Hydrometeorology of tropical montane cloud forests: emerging patterns

Bruijnzeel LA, Mulligan M, Scatena FN. 2010. Hydrometeorology of
tropical montane cloud forests: emerging patterns. Hydrological
Processes. DOI: 10.1002/hyp.7974.

altitudinal limits between which TMCF generally occur (800–3500 m.a.s.l. depending on mountain size and distance to coast) their current areal extent is estimated at ¾215 000 km2 or 6Ð6% of all montane tropical forests. Alternatively, on the basis of remotely sensed frequencies of cloud occurrence, fog-affected forest may occupy as much as 2Ð21 Mkm2. Four hydrologically distinct montane forest types may be distinguished, viz. lower montane rain forest below the cloud belt (LMRF), tall lower montane cloud forest (LMCF), upper montane cloud forest (UMCF) of intermediate stature and a group that combines stunted sub-alpine cloud forest (SACF) and ‘elfin’ cloud forest (ECF). Average throughfall to precipitation ratios increase from 0Ð72 š 0Ð07 in LMRF (n D 15) to 0Ð81 š 0Ð11 in LMCF (n D 23), to 1Ð0 š 0Ð27 (n D 18) and 1Ð04 š 0Ð25 (n D 8) in UMCF and SACF–ECF, respectively. Average stemflow fractions increase from LMRF to UMCF and ECF, whereas leaf area index (LAI) and annual evapotranspiration (ET) decrease along the same sequence. Although the data sets for UMCF (n D 3) and ECF (n D 2) are very limited, the ET from UMCF (783 š 112 mm) and ECF (547 š 25 mm) is distinctly lower than that from LMCF (1188 š 239 mm, n D 9) and LMRF (1280 š 72 mm; n D 7). Field-measured annual ‘cloud-water’ interception (CWI) totals determined with the wet-canopy water budget method (WCWB) vary widely between locations and range between 22 and 1990 mm (n D 15). Field measured values also tend to be much larger than modelled amounts of fog interception, particularly at exposed sites. This is thought to reflect a combination of potential model limitations, a mismatch between the scale at which the model was applied (1 ð 1 km) and the scale of the measurements (small plots), as well as the inclusion of near-horizontal wind-driven precipitation in the WCWB-based estimate of CWI. Regional maps of modelled amounts of fog interception across the tropics are presented, showing major spatial variability. Modelled contributions by CWI make up less than 5% of total precipitation in wet areas to more than 75% in low-rainfall areas. Catchment water yields typically increase from LMRF to UMCF and SACF–ECF reflecting concurrent increases in incident precipitation and decreases in evaporative losses. The conversion of LMCF (or LMRF) to pasture likely results in substantial increases in water yield. Changes in water yield after UMCF conversion are probably modest due to trade-offs between concurrent changes in ET and CWI. General circulation model (GCM)-projected rates of climatic drying under SRES greenhouse gas scenarios to the year 2050 are considered to have a profound effect on TMCF hydrological functioning and ecology, although different GCMs produce different and sometimes opposing results. Whilst there have been substantial increases in our understanding of the hydrological processes operating in TMCF, additional research is needed to improve the quantification of occult precipitation inputs (CWI and wind-driven precipitation), and to better understand the hydrological impacts of climate- and land-use change. Copyright  2010 John Wiley & Sons, Ltd.

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.

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.

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.

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.
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