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[1] Castellví et al. [2002] proposed a new approach for estimating sensible heat flux that combined surface renewal analysis and similarity theory. The approach used a calibration parameter (here referred as b) which was introduced by scaling the mean local gradient of air temperature with the ramp amplitude of air temperature over the mean volume of air parcel renewed per unit ground area, traditionally denoted as (az), with z as the measurement height. Parameter b is explained and determined for half-hourly samples. It is shown that (kb) $ 0.1 is appropriate under unstable conditions over a variety of canopies, with k as the von Kármán constant. This value is rather robust with regard to height and when measuring in both the roughness and inertial sublayers. This understanding of parameter b allowed a better understanding of parameter a and permitted the derivation of a modified approach for estimating sensible heat flux. In practice, it was possible to consider the new approach exempt from calibration. It was attractive for field applications and showed excellent performance under both stable and unstable conditions. A test was carried out for canopies where fetch requirements and full surface cover were not adequate. It was also shown that the flux variance method required calibration and did not perform as well as the proposed approach under such field conditions.

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Abstract:This paper examines a model for estimating canopy resistance r c and reference evapotranspiration ET o 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 ET o 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 r c as an empirical function of the square root of a climatic resistance r In the proposed model, the results show that r c /r a (where r a is the aerodynamic resistance) presents a dependence on the square root of r Ł /r a , as the best approach with empirically derived global parameters. When estimating hourly ET o values, we compared the performance of the PM equation using those estimated variable r c values with the PM equation as proposed by the Food and Agriculture Organization, with a constant r c D 70 s m 1 . The results confirmed the relative robustness of the PM method with constant r c , but also revealed a tendency to underestimate the measured values when ET o is high. Under the semiarid conditions of the two experimental sites, slightly better estimates of ET o were obtained when an estimated variable r c was used. Although the improvement was limited, the best estimates were provided by the Todorovic and the proposed methods. The proposed approach for r c as a function of the square root of r Ł may be considered as an alternative for modelling r c , 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.

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