Estimation of reference evapotranspiration in the Maule Region of Chile:: A comparison between the FAO Penman-Monteith and Bowen ratio methods

A study was carried out in order to evaluate the FAO Penman-Monteith (FAO-PM) model for computing hourly reference evapotranspiration (ETo) over well-irrigated grass at the Panguilemo Experimental Station (PES), which is located 11 km north of Talca, Maule Region of Chile (35 degrees 23' S and 71 degrees 41' W; 110 m above the sea level). An automatic weather station and Bowen ratio system were installed in the center of grass canopy to measure climatic variables (air temperature, relative humidity, and wind speed) and energy balance components (solar radiation, net radiation, latent heat flux, and soil heat flux), respectively. Hourly values of ETo estimated by the FAO-PM equation were tested with reference evapotranspiration measurements obtained from the Bowen ratio system (ETB). Results indicated that FAO-PM model was able to predict ETo with a standard error of estimate (SEE) of 0.07 mm h(-1) (48 W m(-2)) and an absolute error (Ea) less than 5 %. In this case, daytime and nighttime soil heat fluxes (G) were estimated as 10 % and 50 % of net radiation, respectively. When G was assumed equal to zero, the FAO-PM model computed ETo with SEE and Ea values of 0.08 mm h(-1) (54 W m(-2)) and 9 %, respectively. Furthermore, the net radiation model recommended by FAO overestimated the observed values with Ea value of 9.0 %.