Balance Method to Estimate Evapotranspiration of Irrigated Corn (Maize).

Irrigated agriculture remains a primary regional water user, and experiments at different irrigation levels have investigated crop growth, grain yield, and water use. Evapotranspiration (ET) must be quantified to estimate crop water use, but ET is typically not measured directly. Latent heat flux can be estimated as a residual component of the surface energy balance (EB). Theory, including concepts of estimating sensible heat flux and ground heat flux, will be discussed. EB data were collected at the Limited Irrigation Research Farm near Greeley, Colorado, USA during three growing seasons (2009-2011) in fully and deficit-irrigated corn (maize). Although the atmospheric conditions above and within the canopy of a plot (9 m wide) are likely affected by neighboring plots at different irrigation levels (randomized design), EB sensors measure the actual plot conditions in each plot. Thus, estimated differences in ET reflect actual conditions experienced by plants within each of the measured plots. Differences in estimated fluxes between full and low (55% of full) irrigation treatments are significant and reasonable. On average, estimated ET under low irrigation was 85% of ET under full irrigation, and the seasonal differences for years 2009, 2010 and 2011 were 42, 135 and 153 mm, respectively.  These estimates of ET using the EB method could help others improve the basic theory and process models used to address water stress effects on crop water use and production.