I am sure you have heard about the farmer in Sidell, Illinois. After that fiasco with the cheese being left standing alone taking the blame for polluting the nearby stream, he decided to build a channel to transport the runoff from his feedlots to a

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I am sure you have heard about the farmer in
Sidell, Illinois. After that fiasco with the
cheese being left standing alone taking the blame
for polluting the nearby stream, he decided to
build a channel to transport the runoff from his
feedlots to a treatment pond. He was advised to
size the channel based on the expected rainfall
in April. However, he is a bit confused. How much
rain falls in Sidell in April? He found historic
rainfall data for Sidell online at an Illinois
State Water Survey site, and found that April
rainfall varies from year to year. Please help
the farmer and save him from another scandal.
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Estimating Rainfall Quantity for Design The
design of water management systems is based more
on extreme values than on average values. If the
mean value is used in the design of an irrigation
system then on average, in one out of every two
years there will not be enough water to meet the
demands of the crop and yield will be reduced. If
the mean is used in drainage design, then one out
of every two years the crops will be flooded. It
is better to use design values with lower
associated risk.
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Estimating 80 Dependable Rainfall and 80
Maximum Rainfall from mean and standard If only
the mean and standard deviation of monthly
rainfall are known then 80 Dependable Rainfall
Mean - 0.84 x Standard Deviation 80 Maximum
Rainfall Mean 0.84 x Standard Deviation.
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80 Dependable Rainfall The value of period
rainfall (monthly, seasonal, etc.) that will be
exceeded 80 of the time. This value ensures that
on average, there will be enough water to meet
the crop's need four out of every five years.
80 Maximum Rainfall The value of period
rainfall that on average, will not be exceeded
80 of the time. This value ensures that on
average, a drainage system or a sedimentation
pond will have adequate capacity four out of
every five years.
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Example For Sidell the mean rainfall for
April is 3.75" and the standard deviation is
1.78 80 Dependable Rainfall 3.75 - 0.84 x
1.78 2.25 80 Maximum Rainfall 3.75
0.84 x 1.78 5.25"
-0.84s
0.84s
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20
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10 Step Procedure for Rainfall Frequency Analysis
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1. Locate Data Source
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2. Extract as Specific Data as Required
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3. Import into Excel and convert to columns
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4. Sort, and Extract Targeted Data
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5. Graph, and check for jumps, trends or cycles
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6. Sort the data in ascending order and
determine the non-exceedance probability of each
data value
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7. Plot Probability of Non-exceedance vs
Precipitation (Empirical Distribution Function)
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8. Determine the mean and standard deviation of
the logs of the precipitation values
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9. Determine the cumulative log normal values
for the precipitation data
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10. Plot the cumulative distribution function
for the fitted logNormal Distribution
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Return Period (Recurrence Interval)
The frequency with which, on average, a given
precipitation event is equaled or exceeded.
P probability of exceedance
Example If there is a 12.5 percent chance that a
storm of a certain magnitude will occur, the
return period for that storm is
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Multi-year Chance of Exceedance (R)
The probability of a given return period storm
being equaled or exceeded within a given number
of years.
Example The chance that an 8-year return period
storm will occur over the 5 year life of a
project is