1,721,009 research outputs found
The chemical composition and sources of the bulk deposition on Lake Kinneret (The Sea of Galilee), Israel
No full textThe chemical composition of the bulk deposition into Lake Kinneret, Israel was determined for the three hydrological years from 1992 to 1995. The fluxes of the elements and ions in the bulk deposition around the lake are fairly uniform although some local effects due to anthropogenic activity are observed; temporal effects are more pronounced. Na and Cl fluxes are greatly affected by rainfall that has passed over the Mediterranean Sea whilst Ca fluxes increase during dust storm episodes. The amount of each of the determined constituents entering the lake was calculated. The major chemical constituent entering the lake (except Ca) is SO(4) whose main source is long-range transport from Europe. Enrichment factor analysis shows that the insoluble portion of the bulk deposition has the same characteristics as regional (and Saharan) dust storms, whilst the soluble portion is strongly affected by the marine environment. Significant additions to the previously known input of nitrogen and phosphorous enter the lake due to bulk deposition: 10% for nitrogen and 40% for phosphorous. The Pb content of the lake sediments correlates with Pb usage in petrol. Full experimental results are given in the electronic appendices. (C) 2008 Elsevier Ltd. All rights reserved
The amount and nature of the dustfall on Lake Kinneret (the Sea of Galilee), Israel: flux and fractionation
No full textThe dustfall flux on Lake Kinneret, Israel was measured by a network of 15 sampling stations around the lake over a 3 year period, with partial measurements over a further 2 years. The dustfall was characterized by dividing it into fractions including inorganic water insoluble matter (IWI), i.e. insoluble minerals; inorganic water soluble matter (IWS); and total organic matter (TORG). The total dustfall was fairly constant over the years with a mean input of 12,000 t on the 170 km(2) lake surface. The proportion of atmospheric input to total solids entering the lake is inversely dependent on the amount of rain in a particular year and varied between 17% and 46% (mean = 34%). The dustfall is composed of roughly equal proportions of TORG, IWI and IWS. The organic matter itself is about 50% water soluble. The most variable fraction of the dustfall is the insoluble mineral content (IWI) which is dependent on violent dust storms usually occurring in spring and autumn. The amount of aeolian matter entering the lake is sufficiently large, compared to the fluvial input, to have a significant effect on the water quality. Much of the dustfall entering the lake is more finely divided than the fluvial suspended matter and this will presumably increase its relative effect. (C) 2003 Elsevier Ltd. All rights reserved
Input of atmospheric particles into forest stands by dry deposition
No full textIn this study the dry input of atmospheric particles into a forest stand is quantified. A wash-off-method using the natural leaf surfaces as collectors of the dry deposition was chosen. The direct on-site-measurement on living branches were achieved in a spruce stand (Picea abies (L.) Karst) at Solling, Germany. The ion exchange processes occurring on natural branches can reliably be quantified through immediate sequential washings. In order to calculate also the gas dry deposition of those trace elements which occur in both particle and gas phases, a resistance model was used. From these results the deposition velocity of particulate aerosol components into the forest stand was calculated. Dry particle deposition constitutes an important part of the total matter input into the forest ecosystem. Just the nitrogen input into Solling only by dry deposition (from particle-, mist-, and gas-deposition) with about 30 kg N ha(-1) a(-1) already exceeds the critical load of 20 kg N ha(-1) a(-1) by far, and this is without even considering the additional load by wet deposition which amounts to 15 kg ha(-1) a(-1). These findings are of greatest ecological importance, as the damage to the stability of the forest ecosystem caused by increased nitrogen input is considerable. Only a quick and drastic reduction of sulphur and nitrogen emissions could stop the further increase of the nutritient imbalance and the progressing acidification of this ecosystem
Forests as protection against airborne immissions
No full textThe effect of a spruce forest in the Solling-hills (Germany) on the concentration of airborne trace compounds in the atmospheric boundary layer is discussed. The discussion is based on field measurements of vertical concentration profiles and vertical fluxes of reactive trace gases and particles in and above a spruce forest and on numerical modelling. Measured SO2-, O-3- and PAN-concentrations are 10 % to 20 % lower near the forest floor than just above the canopy. NO2 is emitted from the forest into the atmospheric boundary-layer and NO is transported both from the atmosphere and from the forest floor into the spruce canopy air space. The net NOx-flux between the atmosphere and this spruce forest can be neglected compared quantitatively to other N-fluxes. Numerical experiments using two models show that a 1000 m long spruce forest reduces the near surface concentration of an airborne trace substance with a deposition velocity of about 0.7 cm/s by up to 6 % as compared to the upwind SO2-concentration. If a forest is replaced by a meadow the SO2-concentration at the former downwind side of a forest increases by about 10 %
Numerical analysis of flux footprints for different landscapes
No full textA model for the canopy - planetary boundary layer flow and scalar transport based on E-is an element of closure was applied to estimate footprint for CO2 fluxes over different inhomogeneous landscapes. Hypothetical heterogeneous vegetation patterns - forest with clear-cuts as well as hypothetical heterogeneous relief - a bell-shaped valley and a ridge covered by forest were considered. The distortions of airflow caused by these heterogeneities are shown - the upwind deceleration of the flow at the ridge foot and above valley, acceleration at the crest and the flow separation with the reversed flow pattern at lee slopes of ridge and valley. The disturbances induce changes in scalar flux fields within the atmospheric surface layer comparing to fluxes for homogeneous conditions: at a fixed height the fluxes vary as a function of distance to disturbance. Correspondingly, the flux footprint estimated from model data depends oil the location of the point of interest (flux measurement point) and may significantly deviate from that for a flat terrain. It is shown that proposed method could be used for the choice of optimal sensor position for flux measurements over complex terrain as well as for the interpretation of data for existing measurement sites. To illustrate the latter the method was applied for experimental site in Selling, Germany, taking into account the complex topography and vegetation heterogeneities. Results show that in certain situations (summer, neutral stratification, south or north wind) and for a certain sensor location the assumptions of idealized air flow structure could be used for measurement interpretation at this site, though in general, extreme caution should be applied when analytical footprint models are used in the interpretation of flux measurements over complex sites
Estimations of total ecosystem carbon pools distribution and carbon biomass current annual increment of a moist tropical forest
No full textApplication of a three-dimensional model for assessing effects of small clear-cuttings on radiation and soil temperature
No full textA three-dimensional model Mixfor-3D of soil–vegetation–atmosphere transfer (SVAT) was developed and applied to estimate possible effects of tree clear-cutting on radiation and soil temperature regimes of a forest ecosystem. The Mixfor-3D model consists of several closely coupled 3D sub-models describing: forest stand structure; radiative transfer in a forest canopy; turbulent transfer of sensible heat, H2O and CO2 between ground surface and the atmospheric surface layer; evapotranspiration of ground surface vegetation and soil; heat and moisture transfer in soil. The model operates with the horizontal grid resolution, 2 m × 2 m; vertical resolution, 1 m and primary time step, 1 h. The model was tested against meteorological data obtained at a small clear-cutting area in Otterbach in central Germany during summer 2005. The meteorological data including air temperature and humidity, precipitation, solar radiation, wind speed and direction, soil temperatures at 10 and 20 cm depth were measured by five automatic stations within the clear-cut area. One reference station was placed about 100 m from the clear-cut inside the forest stand. Comparisons of modelled and measured solar radiation fluxes and soil temperature profiles showed that the model adequately describes the spatial heterogeneity and dynamics of these variables under different weather conditions. The model can be used to explore solar radiation and soil temperature patterns within heterogeneous forest plots, with applications to various silvicultural tasks
The role of canopy structure in the spectral variation of transmission and absorption of solar radiation in vegetation canopies
No full textThis paper presents empirical and theoretical analyses of spectral hemispherical reflectances and transmittances of individual leaves and the entire canopy sampled at two sites representative of equatorial rainforests and temperate coniferous forests, The empirical analysis indicates that some simple algebraic combinations of leaf and canopy spectral transmittances and reflectances eliminate their dependencies on wavelength through the specification of two canopy-specific wavelength-independent variables. These variables and leaf optical properties govern the energy conservation in vegetation canopies at any given wavelength of the solar spectrum. The presented theoretical development indicates these canopy-specific wavelength-independent variables characterize the capacity of the canopy to intercept and transmit solar radiation under two extreme situations, namely, when individual leaves 1) are completely absorptive and 2) totally reflect and/or transmit the incident radiation. The interactions of photons with the canopy at red and near-infrared (IR) spectral bands approximate these extreme situations well. One can treat the vegetation canopy as a dynamical system and the canopy spectral interception and transmission as dynamical variables, The system has two independent states: canopies with totally absorbing and totally scattering leaves. Intermediate states are a superposition of these pure states, Such an interpretation provides powerful means to accurately specify changes in canopy structure both from ground-based measurements and remotely sensed data, This concept underlies the operational algorithm of global leaf area index (LAI), and the fraction of photosynthetically active radiation absorbed by vegetation developed for the moderate resolution imaging spectroradiometer (MODIS) and multiangle imaging spectroradiometer (MISR) instruments of the Earth Observing System (EOS) Terra mission
Energy and water fluxes above a cacao agroforestry system in Central Sulawesi, Indonesia, indicate effects of land-use change on local climate
No full textRapid conversion of tropical rainforests to agricultural land-use types occurs throughout Indonesia and South-East Asia. We hypothesize that these changes in land-use affect the turbulent heat exchange processes between vegetation and the atmosphere, and the radiative properties of the surface, and therefore, induce an impact on local climate and water flows. As part of the international research project (SFB 552, Stability of Rainforest Margins in Indonesia, STORMA) the turbulent heat fluxes over a cacao agroforestry system (AFS) were investigated, using the eddy covariance technique. These first heat flux observations above a cacao AFS showed an unexpectedly large contribution of the sensible heat flux to the total turbulent heat transport, resulting in an averaged day-time Bowen ratio of beta = H/lambda E approximate to 1. Seasonality of beta did mainly coincide with the seasonal course of precipitation, which amounted to 1970 mm yr(-1) during the investigated period. The findings are compared to invastigations at four neotropical rain forests where daytime beta were substantially smaller than 1. All discussed sites received similar incident short wave radiation, however, precipitation at the neotropical sites was much higher. Our first observations in a nearby Indonesian upland rain forest where precipitation was comparable to that at the cacao AFS showed an intermediate behaviour. Differences in beta between the cacao AFS and the tropical forests are discussed as a consequence of differing precipitation amounts, and albedo. From these comparisons we conclude that conversion from tropical forests to cacao AFS affects the energy fluxes towards increased heating of the day-time convective boundary-layer
Transpiration of a mixed forest stand: field measurements and simulation using SVAT models
No full textTranspiration of a mixed spruce-aspen-birch forest at the Valday Hills in Russia was determined using sap flow measurements and two different SVAT (Soil-Vegetation-Atmosphere-Transfer) models. The more sophisticated Mixed Forest multi-layer SVAT model (MF-SVAT) considers water uptake and transpiration of each tree species individually, and the simple Multi-Layer (ML-SVAT) describes the forest stand using averaged effective parameters of canopy structure and tree physiology. Comparisons of modelled and measured transpiration rates under sufficient soil moisture conditions did not show any significant differences between two models. Under limited soil moisture conditions MF-SVAT described forest transpiration still realistically whereas ML-SVAT overestimated it by up to 50%. Drought in the upper soil layers reduced transpiration of spruces more than of deciduous trees due to differences in physiological properties and vertical root distribution. Individual regulation of the transpiration of different tree species is typical for mixed forests and cannot be accurately described with averaged parameterisation such as used in ML-SVAT
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