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Wet canopy evaporation from a Puerto Rican lower montane rain forest: The importance of realistically estimated aerodynamic conductance

Holwerda, F., Bruijnzeel, L. A., Scatena, F. N., Vugts, H. F., Meesters, A. G. C. A.
2 012
Journal of Hydrology
Rainfall interception (I) was measured in 20 m tall Puerto Rican tropical forest with complex topography for a 1-year period using totalizing throughfall (TF) and stemflow (SF) gauges that were measured every 2-3 days. Measured values were then compared to evaporation under saturated canopy conditions (E) determined with the Penman-Monteith (P-M) equation, using (i) measured (eddy covariance) and (ii) calculated (as a function of forest height and wind speed) values for the aerodynamic conductance to momentum flux (g(a,M)). E was also derived using the energy balance equation and the sensible heat flux measured by a sonic anemometer (11). I per sampling occasion was strongly correlated with rainfall (P): I = 0.21P + 0.60 (mm), r(2) = 0.82, n = 121. Values for canopy storage capacity (S = 0.37 mm) and the average relative evaporation rate (E/R = 0.20) were derived from data for single events (n = 51). Application of the Gash analytical interception model to 70 multiple-storm sampling events using the above values for S and E/R gave excellent agreement with measured I. For E/R = 0.20 and an average rainfall intensity (R) of 3.16 mm h(-1), the TF-based E was 0.63 mm h(-1), about four times the value derived with the P-M equation using a conventionally calculated g(a.M) (0.16 mm h(-1)). Estimating g(a.M) using wind data from a nearby but more exposed site yielded a value of E (0.40 mm h(-1)) that was much closer to the observed rate, whereas E derived using the energy balance equation and H-s was very low (0.13 mm h(-1)), presumably because H-s was underestimated due to the use of too short a flux-averaging period (5-min). The best agreement with the observed E was obtained when using the measured g(a.M) in the P-M equation (0.58 mm h(-1)). The present results show that in areas with complex topography, g(a.M), and consequently E, can be strongly underestimated when calculated using equations that were derived originally for use in flat terrain; hence, direct measurement of g(a.M) using eddy covariance is recommended. The currently measured g(a.M) (0.31 m s(-1)) was at least several times, and up to one order of magnitude higher than values reported for forests in areas with flat or gentle topography (0.03-0.08 m s(-1), at wind speeds of about 1 m s(-1)). The importance of g(a.M) at the study site suggests a negative, downward, sensible heat flux sustains the observed high evaporation rates during rainfall. More work is needed to better quantify H-s during rainfall in tropical forests with complex topography. (C) 2011 Elsevier B.V. All rights reserved.

Department of Earth and Environmental Science / University of Pennsylvania, 251 Hayden Hall, 240 South 33rd Street, Philadelphia, PA 19104-6316