Meteorological applications at all scales (micro, meso, and global) are adopting a physiologically intensive evapotranpiration estimation scheme. >From the pristine Jarvis-type schemes, the role of assimilation and photosynthesis is being considered in both observational as well as numerical studies. Using observations made during FIFE, for an open well watered canopy, the interactive feedback mechanism between the biotic and abiotic components such as photosynthesis rates, transpiration, air and surface temperature, and humidity is delineated. These interactions are synthesized using a response surface methodology - with nonlinear variations. Using four different stomatal conductane (resistance) schemes, and one set of observational analysis, the interactive strategy and the overlapping compensation, the models tend to predict for different parameter setting is analyzed. Results suggest, nonlinear effects and second order interactions are significantly active in the evapotranpirative mechanism and present day models donot have a consensus in their predictions. A proposal shall be outlined which involves a simple approach for assimilation type studies in SVAT modeuls for regional and climate forecasting.