Regional modeling of evapotranspiration using WRF coupled to the high complexity land surface model ACASA

Estimating evapotranspiration 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. Accurately estimating water losses by evapotranspiration on local and regional scales remains a major challenge. Traditional evapotranspiration measurements are sparse and limited to microscale environments. In this study, we couple the high complexity land surface model ACASA [developed at the University of California, Davis] to the mesoscale atmospheric model WRF [Weather Research Forecasting] for more accurate simulations of evapotranspiration at the regional level.

ACASA (Advanced Canopy-Atmosphere-Soil Algorithm) is a complex multilayer land surface model with interactive biophysical, meteorological and full surface hydrological processes that allows microenvironmental variables such as air and surface temperature, wind speed, humidity, and carbon dioxide concentration to vary vertically. Water vapor, carbon dioxide, energy and momentum fluxes between the atmosphere and the land surface are calculated in ACASA through third order closure turbulence equations and complex canopy physiology, such as 10 leaf classes for better light and precipitation interception. It allows counter-gradient transport that low-order turbulence closure models are unable to simulate. Coupling ACASA with the WRF model bridges the gap between in-situ applications and regional climate research. In this paper, we use WRF-ACASA to simulate evapotranspiration over a transect in Northern California ranging from the coastal area to the Sierra Nevada mountain. The results show that the evapotranspiration, surface and soil temperature, and energy fluxes simulated by the WRF-ACASA coupled model agree better with observational datasets than that of the WRF's pre-existing land surface models. As a result, WRF-ACASA is a promising tool to simulate evapotranspiration under future climate conditions.