Estimation of Actual Evapotranspiration Using Measured and Calculated Values of Bulk Surface Resistance

The Penman-Monteith model (PM) is a useful "one-step" method for evapotranspiration (ET) estimation, if surface resistance (r_s-ms^–1) estimates can be derived. This study has as its objective to evaluate different methods of r_s estimation and the accuracy of the resulting ET estimates in common bean (P. vulgaris L.). The experiment was conducted at the Fortuna Agricultural Experiment Station at Juana Diaz, PR. Four automated weather stations were placed in plots planted with two genotypes of common bean (Phaseolus vulgaris L.). Net radiation, soil heat flux, soil temperature, soil moisture, air temperature, relative humidity, wind speed and direction were recorded at ten second intervals. Each weather station had an elevator system that moved the air temperature and relative humidity sensor between two vertical positions over the crop canopy every two minutes during a complete day. The r_s was derived by stomatal resistance (r_L) and leaf area index (LAI) measurements (PM-1), and by direct micrometeorological variables as follows: inverse of the general PM-model (PM-2), as a function of the soil moisture (PM-3), and as a latent heat flux-λE (PM-4 and ET-Station). The results indicate that PM-1 under-estimated r_s at low LAI, and that r_s and r_L are influenced inversely by the aerodynamic resistance (r_a), which affected the precision of the PM-2 and ET station estimation especially under windy and dry conditions, but not the PM-3 and PM-4 methods.