Development of TMDLs and Implementation Plans Water Quality Analysis
Final Report 63 September 2003
Table 59
SEASONAL RAINFALL DISTRIBUTION
(Source: Huff and Angel, 1992)
Season Annual Contribution Top-Ranked 1-Day Storms Illinois Average Indiana Average Illinois Indiana
Winter 16.7% 18.8% 3.3% 2.4%
Spring 29.1% 28.9% 20.0% 17.1%
Summer 29.8% 29.1% 65.0% 63.4%
Autumn 24.3% 24.3% 11.7% 17.1%
Little data are available on seasonal DO variation in Rayse Creek. For DO, the worst case
condition has been modeled: summer time high temperatures and low flows. As
temperature decreases from summer highs and flow increases from summer low, DO
concentrations will increase.
5.8 Background Concentrations
Background is defined as those loads that represent a baseline or minimum level of water
pollution which are natural and can not be eliminated by local or area-wide water quality
management (Mills et. al. 1985). Background concentrations of suspended sediment for
southern Illinois are between 20 and 50 mg/L (McElroy et al. 1976). Comparing these
background TSS concentrations with the 3- and 10-year storm TSS concentrations
estimated by the model suggests that background concentrations account for between 7
percent and 30 percent of the estimated TSS concentration.
Stream background phosphorous concentrations for undeveloped watersheds in Illinois
are not known. Clark et al. (2000) published the most recent national review of
background nutrient concentrations we are aware of. These scientists reviewed data from
85 sites representing relatively undeveloped watersheds across the nation. Table 60 is an
excerpt from their publication. They admitted that data from the corn belt were scarce,
due to extensive agricultural development. It is reasonable to assume that Illinois’ fertile
prairie soils would have background phosphorus levels at or above the median, 50th
percentile levels of the nationwide data set. Based upon the 75th percentile, background
phosphorus concentrations in Rayse Creek account for between 3 and 8 percent of total
phosphorus.
