This study investigates the impact of canopy representation on regional evapotranspiration using coupled mesoscale WRF model and the complex land surface model ACASA. Accurate estimates of evapotranspiration (both potential and actual) are especially important for regions with limited water availability and high water demand, such as California. Water availability has been and will continue to be the most important issue facing California for years and perhaps decades to come. The terrestrial evapotranspiration are influenced by many processes and interactions in the atmosphere and the biosphere such water, carbon, and momentum exchanges. The need to improve representation within of surface-atmosphere interactions remains an urgent priority within the modeling community. In this study, we couple the Weather Research and Forecasting Model (WRF) with the Advanced Canopy-Atmosphere-Soil Algorithm (ACASA), a high complexity land surface model. The WRF-ACASA model uses a multilayer structure to represent the canopy, consequently allowing micro-environmental variables such as LAI, air and canopy temperature, wind speed and humidity to also vary vertically. The improvement in canopy representation and canopy-atmosphere interactions allow more realist simulation of evapotranspiration in both regional and local scales.