Richard H. Grant

Partitioning of biologically active radiation in plant canopies

Plant germination, growth, maturation, and productivity are heavily influenced by the quality and quantity of the light in its environment. The light environment has traditionally been quantified in terms of radiant heat energy and available photosynthetic radiation (PAR), but detailed spectral irradiance or photon flux distributions have rarely been studied. This information is needed to translate the research that plant photobiologists and photochemists have been conducting with regard to understanding the light controls on plant physiology in the field environment of plant canopies. More interest has recently been generated as the potential impacts of global climate changes on intensively managed and natural terrestrial ecosystems are identified and evaluated. Linkages between the identified impacts of various wavelengths of light on plant physiology and the light environment of the plant canopy are identified, with detailed discussion concerning the impacts of plant canopy structure on the plant light response. Solar radiation in the ultraviolet-B (280–320 nm), ultraviolet-A and blue (350–500 nm), PAR (400–700 nm), blue (400–500 nm), green (500–600 nm) red (600–700 nm), far red (700–800 nm) and near infrared (800–1100 nm) is followed from the top of the plant canopy to the photoreceptor at the cellular level within the plant phytoelement.

Photosynthetically-active radiation: sky radiance distributions under clear and overcast conditions

The photosynthetically active radiation (PAR), defined as the wavelength band of 0.400 μm to 0.700 μm, represents most of the visible solar radiation. Although the proportion of global irradiance that originates from diffuse sky radiation is higher for PAR than for all solar shortwave radiation, it is often assumed that the PAR diffuse sky radiation is distributed identically to that of all shortwave solar radiation. This assumption has not been tested. PAR sky radiance measurements were made in a rural area over a wide range of solar zenith angles. The distribution of PAR sky radiance was modeled using physically-based, non-linear equations. For clear skies, the normalized sky radiance distribution (N) was best modeled using the scattering angle (ψ) and the zenith position in the sky (Θ) as N(Θ,ψ)=0.0361[6.3+(1 + cos2Θ)(1 − cosψ)][1 − e−0.31 secΘ]. The angle Ψ is defined by cos ψ = cosΘcosΘ∗ + sinΘsinΘ∗cosΦ, where solar zenith angle is Θ* and the difference in azimuth between the sun and the position in the sky is Φ. Modeling of the overcast sky depended on the visibility of the solar disk. The translucent middle/high cloud overcast conditions (cloud base greater than 300 m above ground level) were best modeled as: N(Θ∗, ψ) = 0.149 + 0.084Θ∗ + 1.305e−2.5ψ while the translucent low cloud overcast conditions (cloud base less than 300 m above ground level) were best modeled as: N(Θ∗, ψ) = 0.080 + 0.058Θ∗ + 0.652e− 2.1ψ. The obscured overcast sky condition (solar disk obscured) was best modeled as: N(Θ) = 0.441[1 + 4.6cosΘ][1 + 4.6]. The unit of N for all equations is π Sr−1, so that integration of each function over the sky hemisphere yields 1.0. These equations can be applied directly to the sky diffuse irradiance on the horizontal, Idiff, to provide radiance distributions for the sky. Estimates of actual sky radiance distribution can be estimated from Na(Θ,ψ) = IdiffN(Θ,Φ).

Estimation of Ultraviolet-B Irradiance under Variable Cloud Conditions

Abstract Methods to estimate the irradiance of ultraviolet-B (UVB; 280–320 nm) radiation are needed to assess biological effects of changes in atmospheric composition. Measurements of the spatial distribution of sky cloud cover, temporal variability of photon flux density of photosynthetically active radiation (PAR; 400–700 nm), and UVB irradiance (I-UVB) were made on 23 days during the summer of 1993 in a rural area (West Lafayette, Indiana). Prediction equations for the measured UVB irradiance under partly cloudy skies were developed based on the photosynthetically active photon flux density (PPFD), cloud cover fraction, probability of cloud obstruction of the sun, and a semiempirical combination of cloud probability and cloud cover. The I-UVB was linearly related to the PPFD, with the variability in PPFD accounting for 77% of the I-UVB variability. Normalized PPFD (PAR F) and I-UVB (UVB F) values, calculated by dividing the observed value by the expected cloud-free sky PPFD and I-UVB, were also linearl...

Obscured Overcast Sky Radiance Distributions for Ultraviolet and Photosynthetically Active Radiation

Abstract Sky radiance measurements in the wavelength bands of ultraviolet-B (0.28–0.32 μm), ultraviolet-A (0.32–0.40 μm), and photosynthetically active radiation (0.40–0.70 μm) were made under obscured overcast skies in a rural area. Radiance distributions were modeled for seven measurement scans with solar zenith angles varying from 19° to 49°. For the seven scans, the atmospheric transmittance of photosynthetically active photon flux density varied from 0.16 to 0.25. The corresponding fraction of cloud-free sky photosynthetically active photon flux density ranged from 0.21 to 0.32. The corresponding fraction of ultraviolet-B cloud-free sky irradiance was between 0.20 and 0.34, with typically lower fractions of cloud-free sky irradiance in the ultraviolet-B than in the photosynthetically active photon flux density. The sky radiance was modeled from the ensembled measurements according to the standard overcast sky radiance distribution for each of the wavelength bands. Although the ultraviolet wave bands ...

Effects of Supplementary Ultraviolet-B Irradiance on Maize Yield and Qualities: A Field Experiment¶

Abstract Stratospheric ozone depletion has caused an increase in the amount of ultraviolet-B (UV-B) radiation reaching the earths surface. Numerous investigations have demonstrated that the effect of UV-B enhancements on plants includes reduction in grain yield, alteration in species competition, susceptibility to disease and changes in plant structure and pigmentation. Many experiments examining UV-B radiation effects on plants have been conducted in growth chambers or greenhouses. It has been questioned whether the effect of UV-B radiation on plants can be extrapolated to field responses from indoor studies because of the unnaturally high ratios of UV-B/ultraviolet-A radiation (320–400 nm) and UV-B/photosynthetically active radiation (PAR) in many indoor studies. Field studies on UV-B radiation effect on plants have been recommended to use the UV and PAR irradiance provided by natural light. This study reports the growth and yield responses of a maize crop exposed to enhanced UV-B radiation and the UV-B effects on maize seed qualities under field conditions. Enhanced UV-B radiation caused a significant reduction in the dry matter accumulation and the maize yield in turn was affected. With increased UV-B radiation the flavonoid accumulation in maize leaves increased and the contents of chlorophyll a, b and (a + b) of maize leaves were reduced. The levels of protein, sugar and starch of maize seed decreased with enhanced UV-B radiation, whereas the level of lysine increased with enhanced UV-B radiation.

Clear sky radiance distributions in ultraviolet wavelength bands

SummaryIn the ultraviolet, the diffuse sky radiation component is often at least 50% of the global irradiance for middle latitudes. Thus, knowledge of the sky radiance distribution is important for modeling ultraviolet irradiance in vegetative or urban canopies. In this study, the distribution of clear sky radiance was measured and modeled for wavelength bands of ultraviolet-B (280–320 nm) and ultraviolet-A (320–400 nm). Sky radiance measurements were made in a rural area over a wide range of solar zenith angles using radiance sensors mounted on a hand-operated hemispherical rotation mount. The measured sky radiance distribution in the UVA waveband differed greatly from that in the UVB waveband. The sky UVB radiance varied less across the sky hemisphere than the sky UVA radiance, in accordance with theory. A distinct region of minimum sky radiance was commonly found in the UVA waveband, but not the UVB waveband.Nonlinear regression modeling of the sky radiance in the two wavebands showed that the isotropic sky was a poor descriptor of the UV radiance distribution. An anisotropic model of the clear sky UV radiance distributions was developed that used the scattering angle between the sun and the location in the sky and the sky zenith angle as predictor variables. The model coefficients of determination (r2) values were 0.93 and 0.69 for the UVA and UVB waveband models respectively.The model equations can be applied directly to the sky diffuse irradiance on the horizontal to provide radiance distributions for the sky. In addition, these distributions can be used to estimate correction factors for the shadowband method of diffuse irradiance estimation. Since the surface albedo of the measurement location was low and not explicitly used in the model equations, the distributions modeled may not apply to skies over snow cover or other high albedo surfaces.

Estimation of Pedestrian Level UV Exposure Under Trees

Abstract Trees influence the amount of solar UV radiation that reaches pedestrians. A three-dimensional model was developed to predict the ultraviolet-B (UV-B) irradiance fields in open-tree canopies where the spacing between trees is equal to or greater than the width of individual tree crowns. The model predicted the relative irradiance (fraction of above-canopy irradiance) under both sunlit and shaded conditions under clear skies with a mean bias error of less than 0.01 and a root mean square error of 0.07. Both model and measurements showed that the locations people typically perceive as shady, low-irradiance locations in the environment can actually have significant UV-B exposure (40–60% of that under direct sunlight). The relationship of tree cover in residential neighborhoods to erythemal UV-B exposure for children and adults was modeled for the 4 h around noon in June and July. Results showed that human exposures (on the horizontal) in cities located at 15 and 30° latitudes are nearly identical. For latitudes between 15 and 60°, ultraviolet protection factors (UPF) were less than 2 for less than 50% tree cover. A UPF of 10 was possible at all latitudes for tree cover of 90%.

Ammonia emission model for whole farm evaluation of dairy production systems.

Ammonia (NH) emissions vary considerably among farms as influenced by climate and management. Because emission measurement is difficult and expensive, process-based models provide an alternative for estimating whole farm emissions. A model that simulates the processes of NH formation, speciation, aqueous-gas partitioning, and mass transfer was developed and incorporated in a whole farm simulation model (the Integrated Farm System Model). Farm sources included manure on the floor of the housing facility, manure in storage (if used), field-applied manure, and deposits on pasture (if grazing is used). In a comprehensive evaluation of the model, simulated daily, seasonal, and annual emissions compared well with data measured over 2 yr for five free stall barns and two manure storages on dairy farms in the eastern United States. In a further comparison with published data, simulated and measured barn emissions were similar over differing barn designs, protein feeding levels, and seasons of the year. Simulated emissions from manure storage were also highly correlated with published emission data across locations, seasons, and different storage covers. For field applied manure, the range in simulated annual emissions normally bounded reported mean values for different manure dry matter contents and application methods. Emissions from pastures measured in northern Europe across seasons and fertilization levels were also represented well by the model. After this evaluation, simulations of a representative dairy farm in Pennsylvania illustrated the effects of animal housing and manure management on whole farm emissions and their interactions with greenhouse gas emissions, nitrate leaching, production costs, and farm profitability.

The scaling of flow in vegetative structures

A three-prong spruce twig was modeled as three independent circular cylinders as part of a wind tunnel study of the drag and wake characteristics of twigs in flow velocities ranging from 0.5 to 4.5 m s-1. Using this model, the wake structure and twig drag were explained and length scales describing the microscale wake turbulence were proposed.Results indicate that the drag force on the twig is influenced by the two flow regimes within the range of velocities studied. The two flow regimes are separated by an apparent shift in the flow field evidenced by a rapid change in the drag coefficient at approximately 1.5 m s-1. For the low velocity regime (0.5 to 1.5 m s-1), the flow ‘sees’ a highly porous twig-cylinder with pores a function of the inter-needle spacing and a length scale related to the needle length. For the high velocity regime (1.5 to 4.5 m s-1), the flow ‘sees’ a twig-cylinder with low porosity and a dominant length scale related to the twig needle-tip to needle-tip diameter.

Biologically Active Radiation in the Vicinity of a Single Tree

The horizontal photon flux density of photosynthetically active radiation (PAR) and flux density of ultraviolet A (UVA) and ultraviolet B (UVB) radiation were measured in the vicinity of isolated single trees during the summer of 1996. Measurements were made under shade and sunlit conditions along a transect aligned with the solar disk and the tree trunk. Flux density measurements were normalized by the flux density at a reference location away from the tree. Results showed (1) a more rapid decline in the flux density of UVA and UVB radiation than PAR with decreasing distance to the tree trunk on both the sunlit and shaded side of a tree and (2) more rapid changes in the flux density of UVB radiation UVA radiation, and PAR with distance from the tree on the sunlit side of the tree than the shaded side of the tree. The UVB/PAR ratio was found to increase in the shadow of a tree with increasing distance from the tree to between 4 and 6 for the conditions of the study. The potential for detrimental effects by UVB flux density under conditions of the high ratio may be mitigated by sunflecks at a given location over the course of a day.