Understanding the Non-Linear Response of Summer Evapotranspiration to Clouds in a Temperate Forest Under the Impact of Vegetation Water Content

Understanding the cloud impact on forest evapotranspiration (ET) is crucial for studying the interaction of vegetation-cloud-atmosphere. Combining long-term (2003-2010) satellite passive microwave observations and in-situ measurements, a non-linear response of canopy-scale ET to cloud increase was found at a temperate forest in Northeast China. As cloud increased, an initial enhancement (4%-10%) in ET occurred under less cloudy sky, while a significant reduction (>20%) in ET occurred under more cloudy sky. The phenomenon existed under both high and low vegetation water content (VWC) indicated by satellite microwave emissivity difference vegetation index (EDVI). Analysis showed that this was the combined effect from the enhancement (5%-30%) in evaporative fraction (EF) and the reduction (5%-50%) in net radiation under cloud increase. Decoupling analysis based on coefficients (rho) of path analysis model showed that enhanced EF (rho > 0.61) rather than radiation (rho < 0.47) dominated the ET enhancement under less cloudy sky, while the control of reduced radiation became stronger (rho > 0.63) and could not be compensated by increased EF (rho < 0.48) under more cloudy sky. EF enhancement under clouds was strongly correlated with the decline in canopy resistance (Rs) which was dominated by vapor pressure deficit (VPD). Higher VWC increased ET via reducing Rs and enlarging EF. This positive effect of VWC was more noticeable under less cloudy sky. Associated mechanisms could be related to the dynamic controls of plant physiology and environmental conditions induced by VWC and clouds. This study highlighted the dynamic effect of clouds and VWC on forest ET and improved our knowledge of vegetation-cloud interactions.