Canopy Transmittance Research Articles

A panorama-based technique to estimate sky view factor and solar irradiance considering transmittance of tree canopies

Street-view-based techniques for assessing the sky view factor (SVF) and solar irradiance under trees are gaining attention as tools for evaluating trees as nature-based solutions to mitigate urban heat risks. Although these metrics significantly depend on the morphology of trees and resulting canopy transmittance, an existing approach, termed the Solid Canopy Method (SCM), assumes zero transmission and has not accounted for these variations. This paper advances the computation of both metrics, improving their accuracy and application — we developed the Transmissive Canopy Method (TCM), a panorama-based approach that integrates semantic segmentation and binarization to evaluate SVF and solar irradiance while accounting for transmittance of tree canopies. Using a study area on a university campus in Singapore, we collected data on solar irradiance and 360°imagery to validate our method. The results indicated improved accuracy with MAE, RMSE, and R2 values of 77.8 Wm−2, 105.0 Wm−2 and 0.90, respectively — significantly outperforming the SCM. We showcased two use cases of our method: (1) high-resolution mapping of SVF and solar irradiance in a field with trees, and (2) walking route optimization considering sunlight exposure. Our findings highlight the strong capability of our TCM to evaluate the effects of trees in mitigating urban heat more accurately than the existing method. Additionally, the TCM has potential applications in urban planning and management, enabling strategic tree planting prioritizing areas lacking sufficient shading and developing tools for optimizing walking routes to minimize sunlight exposure.

Physically based modelling of spectral transmittance through forest canopies

Abstract Physically based models simulating the spectral transmittance of solar radiation through forest canopies are useful tools for examining the connections between the shortwave radiation environment and the productivity and biodiversity of the forest floor. We report a comprehensive evaluation of two approaches simulating forest canopy spectral transmittance. The approaches were (i) three‐dimensional radiative transfer modelling in canopies composed of individual trees filled with turbid media and (ii) photon recollision probability theory (p‐theory), and were implemented using DART‐FT and PARAS models, respectively. The simulations were evaluated against mean and standard deviation of canopy transmittance spectra measured under clear‐sky conditions in forest plots across central and Northern Europe. In general, both models agreed well with the in situ measurements. They performed equally in conifer forests, while PARAS had a slightly lower accuracy than DART‐FT in broadleaved forests. We conclude that both approaches produce realistic simulations of canopy spectral transmittance at the spatial scale tested in this study, and that p‐theory constitutes a computationally efficient and easy‐to‐parameterize alternative to three‐dimensional radiative transfer.

Photosynthetic capacity and assimilate transport of the lower canopy influence maize yield under high planting density.

Photosynthesis is a major trait of interest for the development of high-yield crop plants. However, little is known about the effects of high-density planting on photosynthetic responses at the whole-canopy level. Using the high-yielding maize (Zea mays L.) cultivars "LY66," "MC670," and "JK968," we conducted a 2-yr field experiment to assess ear development in addition to leaf characteristics and photosynthetic parameters in each canopy layer at 4 planting densities. Increased planting density promoted high grain yield and population-scale biomass accumulation despite reduced per-plant productivity. MC670 had the strongest adaptability to high-density planting conditions. A physiological analysis showed that increased planting density primarily led to decreases in the single-leaf area above the ear for LY66 and MC670 and below the ear for JK968. Furthermore, high planting density decreased chlorophyll content and the photosynthetic rate due to decreased canopy transmission, leading to severe decreases in single-plant biomass accumulation in the lower canopy. Moreover, increased planting density improved presilking biomass transfer, especially in the lower canopy. The yield showed significant positive relationships with photosynthesis and biomass in the lower canopy, demonstrating the important contributions of these leaves to grain yield under dense planting conditions. Increased planting density led to retarded ear development as a consequence of reduced glucose and fructose contents in the ears, indicating reductions in sugar transport that were associated with limited sink organ development, reduced kernel number, and yield loss. Overall, these findings highlighted the photosynthetic capacities of the lower canopy as promising targets for improving maize yield under dense planting conditions.

Evaluation of transpiration in different almond production systems with two-source energy balance models from UAV thermal and multispectral imagery

AbstractA growing number of intensive irrigated production systems of the almond crop have been established in recent years. However, there is little information regarding the crop water requirements. Remote sensing-based models such as the two-source energy balance (TSEB) have proven to be reliable ways to accurately estimate actual crop evapotranspiration. However, few efforts have been made to validate the transpiration with sap flow measurements in woody row crops with different production systems and water status. In this study, the TSEB Priestley-Taylor (TSEB-PT) and contextual approach (TSEB-2T) models were assessed to estimate canopy transpiration. In addition, the effect of applying a basic clumping index for heterogeneous randomly placed clumped canopies and a rectangular hedgerow clumping index on the TSEB transpiration estimation was assessed. The TSEB inputs were obtained from high resolution multispectral and thermal imagery using an unmanned aerial vehicle. The leaf area index (LAI), stem water potential (Ψstem) and fractional intercepted photosynthetically active radiation (fIPAR) were also measured. Significant differences were observed in transpiration between production systems and irrigation treatments. The combined use of the TSEB-2T with the C&N-R transmittance model gave the best transpiration estimations for all production systems and irrigation treatments. The use of in situ PAR transmittance in the TSEB-2T model significantly improved the root mean squared error. Thus, the better agreement observed with the TSEB when using the C&N-R model and in situ PAR transmittance highlights the importance of improving radiative transfer models for shortwave canopy transmittance, especially in woody row crops.

Using just a canopy height model to obtain lidar-level accuracy in 3D forest canopy shortwave transmissivity estimates

This study presents a new model for calculating canopy shortwave radiation transmissivity from synthetic hemispheric images using only information contained within a canopy height model (CHM) – CanopyHeightModel2Radiation (C2R). The enhanced version calculates synthetic hemispherical images based on the geometric arrangement of the surrounding canopy while applying a statistical correction for canopy transmissivity using canopy thickness and tree species leaf area. The simple input data and statistical correction make this model suitable for estimating canopy transmissivity across large spatial extents typical of land surface models for which canopy transmissivity or radiation is a primary input variable. Performance of C2R-enhanced is assessed against hemispherical photographs, and compared to a basic version of C2R without transmissivity correction, and two versions of a Lidar2Radiation model (L2R-enhanced, L2R-basic) with either a basic representation of canopy structure or an enhanced representation including trunks and branches within tree crowns. The two enhanced models (L2R-enhanced and C2R-enhanced) perform best compared to hemispherical photographs, while the L2R-basic and C2R-basic models over- and underestimate canopy transmissivity, respectively. At 1-meter and 10-minute resolution, the two enhanced models perform similarly, but exact timing and location of transmissivity controlled by canopy structure is better represented in the physically explicit L2R-enhanced model. Across hourly and 25 × 25 m grid-averaged scales, both enhanced models achieve similar estimates of canopy transmissivity. Based on these results, it is recommended that the purely physically-based representation in the L2R-enhanced model is used when estimates of canopy transmissivity at high spatial and temporal (meter and minute) resolutions are necessary, while the computationally more efficient C2R-enhanced model is used when calculating canopy transmissivity within spatially aggregated grid cells, for example, as input into coarser-resolution land surface models. Incorporating C2R-enhanced into existing forest energy balance models creates exciting opportunities for investigating forest structure changes on forest hydrology and ecosystems across previously impossible spatial extents.

Continuous ground monitoring of vegetation optical depth and water content with GPS signals

Abstract. Satellite microwave remote sensing techniques can be used to monitor vegetation optical depth (VOD), a metric which is directly linked to vegetation biomass and water content. However, these large-scale measurements are still difficult to reference against either rare or not directly comparable field observations. So far, in situ estimates of canopy biomass or water status often rely on infrequent and time-consuming destructive samples, which are not necessarily representative of the canopy scale. Here, we present a simple technique based on Global Navigation Satellite Systems (GNSS) with the potential to bridge this persisting scale gap. Because GNSS microwave signals are attenuated and scattered by vegetation and liquid water, placing a GNSS sensor under a vegetated canopy and measuring changes in signal strength over time can provide continuous information about VOD and thus on vegetation biomass and water content. We test this technique at a forested site in southern California for a period of 8 months. We show that variations in GNSS signal-to-noise ratios reflect the overall distribution of biomass density in the canopy and can be monitored continuously. For the first time, we show that this technique can resolve diurnal variations in VOD and canopy water content at hourly to sub-hourly time steps. Using a model of canopy transmissivity to assess these diurnal signals, we find that temperature effects on the vegetation dielectric constant, and thus on VOD, may be non-negligible at the diurnal scale or during extreme events like heat waves. Sensitivity to rainfall and dew deposition events also suggests that canopy water interception can be monitored with this approach. The technique presented here has the potential to resolve two important knowledge gaps, namely the lack of ground truth observations for satellite-based VOD and the need for a reliable proxy to extrapolate isolated and labor-intensive in situ measurements of biomass, canopy water content, or leaf water potential. We provide recommendations for deploying such off-the-shelf and easy-to-use systems at existing ecohydrological monitoring networks such as FluxNet or SapfluxNet.

Adjusting line quantum sensing to improve leaf area index measurements and estimations in forests

Rapid and reliable estimation of leaf area index (LAI), a crucial parameter in process-based models of vegetation cover response, is important in ecological studies. The Beer-Lambert law is widely used to calculate forest LAI, but data collection methods are time-consuming and calculations are often inaccurate. Our objective was to improve the accuracy of Beer-Lambert law–based LAI estimation by employing indirect data collection and location-specific light extinction coefficients (K). Canopy transmittance and LAI of two 100 m2 temperate forest stands in southwestern Germany, one managed and one protected, was estimated using line quantum sensing (LQS) at 45,000 points per stand. The Beer-Lambert law was then inverted to estimate LAI using the measured transmittance with a K of 0.53–0.54. Hemispherical reference photographs were used as independent validation data to determine ideal K values. Experimental data demonstrated that LAI values estimated using LQS with adjusted K values were more accurate than those calculated using the basic application of the Beer-Lambert law. LQS results correlated with those determined using hemispherical photography for both the managed (R² = 0.80) and protected (R² = 0.81) stands. Overall, these findings show that adjusting K values for individual forest systems improves the accuracy of LAI estimation.•The modified method is more accurate than that using fixed K ranges.•The modified method accounts for individual ecosystems, with different K values for different environments.•The method can accurately reflect the dynamic changes of forest canopy structure, allowing integration of additional environmental measurements.

Modeling the Microwave Transmissivity of Row Crops

We develop a method to model the microwave transmissivity of row crops that explicitly accounts for their periodic nature as well as multiple scattering. We hypothesize that this method could eventually be used to improve satellite retrieval of soil moisture and vegetation optical depth in agricultural regions. The method is characterized by unit cells terminated by periodic boundary conditions and Floquet port excitations solved using commercial software. Individual plants are represented by vertically oriented dielectric cylinders. We calculate canopy transmissivity, reflectivity, and loss in terms of S-parameters. We validate the model with analytical solutions and illustrate the effect of canopy scattering. Our simulation results are consistent with both simulated and measured data published in the literature.

Modelled effects of channel orientation and tree canopy shape on average shade in streams

ABSTRACT Replanting riparian vegetation can improve ecological health but there are few guidelines that help stakeholders achieve shade targets cost-effectively. A computer model was extended to calculate average shade to direct and diffuse solar radiation in straight channels. It was used to recommend strategies for achieving shade targets, including the 70% guideline shown to prevent nuisance aquatic plant growths and extreme water temperatures. For similar trees on both banks, 70% shade occurs when the ratio of tree height (H) to stream width (W) lies in the range 1.0 < H/W < 2.6 depending on stream orientation and tree shape (canopy overhang and dripline height). With trees on both banks, shade is lower by up to 34% in east–west (EW) than north–south (NS) streams except for tall trees at the equinox. EW streams planted only on the north bank can have a 30% higher shade than NS streams planted on one bank, but low vegetation on the unplanted bank eliminates this difference. For EW streams in the Southern hemisphere, planting tall vegetation on the north bank is cost-effective, with low vegetation on the south bank providing additional benefits. Further work is planned on channel meandering, gaps between trees and canopy transmission.

A single-layer urban canopy model with transmissive radiation exchange between trees and street canyons

Urban trees are one of the most effective strategies to mitigate excessive heat stress in cities. To understand the underlying mechanisms of their cooling effect and to assess their use in urban planning, the accurate simulation of how trees interact with the ambient built environment is critical and imperative. However, the representation of urban trees in existing urban canopy models (in particular the single-layer ones) remains oversimplified. Here we develop a new Monte Carlo ray tracing method to explicitly resolve the canopy transmittance and evaluate its impact on radiative view factors between trees and regular building facets. The new method is highly accurate in reproducing analytical solutions. Sensitivity tests of radiative view factors suggest the importance of canopy transmittance in changing the radiation exchange. We then incorporate the ray tracing algorithm into the new version of the Arizona State University (ASU) Single-Layer Urban Canopy Model (ASLUM v3.1). In addition to radiation transmittance, ASLUM v3.1 explicitly resolves the radiative shading, evapotranspiration, and root water uptake of urban trees in street canyons, with significantly improved performance in predictions (especially latent heat flux) when compared to previous versions. We further apply ASLUM v3.1 to evaluate the impacts of trees with varying characteristics on urban radiation exchange and turbulent heat fluxes. Results show that urban trees reduce the net radiation of ground and wall as well as the daytime temperature via shading and transpiration, but may slightly warm the nighttime street canyons through radiative trapping effect.

High Heterogeneity in Canopy Temperature Among Co‐occurring Tree Species in a Temperate Forest

AbstractTrees regulate canopy temperature (Tc) via transpiration to maintain an optimal temperature range. In diverse forests such as those of the eastern United States, the sensitivity of Tc to changing environmental conditions may differ across species, reflecting wide variability in hydraulic traits. However, these links are not well understood in mature forests, where Tc data have historically been difficult to obtain. Recent advancement of thermal imaging cameras (TICs) enables Tc measurement of previously inaccessible tall trees. By leveraging TIC and sap flux measurements, we investigated how co‐occurring trees (Quercus alba, Q. falcata, and Pinus virginiana) change their Tc and vapor pressure deficit near the canopy surface (VPDc) in response to changing air temperature (Ta) and atmospheric VPD (VPDa). We found a weaker cooling effect for the species that most strongly regulates stomatal function during dry conditions (isohydric; P. virginiana). Specifically, the pine had higher Tc (up to 1.3°C) and VPDc (up to 0.3 kPa) in the afternoon and smaller sensitivity of both ∆T (=Tc − Ta) and ∆VPD (=VPDc − VPDa) to changing conditions. Furthermore, significant differences in Tc and VPDc between sunlit and shaded portions of a canopy implied a non‐evaporative effect on Tc regulation. Specifically, Tc was more homogeneous within the pine canopy, reflecting differences in leaf morphology that allow higher canopy transmittance of solar radiation. The variability of Tc among species (up to 1.3°C) was comparable to the previously reported differences in surface temperature across land cover types (1°C to 2°C), implying the potential for significant impact of species composition change on local/regional surface temperature.

Microclimatic Tipping Points at the Beech–Oak Ecotone in the Western Romanian Carpathians

European beech (Fagus sylvatica) is a drought-sensitive species that likely will retreat from its xeric distribution edges in the course of climate warming. Physiological measurements indicate that the species may not only be sensitive to soil water deficits, but also to high temperatures and elevated atmospheric vapor pressure deficits (vpd). Through microclimatological measurements in the stand interior across near-natural beech forest–oak forest ecotones, we searched for microclimatic tipping points in the contact zone with the aim to define the thermic and hydrometeorological limits of beech more precisely. In three transects in the foothills of the Romanian western Carpathians, we measured in mid-summer 2019 air temperature, relative air humidity, and vpd at 2 m height in the stand interior across the ecotone from pure oak to pure beech forests, and compared the readings to the microclimate in forest gaps. Mean daytime temperature (T) and vpd were by 2 K and 2 hPa, respectively, higher in the oak forests than the beech forests; the extremes differed even more. Especially in the second half of the day, the oak forests heated up and were more xeric than the beech forests. Part of the differences is explained by the elevation difference between oak and beech forests (200–300 m), but species differences in canopy structure, leaf area, and canopy transmissivity enhance the microclimatic contrast. Our T and vpd data point to thresholds at about 30 °C and 25 hPa as maxima tolerated by beech in the lowermost shade canopy for extended periods. In conclusion, the rather sharp stand microclimatic gradient demonstrated here for the xeric distribution limit of beech may well be the decisive factor that hinders the spread of beech into the warmer oak forests.

Direct estimation of photon recollision probability using terrestrial laser scanning

Photon recollision probability p is a spectrally invariant structural parameter and a powerful tool to link canopy optical properties at any wavelengths to model reflectance, transmittance, or absorption of vegetation canopies. The concepts of the p-theory have been reported and examined at the shoot and canopy scales, but not yet for the crown level. Currently, the p-value is estimated indirectly, such as converted from the spherically averaged silhouette to total area ratio (STAR¯) or canopy transmittance measurements. In this work, we first validate the theoretical considerations of the p concept at the crown level (e.g., its relationship with STAR¯), and then provide the first method to directly estimate photon recollision probability using Terrestrial Laser Scanning (TLS) data. The proposed geometric method is data-driven and avoids explicit reconstructions of tree structures. The p-value estimated here is the average recollision probability over spatial locations. We showed that the average recollision probability can be interpreted as the local spherical openness on phytoelement (leaf or needle) surfaces, which enabled a simple visibility calculation by avoiding explicit ray tracing. The developed method was tested on synthetic crowns of needle-leaved tree species, for which the reference p-values were known. Results confirmed the validity of the p-STAR¯ relationship at the crown level, and showed that p-values can be accurately estimated from TLS point clouds with a relative root measure square error of less than 10%. This study displays the distinct advantage of TLS in delineating detailed tree crown structures and highlights its potential in studies of forest reflectance modeling.

HPEval: A canopy shortwave radiation transmission model using high-resolution hemispherical images

Forested environments feature a highly complex radiation regime. The three-dimensional canopy structure masks solar radiation as it penetrates through the forest. This entails sub-canopy radiation to vary in space, over time, season, and with weather conditions, which challenges both measurements and modelling. Here we combine the experimental acquisition of forest structure information with radiation transfer modelling to achieve efficient and accurate estimation of sub-canopy shortwave radiation at very high temporal resolution. For this purpose we have developed a Matlab-based software tool to analyse digital hemispherical imagery for directional canopy transmissivity which yields, in combination with measured or estimated above-canopy shortwave radiation, respective below-canopy radiation. We demonstrate the utility of the tool to accurately model shortwave radiation in comparison to pyranometer measurements at both the point scale and along a transect under heterogeneous canopy cover. Optimized for computational efficiency the software even enables fully-distributed simulations. Without necessitating high performance computing or complex ray-tracing, this tool enables easy access to detailed sub-canopy radiation information. These should be valuable for validating site-scale radiation transfer models, as input to eco-hydrological and snowmelt models, and for researching light-dependant bio-physiological processes.

Rainfall variability, soil heterogeneity, and role of trees in influencing maize productivity—the case from an on-station agroforestry experiment in semi-arid Kenya

ABSTRACTGiven that crop yield in smallholder farms is influenced by rainfall and soil variability, farmers have designed strategies to address water deficit and enhance productivity, including adopting agroforestry. However, there is insufficient evidence demonstrating the combined effect of rainfall variability, soil heterogeneity and agroforestry treatment on maize productivity in semi-arid Kenya. We evaluated performance of agroforestry trees (Vachellia seyal, V. xanthophloea, Cordia africana, Faidherbia albida and Grevillea robusta) in monoculture and mixed-species treatments in a long-term experiment in Kiambu, Kenya, and their influence on maize intercrop. The trial was laid out in Randomized Complete Block Design having a mix of Cambisol and Vertisol soils. Tree growth and phenology was monitored monthly while maize yield assessed in four cropping seasons. Growth was higher in 2014 for all species compared to other years (P < .001). Cordia africana and F. albida in mixed stand and Cambisols exceeded those in monoculture and Vertisols (P < .001). Maize yield grown under C. africana and mixed-species during 2015/16 season was 54% and 44% higher than the control with corresponding canopy transmittance values of 18% and 24% respectively. Maize yield was highest in 2013/14 season (4.2 t ha−1) with rainfall distribution affecting productivity in 2014/15 season whereby 74% of rainfall was received in 1 month resulting in crop failure. These findings demonstrate the reality of climate (rainfall) variability, and the influence of soil properties heterogeneity while supporting call for mixed-species and encouraging efforts aimed at forecasting rainfall patterns as this affects farming operations.

Consequences of environmental heterogeneity for the photosynthetic light environment of a tropical forest

We studied the interplay of atmospheric and canopy structure factors driving the canopy light environment (Photosynthetic Photon Flux Density, PPFD) in primary moist tropical forest in the Tapajós National Forest, Brazil. We quantified the temporal and spatial length scales that characterize intact rain forest inhomogeneities, asking: Are seasonal changes in the canopy radiation balance evident at these scales? We sought to describe the components of intensity, duration and spatial variation of within-canopy PPFD in light of these inhomogeneities. Do fluctuating atmospheric conditions, especially the presence of clouds and precipitation, affect the radiative inputs at both the canopy top and the forest floor?We examined the characteristic scales of heterogeneity in the vertical and the horizontal using a two-part approach. For radiation we combined long-term continuous high-frequency measurements of down- and up-welling short-wave and visible wavebands above the canopy with similarly frequent observations from a dense sensor network at forest floor. Vertical variations in canopy structure, obtained with a ground-based LIDAR, similarly combined intensive observations at the sensor network with occasional large-scale transects along the forest floor. Close similarities in both radiation and canopy structure at both scales support the representativeness of local observations of the wider area.A composite broadband measure similar to the Normalized Difference Vegetation Index (denoted cbNDVI) was constructed from above-canopy observations to compare with reports of remotely sensed canopy reflectivity at this site. We estimated the canopy Leaf Area Index (LAI) by combining observations of the variation in understory transmittance by solar elevation with a commonly-used algorithm. We obtained the conventional whole-canopy extinction coefficient, by applying the Beer-Lambert law.Over the course of a year this forest receives 11,795 mol m−2, only 62% of potential clear-day PPFD – atmospheric transmissivity is reduced by clouds, precipitation, smoke and haze. Very little PAR (≈ 2%) is reflected from the canopy and only 5.7% penetrates to 1 m above the forest floor - overall 92% of PAR is absorbed by the canopy. All radiation balance components closely tracked the dynamics of incoming light, showing little seasonal variation.Understory light observations across the 7.5–28.5 m spanning the understory array sensors showed essentially constant correlation between sensor pairs over time. There was a high degree of local persistence in the understory spatial pattern that varied slowly and directionally with the changing geometry of the sun and canopy structure. Over larger distances (to 1000 m), the patterns of spatial autocorrelation of understory PPFD and outer canopy structure were remarkably similar in shape, both declining rapidly to a more-or-less constant level around 15–20 m, a scale consistent with the dimensions of outer canopy crowns.The vertical pattern of transmission and absorption was estimated by combining understory transmittance with the distribution of canopy surface area obtained from the ground-based LIDAR system. It showed the maximum absorption relatively low in the canopy (8–19 m above ground). Although rather tall (canopy height is at 41.5 m), the extremely elaborate outer surface of the forest suggests the layers highest above ground are of little consequence to the PAR absorption budget.The cbNDVI measure exhibited seasonal variation consistent with other reports (somewhat higher in the wet season) but in contrast to a recent argument that the forest ‘greens up’ during the early dry season. However, except for the variations caused by the angle and intensity of incoming light modulated by atmospheric effects, there was little seasonality in the forest light environment at km67, including: canopy reflectances in several wavebands, all PPFD radiation budget components, the estimated LAI and mid-day PAR extinction coefficient and the length scales of understory PPFD. Canopy transmission was somewhat greater (6.4%) under diffuse skies compared to the least diffuse conditions (6.0%), and slightly more PPFD was absorbed under diffuse (92.7%) versus the least diffuse conditions (92.3%), but was not a significant contribution to the budget.The large diurnal variations in the extinction coefficient seriously affects its utility as a descriptor of canopy radiative properties. We propose an alternative approach for transmitted light: the canopy behaves as: 1.) a constant fraction filter under diffuse conditions, combined with 2.) a variable filter depending on solar elevation for sunfleck conditions. These regimes may be described with simple parameters each having mechanistic relations to canopy structure. In summary, we demonstrate that obtaining the radiation signal at forest floor at high data rate for long periods exploits seasonal sun angle changes to probe canopy structure. Combined with occasional long transect information, temporal and vertical sampling spatially allow improved definition of characteristic scales to describe both the understory light environment and canopy structure.

Spatially Continuous Characterization of Forest Canopy Structure and Subcanopy Irradiance Derived from Handheld Radiometer Surveys

Abstract Small-scale variations in radiative transfer through forest canopies are strongly linked to canopy structural heterogeneity. To date, upscaling of radiative transfer parameterizations developed at the point scale is hampered by (i) poor representation of canopy structure variability and (ii) limited spatially explicit subcanopy irradiance data to assess the performance of radiative transfer schemes at typical resolutions of land surface models. This study presents a novel approach for efficient in situ characterization of canopy structure and subcanopy irradiance over large spatial extents. The method involves a handheld radiometer assembly mounted on a motorized gimbal developed for nonstationary continuous measurements of shortwave and longwave radiation along forest transects. In combination with radiation and temperature data from a stationary reference station, spatially resolved estimates of sky-view fraction, canopy transmissivity, and longwave enhancement could be obtained. Under favorable meteorological conditions, validation against sky-view fraction data from hemispherical photographs yielded an RMSE of 0.03 (i.e., 3%). Irradiance measurements under heterogeneous canopy cover revealed strong spatial coherence between longwave radiation enhancement, shortwave radiation attenuation, and sky-view fraction on overcast days. Under clear-sky conditions, however, sun flecks caused highly variable shortwave radiation transmissivity patterns. This study demonstrates the potential of handheld radiometer surveys to deliver valuable spatially distributed datasets of collocated canopy structure and subcanopy irradiance which can be used (i) as reference data for alternative approaches to derive canopy structure parameters, (ii) to improve modeling of subcanopy radiation across a wide range of canopy distributions, and (iii) to support respective model upscaling efforts.

Observations of a Coniferous Forest at 9.6 and 17.2 GHz: Implications for SWE Retrievals

UWScat, a ground-based Ku- and X-band scatterometer, was used to compare forested and non-forested landscapes in a terrestrial snow accumulation environment as part of the NASA SnowEx17 field campaign. Field observations from Trail Valley Creek, Northwest Territories; Tobermory, Ontario; and the Canadian Snow and Ice Experiment (CASIX) campaign in Churchill, Manitoba, were also included. Limited sensitivity to snow was observed at 9.6 GHz, while the forest canopy attenuated the signal from sub-canopy snow at 17.2 GHz. Forested landscapes were distinguishable using the volume scattering component of the Freeman–Durden three-component decomposition model by applying a threshold in which values ≥50% indicated forested landscape. It is suggested that the volume scattering component of the decomposition can be used in current snow water equivalent (SWE) retrieval algorithms in place of the forest cover fraction (FF), which is an optical surrogate for microwave scattering and relies on ancillary data. The performance of the volume scattering component of the decomposition was similar to that of FF when used in a retrieval scheme. The primary benefit of this method is that it provides a current, real-time estimate of the forest state, it automatically accounts for the incidence angle and canopy structure, and it provides coincident information on the forest canopy without the use of ancillary data or modeling, which is especially important in remote regions. Additionally, it enables the estimation of forest canopy transmissivity without ancillary data. This study also demonstrates the use of these frequencies in a forest canopy application, and the use of the Freeman–Durden three-component decomposition on scatterometer observations in a terrestrial snow accumulation environment.

VARIAÇÃO ESPAÇO-TEMPORAL DA PAR REFLETIDA PELO SOLO E TRANSMITIDA PELO DOSSEL EM FLORESTA INUNDÁVEL NO PANTANAL MATO-GROSSENSE

A cobertura do solo e o tipo de vegetação influenciam nos fluxos radiativos, na refletância do solo abaixo do dossel e na transmitância do dossel. Neste contexto, este trabalho analisou a variação espacial e temporal da radiação fotossinteticamente ativa refletida pelo solo em uma floresta sazonalmente inundável com dominância de cambará (Vochysia divergens Pohl) no Pantanal Mato-grossense. A área de estudo foi dividida em 5 transectos com 11 pontos amostrais, totalizando 55 pontos medidos de dezembro de 2011 a novembro de 2012. A análise da distribuição espacial da radiação fotossinteticamente ativa refletida pelo solo foi realizada utilizando semivariogramas, aplicando-se o modelo esférico. Encontrou-se correlação negativa das variáveis transmitância do dossel, radiação fotossinteticamente ativa incidente abaixo do dossel e refletida pelo solo com o índice de área foliar e positiva entre radiação fotossinteticamente ativa incidente abaixo do dossel e refletida pelo solo. Os resultados espaciais indicaram dependência espacial da radiação fotossinteticamente ativa refletida nos 9 meses analisados. O efeito pepita foi pequeno nas análises e o alcance mais comum entre os semivariogramas foi 100 m em dezembro∕2011, janeiro∕2012 e novembro∕2012, com o maior alcance ocorrendo no mês de junho/2012. As mudanças da radiação fotossinteticamente ativa refletida podem ser consideradas pela diferença da cota topográfica, ocorrendo acúmulo de umidade no transecto mais baixo, e pela diferença de vegetação encontrada na região, alterando a coloração e/ou cobertura do solo, mudando assim a reflexão.

The stochastic Beer–Lambert–Bouguer law for discontinuous vegetation canopies

The 3D distribution of canopy foliage affects the radiation regime and retrievals of canopy biophysical parameters. The gap fraction is one primary indicator of a canopy structure. Historically the Beer–Lambert–Bouguer law and the linear mixture model have served as a basis for multiple technologies for retrievals of the gap (or vegetation) fraction and Leaf Area Index (LAI). The Beer–Lambert–Bouguer law is a form of the Radiative Transfer (RT) equation for homogeneous canopies, which was later adjusted for a correlation between fitoelements using concept of the clumping index. The Stochastic Radiative Transfer (SRT) approach has been developed specifically for heterogeneous canopies, however the approach lacks a proper model of the vegetation fraction. This study is focused on the implementation of the stochastic version of the Beer–Lambert–Bouguer law for heterogeneous canopies, featuring the following principles: 1) two mechanisms perform photon transport- transmission through the turbid medium of foliage crowns and direct streaming through canopy gaps, 2) the radiation field is influenced by a canopy structure (quantified by the statistical moments of a canopy structure) and a foliage density (quantified by the gap fraction as a function of LAI), 3) the notions of canopy transmittance and gap fraction are distinct. The derived stochastic Beer–Lambert–Bouguer law is consistent with the Geometrical Optical and Radiative Transfer (GORT) derivations. Analytical and numerical analysis of the stochastic Beer–Lambert–Bouguer law presented in this study provides the basis to reformulate widely used technologies for retrievals of the gap fraction and LAI from ground and satellite radiation measurements.