Sun, 07/24/2011 - 10:53 — ckass

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.

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