Water losses by evapotranspiration are influenced by many processes and interactions in the atmosphere and the biosphere. However, these atmosphere and terrestrial biosphere exchanges of matter and energy over complex regions such as California are difficult to estimate because many regional models lack interactions with a complex land surface scheme. Traditional evapotranspiration measurements are sparse and limited to microscale environments. Therefore, accurately estimating evapotranspiration on the regional scale remains a major challenge. In this study, we introduce a new framework with a regional atmospheric model, WRF, coupled to a high complexity land surface model, ACASA. Although WRF (Weather Research and Forecasting model) is a state-of-the-art regional atmospheric model with high spatial and temporal resolutions, the land surface schemes available in WRF lack intricate biogeophysical processes that influence evapotranspiration. ACASA (Advanced Canopy-Atmosphere-Soil Algorithm) is a complex multilayer land surface model with interactive canopy physiology and full surface hydrological processes that allows microenvironmental variables such as air and surface temperatures, wind speed, humidity, and carbon dioxide concentration to vary vertically. Carbon dioxide, sensible heat, water vapor, and momentum fluxes between the atmosphere and land surface are estimated in the ACASA model through third order turbulence equations. It includes counter-gradient transport that low-order turbulence closure models are unable to simulate.

The WRF-ACASA model simulations over California are compared with the CIMIS data and the NCEP ADP Operational Global Surface Observations. Overall, the WRF-ACASA results show good agreements with observations on surface temperature, relative humidity and evapotranspiration. Furthermore, GCM data can be used to drive the WRF-ACASA coupled model, thus the model is capable of forecasting evapotranspiration. This is crucial for water budget and management, especially in California, where the agricultural and urban water demand is large and the available water is limited and likely to decrease in the near future as climate conditions change. As a result, WRF-ACASA is a promising tool to simulate as well as to forecast evapotranspiration on a regional scale.