Potential transpiration is a measure' of the rate at which water can be transmitted from evaporation sites in plant leaves to the atmosphere. Potential soil-moisture availabiliy is defined as a measure of the capacity of a soil to transmit water to a root site. A differential equation is presented describing radial flow of soil moisture to a single vertical sink (root) in an infinite soil mass which is initially at a uniform moisture content. The relationship between moisture content and diffusivity for the soils studied may be represented by an exponential function. A numerical solution of the differential equation is used to determine the soil-moisture flux. The results show that for specific soils the decrease in soil moisture with time occurs mainly in the immediate vicinity of the sink. Moisture flux increases with initial moisture content but is essentially time independent. In natural systems the flux would probably decrease with time because of multiple root interference. At large soilmoisture contents, actual transpiration is limited by and equivalent to potential transpiration. At small soil-moisture contents, actual transpiration is limited by and equivalent to potential soil-moisture availability. (
This report describes a sulfur oxides atmospheric pollution model that calculates trajectories using single-layer historical wind data as well as chemical transformation and deposition following discrete contaminant air masses. Vertical diffusion under constraints is calculated, but all horizontal dispersion is a function of trajectory variation. The ground-level air concentrations and deposition are calculated in a rectangular area comprising the northeastern United States and southeastern Canada. Calculations for a 29-day assessment period in April 1974 are presented along with a limited verification. Results for the studies were calculated using a source inventory comprising 61% of the anthropogenic S02 emissions. Using current model parameterization levels, predicted concentration values are most sensitive to variations in dry deposition of S02' wet deposition of sulfate, and transformation of S02 to sulfate. Replacing the variable mixed-layer depth and variable stability features of the model with constant definitions of each results in increased ground-level concentration predictions for S02 and particularly for sulfate.
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