Jun 1, 2012

Because Every Drop Counts

The words below were originally written for
The Greater Oshkosh Editions of the Scene Newspaper

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Shortly after I moved to Oshkosh in 1996 I realized how arousing it would become living here, on the water. Each day I admire our water landscape feeling truly fortunate to call Oshkosh home. The great amount of water resources that border or flow through our city demand respect and continual maintenance and care. Each time rain falls from the sky or a snowpack melt, the water that runs off our roofs, driveways, and yards flow into storm water sewers and creeks that eventually flow directly into the Fox River and Lake Winnebago.

There are an outstanding amount of factors that contribute to a storm water management system. However, ultimately it is the amount of rain that falls on a particular area that brings the land use and soil types to a cohesive head preventing storm water runoff from entering our waterways. The city of Oshkosh uses precipitation data to effectively analyze and implement prudent storm water management. These plans are designed to properly displace water from smaller precipitation events (normal rain events are 0.25 to 1.25 inches). Runoff from these normal precipitation events impacts our agriculture, environment and health. Larger precipitation events often discharge water into non designated areas, our basements and backyards, expanding the potential pollution flow.

The impact of precipitation on storm water management is huge. It makes sense that the planning procedures have great interest in precipitation data, quantity and quality. Sources for precipitation data in the Oshkosh area are hourly observations taken at the airport and daily observations taken by the National Weather Service Cooperative Observer program and the Community Collaborative Rain, Hail & Snow Network also known as CoCoRaHS. Storm water management needs for precipitation data range from documenting and providing warnings on storms in action but also doing post storm analysis and assessing how large a storm was compared to system design. Historic data sets often used come from stations 20-25 miles apart and are not helpful to assess local variations.

For example the 2008 Oshkosh storm water management plan, with guidance from the WDNR, determined a critical rainfall event as 1.67 inches of rainfall in a one hour time span recurring any one year in a ten year span. The daily precipitation data used for this analysis was derived using Green Bay, WI numbers between the years of 1968 and 1972. Observing rainfall hour by hour is difficult without the aid of automated rain gauges. For the sake of making a comparison it is more universal to use an observation period of 24 hours. During the five year period of 1968-1972 the number of critical rainfall events occurring for a 24 hour period in Green Bay was three. In the four years between 2007 and 2011 a critical rainfall event has occurred in Oshkosh seven times, more than doubling the recurrence of the Green Bay numbers from four decades ago the plan was based on.


The frequency of high impact rainfall in the Midwest has increased 40% in the last 60 years. Oshkosh is no stranger to this phenomenon. The Oshkosh 2008 storm water management plan defines a 10-year storm, a storm that dramatically floods the city, as 3.56 inches in 24 hours. In the past four years rain gauges in Oshkosh have surpassed that total three times and nearly surpassed it two more times in 2004 and 2008 respectively. Perhaps the 10-year storm needs to be redefined to help storm water management design criteria for pending disaster floods.

The 2008 Storm water Management plan was completed for three specific locations within Oshkosh, the Northeast side, Northwest side, and South side. Currently in the greater Oshkosh area we have five human precipitation observers. Fortunately for the storm water performance analysis there are now daily precipitation observations being recorded in two of these three areas. The Northwest side is still lacking as is the entire West side of Oshkosh. There is always a need for a greater number of observations, as the sayings go “the rain doesn't fall the same on all” and “every drop counts”. Due to the variability of precipitation, amounts measured can be quite different only a block or two away. The more observations we get the clearer the picture and the better the understanding of where it did and did not rain.

Where storm water management utilities are using automated rain gauges for flood warning systems, CoCoRaHS and NWS cooperative observers data are used extensively for evaluating the automated rain gauge performance and for determining where more gauges may be needed and when they may need to be serviced or replaced. Storm water management utilities also rely heavily on weather radar. Radar precipitation estimates have limitations. Actual reports from CoCoRaHS and NWS cooperative observers are used to track the performance of radar estimations and adjust the radar estimations accordingly. The more years of data we have, the more we can do with assessing if there are preferred storm tracks for high intensity storms. High concentrations of stations would show preferred areas that get hit much more often than others eventually leading to modifications in storm water design criteria.

If anyone out there has an enthusiasm for watching and reporting weather conditions and a desire to learn more about how weather can effect and impact our lives please consider joining CoCoRaHS. CoCoRaHS is a community project. Everyone can help, young, old, and in-between.