A great deal of variability in rainfall occurs between summer 
thundershowers and winter frontal storms. During the summer months this 
area experiences short duration thunderstorms which produce most of the 
rainfall. Approximately 60% of all storm events which occur in the Tampa 
Bay area have a duration of four hours or less (Figure 4). Summer 
thundershowers also exhibit higher intensities than the longer, less 
intense winter frontal storms. Another important characteristic of 
rainfall in this area is the time between storms which relates directly 
to pollutant build up and to the concentration of storms washed off 
developed areas. Thirty percent of all storms occurred with a separation 
of 24 hours or less, 40% had a separation of 48 hours or less and 50% of 
all the storms occurred within 72 hours of each other. 
When compared to runoff quantity and quality data, these weather 
data reveal that during the summer months (June - September) storms of 
short duration, high intensity, and short periods of antecedent dry 
conditions produce high volumes of runoff with generally lower 
constituent concentrations due to very little opportunity for build up of 
particulate matter. On the other hand, winter frontal storms are of 
longer duration, with less intensity and have much longer periods of time 
between storm events. This allows particulate matter to build up with 
concentrations generally higher for winter runoff events. These factors 
are important to the ecology of the bay with regard to mass loadings for 
certain parameters and the event specific toxicity of others. 
LAND USE AND URBANIZATION 
Urbanization in the Tampa and Sarasota Bay areas has resulted in 
the modification of natural watersheds to residential neighborhoods, 
apartment complexes, industrial, commercial, and agricultural land uses. 
As a result of this urbanization, the water quality and quantity entering 
Tampa and Sarasota Bays has been modified. Urbanization began in the St. 
Petersburg, Tampa, and Bradenton/Sarasota areas and spread out from those 
urban cores. Urbanization within Pinellas County has been most rapid, 
encompassing nearly the entire County. Urbanization in Hillsborough 
County has spread out radially to the northwest and east from the City of 
Tampa. Development around Bradenton/Sarasota has been primarily close to 
the coast between the two cities. Current land use distributions within 
the areas tributary to Tampa Bay are shown in Table 1. Only 16.6% of the 
total area of the watershed is forest or natural. Approximately 60% has 
been developed as pasture and crop land, and approximately 25% of the 
entire 1,800 square mile watershed is urbanized. 
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