Winter Precipitation and Forests: Was It Enough?

It's easy to be nonchalant about wet and snowy days in Pennsylvania. It's easy to take water for granted in a state with 84,867 miles of streams and rivers. It's easy to wish for an early end to winter or more bright and sunny days, but don't malign the importance of those grey wet days, the importance of a decent snowpack, the importance of groundwater, and the impacts we have on water.

As someone who loves snow, I always wish for more - get some skiing in, have a snowball fight, curl up by a fire. While snow presents its challenges - slick roads, whiteouts, school delays, missed work, shoveling pains, and more - winter precipitation is one of the most important contributors to our water cycle year for recharge and renewal.

Much of southeastern Pennsylvania is still in mild to moderate drought conditions. In spite of recent snow and rain events, we're still down anywhere between four and ten inches in much of the eastern half of the state. While it may seem like we had a large amount of snow in recent snow events, it takes anywhere between four to eight inches of snow to equal one inch of rain. Across the state, average annual precipitation is around forty inches; based on that, we're down anywhere between 10 to 25% for the last twelve months. If recent years were an aberration, we might make that deficit up, but we're experiencing long periods of no rainfall or snowfall and that has a significant impact on the groundwater on which we all rely.

The forest is a perfect environment to capture and slowly release water into the ground. The soil under forest canopies acts like a sponge to soak up and pass water from the surface into groundwater aquifers, with trillions of gallons of freshwater stored in the pore spaces and cracks in rock beneath the surface. These aquifers serve to maintain the flow of streams throughout the year and provide water that supports industries, businesses, agriculture, and drinking water for millions of Pennsylvania residents. In 2007, Pennsylvania had about one million private water wells supplying water to more than three million rural residents. With an additional 10,000 to 20,000 new private wells drilled each year around the state, those numbers have increased significantly. Where forests are removed, the soil may become compacted or even paved, reducing the amount of water that can infiltrate into the ground to support aquifers. Lawn, buildings, and paved surfaces don't allow water to infiltrate deep into the ground - rain and meltwater run off to streams and sewage systems.

Infiltration of water into the ground occurs most efficiently during times when the forest is dormant - November to March. Snow in winter, steady temperatures, and a gentle spring are the best scenarios for groundwater replenishment. As it melts, the snowpack provides a slow and steady flow of water into the soil. When it rains and the forest soil is unfrozen, which it normally is, we get similar results - water infiltrating into the ground. However, with swings in temperature and cold days followed by warmer precipitation events, rain falling on frozen ground does not give the same benefits. Instead of groundwater replenishment, we get runoff. And while streams are full for a short period of time, the benefit to groundwater is minimal.

This has been a strange winter - minimal precipitation in the form of snow and rain, and warmer days and freeze thaw cycles. We're going into the spring with a groundwater deficit in many parts of the state. Current measurements of ground water in one Centre County monitoring well show that levels are 20 feet below average. Once we hit April and the leaves come out, most precipitation that falls will be quickly taken up by tree roots and go towards tree maintenance and growth. While trees use a lot of water to live in their active seasons, the forested system - the forest soil that allows for maximal infiltration - is the best resource for clean and abundant groundwater.

For streams and groundwater in Pennsylvania, the next two months are critical. If the weather is going to stay warm (as our current ten-day forecast trend seems to predict) we need some days of slow, steady rain. If we're going to cycle back to cold, we need abundant snows and a slow melt.

Do your best "precipitation-inducing" dances and don't malign those days when wet stuff falls from the sky. We rely on it for everything.

For more information about forests and groundwater, check out the Penn State Extension Water Resource publication, "A Quick Guide to Groundwater in Pennsylvania."