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Dew has been recognized as an important source of water for many coastal arid and semiarid ecosystems because of its contributions to daily, seasonal and annual water balance. We measured the frequency, duration and amount of dewfall from January 2011 to December 2016 to assess its contribution to the local water balance in a continental semiarid grassland where Bouteloua gracilis (Blue grama) is the key grass species. Dew was observed with leaf wetness sensors and amounts were estimated with an energy model using micrometeorological data. Dewfall occurred on 39% of the nights with an average duration of 5 ± 4 h. Daily average dewfall deposition during the study period was 0.2 mm with the highest accumulation of 0.7 mm. Annual dewfall ranged between 16.5 and 69 mm, therefore representing between 4.9% and 10.2% of annual precipitation. Dewfall contributed between 7.6% and 33.6% of the total dry season precipitation (winter/spring), which considerably reduced the water deficit. Our results highlight the relevance of dewfall as a consistent and important water source in this semiarid grassland ecosystem. It is of particular importance during dry periods, where it may represent the only water source available to the vegetation at this site.

Tropical montane cloud forests (TMCF) are threatened ecosystems despite their capacity to maintain high dry-season baseflow. A number of conservation policies, including payments for hydrological services, have been implemented to protect these forests. However, since most of the modeling tools used to assess the impacts of these policies were developed for temperate zones, more work is needed to understand and improve the applicability of popular models in tropical contexts. This study uses local evapotranspiration and streamflow datasets to calibrate and validate an improved version of the Soil and Water Assessment Tool model for the Tropics (SWAT-T). Vegetation growth and canopy water storage capacity were calibrated using field data. Three methods provided by SWAT-T to calculate potential evapotranspiration (PET) were compared: Penman-Monteith (SWAT-T-PM), Hargreaves (SWAT-T-HA), and Priestly-Taylor (SWAT-T-PT). Sensitivity analysis and calibration of daily streamflow were conducted at the catchment scale (34 km2). Furthermore, the calibrated models were validated at three sites with evapotranspiration data, and at four distinct micro-catchments (0.137–0.446 km2) with gauged streamflow data. Overall, SWAT-T satisfactorily simulated streamflow during the calibration period producing acceptable goodness of fit indices. However, the model incorrectly predicted the dominance of lateral flow instead of the deep groundwater flow observed from isotope-based studies. SWAT-T-HA performed better than SWAT-T-PM and SWAT-T-PT, but all models underestimated the influence of rainfall interception losses since evaporation is limited by daily PET in forests. Finally, SWAT-T largely over- and underestimated mean annual daily low flow in pastures and forests, respectively. Taken together, these results indicate that improvements in the parametrization of rainfall interception and deep subsurface flow dynamics in SWAT-T are required to improve applicability of this modeling tool in tropical montane areas underlain by permeable substrates.

Stemflow (Sf) measurements in tropical rain and montane forests dominated by large trees rarely include the understory and small trees. In this study, contributions of lower (1- to 2-m height) and upper (>2-m height and 10-cm DBH) to Sf per unit ground area (Sfa) of a Mexican lower montane cloud forest were quantified for 32 days with rainfall (P) during the 2014 wet season. Rainfall, stemflow yield (Sfy), vegetation height, density, and basal area were measured. Subsequently, stemflow funneling ratios (SFRs) were calculated, and three common methods to scale up Sfy from individual trees to the stand level (tree-Sfy correlation, P-Sfy correlation, and mean-Sfy extrapolation) were used to calculate Sfa. Understory woody plants, small trees, and upper canopy trees represented 96%, 2%, and 2%, respectively, of the total density. Upper canopy trees had the lowest SFRs (1.6 ± 0.5 Standard Error (SE) on average), although the lower understory had the highest (36.1 ± 6.4). Small trees and upper understory presented similar SFRs (22.9 ± 5.4 and 20.2 ± 3.9, respectively). Different Sf scaling methods generally yielded similar results. Overall Sfa during the study period was 22.7 mm (4.5% of rainfall), to which the understory contributed 70.1% (15.9 mm), small trees 10.6% (2.4 mm), and upper canopy trees 19.3% (4.4 mm). Our results strongly suggest that for humid tropical forests with dense understory of woody plants and small trees, Sf of these groups should be measured to avoid an underestimation of overall Sf at the stand level.

Relationships between seasonal climate, land cover and surface energy exchange in tropical montane cloud forest environments are poorly understood. The goal of this study was to investigate the seasonality of flux partitioning in lower montane cloud forest (LMCF), shaded coffee (CO) and sugarcane (SU) in central Veracruz, Mexico, as well as to evaluate the changes in surface energy exchange associated with the conversion of LMCF to CO or SU. Sensible ( H ) and latent heat (λ E ) fluxes were measured during the late dry and wet seasons using eddy covariance (CO and SU) and sap flow (LMCF) methods. Other measurements included: meteorological parameters, radiation balance, soil heat flux, soil moisture and vegetation characteristics. During the wet-season month of July, average midday Bowen ratios (βs) for sunny conditions were lowest and least variable among land covers: 0.4 ± 0.2 (SE) in LMCF, 0.5 ± 0.1 in SU and 0.7 ± 0.1 in CO. In contrast, during the late dry-season months of March and April, βs were higher (i.e. higher H and lower λ E ) and more variable. The highest values of β were observed in LMCF, reflecting effects of partial leaf-shedding by dominant deciduous species (2.4 ± 0.8, March) and increased stomatal control (1.4 ± 0.3, April). There was also evidence of stomatal limitation of λ E in CO and SU, having βs of up to 1.0 ± 0.1 in April and March, respectively. As compared to LMCF, the average midday available energy ( A e ) for sunny conditions was very similar in CO (−3 ± 7%) and 15 ± 8% lower in SU. Although not all results were statistically significant, they suggest that for the wet season conversion of LMCF to shaded coffee or sugarcane led to a decrease of 15 ± 14% or 15 ± 17% in midday λ E under sunny conditions, respectively, whereas corresponding values of H increased by 37 ± 38% or remained about the same (−4 ± 40%). In contrast, for the late dry season, conversion of LMCF to shaded coffee or sugarcane appears to have resulted in higher λ E and lower H , with changes of, respectively, +79 (±32)%/–45 (±16)% (CO) or +39 (±32)%/–43 (±16)% (SU) for a partially leafless LMCF in March, and +17 (±16)%/–11 (±16)% (CO) for a fully-leafed LMCF in April. In order to more accurately quantify the changes in surface energy fluxes associated with LMCF conversion, future work should focus on reducing the errors in the flux estimates. Nevertheless, for sunny days during the wet season, potential changes in the moisture and heat content of the local atmosphere due to the conversion of LMCF to CO or SU seem to have been in the same direction as those induced by increased greenhouse gases (drying and warming), whereas for the late dry season the effects appear to have been opposite (moistening and cooling).

Little is known about soil evaporation (Esoil) in shaded coffee plantations. In this study, Esoil and its proportion to total evapotranspiration and rainfall (P) were quantified for a densely shaded Arabica coffee plantation (~80% tree cover) in central Veracruz, Mexico, using eddy covariance (EC) measurements made above the tree canopy and at the boundary between the stem space and canopy of the coffee crop. While measurements above the canopy were made continuously during the 1-year study period (October 2016 to September 2017), this was not possible for the above-soil level due to logistical considerations, and hence, annual Esoil was calculated from a relationship between the ratio of the actual to the equilibrium Esoil and surface soil water content. This empirical model was calibrated using 31 days of EC measurements, representing the range in surface soil water content observed during the study period. Examining the above-canopy and above-soil EC measurements for quality yielded similar results with regard to stationarity (99% and 85% of the time, respectively) and degree of energy balance closure (81% and 96%, respectively). The permanent and relatively dense vegetation cover provided by the shade trees and coffee plants reduced net radiation at the soil surface to only about 12% of the above-canopy value. Consequently, Esoil was a minor fraction of evapotranspiration (10–13%) and P (8–9%), with the spread reflecting uncertainties related to the failure to close the energy balance. In addition to the low-energy availability, Esoil may have been reduced by the presence of a litter layer.

Throughout a single day, the microclimatic conditions in tropical montane cloud forests vary from strong solar radiation with simultaneous strong atmospheric water demand (high vapor pressure deficit, VPD, values), followed quickly by clouds and fog events drastically reducing both of these environmental variables. Due to the complex topography and weather patterns in these regions, microclimate, including fog events, can vary strongly across very small spatial scales as well, leading to a highly dynamic and compelling environment to examine how environmental variables influence tree water use across space and time. Due to this variation, the present study examines how environmental drivers of tree water use varies across three slope positions (upslope, midslope, low-slope) in a tropical montane cloud forest in Veracruz, Mexico. Measurements of sap flow using the heat ratio method were conducted on three dominant canopy species along with simultaneous measurements of microclimate within each site. To assess the relative importance of microclimatic variables in explaining tree water use across diurnal periods, data were separated into day and night periods and fog and clear events. Multiple regression models were conducted for each tree with input variables of VPD, solar radiation, air temperature, shallow soil moisture, deep soil moisture, and leaf wetness. We found that VPD explained a large majority of the variation in tree water use during daytime fog periods, particularly at the upslope and midslope sites. During nighttime periods, VPD was the dominant driver of water use variation during clear periods while a combination of VPD and leaf wetness explained variation during night, fog periods. Additionally, tree water use was more decoupled from environmental variables at the low-slope site. Finally, a separation of model components into fog and clear periods improved model outputs particularly at low flow conditions, highlighting the differential interactions between tree water use and environment during night and fog periods. Results from this study provide new insight into the importance of fog events, low VPD, and leaf surface wetting at controlling tree water use in cloud forests. The variation in drivers of water use across short spatial scales demonstrate the importance of considering individual and species level variation across fog and clear periods in predicting physiological responses of species to climate in cloud forests.

Stream water mean transit time (MTT) is a fundamental hydrologic parameter that integrates the distribution of sources, flow paths, and storages present in catchments. However, in the tropics little MTT work has been carried out, despite its usefulness for providing important information on watershed functioning at different spatial scales in (largely) ungauged basins. In particular, very few studies have quantified stream MTTs or have related these to catchment characteristics in tropical montane regions. Here we examined topographic, land use/cover and soil hydraulic controls on baseflow transit times for nested catchments (0.1–34 km2) within a humid mountainous region, underlain by volcanic soil (Andisols) in central Veracruz (eastern Mexico). We used a 2-year record of bi-weekly isotopic composition of precipitation and stream baseflow data to estimate MTT. Land use/cover and topographic parameters (catchment area and form, drainage density, slope gradient and length) were derived from geographic information system (GIS) analysis. Soil water retention characteristics, and depth and permeability of the soil–bedrock interface were obtained from intensive field measurements and laboratory analysis. Results showed that baseflow MTTs ranged between 1.2 and 2.7 years across the 12 study catchments. Overall, MTTs across scales were mainly controlled by catchment slope and the permeability observed at the soil–bedrock interface. In association with topography, catchment form and the depth to the soil–bedrock interface were also identified as important features influencing baseflow MTTs. The greatest differences in MTTs were found both within groups of small (0.1–1.5 km2) and large (14–34 km2) catchments. Interestingly, the longest stream MTTs were found in the headwater cloud forest catchments.

It is generally assumed that forests improve ecosystem service (ES) provisioning within landscapes. These assumptions drive policies (e.g. Payment for Ecosystem Services) that affect land-use without knowing if the desired services are achieved. Here we use a data-intensive approach to explore the synergies and tradeoffs between three regulating (hydrologic regulation, water quality, carbon storage) and one supporting ES (biodiversity). Using field-based measurements for ten ES indicators collected within eight land use/land cover (LULC) types we assess: (1) the relationship between ES indicators and LULC type and (2) the synergies and tradeoffs across ES indicators. For objective one, we found that primary forests promote more favorable hydrological services, including having greater base flow, flow regulation, and soil conductivity. For objective two, we observed synergies across many ES where management of one would improve provisioning for several other ES, specifically between low flow, carbon storage, and biodiversity. However, many ES parameters (e.g. water quality) had no relationship with other ES parameters. Our results underscore the value of site-specific research in addressing assumptions about the relationship between LULC and ES provisioning. More site-specific data is needed for more informed design of management strategies that can maximize ES benefits.

La provision y regulacion de flujos de agua en cuencas es probablemente el servicio ecosistemico mas importante de los bosques de niebla, sin embargo, su funcionamiento hidrologico y como este es alterado por el cambio en el uso de suelo es aun muy poco entendido. Este estudio evaluo los efectos hidrologicos causados por la conversion del bosque mesofilo de montana sobre suelo volcanico a otros tipos de vegetacion en Veracruz, Mexico. Para ello, se realizaron mediciones micrometeorologicas, ecofisiologicas e hidrologicas combinadas con informacion isotopica. Los resultados mostraron mayores rendimientos hidricos anuales en el pastizal y plantaciones de Pinus patula joven y maduro debido a menores tasas de evapotranspiracion comparados con el bosque maduro y secundario. El caudal total anual y estacional fue similar en los bosques, sugiriendo que con 20 anos de regeneracion natural es posible restaurar la funcionalidad hidrologica de microcuencas. En contraste, la microcuenca de pastizal reporto un mayor caudal anual (10 %), pero flujos 50 % en promedio mas bajos al final de la epoca de estiaje, asociado probablemente a una topografia mas suave y una menor capacidad de infiltracion. Aun en sustratos volcanicos altamente permeables, se observo que la conversion de bosque a pastizal puede conducir a incrementos importantes en los flujos superficiales en respuesta a eventos maximos de precipitacion. El efecto de la reforestacion con P. patula a escala de cuenca se desconoce, pero tasas de infiltracion mas altas que el pastizal sugieren una probable recuperacion hidrica del suelo en el corto a mediano plazo.

Key message Highly variable nighttime transpiration, with higher rates generally observed after preceding fog, is prevalent in dominant tree species of the nutri- ent-poor tropical montane cloud forest environment of central Veracruz, Mexico. Abstract Although nighttime transpiration (En) is pre- valent in a wide range of species from cloud-affected for- ests, its magnitude relative to total daily transpiration (Ed) as reported in the literature is generally small (En/Ed is 0.12 on average). In the present study, we observed high dry- season En/Ed ratios with large night-to-night variation in dominant species from the tropical montane cloud forest (TMCF) zone of central Veracruz, Mexico: 0.22 ± 0.18 for Quercus lancifolia (old-growth TMCF); 0.26 ± 0.14 and 0.16 ± 0.16 for Alchornea latifolia and Alnus jorull- ensis, respectively (regenerating post-fire TMCF); and 0.30 ± 0.20 to 0.12 ± 0.21 for Pinus patula (young and mature pine plantations). En was determined as the dif- ference between observed nocturnal sap flow and estimated refilling of stem water storage, the latter of which was on average: 21-25 % of nocturnal sap flow for Q. lancifolia ;6 and 5 % for A. latifolia and A. jorullensis, respectively; and 21-23 % for P. patula. Night-to-night variation in En was mostly due to large variation in vapor pressure deficit (VPD) related in turn to the alternation of cold fronts (producing fog events) and high pressure weather (pro- ducing nights with VPD up to 2 kPa). Moreover, in the hours following fog events without concurring rainfall, En was often higher as compared to fog-free nights with similar VPD across all species examined. Low-nutrient availability and high water content of the soils in the study area suggest a nutrient uptake benefit associated with the relatively high En rates observed.

Payments for Hydrologic Services (PHS) programs are increasingly used as a policy tool to provide incentives for upstream landowners to adopt land use activities that favor sustainable provision of high-quality water to downstream areas. However, the effectiveness of PHS programs in achieving their objectives and the potential for unintended (often undesirable) consequences remain poorly understood. We integrate results from ecohydrological and socioeconomic research to explore the impact of Mexico's PHS program on the target hydrologic services and people's decisions, behavior, and knowledge regarding forest conservation and water. Using central Veracruz as our case study, we identify areas of both synchrony and disconnection between PHS goals and outcomes. Mature and regenerating cloud forests (targeted by PHS) were found to produce enhanced hydrologic services relative to areas converted to pasture, including reduced peak flows during large rain events and maintenance of dry-season base flows. However, unexpectedly, these hydrologic benefits from cloud forests were not necessarily greater than those from other vegetation types. Consequently, the location of forests in strategic watershed positions (e.g., where deforestation risk or hydrologic recharge are high) may be more critical than forest type in promoting hydrologic functions within watersheds and should be considered when targeting PHS payments. While our results suggest that participation in PHS improved the level of knowledge among watershed inhabitants about forest-water relationships, a mismatch existed between payment amounts and landowner opportunity costs, which may contribute to the modest success in targeting priority areas within watersheds. Combined, these findings underscore the complexity of factors that influence motivations for PHS participation and land use decisions and behavior, and the importance of integrating understanding of both ecohydrological and socioeconomic dynamics into PHS design and implementation. We conclude by identifying opportunities for improving the design of PHS programs and recommending priority areas for future research and monitoring, both in Mexico and globally.

In stands with a broad range of diameters, a small number of very large trees can disproportionately influence stand basal area and transpiration (Et). Sap flow-based Et estimates may be particularly sensitive to large trees due to nonlinear relationships between tree-level water use (Q) and tree diameter at breast height (DBH). Because Q is typically predicted on the basis of DBH and sap flow rates measured in a subset of trees and then summed to obtain Et, we assessed the relative importance of DBH and sap flow variables (sap velocity, Vs, and sapwood depth, Rs) in determining the magnitude of Et and its dependence on large trees in a tropical montane forest ecosystem. Specifically, we developed a data-driven simulation framework to vary the relationship between DBH and Vs and stand DBH distribution and then calculate Q, Et and the proportion of Et contributed by the largest tree in each stand. Our results demonstrate that variation in how Rs is determined in the largest trees can alter estimates up to 26% of Et while variation in how Vs is determined can vary results by up to 132%. Taken together, these results highlight a great need to expand our understanding of water transport in large trees as this hinders our ability to predict water fluxes accurately from stand to catchment scales.

The ecophysiology of tropical montane cloud forest (TMCF) trees is influenced by crown-level microclimate factors including regular mist/fog water inputs, and large variations in evaporative demand, which in turn can significantly impact water balance. We investigated the effect of such microclimatic factors on canopy ecophysiology and branch-level water balance in the dry season of a seasonal TMCF in Veracruz, Mexico, by quantifying both water inputs (via foliar uptake, FU) and outputs (day- and night-time transpiration, NT). Measurements of sap flow, stomatal conductance, leaf water potential and pressure-volume relations were obtained in Quercus lanceifolia, a canopy-dominant tree species. Our results indicate that FU occurred 34% of the time and led to the recovery of 9% (24 ± 9.1 L) of all the dry-season water transpired from individual branches. Capacity for FU was independently verified for seven additional common tree species. NT accounted for approximately 17% (46 L) of dry-season water loss. There was a strong correlation between FU and the duration of leaf wetness events (fog and/or rain), as well as between NT and the night-time vapour pressure deficit. Our results show the clear importance of fog and NT for the canopy water relations of Q. lanceifolia.

Summary 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.

Despite their critical role as freshwater resources and their vulnerability to anthropogenic pressures, our knowledge of the ecohydrology of tropical montane cloud forests remains limited. Here, we use a dual stable isotope approach (δ2H and δ18O) to trace water inputs, fluxes, and pools through a seasonally dry tropical montane cloud forest in central Veracruz, Mexico. We found strong seasonal variation in the isotope composition of precipitation inputs (δ2H ~130‰ and δ18O ~18‰), with significantly more depleted wet-season values as compared with dry-season values. These seasonal differences are subject to inter-annual variation, as reflected by shifts in the local meteoric water line. Even at the peak of the dry season, stable isotope values of plant xylem water from six different deciduous and evergreen species occurring in the understory and canopy of mature and secondary forests were all consistent with the use of shallow soil water (20–60 cm). Alternative limiting factors, such as nutrient distribution along the soil profile, may be strongly contributing to plant rooting patterns. Stable isotope values of soil water were evaporatively enriched compared with precipitation inputs, whereas stream values plotted with precipitation on the local meteoric water line. In contrast to current hydrologic models, this indicates the presence of two distinct soil water pools, one highly mobile pool of precipitation that contributes to streams and a second less mobile pool of soil water that contributes to plant fluxes. This comprehensive perspective can provide the basis for generating process-based models of ecohydrology in the future. Copyright © 2011 John Wiley & Sons, Ltd.

Between 5 March and 10 May 2001, the performance of three types of passive fog gauges (wire harp WH, standard fog collector SC and Juvik gauge JU) was compared at a wind-exposed Puerto Rican elfin cloud forest site. The gauges were used to determine the timing and duration of fog, as well as fog-induced crown drip using gauge to canopy conversions factors. The latter were calculated as the average ratio between throughfall, and fog collected by the respective gauges during periods with fog-only and negligible evaporation loss. The fog gauges indicated very similar timing and fog duration, presumably because dense fog prevailed for 75–80% of the time whereas winds were generally sufficiently strong (4–5 m s−1) and from a nearly constant direction. Resulting gauge to canopy factors were 0·12 for WH and SC, and 0·10 for JU. Estimating fog interception by the elfin forest canopy using these conversion factors gave very similar values for the three gauge types (mean rates of 0·15 ± 0·01 mm h−1). Throughfall typically started about 5 h after the fog gauges indicated the beginning of a fog event. This time lag probably reflects the filling of the canopy storage capacity. Integrating the estimated fog interception rates over the time lag suggested an average value of 0·4 mm for the canopy storage capacity during fog-only events. The present results are expected to apply to sites with comparable wind and fog conditions. Copyright © 2010 John Wiley & Sons, Ltd.

Rainfall and cloud water interception (CWI) were determined for a mature and a 19-year old secondary lower montane cloud forest in central Veracruz, Mexico. Cloud water was measured using a passive fog gauge, and consisted most likely of a mixture of fog and wind-driven drizzle. CWI by the canopy was derived from the wet canopy water budget as throughfall + stemflow + calculated interception loss minus rainfall. Rainfall interception loss was calculated using the Liu model, parameterized for events with rain-only. Precipitation events with cloud water input occurred exclusively during the dry season (November-April), and were primarily associated with cold fronts. CWI was estimated at 6% of dry season rainfall (640 mm on average) for the secondary forest vs. 8% for the mature forest, whereas annual values were ≤2% of total rainfall (3180 mm). Infrequent fog occurrence and low wind speeds were the most important reasons for the observed low values of CWI. Total apparent interception loss (i.e. including CWI) was 17% of annual rainfall for the mature forest and 8% for the secondary forest. Post-event evaporation of intercepted water stored in the canopy rather than within-event evaporation dominated interception loss at both forests. Hence, the higher loss observed for the mature forest is considered to reflect a higher canopy storage capacity, related in turn to a higher Leaf Area Index and larger epiphyte biomass. © 2010 Elsevier B.V. All rights reserved.

Stream water mean transit time (MTT) is a fundamental hydrologic parameter that integrates the distribution of sources, flow paths and storages present in catchments. However, in the tropics little MTT work has been carried out, despite its usefulness for providing important information on watershed functioning at different spatial scales in (largely) ungauged basins. In particular, very few studies have quantified stream MTTs and related to catchment characteristics in tropical montane regions. Here we examined topographic, land use/cover and soil hydraulic controls on baseflow transit times for nested watersheds (0.1–34 km2) within a humid mountainous region, underlain by volcanic soil (Andisols) in central Veracruz (eastern Mexico). We used a 2 year record of bi-weekly isotopic composition of precipitation and stream baseflow data to estimate MTT. Land use/cover and topographic parameters (catchment area and form, drainage density, slope gradient and length) were derived from GIS analysis. Soil water retention characteristics, and depth and permeability of the soil–bedrock interface were obtained from intensive field measurements and laboratory analysis. Results showed that baseflow MTT ranged between 1.2 and 2.7 years across the 12 study catchments. Overall, MTTs across scales were mainly controlled by catchment slope and the permeability observed at the soil–bedrock interface. In association with topography, catchment form, land cover and the depth to the soil–bedrock interface were also identified as important features influencing baseflow MTTs. The greatest differences in MTTs were found at the smallest (0.1–1.5 km2) and the largest scales (14–34 km2). Interestingly, longest stream MTTs were found in the headwater cloud forest catchments.

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 (0! 11 " 0! 05 mm h # 1 ) and rainy conditions (0! 24 " 0! 13 mm h # 1 ) were about three to six times the eddy-covariance-based estimate (0! 04 " 0! 002 mm h # 1 ). 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 $ 40% 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 $ 770 mm year # 1 for the summit cloud forest just below the ridge top (according to the water budget method) and $ 785 mm year # 1 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.

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.

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 ± 50 mm) 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.

The water and energy fluxes of a shaded coffee plantation in the lower montane cloud forest (LMCF) zone of central Veracruz, Mexico, were measured over a two-year period (September 2006–August 2008) using the eddy covariance method. Complementary measurements of throughfall and stemflow were made to study rainfall interception. The sum of the observed sensible ( H ) and latent ( λE ) heat fluxes was almost 95% of the net radiation ( R n ) minus the canopy heat storage fluxes, indicating very good energy balance closure. The mean annual evapotranspiration was 1066 mm, and 95% of the corresponding FAO Penman-Monteith reference evapotranspiration (ET 0 ) of 1117 mm yr −1 . Interception loss was 8% of annual rainfall (1386 mm). Both the eddy covariance, and the throughfall and stemflow measurements showed average wet-canopy evaporation rate to be very low (0.05 mm h −1 ) compared to the corresponding rainfall rate (3.06 mm h −1 ). As a result, and despite the low canopy storage capacity of the coffee plantation ( C m , 0.50 mm), interception was dominated by post-event evaporation of intercepted water rather than by within-event evaporation. Comparing the results for the coffee plantation with interception data from mature and secondary LMCFs in the study area suggests that the conversion of LMCF to shade-coffee may lead to a decrease in interception loss of 8−18% of incident rainfall. This decrease is caused by a three- to seven-fold decrease in C m , probably due to the lower leaf area and smaller epiphyte biomass of the coffee plantation. The mean annual dry-canopy evaporation was 992 mm, and 89% of ET 0 . Comparing the eddy covariance-based estimate of dry-canopy evaporation for the coffee plantation with sapflow-based estimates of transpiration for the LMCFs did not show any clear differences.

During a one-year period, the variability of throughfall and the standard errors of the means associated with different gauge arrangements were studied in a lower montane rain forest in Puerto Rico. The following gauge arrangements were used: (1) 60 fixed gauges, (2) 30 fixed gauges, and (3) 30 roving gauges. Stemflow was measured on 22 trees of four different species. An ANOVA indicated that mean relative throughfall measured by arrangements 1 (77%), 2 (74%), and 3 (73%) were not significantly different at the 0.05 level. However, the variability of the total throughfall estimate was about half as high for roving gauges (23%) as for fixed gauges (48-49%). The variability of stemflow ranged from 36% to 67% within tree species and was 144% for all sampled trees. Total stemflow was estimated at 4.1% of rainfall, of which palms contributed about 66%. Comparative analysis indicated that while fixed and roving gauge arrangements can give similar mean values, least 100 fixed gauges are required to have an error at the 95% confidence level comparable to that obtained by 30 roving gauges. © 2005 Elsevier B.V. All rights reserved.

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