89th American Meteorological Society Annual Meeting

Sunday, 11 January 2009
High flow events in small watersheds in New York State
Phoenix Convention Center
Andrew J. Shook, Cornell Unviersity, Ithaca, NY; and A. DeGaetano
Small watershed streams respond rapidly to intense rainfall and can be an effective tracer for changes in intense rainfall climatology. Rivers that move larger volumes of water will smooth out these intense rainfall events because the percentage of the total water in the watershed is significantly smaller. Therefore, for our study, we started by limiting our watersheds to those with less than 150 square miles. Secondly, in order to identify a trend in the data, we needed to look at stream gages with a record of at least 40 years. This eliminated a large number of gages because often a stream gage would operate for about 20 years, then, for any number of reasons, the gage was discontinued for a few years or moved to a new site along the river. The final cut of gages for consideration was made based on if the location coordinates was available. This allowed us to map the gages on a satellite imaging program such as Google Earth, and determine the level of urbanization and classify streams as those feeling the effects of an urban, urbanizing, or rural environment.

To identify the most intense events for each stream, we derived an equation that would compute a flow index, which would put each event on a level playing field for comparison. When considering just largest single day rises, we acknowledged 24-hour events which had a 50% fall in flow relative to the rise during the previous 24 hours. This isolates the high-rate events that we are most interested in. One issue we quickly discovered was the issue created by snowmelt events. Often a smaller rain event could show up as a massive flood in a small basin if the rain was caused by a system that simultaneously melted the snowpack. In order to purify the record, we only considered storms from April through November and checked each event in those two end months to ensure their validity.

Once we compiled all the events for each station, we ranked the top forty events for each to facilitate our analysis. Assuming there are forty events in forty years, on average, a top forty event should happen each year in each stream. However, this is not what we found. There are some years that are much wetter than those around them, and a couple of years have effectively zero events. It is presumable that year-to-year variability will occur. However, after separating streams into groups by urban characteristics, the variability differs between groups. Further analysis of why events happen when and where they do could lead to correlation with other atmospheric phenomena.

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