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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 h Ϫ1 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. ᭧ 1999 Elsevier Science B.V. All rights reserved.

Abstract:Various complementary techniques were used to investigate the stormflow generating processes in a small headwater catchment in northeastern Puerto Rico. Over 100 samples were taken of soil matrix water, macropore flow, streamflow and precipitation, mainly during two storms of contrasting magnitude, for the analysis of calcium, magnesium, silicon, potassium, sodium and chloride. These were combined with hydrometric information on streamflow, return flow, precipitation, throughfall and soil moisture to distinguish water following different flow paths. Geo-electric sounding was used to survey the subsurface structure of the catchment, revealing a weathering front that coincided with the elevation of the stream channel instead of running parallel to surface topography. The hydrometric data were used in combination with soil physical data, a one-dimensional soil water model (VAMPS) and a three-component chemical mass-balance mixing model to describe the stormflow response of the catchment. It is inferred that most stormflow travelled through macropores in the top 20 cm of the soil profile. During a large event, saturation overland flow also accounted for a considerable portion of the stormflow, although it was not possible to quantify the associated volume fully. Although the mass-balance mixing model approach gave valuable information about the various flow paths within the catchment, it was not possible to distill the full picture from the model alone; additional hydrometric and soil physical evidence was needed to aid in the interpretation of the model results.

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It is becoming increasingly recognized that the cloud forests of tropical mountains are high on the list of the world's most threatened ecosystems. In many countries or regions, their rate of loss exceeds that of the lowland tropical rain forests that have received much public concern. Perhaps 90 percent of these tropical montane cloud forests (TMCFs) in the northern Andes have been lost. During the symposium, for which this publication is the proceedings, visual evidence presented by Dr. Alwyn Gentry and others, and the verbal descriptions given by many participants, attested to the loss not only for the Andes, but for all parts of the world where TMCF occurs. The original estimate (and it was only an estimate) given by Persson (1974) of 50 million ha of cloud forests was probably on the high side. He suggested that TMCF constituted one-quarter of montane and sub montane rain forest. We believe that TMCF is being lost at a rate considerably in excess of that of the muchabused, but much-publicized, lowland tropical rain forest. Indeed the first paper (Doumenge et al.) that follows indicates that annual forest loss in tropical hills and mountains is 1.1 percent compared with 0.8 percent for all forests of the tropics. Those on isolated volcanic mountains, especially where occurring on small oceanic islands, are particularly vulnerable, and their loss is a serious concern. Cutting of forest for firewood or for charcoal production, and conversion to agricultural uses (mainly grazing) are the principal villains. However, the threats and damaging activities are many. They will be discussed in this synthesis and in many of the papers that follow. L. S. Hamilton et al. (eds.), Tropical Montane Cloud Forests