Spatial and temporal variation in wet canopy conditions following precipitation events can influence processes such as transpiration and photosynthesis, which can be further enhanced as upper canopy leaves dry more rapidly than the understory following each event. As part of a larger study aimed at improving land surface modelling of evapotranspiration processes in wet tropical forests, we compared transpiration among trees with exposed and shaded crowns under both wet and dry canopy conditions in central Costa Rica, which has an average 4200 mm annual rainfall. Transpiration was estimated for 5 months using 43 sap flux sensors in eight dominant, ten midstory and eight suppressed trees in a mature forest stand surrounding a 40‐m tower equipped with micrometeorological sensors. Dominant trees were 13% of the plot's trees and contributed around 76% to total transpiration at this site, whereas midstory and suppressed trees contributed 18 and 5%, respectively. After accounting for vapour pressure deficit and solar radiation, leaf wetness was a significant driver of sap flux, reducing it by as much as 28%. Under dry conditions, sap flux rates (Js) of dominant trees were similar to midstory trees and were almost double that of suppressed trees. On wet days, all trees had similarly low Js. As expected, semi‐dry conditions (dry upper canopy) led to higher Js in dominant trees than midstory, which had wetter leaves, but semi‐dry conditions only reduced total stand transpiration slightly and did not change the relative proportion of transpiration from dominant and midstory. Therefore, models that better capture forest stand wet–dry canopy dynamics and individual tree water use strategies are needed to improve accuracy of predictions of water recycling over tropical forests. Copyright © 2016 John Wiley & Sons, Ltd.

Abstract. Measurements of soil volumetric moisture content and temperature were made at 2, 4, 7, 10, and 15 cm below the surface of a bare field soil, over a 1-week period at 20-min intervals. The conductive heat and liquid moisture fluxes were calculated for the soil layer 7-10 cm below the surface, and the water vapor flux was then determined from both the energy transfer and mass transfer equations. Water vapor flux in this layer transported a significant amount of the total energy flux (up to 50%) and an appreciable amount of the total moisture flux (up to 25%). There was reasonable agreement between the water vapor flux calculated by the mass transfer equation and the vapor flux calculated by the energy equation. IntroductionFor at least 80 years it has been recognized that the movement of moisture and heat in the soil are coupled [Boucoyous, 1915]. The total heat flux in the soil occurs not only from simple conduction but also from water movement in both the vapor and liquid states. Likewise, temperature gradients can drive mass transfer. Conceptually, the coupling of the heat and mass transfer equations can be seen as largely resulting from the water vapor flux. The movement of moisture from one location in the soil to another by evaporation and the subsequent recondensation can contribute significantly to the net moisture movement in the soil. Additionally, because of the large value of the latent energy of vaporization of water, the water vapor transports significant energy when it evaporates and condenses.Various authors have examined the significance and magnitude of the water vapor flux as it affects either the mass or energy balances in experimental studies (see Table 1

Coupled heat and water transport in soils has enjoyed extensive focus in soil physics and hydrology and yet, until recently, there has never been a satisfactory comparison of water vapor¯uxes measured in the ®eld with theory. At least two factors have led to this, ®rst, most of the experimental work has been laboratory oriented with steady state boundary conditions imposed and second, there have been relatively few ®eld experiments to test the existing theory. In this paper we review a new theoretical development which explains ®eld observations of water vapor movement. The diurnal warming at the land surface leads to an expansion and contraction of the soil air as it warms and cools resulting in a convective (or``advective'') transport of water vapor. This mechanism has important consequences for the transport of any vapor in the soil air near the landatmosphere interface. #

While it is reasonable to predict that photosynthetic rates are inhibited while leaves are wet, leaf gas exchange measurements during wet conditions are challenging to obtain due to equipment limitations and the complexity of canopy-atmosphere interactions in forested environments. Thus, the objective of this study was to evaluate responses of seven tropical and three semiarid savanna plant species to simulated leaf wetness and test the hypotheses that (i) leaf wetness reduces photosynthetic rates (Anet), (ii) leaf traits explain different responses among species and (iii) leaves from wet environments are better adapted for wet leaf conditions than those from drier environments. The two sites were a tropical rainforest in northern Costa Rica with ~4200 mm annual rainfall and a savanna in central Texas with ~1100 mm. Gas exchange measurements were collected under dry and wet conditions on five sun-exposed leaf replicates from each species. Additional measurements included leaf wetness duration and stomatal density. We found that Anet responses varied greatly among species, but all plants maintained a baseline of activity under wet leaf conditions, suggesting that abaxial leaf Anet was a significant percentage of total leaf Anet for amphistomatous species. Among tropical species, Anet responses immediately after wetting ranged from -31% (Senna alata (L.) Roxb.) to +21% (Zamia skinneri Warsz. Ex. A. Dietr.), while all savanna species declined (up to -48%). After 10 min of drying, most species recovered Anet towards the observed status prior to wetting or surpassed it, with the exception of Quercus stellata Wangenh., a savanna species, which remained 13% below Anet dry. The combination of leaf wetness duration and leaf traits, such as stomatal density, trichomes or wax, most likely influenced Anet responses positively or negatively. There was also overlap between leaf traits and Anet responses of savanna and tropical plants. It is possible that these species converge on a relatively conservative response to wetness, each for divergent purposes (cooling, avoiding stomatal occlusion, or by several unique means of rapid drying). A better understanding of leaf wetness inhibiting photosynthesis is vital for accurate modeling of growth in forested environments; however, species adapted for wet environments may possess compensatory traits that mitigate these effects.

Abstract. This study compares the performance of the Community Land Models (CLM4.5 and CLM5) against tower and ground measurements from a tropical montane rainforest in Costa Rica. The study site receives over 4000 mm of mean annual precipitation and has high daily levels of relative humidity. The measurement tower is equipped with eddy-covariance and vertical profile systems able to measure various micrometeorological variables, particularly in wet and complex terrain. In this work, results from point-scale simulations for both CLM4.5 and its updated version (CLM5) are compared to observed canopy flux and micrometeorological data. Both models failed to capture the effects of frequent rainfall events and mountainous topography on the variables of interest (temperatures, leaf wetness, and fluxes). Overall, CLM5 alleviates some errors in CLM4.5, but CLM5 still cannot precisely simulate a number of canopy processes for this forest. Soil, air, and canopy temperatures, as well as leaf wetness, remain too sensitive to incoming solar radiation rates despite updates to the model. As a result, daytime vapor flux and carbon flux are overestimated, and modeled temperature differences between day and night are higher than those observed. Slope effects appear in the measured average diurnal variations of surface albedo and carbon flux, but CLM5 cannot simulate these features. This study suggests that both CLMs still require further improvements concerning energy partitioning processes, such as leaf wetness process, photosynthesis model, and aerodynamic resistance model for wet and mountainous regions.

Abstract. The scalar roughness length for temperature, Zon, is necessary to estimate the sensible flux from atmospheric surface layer similarity theory in conjunction with skin temperature measurements. A theoretical relationship for Z0h as a function of the roughness Reynolds number z0+ which was developed by Brutsaert [1975] for rough-bluff surfaces is often used to link infrared skin temperature measurements to atmospheric temperature measurements. Measurements of sensible heat flux and temperature at two semiarid sites are used to evaluate and test the temperature roughness length model coefficients. Consideration of the measurement error is important to derive an accurate set of coefficients. These new field-based coefficients correct for some of the underprediction of sensible heat flux at high flux rates that occurred with past formulations.

The time evolution of evaporation from a bare soil, over a 9‐d period following irrigation, is described by a combination of daily and hourly drying patterns. From the second day, the daily evaporation shows a second stage of drying that can be described as a desorptive process (evaporation proportional to (t − to)−1/2, where t is time in days and to is the day when the second stage starts). The short time (hourly) evaporation rate can be modeled on the basis of a type of self‐similarity in the energy balance components. Combining the evaporative flux behavior at the two time scales, desorption at the daily timescale and self‐similarity for the diurnal variations, a robust description of evaporation for drying land surfaces is obtained. This approach is tested using accurate measurements of the different components of the energy balance at the soil surface, obtained at 20‐min intervals. The model accurately describes the time evolution of the evaporative flux and could be used for the disaggregation of daily or weekly evaporation into hourly values.

In this study, we assess whether students and their faculty mentors in a Research Experience for Undergraduates program have similar perceptions about the relative importance of different outcomes of their study abroad experience. Results of a Q-analysis reveal a significant difference of opinion between the students and the faculty mentors. It is argued that the faculty mentors need to spend more time in the field with their students and recognize that undergraduate research provides students with skills and a confidence to conduct research in the future, rather than personal and professional development, and the ability to think like a scientist.