|Porch, Timothy - Tim|
Submitted to: Proceedings of the World Environmental and Water Resources Congress Conference
Publication Type: Proceedings
Publication Acceptance Date: 5/12/2008
Publication Date: 5/16/2008
Citation: Harmsen, E., Ramirez-Builles, V.H., Porch, T.G. 2008. Estimation of actual evapotranspiration using measured and calculated values of bulk surface resistance. Proceedings of the ASCE World Environmental and Water Resources Congress Conference. 316:115. May 13-16, 2008, Honolulu, Hawaii.
Technical Abstract: Currently the United Nations-Food and Agriculture Organization (FAO) recommends using the Penman-Monteith method for estimating ET over all other meteorological methods. The principal limitation of using the generalized form of the Penman-Monteith equation is in obtaining accurate values for the bulk surfaces resistance (rs), because it is highly variable with crop type, atmospheric and soil-moisture conditions. Harmsen et al., (2005) developed the GPM/HG method for estimating rs and actual evapotranspiration based on the generalized Penman-Monteith (GPM) and humidity gradient (HG) methods. The cost of using the methodology is considerably lower than traditional methods (e.g., weighing lysimeter, eddy covariance system, and Bowen Ratio methods). Ortega-Farias et al. (2004) have developed a method for estimating rs based on net radiation, soil heat flux, vapor pressure deficit and soil moisture. The goal of this research was to compare ET estimates from the GPM/HG method and the Ortega-Farias method. The latter method has the advantage that it is relative inexpensive to use. An experiment was conducted at the University of Puerto Rico Agricultural Experiment Station at Lajas, PR, in which ET was estimated using the GPM/HG, Ortega-Farias, and eddy covariance methods. All three methods were in close agreement throughout the two day experiment. Another experiment was conducted at the University of Puerto Rico Fortuna Agricultural Experiment Station near Juana Diaz, PR. Four automated weather stations were placed in plots planted with two genotypes of common beans (Phaseolus vulgaris L.). In this experiment stomatal resistance and leaf area index were measure in order to determine the bulk surface resistance (rs). ET was then calculated using the GPM method. These results were compared with ET estimates from the GPM/HM and Ortega-Farias methods. No significant differences existed between ET estimated using the three methods. This result indicates that the Ortega-Farias method (least expensive) can be used to estimate actual evapotranspiration with reasonable accuracy.