Evaporation losses from a watertight 6.34 ha rain forest catchment under wet maritime tropical conditions in the Luquillo Experimental Forest, Puerto Rico, were determined using complementary hydrological and micrometeorological techniques during 1996 and 1997. At 6.6 mm d−1 for 1996 and 6.0 mm d−1 for 1997, the average evapotranspiration (ET) of the forest is exceptionally high. Rainfall interception (Ei), as evaluated from weekly throughfall measurements and an average stemflow fraction of 2.3%, accounted for much (62–74%) of the ET at 4.9 mm d−1 in 1996 and 3.7 mm d−1 in 1997. Average transpiration rates (Et) according to a combination of the temperature fluctuation method and the Penman-Monteith equation were modest at 2.2 mm d−1 and 2.4 mm d−1 in 1996 and 1997, respectively. Both estimates compared reasonably well with the water-budget-based estimates (ET − Ei) of 1.7 mm d−1 and 2.2 mm d−1. Inferred rates of wet canopy evaporation were roughly 4 to 5 times those predicted by the Penman-Monteith equation, with nighttime rates very similar to daytime rates, suggesting radiant energy is not the dominant controlling factor. A combination of advected energy from the nearby Atlantic Ocean, low aerodynamic resistance, plus frequent low-intensity rain is thought to be the most likely explanation of the observed discrepancy between measured and estimated Ei.

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.

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.

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 location—the Luquillo Experimental Forest in eastern Puerto Rico—66 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 Penman–Monteith 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.