A resource of Biophilia: Pittsburgh, #bioPGH is a weekly blog and social media series that aims to encourage both children and adults to reconnect with nature and enjoy what each of our distinctive seasons has to offer. 

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We all know the feeling of a rainy, gray, sleepy day. The world around us needs the rain, but especially on a late spring or summer day, rain might seem like it puts a damper on sunny plans. However, what if instead of being tired and bored on a rainy day, we used it as an opportunity to explore the science of rainfall? And you don’t even have to wait for the rain to get started; you can start exploring the science of rainfall right now!

As a bit of background, we all know that rain is an important part of the water cycle, also called the hydrologic cycle. Water evaporates from bodies of water such as rivers, oceans, and lakes, and condenses in the atmosphere, forming clouds. Water from the clouds eventually comes back to earth as precipitation, which can take the form of rain, snow, hail, sleet—you get the gist. Once the precipitation lands, it can either evaporate soon after, or be absorbed by plants, or make its way to the next body of water such as a river or ocean. And the marvelous process repeats itself.

For our own scientific inquiry purposes, we can take particular interest in what happens when the rain falls—and where it lands. All surfaces of land are not created equal, and depending on the soil type, rain water might be easily absorbed into the ground, it could absorbed by plants, or it could run across the surface of rock or pavement to the nearest waterway. This process of water making its way across surfaces is called “runoff.” You may have heard the term as a matter of concern, when pollutants and litter on land make it into our rivers—being carried away with surface runoff water after a heavy rain. (This is a common scenario, and everything from our road salt to the soap we wash our cars with in the driveway will eventually make its way into a river or stream!) For now, though, we are just referring to the water that “runs off” across the ground and sloped hillsides on its travels to a waterway. After a heavy rain, you have probably also noticed streams and rivers swelling with the excess runoff water, rising higher than usual and perhaps rushing with more power and speed than usual. It’s all part of natural processes, and it is precisely what we can explore—even from the comfort of the indoors on a rainy day.

Now that we have had a refresher in the basics of rain, below are four different activities to explore the science of a rainy day. The two "activities" are appropriate for all ages, in particular families with younger children. The two "challenges" are a bit more involved—but don’t be alarmed! Just get yourself a notebook, and let’s collect some data!

Rainfall Monitoring Activity—If we have a rainy day, keep track of how much rain we receive with your own rain gauge that you can make at home by reusing an old two-liter bottle. Follow the instructions here, and then test how accurate your rain gauge is by comparing it to your nearest USGS rain gauge.

Watershed Rainfall Activity—A watershed is the land area around a stream or river that ultimately drains into that waterway. Understanding watersheds is very important because what happens on the land will affect the water. This can be simulated at home using a paint pan, a few props, and a watering can. The basin portion of the paint tray can be filled with water to represent a river, but leave the sloping portion of the tray dry—this is your land! If you have rocks or toy houses, feel free to place those on the “land.” Now, sprinkle bits of “litter” on the land—perhaps biodegradable glitter, flavored gelatin powder, or tiny bits paper. Now, using the watering can, create rainfall on the land. What happened to all of the bits of litter? Did they stay on land, or did some get carried by runoff into the river? What are some things we can do to prevent pollution from making its way to rivers as runoff?

Rainfall Monitoring Challenge—Predict how a rainfall will impact streamflow conditions in the rivers or streams around you. Streamflow is the term used to describe how much water is flowing in a given waterway. In the US, this tends to be represented in cubic feet per second. After a significant rainfall, streamflow will increase from all of the extra water within the drainage area.

To determine how rainfall changes your nearest stream, we need to first determine a baseline, or “normal” streamflow. Go to the USGS page for Pennsylvania Daily Stream Conditions, and select the monitored waterway that is nearest to you. Over the course of a few weeks, check the “Discharge” graph for the appropriate day and record that number. Depending on which stream you select, you may notice that a multi-year or even multi-decade median is marked on the graph as well. You are welcome to simply use that number as well, but it’s more fun to have your own data.

After you have a few weeks of a baseline, check the streamflow again after a heavy rain. Check during the rain, a few hours after the rain, the following day after the rain, and two days later. When was the streamflow highest? During the rain? After the rain? How long after the rain?

If the nearest waterway to you is a large river, you may have a bit more trouble teasing out patterns of rainfall. Don’t be discouraged if that is the case, you may be able to keep track of smaller stream even if it is a bit further away from your rain gauge.

Watershed Rainfall Challenge—The USGS page for each waterway will give you the drainage area for that stream, usually in square miles. The drainage area summarizes the size of that waterway’s watershed—the land area that ultimately drains into that stream. For a waterway as large as the Ohio River, the drainage area is roughly 19,500 square miles. For a smaller waterway, like Chartiers Creek, the drainage area is only 257 square miles.

Try to find two waterways with similar drainage areas, but with different slopes around them. Use this shaded-relief map to pick out a stream in a flatter area and then pick another stream that may be between two steeper hillsides or ridges. Keep track of these two waterways changes in streamflow after various rainfall events. Which one would you predict to show a greater change in streamflow?  

What are some of the other variables you might encounter with an activity like this? Will the areas around the two streams have similar soil types, or might they be different? Can different soil types retain water better? What about plant cover? Could the presence or absence of plants impact water runoff?

Connecting to the Outdoors Tip: Hopefully one of these opportunities for inquiry will spice up your next rainy day! Nature is beautiful and hard at work, even during the soggiest of times. Take the opportunity to learn more about it!

Continue the Conversation: Share your nature discoveries with our community by posting to Twitter and Instagram with hashtag #bioPGH, and R.S.V.P. to attend our next Biophilia: Pittsburgh meeting.

Resources

USGS - Runoff

USGS - Surface Runoff: the Water Cycle

Physical Properties of Soil and Soil Water

USGS - The Water Cycle

Photo Credit: Wikimedia User John Morgan CC-BY-2.0