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  1. What is a 100-Year Rainfall Event?

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    Understanding the Extremes

    by Carrie Brown, Engineering Technician

    How often does a “100-year rainfall event” occur? It turns out that the answer is a bit more complicated than you might think.

    It is a common misconception that a “100-year flood” happens once in 100 years. The term can be quite deceiving when taken at face value. In fact, it is possible for two 100-year storms to occur 50, 25, or even one year apart. Let’s break down the terminology a bit and discuss what “100-year” is actually referring to.

    What is a flood?

    Flooding occurs when water overflows onto land that isn’t typically inundated. Floods can result from large rain events, waves from a large body of water coming onto land, rapid snow melt, or when an artificial barrier breaks (such as a dam).

    A flash flood results when heavy rainfall falls at a faster rate than can be absorbed into the ground or stored in a reservoir or lake. Similar to a sponge, the ground eventually becomes saturated, and remaining water is left to flow on the surface. Large impervious areas, such as parking lots, roads, and building roofs, can cause water to accumulate even faster. Flash flood events can be particularly dangerous as there may be inadequate time to warn (and in some cases, evacuate) the public residing within the flooded area.

    A floodplain is the portion of a valley that has historically been inundated by overflowing streams, creeks, and rivers. Consequently, these areas have a higher chance of experiencing flood conditions. The Federal Emergency Management Agency (FEMA) and other government agencies have mapped out these areas affected by past flooding events. These maps are often used to determine flood insurance requirements or to aid communities in regulating future development. Permits are needed for development within a FEMA regulated floodplain. Floodplain maps can be found at the FEMA Flood Map Service Center.

    At Fairfield SWCD, we engineer grassed waterways and stabilization structures, such as the timber box seen above, to handle large rain events. Waterways help to filter water and prevent soil erosion.

    What is a 100-year rainfall event?

    As defined by the National Weather Service, “a 100-year rainfall event (or more accurately the 100-year rainfall amount for a specified duration and at a given location) is an amount that on average is exceeded every 100 years, so its average recurrence interval is 100 years.” This means that it has a 1% chance of occurring in any given year. The actual number of years between flooding events can vary dramatically; it can happen twice in one year, three times in 50 years, or any interval in between.

    Similarly, a 50-year rainfall event has a 1 in 50 or 2% chance of occurring in a year.

    Now you can see why it is incorrect to take these terms literally since the average recurrence interval is simply an average.

    Recurrence Intervals & Probabilities of Occurrences

    Recurrence intervals in yearsProbability of occurrence in any given yearPercent chance of occurrence in any given year
    1001 in 1001%
    501 in 502%
    251 in 254%
    101 in 1010%
    51 in 520%
    21 in 250%

    How much rain is in a 100-year rainfall event?

    The amount of rain that “qualifies” for a 100-year rainfall is determined by studying past floods. Scientists use statistics to observe the frequency of large rain events and the average number of years between these events. From this they determine the probability that a flood of any given size will take place within a year.

    As you might expect, these rainfall amounts vary based on location. The document below, produced by the National Oceanic and Atmospheric Administration (NOAA), shows the average recurrence interval for rainfall durations, ranging from 5-minutes to 60-days for Lancaster, Ohio.

    The Precipitation Frequency Estimate can be selected for any location within the United States.


    Can the amount of rainfall designated as a 100-year rainfall event change for a given location?

    Yes, many flood designations will change over time. When a river basin is altered, including dams or changes in upstream urban development (and increased impervious surfaces), scientists then reevaluate the frequency of flooding and the amount of damage it could produce. This new data may be used to alter a location’s precipitation frequency estimates.

    Want to learn more?

    Check out the following resources for more information on floods, floodplains, and rainfall events.

    Local Information

    Fairfield County Regional Planning Commission: Floodplains

    City of Lancaster Floodplain Information

    City of Pickerington Floodplain Information

    General Information

    FEMA Map Service Center

    National Weather Service’s Hydrometeorological Design Studies Center

    USGS: The 100-Year Flood

    USGS: Daily Streamflow Conditions

    Community Collaborative Rain, Hail & Snow Network

  2. Caught Up In Cicada Mania? 10 Things You Should Know About These Mystic Creatures

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    by Carrie Brown, Engineering Technician

    If you are an Ohio resident, you are likely aware of this year’s mass emergence of periodical cicadas, Brood X edition. Whether you’ve heard about the 17-year surfacing on television or social media or have had the honor of experiencing the immersive nature of their collective drone firsthand, there’s no denying the astonishing character of this generational phenomenon.

    Also known as the Great Eastern Brood, Brood X (pronounced Brood Ten) is one of 15 broods of periodical cicadas that emerge throughout the eastern United States. Brood X is one of the largest groups, gracing 15 states, including central and western Ohio, with their collective presence.

    Periodical cicadas spend most of their lifecycle under the ground, feeding on juices from tree roots in an immature state known as a nymph. Depending on the brood, these juveniles clamber upwards every 13 or 17 years, where they emerge synchronously en masse over a two week period, shed their outer shells, and commence what can only be described as a breeding frenzy.

    With a population numbering in the billions, periodical cicadas only live a few weeks as an adult, and during this time they all share the same reproductive mission. Once this quest is fulfilled and the females have deposited their eggs in a nearby tree branch, their life cycle is complete.

    Whether you love them or loathe them, one must admire the many unique qualities of these remarkable critters. Let’s take a look at 10 traits that will make sure-fire table talk. (Family dinners are about to get a whole lot more interesting.)


    17-year periodical cicada broods consist of three different cicada species (Magicicada septendecim, Magicicada cassini, and Magicicada septendecula), while 13-year broods include four different species (Magicicada tredecim, Magicicada neotredecim, Magicicada tredecassini, and Magicicada tredecula). They use their distinctive calls to find a mate within their same species.


    Despite the fact that these bug-eyed beauties look remarkably similar to a creature from your favorite science fiction movie, cicadas pose no physical threat to humans and do not bite. (However, since they aren’t exactly graceful flyers, an inadvertent collision with your head is always a possibility.)


    Cicadas emerge from the depths of the ground when the soil reaches 64 degrees Fahrenheit.


    A cicada chorus is made up entirely of fellas! Males make their signature calls (“FAAAIR-oh” in the case of the most common species, M. septendecim and a constant “SHHHHH” by the smaller species M. cassini) using two organs called tymbals. The song resonates in their mostly hollow abdomen and is amplified, similar to the belly of a guitar. In response to this sweet serenade (a mass chorus can reach upwards of 120 decibels in volume), females flick their wings to show their mutual interest.


    The female uses a structure called an ovipositor to slice the end of a small tree branch and lays her eggs within the slit. Recent studies show that not only is the ovipositor reinforced with metals, the structure is serrated like a tiny steak knife.


    On average, females lay between 400 and 600 eggs, making multiple “nests,” each containing ten to twenty eggs.


    Cicada eggs typically hatch towards the end of July, weeks after the demise of their parents. From hungry beetles to ravenous ants, the immature nymphs are extremely vulnerable to predators, so as they fall to the earth, they scurry to the nearest crack in the ground and begin their descent. They slowly make their way down, reaching their final destination 10-12 inches below the earth’s surface by New Year’s Day.


    Like other insects, a cicada wears its skeleton on the outside of its body. The empty shells often found on tree trunks represent their final molting and entrance into adulthood. However, this isn’t the only time within their lifecycle that they shed their exoskeleton. They actually undergo five molting events (also called instar stages) as they slowly grow underground.


    Due mainly to activities such as deforestation, entire broods have been known to disappear completely. For instance, Brood XI was recorded to have emerged in great numbers in 1699 just outside of Boston. Unfortunately, this brood hasn’t been spotted since 1954, their extinction assumed to largely be due to urbanization.


    Want to take part in this generational insect shindig? (In the case of Brood X, your next chance won’t be until 2038!) Download the Cicada Safari Smartphone mapping app and tell researchers where you are seeing periodical cicadas!

    Cicada nymphs emerge from the ground when soil temperatures reach 64 degrees Fahrenheit
    A periodical cicada molts and emerges as an adult
    An adult periodical cicada perches on a leaf, likely serenading a nearby female.
    A female periodical cicada uses its ovipositor to slice and deposit eggs on small branches
  3. New Supervisors Elected

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    David Ochs and Gregg Pontius, both of Lancaster, were elected to serve on the Fairfield SWCD Board of Supervisors starting January 1, 2021, and will serve three-year terms.

  4. 2016 Healthy Soils Mini-Grant

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    The FSWCD, through a 2016 Healthy Soils Mini-Grant, partnered with several schools, community gardens, Fairfield DD and OSU Extension Master Gardners to plant cover crops to demonstrate soil quality improvements that cover crops can provide. Thank you to ODNR for helping us educate the non-ag community about the benefits of cover crops.