Canopy Net Radiation Research Articles

Radiative balance and temperature of differently pigmented cotton canopies.

Pigments are known to modify the spectral properties of foliage, which in turn affect the amount of radiant energy stored by the plant canopy. Studies have shown that red pigments (anthocyanin) increase leaf absorptivity of solar radiation, but little is known about their effect on canopy net radiation and temperature. We hypothesized that increased absorptivity of solar radiation caused by red pigments would result in higher canopy temperature when compared to that of a green canopy. To better understand the role of red pigments on canopy net radiation and temperature, we conducted a study where we measured leaf spectral properties, canopy spectral reflectivity, stomatal conductance, net radiation, and leaf and canopy temperature of red and green cotton (Gossypium hirsutum L.) canopies. On average, albedo of the red canopy was 0.02 lower than that of the green canopy, and most of the differences in reflected solar irradiance were in near-infrared wavelengths. Red canopy had greater net radiation than the green canopy throughout the measurement period, and that was due to its lower albedo. Red canopy was about 1°C warmer than the green canopy on average; however, computer simulation indicates that albedo was of secondary importance in controlling canopy temperature. Contrary to our hypothesis, results suggest that lower stomatal conductance in the red leaves was the main driver of canopy temperature differences between red and green canopies.

Plastic Film Mulch Performed Better in Improving Heat Conditions and Drip Irrigated Potato Growth in Northwest China than in Eastern China

Plastic film (i.e., transparent and black films) is widely applied in mulching to improve thermal environments and plant growth in China. However, choosing suitable film for potato cultivation in different areas is still a problem. A four-year field experiment was conducted to explore the influences of plastic mulch on heat transfer (soil heat flux, G; soil temperature; and canopy net radiation, Rn) and potato growth in two typical areas of China: Experiment 1 in Northwest China (2014 and 2015) and Experiment 2 in Eastern China (2016 and 2017). Three mulch treatments were designed as follows: (1) transparent film (TF), (2) black film (BF), and (3) no film (NF). Results showed that the Rn and G differences between TF, BF, and NF varied at different areas. However, the Rn in TF was always smaller (average 12.4 W/m2 in 2014–2017) than in BF in the two study areas. The differences of soil temperature between TF, BF, and NF in Experiment 1 were greater than in Experiment 2. The average soil temperature difference between TF and NF was 1.4 °C higher in Experiment 1 than in Experiment 2. TF had 23% and 19% more jumbo tubers than BF in weight and number on average with no significant difference in 2015–2017. TF and BF significantly had greater potato yield and irrigation water use efficiency than NF on average in Experiment 1, while no significant differences were found in Experiment 2. The transparent film was more beneficial to increase soil temperature and jumbo tubers than the black film. Film mulch functioned better for improving heat conditions and drip-irrigated potato growth in Northwest China than in Eastern China.

Canopy radiation transmission for an energy balance snowmelt model

To better estimate the radiation energy within and beneath the forest canopy for energy balance snowmelt models, a two stream radiation transfer model that explicitly accounts for canopy scattering, absorption and reflection was developed. Upward and downward radiation streams represented by two differential equations using a single path assumption were solved analytically to approximate the radiation transmitted through or reflected by the canopy with multiple scattering. This approximation results in an exponential decrease of radiation intensity with canopy depth, similar to Beer's law for a deep canopy. The solution for a finite canopy is obtained by applying recursive superposition of this two stream single path deep canopy solution. This solution enhances capability for modeling energy balance processes of the snowpack in forested environments, which is important when quantifying the sensitivity of hydrologic response to input changes using physically based modeling. The radiation model was included in a distributed energy balance snowmelt model and results compared with observations made in three different vegetation classes (open, coniferous forest, deciduous forest) at a forest study area in the Rocky Mountains in Utah, USA. The model was able to capture the sensitivity of beneath canopy net radiation and snowmelt to vegetation class consistent with observations and achieve satisfactory predictions of snowmelt from forested areas from parsimonious practically available information. The model is simple enough to be applied in a spatially distributed way, but still relatively rigorously and explicitly represent variability in canopy properties in the simulation of snowmelt over a watershed.

Radiation balance of coffee hedgerows

The radiation balance of hedgerows is an important variable in studies of mass and energy exchanges between parcial ground cover crops and the atmosphere. This paper describes a device with eight net radiometers encompassing the plants of a hedgerow. The radiometers were moved along a length of hedgerow, in a continuous and reversible movement. The canopy net radiation in this length (Rnc) was found by integration of the measurements over the notional cylinder formed. The device showed good performance and provided reliable measurements of Rnc of coffee hedgerows, showing itselfto be an useful technique of measurement in field conditions. Good correlations between Rnc and global solar radiation, turfgrass and coffee crop net radiation were found in 15-min, daytime and 24-hr periods, allowing the possibility of estimating Rnc from these simple measurements. Beer’s law was also used to have an independent estimation of Rnc. A good agreement was found between values of Rnc estimated by this law of attenuation and those integrated by the device in periods of 15 min, with overestimation of 10%, whereas for values integrated over daytime periods the agreement was not satisfactory.

Comment on “Relationships between single tree canopy and grass net radiations”

Pereira et al. [Pereira, A.R., Green, S., Villa Nova, N.A., 2007. Relationships between single tree canopy and grass net radiations. Agric. Forest Meteorol. 142 (1), 45–49] have proposed an empirical relationship between whole tree canopy net radiation and grass net radiation as a function of tree leaf area. In this work, I show that the relationship does not hold for either isolated trees or trees in orchards. A simple model of tree interception is used to analyze the scaling up of radiation interception by isolated trees while a published radiation interception model is used for trees in canopies. The two models are able to predict some of the data presented by Pereira et al. and explain the main causes of variation. For isolated trees radiation interception is proportional to leaf area but the coefficient is much higher than that found by the authors. For trees in orchards, net radiation absorbed might be calculated as a function of intercepted radiation using available models.

Sap flow, leaf area, net radiation and the Priestley–Taylor formula for irrigated orchards and isolated trees

This paper describes the goodness of fit for two simple methods to estimate the daily sap flow of irrigated, non-stressed apple and olive trees in an orchard, and a walnut tree in isolation. The required inputs for the calculation are the tree leaf area ( L A, in m 2 tree −1), the net (all-wave) radiation over grass ( R N, in MJ m −2 day −1) and the average air temperature. Data are presented for mid-summer when daily R N ranged between 2 and 20 MJ m −2 day −1. Tree leaf area ranged between 8.65 m 2 for a dwarf apple and 35.5 m 2 for a large apple. With the first method, daily sap flow ( S, MJ tree −1 day −1) was empirically found to equal approximately 1/4 of R N times L A ( R 2 = 0.92, n = 72 days). The second method used the Priestley–Taylor equation with tree canopy net radiation term ( A, in MJ tree −2 day −1) empirically computed as A = 0.32 R N L A. Estimates of S based on the original α value of 1.26 did not differ significantly from a linear relationship ( R 2 = 0.91; n = 72; p < 0.05), for sap flows up to 56 L tree −1 day −1. However, there was a small leaf-area dependence for the ‘best-fit’ α value i.e., α = 1.41 − 0.0064 L A ( R 2 = 0.94; n = 4 trees). On average, the daily sap flow equated to about 2/3 of A. Both relationships appear robust and capable of providing a simple working alternative to the traditional crop-coefficient approach that relates crop water use to the potential evapotranspiration rate. The problem then shifts to that of obtaining a reliable estimate of tree leaf area either by destructive sampling or using a remote sensing method such as light transmission.

Relationships between single tree canopy and grass net radiations

It is reported a simple approach to transform daily values of grass net (all-wave) radiation ( R n, MJ m −2 day −1), as measured over standard grass surface at meteorological stations, into whole tree canopy net radiation ( A, MJ tree −1 day −1). The revolving Whirligig device [McNaughton, K.G., Green, S.R., Black, T.A., Tynam, B.R., Edwards, W.R.N., 1992. Direct measurement of net radiation and photosynthetically active radiation absorbed by a single tree. Agric. For. Meteorol. 62, 87–107] describing a sphere about the tree measured A in five trees of different species (walnut, dwarf apple, normal apple, olives and citrus), with leaf area L A varying from 8.65 to 40 m 2. For each tree, A and R n were linearly related ( A = bR n), as previously reported elsewhere, but it was found that the slope of such regression was also a linear function of L A or, b = 0.303 (±0.032) L A. Consequently, the hypothesis that total daily tree canopy net radiation per unit leaf area is linearly related to grass net radiation could not be rejected after 86 days of measurements in five locations, and the empirical relationship is A = 0.303 (±0.032) R n L A ( R 2 = 0.9306).

Microclimatological characteristics of a miscanthus (Miscanthus cv. giganteus) stand during stable conditionsat night in the nonvegetative winter period

¶Microclimatological data obtained during a field experiment in the nongrowing winter period were used to study the microclimatologically stable night conditions of a 200 × 150 m miscanthus (Miscanthus cv. giganteus) stand and compared to open field conditions. The microclimatological pattern within the miscanthus canopy was characterized by long-wave radiative cooling of the plant stand and by an established temperature inversion within the canopy at calm nights. The results show that there are significant differences in air temperature and energy balance components between the open field and the miscanthus field during calm and clear nights. In general, net radiation difference during the cold and calm nights was relatively constant and about 20 W m−2 less negative in miscanthus (because of lower surface temperatures) than at the open field. Air temperature differences also remained fairly constant and were up to 3 °C lower than at the open field (at the height of 1 m). Through thermal inversion cold air accumulated in the lower parts of the canopy as shown by the vertical air temperature profiles. They showed a greater amplitude within the diurnal cycle in the miscanthus stand than in the open field. Through the onset of wind, temperature profiles changed rapidly and differences diminished. Vertical katabatic air drainage into the canopy layers was estimated indirectly by using the energy balance approach. It was calculated from the significant energy balance closure gap and showed a mean air exchange rate of up to 22 m3 m−2 h−1, related to a stand volume of 1 m2 area and 4 m height, during the mostly calm and clear nights, depending on the canopy net radiation and turbulent heat exchange forced by slight wind spells. Quantitative uncertainties in calculated cold air drainage which are introduced by the measurement method and certain assumptions in the calculations, were considered in a sensitivity analysis. In spite of these uncertainties evidence of katabatic cold air flow is given.

Micrometeorology, biophysical exchanges and NEE decomposition in a two-story boreal forest — development and test of an integrated model

An integrated model of canopy micrometeorology and exchanges of mass and energy was developed and tested for a two-story boreal forest. Roles of different elements of this forest ecosystem in determining net ecosystem exchanges (NEEs) of sensible heat, water vapor and CO 2 were analyzed by using the model. In this model, plant canopies are divided vertically into multiple layers. It first predicts profiles of air temperature, water vapor and CO 2 partial pressures inside plant canopies by using the localized near-field (LNF) theory. Then from these predicted profiles, exchanges of sensible heat, water vapor and CO 2 in each layer are computed. Canopy-scale fluxes are obtained by integrating these exchanges over the canopy depth. The model was tested against measurements for diurnal cycles of canopy net radiation, sensible heat flux, water vapor flux, CO 2 flux, friction velocity, and profiles and diurnal cycles of air temperature, water vapor partial pressure and CO 2 concentration. Once tested, the model was used to decompose NEEs into contributions from different ecosystem elements. The results showed that daytime exchanges of energy and mass in this two-story boreal forest were largely controlled by the overstory even through its LAI was smaller than that of the understory. However, the degree of dominance varied for sensible heat, water vapor and CO 2, and from daytime to nighttime. Relative contributions of different ecosystem elements to NEEs of sensible heat and water vapor remained largely unchanged from day to day during the testing period. In contrast, relative contributions of different ecosystem elements to NEE of CO 2 fluctuated significantly from day to day in responses to changes in environmental conditions. The role of the understory was most significant for the CO 2 exchange and least significant for the sensible heat exchange with the water vapor exchange in the intermediate. The soil and stem respirations balanced much of the foliage CO 2 absorption during the daytime while during the nighttime they dominated the CO 2 exchange. The contribution from soil to the NEEs of sensible heat and water vapor was trivial.

Applying single and two layer canopy models to derive conductances of a Scots pine plantation from micrometeorological measurements

A single layer (Penman-Monteith) and a two layer (modified Shuttleworth-Wallace) evapotranspiration (ET) model are used alternatively to derive conductances related to the dominant fluxes of water vapor from a semi-closed Scots pine plantation. The derivations are based on micrometeorological measurements of above canopy energy flux densities and a simple resistance network. For a period of consecutive fine weather days, below canopy net radiation and below canopy ET were about 20 percent of the corresponding above canopy values. Resulting conductances for latent heat flux agreed well with porometric measurements of pines and understory scaled to canopy level. The shift from single to two layer modelling reduced the canopy conductance to pine conductance by the fraction of understory ET.

Summertime Radiation Balances of Clearcut and Shelterwood Slopes in Southwest Oregon

Abstract Measurements of incoming solar radiation, reflected solar radiation, net radiation, and outgoing longwave radiation were made on three clearcuts and below three shelterwood canopies in southwest Oregon. This dataset was used to characterize the radiation balance of reforestation sites. The fraction of incoming solar energy (K ↓) producing the near surface microclimate (i.e., the net radiation, Q*) was 0.5 ± 0.06 for all sites studied. This value is appreciably lower than commonly measured values of uncut forest (Q*/K ↓ = 0.7 to 0.9). Although Q*/K ↓ is the same for clearcuts and shelterwoods, shelterwood understories have smaller net radiation because the overstory partially intercepts incoming solar radiation. Shelterwood data also exhibit the expected decrease in longwave radiation loss at night. The major factor affecting site albedo in both shelterwoods and clearcuts was slashburning, which decreased albedo to 0.13 as compared to 0.21 for unburned sites. Net radiation for entire sites was estimated with a simple model to sum ground surface net radiation and shelterwood canopy net radiation. Model calculations suggest that harvested sites average a 36% reduction in total net radiation when compared to unharvested stands. Shelterwood understory environments are less harsh than clearcut environments because much of the total site net radiation is dissipated in the overstory canopy. The model is also used to estimate the effect of canopy density on net radiation below the canopy. For. Sci. 32(2):504-516.