Precipitation & Storm Recurrence Trends in the Hudson Valley and Catskill Mountain Regions in NY State

Summary and data compilation by Simon Gruber, April 5, 2018

This fact sheet summarizes information about precipitation trends and the changing size and frequency of rain and snow storms and flood events in the southern Hudson Valley and Catskill mountain regions, based on measured precipitation records used in recent studies by scientists at Cornell University, the City University of New York, and the NY City Department of Environmental Protection. These are the key findings highlighted in this fact sheet regarding the changing size of precipitation events in the last 60-70 years:

In the northeast U.S., there has been a 71% increase in the amount of precipitation falling in very heavy precipitation events from 1958-2012 (see map, citation and web link below from one source where this map is published, Climate Change Impacts in the United States: The Third National Climate Assessment report).

A recent study of the frequency of larger storms and regional streamflow trends found that the 10-year flood from the period 1960-1990 became the 5-year flood during 1980-2011, and the 25-year flood from 1960-1990 became the 10-year flood for 1980-2011 (Matonse & Frei 2013, see graph and full citation below).

Smaller storms have also become more frequent – in recent years, storms of 2 inches or more are occurring on average more than twice every year, up from an average of about once each year in 1950 (personal communication with Dr. Art DeGaetano, Cornell University, on August 18, 2014).

Observed Change in Very Heavy Precipitation

The map shows percent increases in the amount of precipitation falling in very heavy events (defined as the heaviest 1% of all daily events) from 1958 to 2012 for each region of the continental United States. These trends are larger than natural variations for the Northeast, Midwest, Puerto Rico, Southeast, Great Plains, and Alaska. The trends are not larger than natural variations for the Southwest, Hawaii, and the Northwest. The changes shown in this figure are calculated from the beginning and end points of the trends for 1958 to 2012. (Figure source: updated from Karl et al. 20091). More details about this graphic and its source are available at this page.

In two areas close to each other in southeastern New York, the 10-year flood from the period 1960-1990 became the 5-year flood during 1980-2011, and the 25-year flood from 1960-1990 became the 10-year flood for 1980-2011. (Also, the 25-year flood during the period 1980-2011 was only slightly smaller than the 100-year flood in 1960-1990.) This is illustrated in the graph above, a slightly modified version of Figure 5a from a study published in the American Meteorological Association Journal of Climate in December, 2013. (It was modified by adding the horizontal blue bars to highlight the comparison between streamflow during storm events in two data periods.) This research used data from the Wallkill River and part of the Catskill mountains. These results are based on flood frequency estimates from the following six streamflow gauges: Schoharie Creek at Prattsville, Esopus Creek at Coldbrook, Wallkill River at Gardiner, Mill Brook near Dunraven, Tremper Kill near Andes, and Neversink River near Claryville. The abstract is available at this page. (Citation: Matonse, Adão H, Allan Frei, 2013: A Seasonal Shift in the Frequency of Extreme Hydrological Events in Southern New York State. J. Climate, 26, 9577–9593.)

Summary of Findings

These and other analyses of precipitation trends by climatologists and other authorities indicate that our region is experiencing a significantly larger number of bigger rainstorms in recent decades as compared to 50-60 years ago. The total annual average precipitation – the amount of rain and snow we get each year — has gone up only slightly during this period. In other words, while we have not experienced a dramatic increase in the total amount of precipitation we get each year, on average, there has been a very marked change in the pattern of how this water comes. We are getting less rain coming in long, gradual storms (known by some as “farmer’s rain”), and more is coming in shorter, more intense storms. As a result, rain has less time to seep into the soil and groundwater, and more of the rain runs off quickly. This creates higher risks for soil erosion and localized flooding. It also reduces the amount of water that recharges our groundwater aquifers, because more runs off to streams that drain away more quickly.

Context and Implications

To be clear, all of the forgoing information refers only to changes that have already occurred – this is what has been measured in actual precipitation and stream flow observations. It is also important to consider the predicted changes that will occur in the next 50 years and beyond based on climate change studies, which indicate a strong likelihood that these patterns and changes will become even more pronounced. More of the precipitation will come in fewer, larger storms and we are expected to get somewhat more total annual precipitation in this part of the U.S. Notably, some engineering standards for sizing stormwater infrastructure, and the underlying data used for FEMA flood insurance maps, are based on historical data, not on more recent conditions.

As we are in the midst of these changes in recent decades, the changes that have already occurred create new challenges for site and infrastructure design. Key questions for property owners, municipal officials involved in planning and development, and agencies involved in managing municipal infrastructure others is whether sizes and designs of existing and proposed culverts, retention ponds, and other stormwater infrastructure are based on relatively recent data on storm sizes and recurrence intervals. More broadly, areas that have not been at risk of flooding in the past may be at higher risk now than existing FEMA flood maps indicate, and these risks may increase even more in the future. These trends also have major implications for water supply planning and management of drinking water sources – increased erosion of sediment into water bodies will affect water quality in terms of turbidity and other issues, and reduced groundwater recharge may affect availability of water from wells.

Simon Gruber
Environmental Planning & Communications
Fellow, CUNY Institute for Sustainable Cities
Town of Cornwall representative, Moodna Creek Watershed Intermunicipal Council
PO Box 202, Cornwall NY 12518


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