Impact of Air Temperature and Relative Humidity Measured over Rice and Grass Canopies on Penman-Monteith Reference Evapotranspiration Estimates

The adoption of the Penman-Monteith reference evapotranspiration (ETo) method necessitates the use of climatic data and parameters measured above a reference surface (e.g., reference grass), which is rarely available across agro-ecosystems in all countries. In such cases, alternatively, climate data collected above other vegetation surfaces can be potential sources of data for ETo estimations. The objective of this study was to determine whether microclimatic data measured above irrigated lowland rice can be used in estimating ETo using the Penman-Monteith model in conditions where climatic data measured in reference weather stations do not exist. Maximum and minimum air temperatures (Tmax and Tmin, respectively) and maximum and minimum relative humidity (RHmax and RHmin, respectively) were measured above a rice canopy in 2014, 2015, and 2016, when a full data set was also collected above well-maintained grass canopy, which was 100 m from the rice research plots. Daily average Tmin and Tmax for rice and Tmin and Tmax for grass were linearly and strongly correlated with regression slopes of 1.0085 and 0.9829, $R^2$ greater than 0.83, and root-mean square error (RMSE) of 1.30 and 1.53°C for Tmin and Tmax, respectively. Dew point temperature (Td) varied from $-3.73$ to 33.44°C over grass and from $-0.96$ to 34.26°C above rice canopy, with an average of 19.32 and 21.10°C above grass and rice, respectively. The observed RMSE and mean bias error (MBE) for RHmax and RHmin were 4.1% and 4.8% and 2.2% and 4.1%, respectively. The relative humidity difference of 3.2% was observed above the rice canopy. RHmin, RHmax, and RHmean measured over the rice canopy were linearly correlated with those measured over grass with regression slopes of 1.13, 1.03, and 1.06, respectively, and $R^2$ values of 0.98, 0.96, and 0.98, respectively. The vapor pressure deficit (VPD) difference was 9.4% greater over the grass canopy and averaged 2.15 kPa, while it averaged 1.97 kPa over rice with a very low RMSE of 0.27 kPa between VPD values measured above the two surfaces. The estimated net radiation (Rn) values above grass and rice canopies were highly correlated, with a regression slope of 1.017, $R^2$ of 0.9976 and a RMSE as low as $0.25\mathrm{MJ}/{\mathrm{m}}^2$, which would result in only $0.1\mathrm{mm}/\mathrm{d}$ of water. Daily ETo computed using the weather data measured above grass and flooded rice canopies showed that daily ETo-grass varied from 1.66 to 10.78 mm and averaged 5.38 mm. Similarly, daily ETo-rice varied from 1.53 to 10.16 mm, averaging 5.21 mm. The MBE between ETo-grass and ETo-rice was $0.17\mathrm{mm}/\mathrm{day}$ and the RMSE was as low as $0.24\mathrm{mm}/\mathrm{day}$, representing a relative error of only 4.5%. There was strong agreement between ETo-rice and ETo-grass and the simple linear regression slope between ETo-rice and ETo-grass was 0.943, 0.966, and 0.977 in 2014, 2015, and 2016, respectively, with an $R^2\ge 0.98$. Overall, it was determined that $\mathrm{ETo}\text{-}\mathrm{rice}=0.9687$ ETo-grass with an $R^2$ of 0.99. The good fit between ETo-rice and ETo-grass demonstrated the possibility of using weather data measured above a rice canopy to compute ETo as a viable alternative for data sources in conditions where grass-reference weather stations are not available.