A simple parameterization of bulk canopy resistance from climatic variables for estimating hourly evapotranspiration
Lecina Brau, Sergio
Villalobos, F. J.
MetadataShow full item record
This paper examines a model for estimating canopy resistance rc and reference evapotranspiration ETo on an hourly basis. The experimental data refer to grass at two sites in Spain with semiarid and windy conditions in a typical Mediterranean climate. Measured hourly ETo values were obtained over grass during a 4 year period between 1997 and 2000 using a weighing lysimeter (Zaragoza, northeastern Spain) and an eddy covariance system (Córdoba, southern Spain). The present model is based on the Penman-Monteith (PM) approach, but incorporates a variable canopy resistance rc as an empirical function of the square root of a climatic resistance r* that depends on climatic variables. Values for the variable rc were also computed according to two other approaches: with the rc variable as a straight-line function of r* (Katerji and Perrier, 1983, Agronomie 3(6): 513-521) and as a mechanistic function of weather variables as proposed by Todorovic (1999, Journal of Irrigation and Drainage Engineering, ASCE 125(5): 235-245). In the proposed model, the results show that rc/ra (where ra is the aerodynamic resistance) presents a dependence on the square root of r*/ra, as the best approach with empirically derived global parameters. When estimating hourly ETo values, we compared the performance of the PM equation using those estimated variable rc values with the PM equation as proposed by the Food and Agriculture Organization, with a constant rc = 70 s m-1. The results confirmed the relative robustness of the PM method with constant rc, but also revealed a tendency to underestimate the measured values when ETo is high. Under the semiarid conditions of the two experimental sites, slightly better estimates of ETo were obtained when an estimated variable rc was used. Although the improvement was limited, the best estimates were provided by the Todorovic and the proposed methods. The proposed approach for rc as a function of the square root of r* may be considered as an alternative for modelling rc, since the results suggest that the global coefficients of this locally calibrated relationship might be generalized to other climatic regions. It may also be useful to incorporate the effects of variable canopy resistances into other climatic and hydrological models.
Is part ofHydrological Processes, 2006, vol. 20, núm. 3, p. 515-532
European research projects
Showing items related by title, author, creator and subject.
Fixed versus variable bulk canopy resistance for reference evapotranspiration estimation using the Penman-Monteith equation under semiarid conditions Lecina Brau, Sergio; Martínez-Cob, Antonio; Pérez García, Pedro J.; Villalobos, F. J.; Baselga, J. J. (Elsevier, 2003)
A comparison of new and existing equations for estimating sensible heat flux using surface renewal and similarity concepts Castellví Sentís, Francesc; Snyder, R. L.; Baldocchi, D. D.; Martínez-Cob, Antonio (American Geophysical UnionWiley, 2006)This paper describes two approaches for estimating sensible heat flux, using surface renewal and similarity concepts. One approach depends on a temperature structure function parameter and is valid in the inertial sublayer. ...
Estimating sensible heat flux using surface renewal analysis and the flux variance method: A case study over olive trees at Sástago (NE of Spain) Castellví Sentís, Francesc; Martínez-Cob, Antonio (American Geophysical UnionWiley, 2005)The eddy covariance technique for measuring surface fluxes is often not affordable outside experimental research institutes. Therefore knowledge of the performance of alternative methods for determining surface fluxes is ...