The hydrology and geochemistry of groundwater in tropical island aquifers, such as Barbados, Guam and Puerto Rico, are significantly influenced by tropical climatic conditions. Recharge to these aquifers is the product of regional and local climate patterns that control rainfall. Oxygen isotopes can be used to estimate the amount and timing of recharge on these islands because seasonal fluctuations of rainwater oxygen isotopic compositions are related to the amount of rainfall. The karst aquifers on Barbados, Guam and Puerto Rico have similar rainwater and groundwater oxygen isotopic compositions. Comparison of groundwater and rainwater oxygen isotopic compositions in the three aquifers indicates that: (1) recharge occurs by rapid infiltration with little evaporation prior to recharge; and (2) recharge is associated with similar monthly rainfall thresholds of 190–200 mm. These rainfall thresholds are remarkably similar for three aquifers in different geographic locations. Differences between the spatial variations of groundwater oxygen isotopic compositions on Barbados and Puerto Rico can be attributed to the more complex groundwater flow system on Puerto Rico. The surprising similarities of hydrologic conditions under which recharge will take place can be attributed to similarities in climate and geologic conditions, such as soils and limestone bedrock, that exist on the three islands. We therefore speculate that similar recharge-rainfall thresholds may be observed in other tropical karst aquifers.

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).

Water is a natural resource important for life, growth and development of cities and towns. It's "Not just the most basic of necessities it's also the basis of sustainable development." The spatial and temporal distributions varies in such a way that its in abudnant in some regions and times while is is vary limited in others. While it is considers a renewable resource it is no less correct that improper management can lead to a reduced quantity and quality of available water. It's also the case that contamination can also render the resource unusable. For these reasons the availability, quality, and management of water resources represents a grand challenge for Puerto Rico and most of the world. El agua es un recurso natural de vital importancia para la vida, crecimiento y desarrollo de los pueblos. Es “[n]o sólo la más básica de las necesidades, sino también el núcleo del desarrollo sostenible”1. Su distribución espacial y temporal varía de forma que, mientras es abundante en unas regiones o épocas, en otras es muy limitado. A pesar de que se considera un recurso renovable no es menos cierto que su manejo inapropiado, puede tener el efecto de reducir la cantidad disponible y utilizable del recurso. De igual forma, la contaminación irreversible del recurso puede convertirlo en uno agotable. Es por ello que la disponibilidad, calidad y manejo adecuado del recurso representa un gran desafío para Puerto Rico y la mayor parte del mundo.