Reference and crop evapotranspiration in South Central Nebraska. II: Measurement and estimation of actual evapotranspiration for corn

In planning, designing, and managing of surface and groundwater supply, it is essential to accurately quantify actual evapotranspiration (ET_c) from various vegetation surfaces within the water supply areas to allow water management agencies to manipulate the land use pattern alternatives and scenarios to achieve a desired balance between water supply and demand. However, significant differences among water regulatory agencies and water users exist in terms of methods used to quantify ET_c. It is essential to know the potential differences associated with using various empirical equations in quantifying ET_c as compared with the measurements of this critical variable. We quantified and analyzed the differences associated with using 15 grass (ET_o) and alfalfa-reference (ET_r) combination, temperature and radiation-based reference ET (ET_ref) equations in quantifying grass-reference actual ET (ET_co) and alfalfa-reference actual ET (ET_cr) as compared with the Bowen ratio energy balance system (BREBS)-measured ET_c (ET_c-BREBS) for field corn (Zea mays L.). We analyzed the performance of the equations for their full season, irrigation season, peak ET month, and seasonal cumulative ET_c estimates on a daily time step for 2005 and 2006. The step-wise K_c values instead of smoothed curves were used in the ET_c calculations. The seasonal ET_c-BREBS was measured as 572 and 561 mm in 2005 and 2006, respectively. The root-means-quare difference (RMSD) was higher for the full season than the irrigation season and peak ET month estimates for all equations. The standardized ASCE Penman-Monteith (PM) ET_co had a RMSD of 1.37 mm d^-1 for the full growing season, 1.05 mm d^-1 for the irrigation season, and 0.76 mm d^-1 for the peak month ET. The ASCE-PM, 1963 and 1948 Penman ET_c estimates were closest to the ET_c-BREBS. The FAO-24 radiation and the HPRCC Penman ET_c estimates also agreed well with the ET_c-BREBS. Most combination equations performed best during the peak ET month except the temperature and radiation-based equations. There was an excellent correlation between the ASCE-PM ET_co and ETcr with a high r² of 0.99 and a low RMSD of 0.34 mm d_-1. The difference between the ET_cr and ET_co was found to be larger at the high ETc range (i.e., >8mm), but overall, the ET_cr and ET_co values were within 3%. Significant differences were found between the cumulative ET_co-METHOD and ET_cr-METHOD versus ET_c-BREBS. Most combination equations, including the standardized ASCE-PM ET_co and ET_cr underestimated ET_c-BREBS during the early periods of the growing season where the soil evaporation was the dominant energy flux of the energy balance and in the late season near and after physiological maturity when the transpiration rates were less than the midseason. The underestimations early in the season can be attributed to the lack of ability of the physical structure of the ET_ref × crop coefficient approach to "fully" account for the soil surface conditions when complete canopy cover is not present. The results of this study can be used as a reference tool by the water resources regulatory agencies and water users and can provide practical information on which method to select based on the data availability for reliable estimates of daily ET_c for corn.