Analysis of effective resistance calculation methods and their effect on modelling evapotranspiration in two different patches of vegetation in semi-arid SE Spain

Abstract

Abstract. Effective parameters are of major importance in modelling surface fluxes at different scales of spatial heterogeneity. Different ways to obtain these effective parameters for their use in meso-scale and GCM models have been studied. This paper deals with patch-scale heterogeneity, where effective resistances were calculated in two patches with different vegetation (Retama sphaerocarpa (L.) Boiss shrubs, and herbaceous plants) using different methods: aggregating soil and plant resistances in parallel, in series or by an average of both. Effective aerodynamic resistance was also calculated directly from patch fluxes. To assess the validity of the different methods used, the Penman-Monteith equation was used with effective resistances to estimate the total λ E for each patch. The λ E estimates found for each patch were compared to Eddy Covariance system measurements. Results showed that for effective surface resistances, parallel aggregation of soil and plant resistances led to λ E estimates closer to the measured λ E in both patches (differences of around 10%). This may be due to the fact that in semi-arid areas, with very sparse vegetation, soil resistances are much higher than plant resistances, and therefore parallel aggregation attenuates the effect of the high soil resistances on λ E modelling. Results for effective aerodynamic resistances differed depending on the patch considered and the method used to calculate them. The use of effective aerodynamic resistance calculated from fluxes provided less accurate estimates of λ E compared to the measured λ E, than the use of effective aerodynamic resistances aggregated from soil and plant resistances. The results reported in this paper show that the best way of aggregating soil and plant resistances depend on the type of resistance, and the type of vegetation in the patch.