Impact of Microclimatic Data Measured above Maize and Grass Canopies on Penman-Monteith Reference Evapotranspiration Calculations

Estimation of reference evapotranspiration (ETref) using measured microclimatic data and the Penman-Monteith (PM) method provides a powerful means of quantifying actual plant evapotranspiration (ETa) needed for use in various disciplines. When applying the PM method to estimate ETref, it is desirable to measure the required microclimatic data over a reference grass or alfalfa surface rather than above non-reference surfaces. However, in reality, establishing and maintaining a reference surface for long periods of time is a difficult task. Other surface energy balance systems, such as the Bowen ratio energy balance system (BREBS), eddy covariance system, and surface renewal, are increasingly used to measure surface energy fluxes along with the microclimatic data above various plant canopies. These systems could be another source of data for ETref estimations when reference weather station data are not available due to logistical difficulties associated with establishing and maintaining a separate reference weather station. In many cases, data measured above other vegetation surfaces using the surface energy balance systems are the only source of data for ETref and ETa estimations due to the absence of reference weather stations. There is little information on how microclimatic data measured above different plant canopies impact the calculated ETref if used in the PM method in place of data collected from a reference surface. This study compares data measured above grass and maize (Zea mays L.) canopies and assesses how the variables measured above two canopies impact ETref calculated using the ASCE standardized Penman-Monteith (ASCE-EWRI PM) equation. Two years (2005 and 2006) of hourly microclimatic data measured above a grass surface using an automated weather station and above a maize canopy using BREBS installed on a well-watered maize field were used. The results obtained indicate very good agreements between the microclimatic variables measured above grass and maize, and between ETref calculated with data measured above the two surfaces. The measured rainfall was the same for both sites (316 and 323 mm in 2005 for the weather station and BREBS, respectively, and 368 and 366 mm in 2006). The main difference between the two surfaces was in u2 and aerodynamic resistance (ra). On a seasonal average basis, wind speed (u2) was 15% and 20% higher over the grass canopy than the maize canopy for 2005 and 2006, respectively. The maximum difference in ra between the two surfaces occurred when the maize was at its maximum height (2.45 m). On a seasonal average, the ra above the maize canopy was 37 s m-1 higher than the ra above the grass surface. However, the impact of u2 and ra on ETref was insignificant. The grass and alfalfa-reference ET (ETo and ETr) estimated using the data measured above maize (ETo-maize and ETr-maize) and above grass (ETo-grass and ETr-grass) were very similar in both years. In 2005, ETo-maize (816 mm) and ETo-grass (824 mm) were within 1%, and ETr-maize (1,033 mm) and ETr-grass (1,070 mm) were within 3%. The same percentages were obtained in 2006 (ETo-maize = 671 mm, ETo-grass = 675 mm, ETr-maize = 838 mm, ETr-grass = 868 mm). Thus, in practice, data measured above a well-watered maize canopy can be a substitute for the microclimatic data measured above a reference surface in ETref estimations when "reference" weather station data are not available to solve the PM equation in areas with similar rainfall (>300 mm) during the growing season, as observed in this study.