Comparison of two evapotranspiration schemes on a sub-urban site

Abstract

Two different urban evapotranspiration (ET) schemes are compared with the dataset of an instrumented sub-urban site, during 5 years at the annual, monthly and hourly time scales. The first scheme comes from atmospheric sciences and details the exchanges with the atmosphere; it is used in the soil model SM2-U. The second one comes from hydrological studies and details the surface and soil water content; it is used in the hydrological model UHE. For both models, the annual ET from the site comes mainly from natural surfaces, which cover 55% of the surface area. At the hourly scale, the ET predominance from natural surfaces is not always verified, especially just after rainfall events. At the annual scale, both models indicate also that the soil water status is not a limiting factor for the ET above this site. However, a large amplitude difference appears between ET simulated by the two models: simulated with UHE the annual ET is 37% higher than with SM2-U. This is explained by uncertainties in some parameter values and by differences in some parameterizations of common variables. These results, illustrate the strong remaining uncertainties in ET estimation above urban areas and the need of an enhanced research effort to measure and model the urban ET factors.