Transport Model Details

The Integrated Aerobiology Modeling System (IAMS) box-parcel model was constructed by ZedX programmers accounting for spore release and canopy escape in source areas, mortality due to exposure to solar radiation during atmospheric transport, and wet deposition in destination regions. The domain of the model includes most of the continents of North and South America, Africa, and Europe. Computations are conducted on a ˜ 14 x 14 km grid throughout the model domain using a 6-hr time step and the National Weather Service Reanalysis 2 datasets.

Spore release and escape from the crop canopy in a source area is computed using the following assumptions: (1) 25% of the soybean crop is heavily infected with soybean rust (personal communications with South American growers and researchers), 2) 6 million spores are released per day per heavily infected soybean plant (Melching et al. 1979, Yang et al. 1990), 3) planting density is 500,000 soybean plants/ha (personal communications with South American growers and researchers, 4) 33% of soybean rust spores are released during the late-morning to noon optimal transport period (applicable for Peronospora tabacina spores, Aylor 1986, Davis and Main 1989), and 5) 15% of spores released escape from the soybean canopy (applicable for Peronospora tabacina spores, Aylor and Taylor 1983).

The model spreads the spores that have escaped from the soybean canopy first in the horizontal or down wind direction, and then vertically between pressure levels. This procedure is followed for each point source (grid cell containing spores). After the trajectories are computed, the spores arriving in the air column at each pressure level above each grid cell (potentially from multiple point sources) are summed in preparation for the solar radiation mortality and wet deposition computations.

Spore mortality due to UVB radiation exposure in the atmosphere is proportional to cloud-adjusted, surface total incoming solar radiation (Aylor 1999) calculated for each grid cell and time step. Wet deposition of viable spores is calculated as a function of the observed surface precipitation total for the grid cell and time step. Consequently, the number of spores poised to move during the next iteration of the model is the total number of spores that arrived minus the sum of those that expired and were deposited during the previous time step.

For display purposes, total values of UV expired spores, deposited viable spores, and viable spores continuing in transit associated with each day are obtained by integrating the spore concentrations over the pressure levels in the air column above each grid cell. Wet deposition is also accumulated through time for each grid cell and release date(s) for mapping. Spore densities are presented as number of spores/ha/day on maps.