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Research
Seasonal Forecast of St. Louis
Encephalitis Virus Transmission, Florida
Jeffrey Shaman,*
Jonathan F. Day,† Marc Stieglitz,* Stephen Zebiak,‡ and Mark Cane*
*Columbia University, New York, New York, USA; †University of Florida,
Gainesville, Florida, USA; and ‡International Research Institute for Climate
Prediction, Palisades, New York, USA
Appendix A (Online Only)
The Topographically-Based
Hydrology (TBH) Model
We use a dynamic hydrology model (A1), here
referred to as the Topographically-Based Hydrology (TBH) model, to simulate
variations in WTD in the Vero Beach, Indian River County area. Mean area
WTD provides an integrated measure of near surface soil wetness conditions.
It is the rise and fall of the water table that determines where and when
pools of water form at the land surface thus creating potential mosquito
breeding habitats. To model WTD, a suite of meteorological variables,
including precipitation and temperature, area soil and vegetation type,
and antecedent conditions must be accounted for so that evapotranspiration,
water movement within the soil column, and river runoff can be quantified.
Topography must also be constrained if the flow of water across the land
surface, runoff rates, and the local convergence of water in lowlands
(surface pooling) are to be modeled accurately. By using the TBH model
we are able to track these variables and simulate variations in WTD.
The TBH model combines a soil column, which simulates the vertical movement
of water and heat within the soil and between the soil surface plus vegetation
and the atmosphere, with the TOPMODEL approach (A2-A4),
which incorporates the statistics of topography to track the horizontal
movement of shallow groundwater from the uplands to the lowlands. TOPMODEL
formulations permit calculation of both the saturated fraction within
the watershed (partial contributing area), and the groundwater flow that
supports this area, from knowledge of the mean WTD and a probability density
function for soil moisture deficit derived from topographic statistics.
Using the TBH model, we can produce a three-dimensional picture of soil
moisture distribution within a catchment. This approach to modeling the
land surface has been validated at several catchments, ranging in scale
from the Red Arkansas Basin (570,000 km2) (A5)
to the Black Rock Forest catchment (1.34km2) (A6).
Model Input and Validation
Data
Hourly meteorological data were assembled from National Climate Data
Center (NCDC) archives for Vero Beach, Florida for 1984-95. Gaps in the
record were filled by direct substitution with hourly data from NCDC archives
for nearby Melbourne and West Palm Beach. Solar radiation data were provided
by the Northeast Regional Climate Center (NRCC) from analysis of the NCDC
data using the NRCC solar energy model (A7).
A resampling procedure was then used to further extend the hourly meteorological
data set using daily NCDC temperature and precipitation data from January
1949-March 2002 (see Shaman et al. (A8) for
details). Additional, near-real-time observations of hourly meteorological
conditions in Indian River County for April through June 2002 were derived
from Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale
International Project (GCIP) Land Data Assimilation System forcing datasets
(A9).
Using TarDEM Version 4 routing freeware (A10)
and subsequent analysis, topographic statistics for the Vero Beach area
were generated from a 30m cell USGS National Elevation Dataset Digital
Elevation Model (DEM) of south-central Florida. Soil and vegetation types
were derived from U.S. Department of Agriculture sources and personal
inspection of the Vero Beach landscape.
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