Holwerda F., Bruijnzeel L.A., Scatena F.N., Vugts H.F., Meesters A.G.C.A 2011. Wet canopy evaporation from a Puerto Rican lower montane rain forest: the importance of realistically estimated aerodynamic conductance. In press Journal of Hydrology

Abstract:

Rainfall interception (I) was measured in 20 m tall Puerto Rican tropical forest with complex topography for a one-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 (ga,M). E was also derived using the energy balance equation and the sensible heat flux measured by a sonic anemometer (Hs). I per sampling occasion was strongly correlated with rainfall (P): I = 0.21P + 0.60 (mm), r2 = 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 ga,M (0.16 mm h-1). Estimating ga,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 Hs was very low (0.13 mm h-1), presumably because Hs 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 ga,M in the P-M equation (0.58 mm h-1). The present results show that in areas with complex topography, ga,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 ga,M using eddy covariance is recommended. The currently measured ga,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 ga,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 Hs during rainfall in tropical forests with complex topography.

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 23 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.