Partitioning Of Net Radiation Research Articles

A comparative study of mass and energy exchange over a closed C 3 (wheat) and an open C 4 (corn) canopy: I. The partitioning of available energy into latent and sensible heat exchange

Eddy correlation measurements of mass and energy were made simultaneously over a closed wheat and an open corn crop. This combination of crops, differing in canopy coverage and stomatal physiology, facilitated an investigation on how surface and environmental variables control the partitioning of net radiation into sensible, latent and soil heat exchange. Contrasts in canopy closure had a minor affect on the flux densities of net radiation ( R n) measured over open corn and closed wheat canopies. On the other hand, differences in canopy closure perturbed the partitioning of net radiation into sensible, latent and soil heat exchange. On average, measurements of latent heat flux densities ( LE) over the sparse corn canopy were 59% less than LE measured over the closed wheat crop. Compensation for this bias caused greater flux densities of sensible ( H) and soil ( G) heat to be generated from the sparse corn canopy. Sensible heat flux densities were, on average, 22% greater over the open corn canopy. During the day, soil heat flux densities were, on average, 64% greater under the corn canopy than under the wheat. Available energy ( A) governed daytime latent heat flux densities over the closed wheat canopy. Consequently, LE was proportional to equilibrium evaporation rates ( LE eq). When the corn canopy was dry, latent heat flux densities were less coupled to available energy ( A). This response occurred because exposed soil and the corn's C 4 physiology increased the corn's canopy resistance to water vapor transfer. When the sparse corn canopy was wet, LE approached values experienced by the closed wheat canopy. A rule of thumb, explaining the surface control of latent heat exchange, was extracted from these data. The ratio between LE and LE eq diminished linearly with logarithmic increases of surface resistance. Different processes controlled canopy evaporation at night. Nocturnal evaporation flux densities measured over the closed wheat crop were independent of available energy, and instead were a function of the atmosphere's vapor pressure deficit. Evaporation flux densities measured over the sparse crop were weakly dependent on available energy.

The impact of soil moisture availability upon the partition of net radiation into sensible and latent heat fluxes

The impact of soil moisture availability on the Bowen ratio and on the partition of net radiation flux into sensible, latent and soil heat fluxes was investigated by using one-dimensional primitive equations with a refined soil parameterization scheme. Simulation results presented that as soil moisture availability increases, the Bowen ratio and the partition of net radiation flux into sensible and soil heat fluxes decrease. The partition of net radiation flux in to latent heat flux, however, increases. Quantitative relationships between Bowen ratio and the partitions with soil moisture availability were also given in this study.

The Priestley-Taylor parameter α for the Amazon forest

Priestley and Taylor provided a practical formulation of the partitioning of net radiation between heat flux and evaporation contained within a parameter α. Their model (PTM) needs verification under a range of environmental conditions. Micrometeorological data sets collected over the Amazon forest at the Ducke Reserve site (2°57′S; 59°57′W) gave an opportunity to evaluate α. Evidence presented here and by others shows that there is pronounced diurnal variation in α, with minimum values around midday and maximum values in the morning and evening hours. During unstable and stable conditions in the daylight hours, the Bowen ratio ( B) varied from 0.10 to 0.57 and −0.71 to −0.08, respectively, whereas α varied from 0.67 to 1.16 and 1.28 to 3.12, respectively. A mean value of α = 1.16±0.56 was obtained from daytime hourly values for two days. The daily data sets from three expeditions gave a mean of α = 1.03±0.13. This work confirms that α is a function of atmospheric stability over the Amazon forest. Thus the PTM should be applied with caution over time-intervals of one day or less because of the sensitivity to variation in α. The calculated values of α are in general agreement with those reported in literature.

Soil Surface Roughness Effects on Radiation Reflectance and Soil Heat Flux

AbstractSoil surface roughness provides a mechanism to alter soil reflectance and the surface energy balance. A field study was conducted to determine the effect of surface roughness on energy absorption and energy partitioning at the soil surface. A range of surface roughness conditions was created by varying the intensity of secondary tillage after moldboard plowing. Parameters measured included spectral reflectance, net radiation, soil temperature, and soil heat flux. Reflectance of solar radiation decreased with increasing surface roughness, with the greatest differences occurring among radiation wavelengths of 850 and 1350 nm. Reflectance was similar between 400 and 850 nm. Reflectance from a visibly dry surface was increased up to 25% in some wavelengths 5 d after a 47‐mm rainfall event, possibly because of decreased surface roughness. Although net radiation increased with increased surface roughness, soil heat flux at 0.01‐m was similar for all roughness conditions. This indicates that the latent and/or sensible heat flux was increased by increasing surface roughness. Theoretical considerations indicate that increasing surface roughness results in greater transport of energy from the soil surface to the atmosphere. Relationships between the change in net radiation due to surface roughness and the partitioning of net radiation among the surface energy balance components are developed and presented graphically.

Water use efficiency in a soybean field: influence of plant water stress

Measurements were made in 1980 over a fully-developed soybean ( Glycine max (L.) Merrill) canopy at Mead, Nebraska to determine how crop water status influences photosynthesis, evapotranspiration and water use efficiency. Water use efficiency was calculated in terms of the CO 2—water flux ratio ( CWFR). Micrometeorological techniques were used to measure the exchange rates of CO 2 and water vapor above the crop canopy. Crop water status was evaluated by reference to volumetric soil moisture ( θ v), stomatal resistance ( r s), and leaf water potential (ψ) measurements. Stomatal resistance ( r s) was independent of ψ when the latter was greater than −1.1 MPa. r s increased sharply as ψ dropped below this threshold. Canopy CO 2 exchange ( F c) decreased logarithmically with increasing r s under strong irradiance. Although F c was found to be strongly correlated with r s, the influence of low values of ψ and of high air temperature cannot be discounted since these factors affect the enzymatic reactions associated with photosynthesis. Stomatal closure also reduced evapotranspiration and influenced the partitioning of net radiation. Under strong irradiance the CO 2 water flux ratio (CWFR) decreased with increasing stomatal resistance. This observation is at variance with predictions of certain early ‘resistance’ models, but substantiates predictions of some recent models in which leaf energy balance considerations are incorporated.

Leaf Pubescence Effects on the Mass and Energy Exchange Between Soybean Canopies and the Atmosphere1

AbstractMass and energy exchanges with the atmosphere were compared in two soybean (Glycine max L. Merr. cv. Harosoy) isolines differing in pubescence density. The study was conducted in a field with a Sharpsburg silty clay loam soil (fine, montmorillonitic, mesic Typic Argiudoll) during the summer of 1980 at Mead, Nehr. Mass and energy exchanges were determined by means of micrometeorological techniques. Evapotranspiration (reported in terms of latent heat flux) was reduced in the densely pubescent isoline. Canopy CO2 exchange was unchanged on a per unit land area basis. Water use efficiency (reported in terms of the CO2‐water flux ratio) was, accordingly, greater in the densely pubescent isoline. The increase in pubescence did not significantly alter the net radiation balance, turbulent mixing, canopy CO2 exchange, or plant water status. Observed differences in the partitioning of net radiation into latent and sensible heat can be explained by greater penetration of solar radiation into the densely pubescent canopy. Leaf pubescence appears to alter the spectral characteristics of the leaf and, thus, to facilitate the penetration of solar radiation into the canopy.

The Energy and Mass Exchange Characteristics of a Loblolly Pine Plantation

(1) Energy balance measurements based on Bowen ratio techniques were used in conjunction with the big-leaf model to characterize energy and mass exchange between the atmosphere and the forest canopy in a loblolly pine plantation. (2) The partitioning of net radiation into latent and sensible heat by the Bowen ratio varied greatly with time of day and from day to day even when measured at the same time of day. Such variation results from variations in net radiation, canopy resistance, and air vapour density deficit. The maximum Bowen ratio during daylight hours was 2.3 and the minimum 0.1 1. (3) Canopy conductance was significantly correlated with solar radiation flux density in the canopy. (4) Significant correlations were found between average stomatal conductance (calculated from canopy resistance and leaf area index) and leaf temperature, vapour density deficit, and solar radiation flux in the canopy.