Cloud Water Interception Research Articles

THEORETICAL AND METHODOLOGICAL ASPECTS OF CLOUD WATER INTERCEPTION

Cloud water interception (CWI) occurs when water contained in fog and wind-driven rain collides with vegetation, merges into larger droplets, and precipitates to the ground. CWI has an important function as an additional source of water and its relationships with tropical cloud forests have often been emphasized. Despite its importance, there is no standardization of measurement methods, nor of the terms that designate the process in Portuguese. Therefore, a systematic analysis of research on CWI is necessary. To this end, the present study carried out a review of the theoretical and methodological aspects of CWI through description and analysis of terminology; history and chronology of studies on the topic; survey of the environmental conditions necessary for the CWI process to occur; analysis of methodological aspects relating to the measurement of CWI; and synthesis and discussion of magnitudes described in scientific literature. As a result, of the 31 publications reviewed, 14 different words were found, the most common being “Cloud Water Interception” (19.4%) and “Fog Drip” (16.1%). In general, CWI is more common in places such as continental edges and islands that are constantly subject to sea breezes. In most cases, the below-canopy measurement approach can be considered more accurate than those obtained by fog collectors. CWI is on average responsible for 42% of effective precipitation (n:41). The values listed show a large variation, between 0.5% and 462%, probably due to the different environmental characteristics of the sampled locations as well as variations in sample sizes.

Quantifying rainfall and cloud water interception in upland forests of Norfolk Island

AbstractThe higher elevation (>200 m ASL) forests of Norfolk Island are regularly immersed in the clouds and scientific and anecdotal evidence suggests that in addition to rainfall, water is likely to be collected as cloud droplets are intercepted by the forest canopy. This water is likely to be important for the local hydrology and ecology, yet it has never been quantified. To address this, a field measurement campaign was established to measure hydrological inputs to the forest floor at two elevated (290 and 310 m ASL) forest sites in the Norfolk Island National Park over a 524‐day period. Instrumentation included throughfall and stemflow measurement systems and recording rain gauges in the open in nearby clearings. Sites exhibited very high stem density and basal area and delivery of water to the forest floor was dominated by stemflow because of the funnelling characteristics of the dominant palm and pine trees. Both sites showed similar hydrological behaviour with stemflow and throughfall of around 48% and 32% of total atmospheric inputs, respectively. Stemflow contributions of 48% far exceed observations from the literature on cloud‐affected ecosystems which are typically less than 10%. Rainfall rarely occurred in the absence of low‐level cloud and some cloud immersion events lasted for many days with hydrologic inputs continuing for extended periods despite rainfall not being observed in the open. Cloud water interception accounted for approximately 20% of total water input at both sites which is equivalent to 25% extra water on top of rainfall measured in the open. From an island‐wide perspective, the calculated additional hydrological input is only small due to the limited spatial extent of upland forest; however, the additional water is likely to be very important to local hydrological processes and the unique plants, insects and animals which inhabit the upland forests of Norfolk Island.

Fog harvesting potential for domestic rural use and irrigation in San Cristobal Island, Galapagos, Ecuador

The Galapagos Archipelago is a semiarid zone scarce of superficial water resources. Due to its geographic location and the presence of oceanic currents, these islands are covered by fog during seven months along the year which could represent a potential source of water supply. In this study, the feasibility of fog collection for domestic rural use and irrigation was investigated using two Standard Fog Collectors (SFC) of 50% and 35% shade coefficient of 1 m2 and a cylindrical fog net (CFN) of 0.15 m2 at 600 m of elevation in the windward side of San Cristobal Island (Galapagos). The methodology applied consists in the quantification of the monthly and annual water balance at different altitudes in an average and dry years, using the orographic gradients of rainfall, evapotranspiration and cloud water interception. The latter was used to estimate the fog water availability to satisfy any water deficit. Fog water interception was estimated using a geometric model and the use of different climatic variables. On the other hand, water demand in the rural zone for domestic consumption and livestock was estimated with the quota method, and the water demand for agriculture was estimated through the water balance. Results show that the amount of fog captured reaches 7.9, 5.9 and 3.4 mm/d with 50-SFC, 35-SFC and CFN, respectively. Comparing these results with other locations in the world, fog water collection in Galapagos is above the average. A fog gauge system of 50-SFC was designed at different elevations to cover the 25% and 15% of the water deficit in the average and dry year, respectively.

Quantification of cloud water interception in the canopy vegetation from fog gauge measurements

AbstractWith changes in climate looming, quantifying often‐overlooked components of the canopy water budget, such as cloud water interception (CWI), is increasingly important. Commonly, CWI quantification requires detailed continuous measurements, which is extremely challenging, especially when throughfall is included. In this study, we propose a simplified approach to estimate CWI using the Rutter‐type interception model, where CWI inputs in the canopy vegetation are proportional to fog interception measured by an artificial fog gauge. The model requires the continuous acquisition of meteorological variables as input and calibration datasets. Throughfall measurements below the forest are used only for calibration and validation of the model; thus, CWI estimates can be provided even after the cessation of throughfall monitoring. This approach provides an indirect and undemanding way to quantify CWI by vegetation and allows the identification of its controlling factors, which could be useful to the comparison of CWI in contrasting land covers. The method is applied on a 2‐year dataset collected in an endemic highland forest of San Cristobal Island (Galapagos). Our results show that CWI reaches 21% ± 6% of the total water input during the first year, and 9% ± 2% during the second one. These values represent 32% ± 10% and 17% ± 5% of water inputs during the cool foggy season of the first and second year, respectively. The difference between seasons is attributed to a lower fog liquid water during the second season.

The role of epiphytic bryophytes in interception, storage, and the regulated release of atmospheric moisture in a tropical montane cloud forest

Epiphytes in tropical montane cloud forests (TMCF) intercept atmospheric water and, as a result, form a vital part of the hydrological cycle of this ecosystem. Our study investigates the role of bryophytes in such systems on La Réunion Island (Mascarenes). To better understand ecohydrological functioning of the forest, we investigated cloud water interception (CWI) by two locally abundant liverwort species (Bazzania decrescens and Mastigophora diclados) using a novel lysimetric approach. We also evaluated biomass and water storage capacity of our study species, as well as of the entire bryophyte community in our plots, which we extrapolated to the forest community level. Both study species exhibited excellent abilities to intercept and store cloud water, and showed distinct diurnal variation in this ability according to varying climatic conditions. The two liverwort species’ response to climatic conditions differed dramatically from one another, with B. decrescens storing double the mean and maximum litres of water per hectare despite having less than half the abundance of M. diclados. Despite its lower water storage capacity, M. diclados had a greater ability to intercept atmospheric moisture than B. decrescens. The differences in CWI were attributed to differences in plant structure of these two species, which explains their microhabitat requirements in this system. Our two species in this system were estimated to store 34,569 l.ha−1 of water, the equivalent of 3.46mm of rainfall. The abundance of our study species combined with their atmospheric water interception, storage, and regulated release ability make both species ecologically important in the forest’s microhydrological cycle. For the first time these data allow us to better understand the role of these plants in the microhydrological cycle of tropical montane cloud forests and to determine whether the diversity and functioning of these and similar systems will be at risk from predicted cloud layer/coverage lifting.

Adjustment of global precipitation data for enhanced hydrologic modeling of tropical Andean watersheds

Global gridded precipitation is an essential driving input for hydrologic models to simulate runoff dynamics in large river basins. However, the data often fail to adequately represent precipitation variability in mountainous regions due to orographic effects and sparse and highly uncertain gauge data. Water balance simulations in tropical montane regions covered by cloud forests are especially challenging because of the additional water input from cloud water interception. The ISI-MIP2 hydrologic model ensemble encountered these problems for Andean sub-basins of the Upper Amazon Basin, where all models significantly underestimated observed runoff. In this paper, we propose simple yet plausible ways to adjust global precipitation data provided by WFDEI, the WATCH Forcing Data methodology applied to ERA-Interim reanalysis, for tropical montane watersheds. The modifications were based on plausible reasoning and freely available tropics-wide data: (i) a high-resolution climatology of the Tropical Rainfall Measuring Mission (TRMM) and (ii) the percentage of tropical montane cloud forest cover. Using the modified precipitation data, runoff predictions significantly improved for all hydrologic models considered. The precipitation adjustment methods presented here have the potential to enhance other global precipitation products for hydrologic model applications in the Upper Amazon Basin as well as in other tropical montane watersheds.

Using stable isotopes to characterize groundwater recharge sources in the volcanic island of Madeira, Portugal

Summary The hydrogeology of volcanic islands remains poorly understood, despite the fact that populations that live on them rely on groundwater as a primary water source. This situation is exacerbated by their complex structure, geological heterogeneity, and sometimes active volcanic processes that hamper easy analysis of their hydrogeological dynamics. Stable isotope analysis is a powerful tool that has been used to assess groundwater dynamics in complex terrains. In this work, stable isotopes are used to better understand the hydrogeology of Madeira Island and provide a case-study that can serve as a basis for groundwater studies in other similar settings. The stable isotopic composition (δ 18 O and δ 2 H) of rain at the main recharge areas of the island is determined, as well as the sources and altitudes of recharge of several springs, groundwater in tunnels and wells. The water in tunnels was found to be recharged almost exclusively by rain in the deforested high plateaus, whilst several springs associated with shallow perched aquifers are recharged from rain and cloud water interception by the vegetated slopes. Nevertheless some springs thought to be sourced from deep perched aquifers, recharge in the central plateaus, and their isotopic composition is similar to the water in the tunnels. Recharge occurs primarily during autumn and winter, as evidenced by the springs and tunnels Water Lines (WL). The groundwater in wells appears to originate from runoff from rain that falls along the slopes that infiltrates near the streams’ mouths, where the wells are located. This is evident by the evaporation line along which the wells plot. Irrigation water is also a possible source of recharge. The data is compatible with the hydrogeological conceptual model of Madeira. This work also shows the importance of cloud water interception as a net contributor to groundwater recharge, at least in the perched aquifers that feed numerous springs. As the amount of rainfall is expected to decrease until the end of the century and water supply to become scarcer, cloud water interception might become an increasingly important aspect of Madeira Island hydrology.

Life in the clouds: are tropical montane cloud forests responding to changes in climate?

The humid tropics represent only one example of the many places worldwide where anthropogenic disturbance and climate change are quickly affecting the feedbacks between water and trees. In this article, we address the need for a more long-term perspective on the effects of climate change on tropical montane cloud forests (TMCF) in order to fully assess the combined vulnerability and long-term response of tropical trees to changes in precipitation regimes, including cloud immersion. We first review the ecophysiological benefits that cloud water interception offers to trees in TMCF and then examine current climatological evidence that suggests changes in cloud base height and impending changes in cloud immersion for TMCF. Finally, we propose an experimental approach to examine the long-term dynamics of tropical trees in TMCF in response to environmental conditions on decade-to-century time scales. This information is important to assess the vulnerability and long-term response of TMCF to changes in cloud cover and fog frequency and duration.

Cloud water interception and element deposition differ largely between Norway spruce stands along an elevation transect in Harz Mountains, Germany

AbstractCloud deposition can contribute a significant amount of water and elements to montane forests and thus play an important role in the hydrological and biochemical cycles of these ecosystems. However, elevational pattern in cloud and throughfall input to forest ecosystems and the associated chemical element fluxes are poorly understood so far. We studied gross precipitation, throughfall and cloud water deposition, and the associated fluxes of elements (N, P, Ca, K, Mg and Na) in five mature stands of Norway spruce along an elevation transect from lowlands to the alpine treeline (420–1060 m) at Mt Brocken (Harz Mountains, Germany). We tested the hypotheses that water input increases with elevation despite a reduction in intercepting plant surfaces, leading to higher inputs of nitrogen and other elements to high‐elevation forests than in lower‐elevation stands. Gross precipitation (only snow‐free period) rose by 75% towards the treeline, but water input with throughfall increased by 350% because of a large increase in cloud water deposition (from 2 mm at 420 m to 160–200 mm at 1000 m in the study period). Element fluxes with gross precipitation were not uniformly related to elevation across all six elements. However, element fluxes via throughfall showed a positive correlation with elevation for all elements. Thus, the upper montane spruce forests received ~50–150% higher atmospheric inputs of base cations and inorganic N and ~200% higher P inputs than lower‐elevation forests. We conclude that elevation does have a large influence on the atmospheric water and element inputs in temperate mountain forests. Copyright © 2014 John Wiley & Sons, Ltd.

Stable isotopes in rain and cloud water in Madeira: contribution for the hydrogeologic framework of a volcanic island

Madeira is a volcanic island whose economy is highly dependent on groundwater. Previous studies suggest that, besides precipitation, cloud water interception contributes to groundwater recharge. As such, the isotopic characterization of the sources of recharge is useful for the hydrogeological framework of the island. The δ 18O and δ 2H content of rain and cloud water was analyzed during three campaigns. Rain plots over a local meteoric water line (LMWL) whose equation is δ 2H = 7.92 δ 18O + 10.49 (R 2 = 0.97, n = 43). The volume-weighted average meteoric line (WLMWL) is given by δ 2H = 7.97 δ 18O + 10.57 (R 2 = 0.98, n = 20). Both are similar to the global meteoric water line which indicates, along with the d-excess values (10.8 and 11 ‰ for the LMWL and WLMWL, respectively), that rain originates, essentially in the Atlantic. Seasonal variations were also observed and rain in Campaign II was isotopically enriched and had a higher d-excess (δ 18Omean −3.63 ‰; δ 2Hmean −16.3 ‰; d-excess 12.7 ‰), than rain in Campaign III (δ 18Omean −7.01 ‰; δ 2Hmean −46.5 ‰; d-excess 9.5 ‰). Campaign I stood in between. These variations were probably the result of differences in temperature, water vapor source areas and rain forming processes. Rain was found to become increasingly depleted with altitude (δ 18O), at a rate of −0.15 ‰/100 m and −0.11 ‰/100 m in the windward and leeward sides, respectively. Cloud water was always isotopically enriched when compared with rain at the same altitude. This was related to constant water vapor replenishment of orographic clouds, to the fact that it usually represents an early stage condensation from air moisture and to the smaller size of cloud water droplets when compared to rain droplets. These results will be useful to refine Madeira’s hydrogeological conceptual model. Future extended sampling during more years, with smaller sampling periods and single rainfall events should be useful to support the conclusions of this study.

Suppression of transpiration due to cloud immersion in a seasonally dry Mexican weeping pine plantation

Cloud immersion affects the water budget of fog-affected forests not only by introducing an additional source of water (via cloud water interception by the canopy), but also by suppressing plant transpiration. The latter effect is often overlooked and not routinely quantified, restricting a complete understanding of the net hydrological effect of cloud immersion and the possible consequences of projected reductions in cloud immersion under drier and warmer climates in tropical montane regions in the coming decades. This paper describes an approach to quantify the suppression of stand-level tree transpiration (Et) due to cloud immersion using measurements of sapflow, fog occurrence (visibility), leaf wetness, and near-surface climate. Estimates of fog-induced Et suppression in a 10-year-old Pinus patula plantation in the montane cloud belt of central Veracruz, Mexico, are presented for two contrasting dry seasons and a wet season. Fog occurred for 32% of the total study period, although showing pronounced seasonal variation (e.g. 44% during the second dry season). When fog occurred it was accompanied by rainfall during three quarters of the total time. Although the canopy was wet for almost a third of the time, fog-induced canopy wetness constituted only a very small portion of this total (2%). Relative to sunny conditions, Et was suppressed by 90±7% under conditions of dense fog versus 83±7% under light fog and 78±10% during overcast conditions. Quantification of the potential change in annual Et associated with two scenarios for future cloud immersion at the study site revealed that: (i) when all fog occurrence is replaced by overcast conditions, mean annual Et (645±50mm) is likely to increase by only 2±1%; and (ii) when sunny conditions replace all foggy conditions, the likely increase in annual Et is 17±3%. As the rise in the regional lifting condensation level is likely to be on the order of only a couple of hundred meters and will probably result in a shift to overcast rather than clear-sky conditions, the present results suggest that the corresponding impact on Et may be relatively small. Consequently, a climate change-related reduction in dry-season precipitation, through the associated potential reductions in soil water reserves, presents a more worrisome prospect for plant–water relations and water yield from headwater catchments than diminishing cloud immersion alone. The present results highlight the need for better projections of climate change-related alterations in cloud cover and immersion, as well as rainfall patterns for tropical montane regions.

Cloud water interception in the temperate laurel forest of Madeira Island

A cloud belt frequently forms on the windward side of Madeira Island, between 800 and 1600 m a.s.l., as a result of adiabatic cooling of the northeastern trade winds that are forced upward. Temperate laurel forest is the most common vegetation inside that cloud belt altitudinal range. Cloud water interception was estimated by comparing precipitation and throughfall during a hydrological year. It totalled 200 mm (8% of rainfall) during 65 days (3 mm d−1) and seems to constitute a larger fraction of water input during drier months. Multiple linear regression between gauge standard deviation and throughfall throughout rain events shows that cloud interception is common before the onset of rainfall. Its role in the ecohydrology of laurel forest and in the island's hydrology should be acknowledged. Further studies on this issue should be a priority in order to better understand these dynamics and provide tools for the correct management of this protected forest and the island's groundwater resources. Editor Z.W. Kundzewicz Citation Figueira, C., Menezes de Sequeira, M., Vasconcelos, R., and Prada, S., 2013. Cloud water interception in the temperate laurel forest of Madeira Island. Hydrological Sciences Journal, 58 (1), 1–10.

Quantification of cloud water interception along the windward slope of Santa Cruz Island, Galapagos (Ecuador)

The Galapagos Archipelago is nearly devoid of freshwater resources, but during six months of the year, a fog layer covers the windward slopes of the main islands. In order to investigate the hydrological importance of this phenomenon, a monitoring network was set up on Santa Cruz Island, at the center of the archipelago. Meteorological parameters were monitored together with throughfall and stemflow at two stations: a first in a secondary forest at the lowest fringe of the fog layer (400ma.s.l.), and a second in shrub lands of the Galapagos National Park, at the center of the fog layer (650ma.s.l.). Cloud water interception was quantified from the wet canopy water budget, based on a modified Rutter-type canopy interception model. This methodology allowed the estimation of fog interception for short time intervals (15min) and avoided the subjective separation into individual rainfall events. Fog was found to be a negligible water input at the lower site, but was estimated at 26±16% of incident rainfall at the higher site. Wind was shown to enhance fog interception, but this alone could not explain the difference in fog catch between the two sites. Higher liquid water content and more frequent fog occurrence contributed to the difference as well. This study highlights that the presence of fog may induce a marked increase of net precipitation, but this effect is restricted to the summit areas exposed to winds, located in the center of the cloud belt.

Dew, where and when? ‘There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy …’

Dew, where and when? ‘There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy …’

Land cover effects on groundwater recharge in the tropics: ecohydrologic mechanisms

ABSTRACTManaging groundwater recharge to ensure sufficient water supply is a growing concern for many communities. We explore land cover effects on recharge processes and rate in Hawai'i, where precipitation on the mountain slopes of leeward Hawai'i Island recharges the aquifer that is the sole source of water for coastal communities. Applying a water balance, we quantify the influences of native forest and cattle pasture on recharge using measurements of precipitation and the drivers of evapotranspiration taken over 18 months. Surface runoff is negligible in these highly permeable basalt watersheds, simplifying fluxes into and out of the recharge area. On the basis of our analysis of measured fluxes, groundwater recharge is 96% of rainfall in pasture, 87% of above‐canopy rainfall in open forest, and 106% of above‐canopy rainfall in dense forest. Differences in recharge among vegetation types result largely from direct interception of cloud water by native Hawaiian forest. The majority of rainfall occurs in infrequent, large storms, so water moves quickly beyond plant rooting depth, limiting its availability for evapotranspiration. Potential evapotranspiration is low, as its drivers are modest and constant throughout the year. Existing estimates of submarine groundwater discharge support our conclusion that the rainfall‐to‐recharge ratio is close to one for all sites. Identifying primary influences on the recharge‐to‐rainfall ratio is key to predicting the types of changes that will have substantial effects on recharge. In any tropical setting with young, porous substrates, precipitation is likely to play a dominant role in the magnitude and variability of groundwater recharge; here, land‐use changes that affect local precipitation, such as an increase in fog interception, will play a larger role than vegetation shifts that affect evapotranspiration. Copyright © 2011 John Wiley & Sons, Ltd.

Water balances of old-growth and regenerating montane cloud forests in central Veracruz, Mexico

► We compare the catchment water budgets of old-growth and regenerating cloud forests. ► Water use was higher in the mature forest mainly because of greater water canopy storage capacity. ► Streamflow was greater in the regenerating forest due to lower rainfall interception. ► Storm runoff was low in both forest attributed to the high permeability of the soils. ► We conclude that 20 years of forest growth is sufficient to restore the original hydrology. This paper compares the water budgets of two adjacent micro-catchments covered by mature (MAT) and 20-year-old secondary (SEC) lower montane cloud forests, respectively, in central Veracruz, Mexico over a 2-year period. Rainfall ( P ) and streamflow ( Q ) were measured continuously, whereas dry canopy evaporation (transpiration E t ), wet canopy evaporation (rainfall interception I ), and cloud water interception ( CWI ) were quantified using a combination of field measurements and modeling. Mean annual P was 3467 mm, of which typically 80% fell during the wet season (May–October). Fog interception occurred exclusively during the dry season (November–April), and was ⩽2% of annual P for both forests. Rainfall interception loss was dominated by post-event evaporation of intercepted water rather than by within-event evaporation. Therefore, the higher overall I (i.e. including CWI ) by the MAT (16% of P vs. 8% for the SEC) reflects a higher canopy storage capacity, related in turn to higher leaf area index and greater epiphyte biomass. Annual E t totals derived from sapflow measurements were nearly equal for the MAT and SEC (∼790 mm each). Total annual water yield calculated as P minus ( E t + I ) was somewhat higher for the SEC (2441 mm) than for the MAT (2077 mm), and mainly reflects the difference in I . Mean annual Q was also higher for the SEC (1527 mm) than for the MAT (1338 mm), and consisted mostly of baseflow (∼90%). Baseflow recession rates were nearly equal between the two forests, as were stormflow coefficients (4% and 5% for MAT and SEC, respectively). The very low runoff response to rainfall is attributed to the high infiltration and water retention capacities of the volcanic soils throughout the ∼2 m deep profile. The water budget results indicate that ∼875 and 700 mm year −1 leave the SEC and MAT as deep groundwater leakage, which is considered plausible given the fractured geology in the study area. It is concluded that 20 years of natural regeneration following cloud forest disturbance in central-eastern Mexico is capable of producing near-original hydrological behavior.

Hydrometeorology of tropical montane cloud forests: emerging patterns

AbstractTropical montane cloud forests (TMCF) typically experience conditions of frequent to persistent fog. On the basis of the 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 ∼215 000 km2or 6·6% of all montane tropical forests. Alternatively, on the basis of remotely sensed frequencies of cloud occurrence, fog‐affected forest may occupy as much as 2·21 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 0·72 ± 0·07 in LMRF (n= 15) to 0·81 ± 0·11 in LMCF (n= 23), to 1·0 ± 0·27 (n= 18) and 1·04 ± 0·25 (n= 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= 3) and ECF (n= 2) are very limited, the ET from UMCF (783 ± 112 mm) and ECF (547 ± 25 mm) is distinctly lower than that from LMCF (1188 ± 239 mm,n= 9) and LMRF (1280 ± 72 mm;n= 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= 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 × 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.

Canopy water balance of windward and leeward Hawaiian cloud forests on Haleakalā, Maui, Hawai'i

AbstractThe contribution of intercepted cloud water to precipitation at windward and leeward cloud forest sites on the slopes of Haleakalā, Maui was assessed using two approaches. Canopy water balance estimates based on meteorological monitoring were compared with interpretations of fog screen measurements collected over a 2‐year period at each location. The annual incident rainfall was 973 mm at the leeward site (Auwahi) and 2550 mm at the windward site (Waikamoi). At the leeward, dry forest site, throughfall was less than rainfall (87%), and, at the windward, wet forest site, throughfall exceeded rainfall (122%). Cloud water interception estimated from canopy water balance was 166 mm year−1 at Auwahi and 1212 mm year−1 at Waikamoi. Annual fog screen measurements of cloud water flux, corrected for wind‐blown rainfall, were 132 and 3017 mm for the dry and wet sites respectively. Event totals of cloud water flux based on fog screen measurements were poorly correlated with event cloud water interception totals derived from the canopy water balance. Hence, the use of fixed planar fog screens to estimate cloud water interception is not recommended. At the wet windward site, cloud water interception made up 32% of the total precipitation, adding to the already substantial amount of rainfall. At the leeward dry site, cloud water interception was 15% of the total precipitation. Vegetation at the dry site, where trees are more exposed and isolated, was more efficient at intercepting the available cloud water than at the rainy site, but events were less frequent, shorter in duration and lower in intensity. A large proportion of intercepted cloud water, 74% and 83%, respectively for the two sites, was estimated to become throughfall, thus adding significantly to soil water at both sites. Published in 2010 by John Wiley & Sons, Ltd.

Rainfall partitioning and cloud water interception in native forest and invaded forest in Hawai'i Volcanoes National Park

AbstractIn many Hawaiian forests, including cloud forests, native plant communities are being displaced by invasive tree species, possibly affecting rainfall partitioning and direct harvesting of cloud droplets by vegetation. In this study, the hydrological impacts of invasive species are examined, using measurements of rainfall (RF), throughfall (TF) and stemflow (SF), and estimation of wet‐canopy evaporation and cloud water interception (CWI) by the canopy water balance approach in both native Metrosideros polymorpha‐dominated and invaded, Psidium cattleianum‐dominated forests within Hawai'i Volcanoes National Park (HAVO). Canopy water storage capacity was found to be more than twice as great at the native site (1·86 mm) compared to the invaded site (0·85 mm). Annual RF, CWI, TF and SF were 3233, 1188, 3700 and 261 mm, respectively, for the native site; and 3735, 734, 3033 and 1091 mm, respectively, for the invaded site. Net RF (TF + SF) was 123 and 110% of RF, respectively, at the two sites. Annual evaporation of water from the wet canopy was also greater at the native site (464 mm) than at the invaded site (347 mm). Low canopy water storage capacity and the exceptionally high SF total at the invaded site are related to morphological characteristics and high stem density of the invasive P. cattleianum tree, which favour efficient transport of intercepted water to the ground via the stems. Despite its more peripheral location near the edge of the orographic cloud, CWI at the native site was higher. The characteristics of the native M. polymorpha tree may facilitate more effective harvesting of cloud water droplets, enhancing CWI at the site. Species invasion results in a lower proportion of RF reaching the forest floor (110 vs 123%) and becoming available for groundwater recharge, suggesting that invasion by P. cattleianum may have significant negative effects on Hawai'i's aquatic ecosystems and water resources. Copyright © 2010 John Wiley & Sons, Ltd.

A 50th anniversary reassessment of the seminal ‘L ana'i fog drip study’ in Hawai'i

AbstractDuring the 1950s, the Pineapple Research Institute (PRI) in Hawai'i launched one of the first, and still widely cited, quantitative studies of direct cloud water interception (CWI) and its contribution to throughfall (TF) (‘fog drip’) under Cook Pine trees (Araucaria columnaris) on a trade‐wind exposed, mountain ridge on the island of Lāna'i. That study concluded that fog drip under a tree canopy supplemented rainfall (RF) (average 1270 mm year−1) by 762 mm or 60%, even after throughfall was corrected for presumed non‐vertical rainfall also intercepted by the tree. A resurvey of fog drip in the upper Lāna'i watershed was undertaken in 2006–2008. Open‐site automated weather stations were paired with throughfall, stemflow and soil moisture measurements for adjacent Araucaria trees. For upper ridge sites, these results indicate fog drip rates beneath individual Araucaria trees to be substantially greater than found in the earlier study. Fully exposed trees in the frequently cloud‐immersed summit region collected annualized fog drip totals of 901–2883 mm, representing 181–462% of measured rainfall. The increased fog drip may be largely attributed to: (i) a more robust estimate of the indirect (non‐vertical) rainfall contribution; (ii) the increased fog‐capturing capacity of the Araucaria trees which had a height of ∼10 m in the 1950s versus ∼20 m during this study and (iii) the presence of atmospheric conditions favouring much increased CWI at higher ridge elevations (921–1021 m.a.s.l.) compared to the site of the original PRI study at 838 m. The fog drip values reported here are (much) higher than those measured in most other tropical montane cloud forest studies, and reflect the unique site conditions associated with fully exposed isolated trees on a knife‐edged ridge. Copyright © 2010 John Wiley & Sons, Ltd.