Modeling evapotranspiration for three-dimensional global climate models

Abstract

A parameterization is developed for the calculation of evapotranspiration in three-dimensional atmospheric models. It distinguishes separately between evaporation from the ground and evapotranspiration from plant foliage. Soil water is stored in an active layer of 1 m depth and a 10 cm surface layer is separately distinguished. The evaporation from this soil is parameterized using a high resolution multilayer model for comparison. This parameterized evaporation from the soil is defined by either the potential evaporation rate or by the maximum rate at which water can diffuse to the surface, depending on which rate is smaller. The maximum rate is obtained empirically in terms of various soil-hydraulic parameters. The evapotranspiration from plants occurs either as the evaporation of water stored on the surface of the foliage or as the transpiration of water extracted by roots from the soil. The flux of water from the outer surface of foliage to the atmosphere above the canopy is determined by the decrease in water vapor concentration from the foliage surface to the overlying atmosphere and by the resistance of the foliage molecular boundary layers and the bulk aerodynamic resistance of the canopy. Transpired water encounters an additional stomatal resistance in passing from the inside to the outside of leaves. The foliage temperature and saturation vapor pressure are calculated from a model of the plant canopy energy balance. Soil moisture determines the maximum rate at which roots can extract water from the soil, and if the transpiration demand exceeds this maximum rate, stomatal closure occurs until the demand matches the root supply. A parameterization of land evapotranspiration at the level of detail described in this paper may be required to obtain a realistic diurnal cycle of surface temperature and evapotranspiration for use in mesoscale or global climate models. However, application of the developed procedures is made difficult by the extreme complexity and small-scale detail of surface processes, the lack of adequate data sets for surface parameters, and the need for satisfactory parameterizations of other components of. GCMs such as their rainfall and planetary boundary layer treatments. Because of these difficulties, the development of validated models of land surface processes first requires detailed sensitivity studies to establish what further data sets are most urgently required and what model improvement should be given highest priority.