Detecting rainfall interception in an Amazonian rain forest with eddy flux measurements

Abstract

Summary We introduce a new method to estimate rainfall interception and demonstrate its use for data obtained in an old-growth rain forest in the eastern Amazon basin. The approach is to use eddy covariance evaporation observations to estimate the ‘excess’ evaporation that occurs following individual events. Ensemble averaged water vapor fluxes were calculated from original high frequency data both for rain event and for base state dry days. Interception was inferred from the difference between observed evaporation for selected times during and following rainfall events from baseline evaporation estimates. This method allows the interception evaporation to be directly measured rather than determined from the residual of incident precipitation and throughfall. In conventional studies, large differences in throughfall can occur on a site due to varying forest canopy density, structure and the appearance of canopy gaps. This problem is mitigated when using the current approach, which provides an average interception value over the flux footprint area. Identification of light rainfall events not detected by an on-site tipping bucket rain gauge was aided by the use of a ceilometer. The mean interception for all events in the study (daytime and nocturnal) was 11.6%, comparable to some recent conventional studies in this region. We found an approximately 15% increase of evaporative fraction on the rain days as compared to dry baseline days, with the energy being supplied by a corresponding decrease in the canopy heat storage. Since net radiation is used to scale the evaporation in this method, this method may be applicable to data from other tower sites in varying surface and climatic types. We did not find that bulk stomatal resistance vanishes just after rainfall. The effective bulk stomatal resistance can be used as the observational equivalent of the wet fraction of canopy parameter used in interception models.