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.

Schellekensa, J.; Scatenab,F.N.; Bruijnzeela,L.A.; Wickela,A.J. 1999. Modelling rainfall interception by a lowland tropical rain forest in northeastern Puerto Rico. Journal of Hydrology 225 :168-184.

Abstract:

Recent surveys of tropical forest water use suggest that rainfall interception by the canopy is largest in wet maritime locations. To investigate the underlying processes at one such locationthe Luquillo Experimental Forest in eastern Puerto Rico66 days of detailed throughfall and above-canopy climatic data were collected in 1996 and analysed using the Rutter and Gash models of rainfall interception. Throughfall occurred on 80% of the days distributed over 80 rainfall events. Measured interception loss was 50% of gross precipitation. When PenmanMonteith based estimates for the wet canopy evaporation rate (0.11 mm h21 on average) and a canopy storage of 1.15 mm were used, both models severely underestimated measured interception loss. A detailed analysis of four storms using the Rutter model showed that optimizing the model for the wet canopy evaporation component yielded much better results than increasing the canopy storage capacity. However, the Rutter model failed to properly estimate throughfall amounts during an exceptionally large event. The analytical model, on the other hand, was capable of representing interception during the extreme event, but once again optimizing wet canopy evaporation rates produced a much better fit than optimizing the canopy storage capacity. As such, the present results support the idea that it is primarily a high rate of evaporation from a wet canopy that is responsible for the observed high interception losses.

Holwerda, F., R. Burkard, W. Eugster, F. N. Scatena, A. G. C. A. Meesters,

and L. A. Bruijnzeel (2006), Estimating fog deposition at a Puerto

Rican elfin cloud forest site: Comparison of the water budget and eddy

covariance methods, Hydrol. Processes, 20, 2669– 2692.

Abstract:

The deposition of fog to a wind-exposed 3 m tall Puerto Rican cloud forest at 1010 m elevation was studied using
the water budget and eddy covariance methods. Fog deposition was calculated from the water budget as throughfall
plus stemflow plus interception loss minus rainfall corrected for wind-induced loss and effect of slope. The eddy
covariance method was used to calculate the turbulent liquid cloud water flux from instantaneous turbulent deviations
of the surface-normal wind component and cloud liquid water content as measured at 4 m above the forest canopy. Fog
deposition rates according to the water budget under rain-free conditions (0Ð11 š 0Ð05 mm h1) and rainy conditions
(0Ð24 š 0Ð13 mm h1) were about three to six times the eddy-covariance-based estimate (0Ð04 š 0Ð002 mm h1). Under
rain-free conditions, water-budget-based fog deposition rates were positively correlated with horizontal fluxes of liquid
cloud water (as calculated from wind speed and liquid water content data). Under rainy conditions, the correlation
became very poor, presumably because of errors in the corrected rainfall amounts and very high spatial variability in
throughfall. It was demonstrated that the turbulent liquid cloud water fluxes as measured at 4 m above the forest could
be only ¾40% of the fluxes at the canopy level itself due to condensation of moisture in air moving upslope. Other
factors, which may have contributed to the discrepancy in results obtained with the two methods, were related to effects
of footprint mismatch and methodological problems with rainfall measurements under the prevailing windy conditions.
Best estimates of annual fog deposition amounted to ¾770 mm year1 for the summit cloud forest just below the
ridge top (according to the water budget method) and ¾785 mm year1 for the cloud forest on the lower windward
slope (using the eddy-covariance-based deposition rate corrected for estimated vertical flux divergence). Copyright
2006 John Wiley & Sons, Ltd.