Analysis of Microclimate Data Measured over Grass and Soybean Canopy and Their Impacts on Penman-Monteith Grass and Alfalfa Reference Evapotranspiration

The use of Penman-Monteith (PM)-type combination-based energy-balance equations to estimate reference evapotranspiration (${\mathrm{ET}}_{\mathrm{ref}}$) requires climatic data measured over well-watered and well-maintained reference grass or alfalfa vegetation surfaces. However, establishing and maintaining reference weather stations for a long period of time is a very formidable and expensive process. Thus, expansion of the microclimate data available for use in the PM equation for estimating ${\mathrm{ET}}_{\mathrm{ref}}$ is needed. In the absence of reference weather stations, one alternative is using microclimatic data measured over other well-watered vegetation surfaces as inputs to the PM equation. This study determines if weather data collected from a well-watered soybean surface in a semihumid climate can be used for this purpose. Measured and estimated microclimate variables, including net radiation ($R_n$), average air temperature ($T_{\mathrm{ave}}$), dew-point temperature ($T_d$), average relative humidity (${\mathrm{RH}}_{\mathrm{ave}}$), aerodynamic resistance ($r_a$), and wind speed at 3 m ($u_3$) of a soybean and a grass canopy in South Central Nebraska, were analyzed and compared. The aerodynamic resistances of the soybean and grass canopies showed the largest percent difference of any of the microclimate variables for both 2007 and 2008. Wind speed was the primary microclimate variable with the largest percent difference between the two fields. The average percent differences in $u_3$ between the soybean and grass field were 9.0 and 9.8% for 2007 and 2008. Although $T_{\mathrm{ave}}$, ${\mathrm{RH}}_{\mathrm{ave}}$, and $T_d$ percent differences were not that large, there were distinct seasonalities to the differences. Grass and alfalfa reference evapotranspiration (${\mathrm{ET}}_o$ and ${\mathrm{ET}}_r$, respectively) calculations using data from the soybean (${\mathrm{ET}}_{o\mathrm{\text{-}}s}$ and ${\mathrm{ET}}_{r\mathrm{\text{-}}s}$) and grass (${\mathrm{ET}}_{o\mathrm{\text{-}}g}$ and ${\mathrm{ET}}_{r\mathrm{\text{-}}g}$) canopies were compared daily and seasonally. Seasonal total ${\mathrm{ET}}_o$ and ${\mathrm{ET}}_r$ estimates using soybean and grass microclimate data were very close, and within 1 and 2% during 2007 (${\mathrm{ET}}_{o\mathrm{\text{-}}g}=583\mathrm{mm}$ and ${\mathrm{ET}}_{o\mathrm{\text{-}}s}=576\mathrm{mm}$; ${\mathrm{ET}}_{r\mathrm{\text{-}}g}=751\mathrm{mm}$ and ${\mathrm{ET}}_{r\mathrm{\text{-}}s}=733\mathrm{mm}$), and 4 and 5% during 2008 (${\mathrm{ET}}_{o\mathrm{\text{-}}g}=554\mathrm{mm}$ and ${\mathrm{ET}}_{o\mathrm{\text{-}}s}=531\mathrm{mm}$; ${\mathrm{ET}}_{r\mathrm{\text{-}}g}=707\mathrm{mm}$ and ${\mathrm{ET}}_{r\mathrm{\text{-}}s}=669\mathrm{mm}$). In 2007, differences in temperature variables were most correlated to differences in ${\mathrm{ET}}_{\mathrm{ref}}$ estimates. In 2008, the greatest correlations of differences in ${\mathrm{ET}}_o$ and ${\mathrm{ET}}_r$ were with differences in $T_{\mathrm{ave}}$, ${\mathrm{RH}}_{\mathrm{ave}}$, and $u_3$. The results indicated that the microclimate data measured over an irrigated soybean canopy during normal or wet years ($\mathrm{\text{rainfall}}\ge 300\mathrm{mm}$ during the growing season) can be used in place of measurements taken over a grass canopy to estimate ${\mathrm{ET}}_o$ and ${\mathrm{ET}}_r$ in climatic conditions similar to semihumid climatic conditions of South Central Nebraska when reference weather station data are not available to solve the standardized PM equation.