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

Abstract: 
Rainfall interception (I) was measured in 20 m tall Puerto Rican tropical forest with 4 complex topography for a one-year period using totalizing throughfall (TF) and stemflow 5 (SF) gauges that were measured every 23 days. Measured values were then compared to 6 evaporation under saturated canopy conditions (E) determined with the Penman-Monteith 7 (P-M) equation, using (i) measured (eddy covariance) and (ii) calculated (as a function of 8 forest height and wind speed) values for the aerodynamic conductance to momentum flux 9 (ga,M). E was also derived using the energy balance equation and the sensible heat flux 10 measured by a sonic anemometer (Hs). I per sampling occasion was strongly correlated with rainfall (P): I = 0.21P + 0.60 (mm), r2 11 = 0.82, n = 121. Values for canopy storage 12 capacity (S = 0.37 mm) and the average relative evaporation rate (E/R = 0.20) were 13 derived from data for single events (n = 51). Application of the Gash analytical 14 interception model to 70 multiple-storm sampling events using the above values for S and 15 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-116 , about four times the value derived with the P-M equation using a conventionally calculated ga,M (0.16 mm h-117 ). 18 Estimating ga,M using wind data from a nearby but more exposed site yielded a value of E (0.40 mm h-119 ) that was much closer to the observed rate, whereas E derived using the energy balance equation and Hs was very low (0.13 mm h-120 ), presumably because Hs was 21 underestimated due to the use of too short a flux-averaging period (5-min). The best 22 agreement with the observed E was obtained when using the measured ga,M in the P-M equation (0.58 mm h-123 ). The present results show that in areas with complex topography, 1 strongly underestimated when calculated using 2 equations that were derived originally for use in flat terrain; hence, direct measurement of ga,M using eddy covariance is recommended. The currently measured ga,M (0.31 m s-13 ) 4 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-15 , at wind speeds of about 1 m s-16 ). The importance of ga,M at the study site suggests a negative, downward, 7 sensible heat flux sustains the observed high evaporation rates during rainfall. More work 8 is needed to better quantify Hs during rainfall in tropical forests with complex 9 topography.