Modelling potential evapotranspiration of potential vegetation

Abstract

A method to estimate the potential evapotranspiration (PET) rate of potential vegetation is presented. The method is based on the Penman–Monteith equation which includes fixed parameters and the variables vegetation height and leaf area index (LAI). A fixed starting LAI value of 0.2 is used to calculate vegetation height, aerodynamic resistance, vegetation canopy resistance and finally, the PET rate. The LAI is successively increased by 0.1 and the calculation process repeated to obtain corresponding PET values. When the annual PET exceeds yearly available water reserves, the cycle is interrupted leading to the definitive PET and corresponding LAI. The evaporation of rainfall intercepted by the plant canopy and soil evaporation are then added to the PET estimated in this manner. These variables in turn vary according to the LAI. The PET distribution of the Spanish peninsula calculated by several methods is represented on maps which may be easily compared. A comparison with those published by other authors was also undertaken. The estimated PET distribution of potential vegetation differed considerably from that of the reference crop (grass). In rainy areas (irrespective of temperature) the PET exceeded that of the reference due to the evaporation of water intercepted by the plant canopy. In dry areas, potential vegetation PET was lower with respect to the reference PET. This may be explained by the fact that when the LAI is small, transpiration and evaporation of intercepted rain are consequently reduced. Furthermore, the LAI values obtained are reasonably consistent with the characteristics of the potential vegetation. It is proposed that this type of method may be of use in the study of potential vegetation and climate in general.