Holwerda F.

Water and energy budgets of rain forests along an elevation gradient under maritime tropical conditions

Holwerda, F., 2005. Water and Energy Budgets of Rain Forests Along an Elevational Gradient Under Maritime Tropical Conditions. PhD Thesis, VU University, Amsterdam, The Netherlands.

Abstract: 
From the hydrological point of view, mountains present somewhat of a paradox. Although they provide the bulk of the Earth’s freshwater resources, knowledge of the hydrological functioning of mountainous areas is generally much less extensive, reliable, and precise than that of other, often more easily accessible physiographic regions. Indeed, mountain regions have been referred to as ‘the blackest of black boxes in the hydrological cycle’ (Bandyopadhyay et al., 1997). Data collection networks are more difficult to set up and maintain in complex mountainous terrain, particularly in uninhabited forested headwater areas without road access, and minimum recommended instrumental densities are rarely met (Manley and Askew, 1993). Whilst the hydrological knowledge base on mountains in general has increased considerably in the last few decades, most montane research work has focused on determining catchment water and sediment outputs and their distribution in time and space; snow cover and glacier dynamics; or flood frequencies (Molnar, 1990; Lang and Musy, 1990; Bergmann et al., 1991; Young, 1992; Hofer, 1998), as opposed to the underlying hydrological processes (cf. Bonell, 1993). Until very recently (e.g. Motzer, 2003; Schellekens et al., 2004; Goller et al., 2005), the vast majority of this work dealt with mountains in the temperate zone, with very little pertaining to forested tropical mountains (see summaries of early research by Bruijnzeel and Proctor (1995) and Bruijnzeel (2001)). Knowledge of such processes would serve as a basis for increased understanding of how streamflows emanating from tropical mountains might change as a result of changes in climate, including the lifting condensation level, frequency and density of clouds and, by implication, water inputs and evaporative losses (Bruijnzeel, 2001). The average cloud condensation level on tropical islands can be as low as 600-800 m (Malkus, 1955), although on larger mountains situated further inland this may be closer to 2,000 m (Stadtmüller, 1987). Above this condensation level, the hydrology of the forest changes profoundly because of contributions of cloud water (i.e. fog) deposited to the forest canopy (Bruijnzeel, 2001). There is circumstantial evidence that complete conversion of these ‘tropical montane cloud forests’ (TMCF) to pasture or vegetable cropping may have an adverse effect on dry season flows or even on total water yield because of strongly diminished fog interception after clearing (Ingwersen, 1985; Brown et al., 1996). Similar effects may be expected when the average cloud condensation level is raised because of warming of the atmosphere due to global climate change (Still et al., 1999; Foster, 2001), or clearing of forest at lower elevations (Lawton et al., 2001; Van der Molen, 2002).

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 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.

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.

Comparison of passive fog gages for determining fog duration and fog interception by a Puerto Rican elfin cloud forest

Holwerda, F.; Bruijnzeel, L.A.; Scatena, F.N. 2010. Comparison of passive fog gages for determining fog duration and fog interception by a Puerto Rican elfin cloud forest. Bruijnzeel, L.A.; Scatena, F.N.; Hamilton, L.S., eds. Tropical Montane Cloud Forests: Science for Conservation and Management. Cambridge, UK: Cambridge University Press. p. 275-281.

Abstract: 
Rates and amounts of fog interception by vegetation depend on wind speed, fog liquid water 4 content (LWC) and duration, as well as surface area and geometry of the vegetation 5 (Schemenauer, 1986). Information on the timing and duration of fog can be obtained with 6 passive fog gages, provided these are protected from rainfall and equipped with a recording 7 device (Bruijnzeel et al., 2005). Fog LWC may also be evaluated from collections by passive 8 gages when information on their collection efficiency and prevailing wind speeds is available 9 (e.g. Schemenauer and Joe, 1989). A variety of passive gages is available, and there has been 10 some discussion as to what is the most suitable type of gage to characterize local fog 11 conditions (Juvik and Nullet, 1995a; Schemenauer and Cereceda, 1995; cf. Delay and 12 Giambelluca, in press; Frumau et al., this issue). For example, a cylindrical gage is considered 13 superior to a flat screen, because it has uniform exposure to all wind directions (Juvik and 14 Nullet, 1995a; cf. García Santos and Bruijnzeel, this issue; Giambelluca et al., this issue). On 15 the other hand, a flat screen generally has a much larger collection area than a cylindrical 16 gage, and may thus measure fog when LWC or wind speeds are low (Schemenauer and 17 Cereceda, 1995).

Characteristics of fog and fogwater fluxes in a Puerto Rican elfin cloud forest

Eugster, Werner ; Burkard, Reto; Holwerda, Friso; Scatena, Frederick N.; Bruijnzeel, L.A.(Sampurno) 2006. Characteristics of fog and fogwater fluxes in a Puerto Rican elfin cloud forest.. Agricultural and Forest Meteorology 139 :288-306.

Abstract: 
The Luquillo Mountains of northeastern Puerto Rico harbours important fractions of tropical montane cloud forests. Although it is well known that the frequent occurrence of dense fog is a common climatic characteristic of cloud forests around the world, it is poorly understood how fog processes shape and influence these ecosystems. Our study focuses on the physical characteristics of fog and quantifies the fogwater input to elfin cloud forest using direct eddy covariance net flux measurements during a 43-day period in 2002.We used an ultrasonic anemometer–thermometer in combination with a size-resolving cloud droplet spectrometer capable of providing number counts in 40 droplet size classes at a rate of 12.5 times per second. Fog occurred during 85% of the time, and dense fog with a visibility <200 m persisted during 74% of the period. Fog droplet size depended linearly on liquid water content(r2 ¼ 0:89) with a volume-weighted mean diameter of 13.8 mm. Due to the high frequency of occurrence of fog the total fogwater deposition measured with the eddy covariance method and corrected for condensation and advection effects in the persistent upslope air flow, averaged 4.36 mm day1, rainfall during the same period was 28 mm day1. Thus, our estimates of the contribution of fogwater to the hydrological budget of elfin cloud forests is considerable and higher than in any other location for which comparable data exist but still not a very large component in the hydrological budget. For estimating fogwater fluxes for locations without detailed information about fog droplet distributions we provide simple empirical relationships using visibility data.

Estimating fog deposition at a Puerto Rican elfin cloud forest site: comparison of the water budget and eddy covariance methods

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.

Throughfall in a Puerto Rican lower montane rain forest: A comparison of sampling strategies

Holwerda, F.; Scatena, F.N.; Bruijnzeel, L.A. 2006. Throughfall in a Puerto Rican lower montane rain forest: A comparison of sampling strategies. Journal of Hydrology 327, :592- 602.

Abstract: 
During a one-year period, the variability of throughfall and the standard errors of the means associated with different gauge arrangements were studied in a lower montane rain forest in Puerto Rico. The following gauge arrangements were used: (1) 60 fixed gauges, (2) 30 fixed gauges, and (3) 30 roving gauges. Stemflow was measured on 22 trees of four different species. An ANOVA indicated that mean relative throughfall measured by arrangements 1 (77%), 2 (74%), and 3 (73%) were not significantly different at the 0.05 level. However, the variability of the total throughfall estimate was about half as high for roving gauges (23%) as for fixed gauges (48–49%). The variability of stemflow ranged from 36% to 67% within tree species and was 144% for all sampled trees. Total stemflow was estimated at 4.1% of rainfall, of which palms contributed about 66%. Comparative analysis indicated that while fixed and roving gauge arrangements can give similar mean values, least 100 fixed gauges are required to have an error at the 95% confidence level comparable to that obtained by 30 roving gauges.
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