A new gas - exchange/assimilation surface evapotranspiration model (GEM) for mesoscale applications

A new gas - exchange / assimilation surface evapotranspiration model, called GEM, is developed for mesoscale applications. GEM incorporates carbon dioxide assimilation (photosynthesis) and soil moisture control of evapotranspiration. The parameterization is based on physiologically intensive representation of stomatal response coupled with prognostic equations for soil moisture and subsurface temperature through surface energy balance. This parameterization is dynmically coupled with a columnar version of a mesoscale model and tested with observations made during FIFE. The surface energy balances from more traditional evapotranspirative approaches (Jarvis-type) are also computed for the coupled simulations. Results indicate a good performance of the new parameterization with default gas exchange coefficients to generate dynamic responses for soil moisture as well as carbon dioxide variability over the traditional (Jarvis-type) approach. Based on these validation results as well as additional constraints such as, generalization for a larger landuse pattern, as well as better estimation or measurement of the constants involved in the model initial conditions, this study concludes mesoscale models can benefit by adopting the physiological approach over the traditional Jarvis-type schemes. Results from the sensitivity of various CO2 and related second-order changes on local scale hydrology (precipitation, cloudiness, and evapotranpiration) at short and long term will also be presented.