Soil-Vegetation-Atmosphere Transfer

Water balance models have frequently been used to examine the surface runoff from watersheds. Some of these models, focused more on climate change, are called Soil-Vegetation-Atmosphere Transfer Schemes (SVATs) (Vordsmarty and Peterson, 2000). These model simulations use different parameters such as vegetation cover, soil texture (different sizes of mineral particles), water-holding capacity of soils, surface roughness, and albedo (the fraction of light reflected by a body or surface), to make predictions on... 

Soil-Vegetation-Atmosphere Transfer Schemes (SVATs) simulated models that use parameters such as vegetation cover, soil texture, water-holding capacity of soils, surface roughness, and albedo, to make predictions on soil moisture, runoff, evapotranspiration, and runoff. 

Deposition is the atmospheric removal process by which gaseous and particulate contaminants are transferred from the atmosphere to surface receptors - soil, vegetation, and surface waters (22,27,28, 32). This process has been conveniently separated into two categories dry and wet deposition. Dry deposition is a direct transfer process that removes contaminants from the atmosphere without the intervention of precipitation, and therefore may occur continuously. Wet deposition involves the removal of contaminants from the atmosphere in an aqueous form and is therefore dependent on the precipitation events of rain, snow, or fog. 

Vegetation plays an important role in the fate of many chemicals. Plants are the first link in terrestrial food chains, and hence the accumulation of chemicals in plants is a crucial step determining exposure of higher terrestrial organisms, including humans, to environmental chemicals. Plants affect the atmospheric transport of chemicals by scavenging them from the air, and they also serve as a medium for transfering chemicals between the soil and the... 

There are two possible sources of PCDD/Fs to vegetation the atmosphere and soil. Initially it was thought that PCDD/Fs would not be present in the atmosphere in quantities sufficient to contaminate plants owing to their low volatility, and early research in this area focused on uptake from soil. There are three possible pathways of soil-bound PCDD/Fs to aerial plant parts root uptake and translocation, volatilization followed by adsorption to foliage, and transfer of soil particles (see Figure 1). The first of these pathways has received the most attention. 

Of particular importance is the contamination of soil, because it receives pollutants from the atmosphere (e.g., sulfates and nitrates resulting from oxidation of nitrogen and sulfur oxides, and metals from smelters) and from the hydrosphere (e.g., sediments that concentrate heavy metals from aqueous bodies and mining operations). In multimedia mass-balance models, soil is an important sink as well as a conduit for mass transfer to vegetation and shallow groundwater. 

NPP is the net carbon gain by vegetation over a particular time period— typically a year. It is the balance between the carbon gained by photosynthesis and the carbon released by plant respiration. NPP includes the new biomass produced by plants, the soluble organic compounds that diffuse or are secreted by roots into the soil (root exudation), the carbon transfers to microbes that are symbiotically associated with roots (e.g., mycorrhizae and nitrogen-fixing bacteria), and the volatile emissions that are lost from leaves to the atmosphere (Clark et al., 2001). 

The first attempts to model flow and transport in plant canopies that accommodated (i) the distinct microclimates of different stands of vegetation (ii) the separation of soil surface and layers of canopy as distinct sources and sinks of heat and mass and (iii) the influence of atmospheric stability or advection effects, applied gradient transfer to diffusion within the canopy space ([493]). In this procedure, a flux density is expressed as the product of a diffusion coefficient (turbuient or eddy diffusivity) and the gradient of the time average of the quantity of interest, as in the following examples ... 

The SM2-U model (Dupont and Mestayer, 2004 [158] Dupont et al., 2005 [160]) is based on the force-restore model of Noilhan and Planton 1989 [469] for the transfers between the atmosphere, one vegetation layer, and three soil layers in its most recent version, ISBA-3L (Boone et al., 1999 [66]). It keeps the principal characteristics of this soil model and was developed as a pre-processor for fine resolution sub-mesoscale simulations. The surface dynamic influence is represented through roughness lengths... 

Pollutants are removed from the atmosphere by rain and snow and are transferred to soils, natural waters, and vegetation by wet and dry deposition processes. Through these processes, plants are exposed periodically to substances dissolved in atmospheric precipitation and to gaseous pollutants. The major soluble constituents in rain and snow in the eastern U.S. are hydrogen, sulfate, and nitrate ions and there is concern over the environmental influence of these substances, particularly acidity (Jacobson et al., 1976). Knowledge of plant nutrition and response to pollutants raises the question of whether it is necessary to consider the supply of nitrate and sulfate in rain and the concentration of ozone in the atmosphere when determinations are made of the effects of acidic precipitation on vegetation of the eastern U.S. 

In the following discussion, factors used for the transfer of from soil to plants and from animal feeds to animal products are those assembled by Soldat for the study of the potential doses to people from a nuclear power complex in the year 2000 (Fletcher and Dotson, 1971). Although a number of models for the environmental transport of radioiodine exist in the scientific and technical literature (many of them for the atmosphere — vegetation -> cow — milk — person pathway for 1), the concepts and values from the many other available models are similar to those presented in the Fletcher and Dotson report. 

In all cases, we assume that the air phase is well mixed vertically, except in the boundary layer immediately above the soil surface in which there is a resistance to mass transfer. The rate of transfer from air to soil influences the chemical levels in the soil and is included in these models, but the reader is referred to Chapter 6 for a more detailed treatment of atmospheric deposition and absorption processes and to Chapter 7 for treatment of absorption to vegetation and subsequent transport to the soil surface. 

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