Detecting the evaporation of intercepted water over an old-growth rain forest in the eastern Amazon using eddy flux measurements

We introduce and demonstrate a new method to measure the interception evaporation over an old-growth rain forest in the eastern Amazon using eddy covariance. The approach is to estimate the ‘excess' evaporation that occurs following individual events, using baseline evaporation time series obtained from long time series of flux data and creating ensemble averages from these precipitation events and base-state dry days. One advantage of this method over the traditional techniques of estimating interception using rain gauges alone is that the interception evaporation is directly measured and not determined as the residual of incident precipitation and throughfall. Furthermore, the large differences in interception that can occur on a site due to varying forest canopy density, structure and the appearance of canopy gaps is smoothed out using the eddy covariance method as the size of the flux footprint area incorporates these variations, and 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.

A total of over 200 combined daytime and nighttime rainfall events were analyzed over a three-year period. For daytime events, mean interception for events of all rainfall intensities ranged from 5 to 9 percent of total event precipitation. Light rainfall events (less than 1 mm total) resulted in higher interception values of up to 20 percent. During nighttime precipitation events occurring under high winds, approximately 20 percent of the incoming precipitation goes to interception evaporation in the two hours following the start of precipitation. For nighttime light wind events only about 5 percent of the incoming precipitation is evaporated after rainfall, with the remainder evaporating the following morning.

Changes in the energy balance also occur during precipitation events. On days with afternoon rainfall, the evaporative fraction (LE/-Q*) increases by over 15 percent from the pre-rain morning period to the rainy afternoon period. Most of the energy required to supply this enhanced evaporative fraction during and after rainfall comes from the canopy heat storage. This method to estimate interception may be applicable to other tower sites worldwide in varying types and climates as net radiation is used to scale the evaporation in this method.