This spring has seemed customarily soggy. Damp spring days give the appearance that far more precipitation falls during April and May than during other times of the year. But data collected over the past 40 years at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire tell another story. On average, we receive nearly the same amount of precipitation each month of the year. That’s right – nearly as much rain falls in August as in April.
Not only does this simple fact contradict our sense of weather but also it runs counter to the changes in river flows that we observe throughout the year. If the same amount of water falls each month, why does streamflow change so much with the seasons? While melting snow obviously is responsible for high water flow in spring, forests play an important but more subtle role the remainder of the year.
From its highest leaves to its deepest roots, a tree influences the path of water falling as snow and rain. During the growing season, the tree actively catches and uses water to support its growth. A tree’s architecture is well suited to capturing water: its spreading limbs and dense foliage create an umbrella that intercepts rain and snow. Hardwood trees (such as maples) leaf out in springtime and are then ready to collect precipitation. Conifers (such as white pine) capture precipitation year-round with their ever-present needles. According to data from Hubbard Brook, the forest umbrella catches 40 percent of the rain and snow that falls on the forest.
Most of the water the forest captures evaporates back to the atmosphere and never makes it to the stream. The rain or snow that passes through the canopy reaches the ground by either falling through the leaves (aptly termed “throughfall”) or by flowing down the stem of the tree (referred to as – you guessed it – “stemflow”). After traversing these various paths, the water make its way to the forest floor where it encounters soil and roots.
Tree roots extract water from the soil through a process known as transpiration. By drawing water from the soil, trees control their temperature and conduct photosynthesis. After moving through the tree, the water evaporates back to the atmosphere through small pores (or “stomata”) on leaves and needles. As you can see, trees exert a strong influence on the water cycle, and, during the growing season, when trees are making use of the water around them, streams experience their lowest flow.
When fall arrives, hardwood trees shed their leaves and the forest enters dormancy. At this time of year, more water passes through the forest to the stream, and streamflows rebound in October and November. Then comes winter, with its snow and cold, and as water becomes locked up in the form of snow and ice, streamflows decrease again until spring.
This connection between forest cover and streamflow can vary with changes in the landscape. The most obvious alteration comes from forest harvesting. Many studies in the U.S. have shown conclusively that the reduced forest cover that immediately follows logging increases streamflow, while the early years of reforestation reduce streamflow. The increase in streamflow following logging is approximately proportional to the amount of forest cover removed. This makes good sense, because, when trees are cut, the canopy of the tree is removed, less water is transpired, and more water is available to pass through the soil to the stream. As the forest regenerates, the young, vigorous trees use more water than their predecessors, reducing the amount of water that makes its way to the stream.
An even more profound effect on the water cycle is the loss of forests to urbanization. As forests are converted to pavement, the canopy is permanently lost and precipitation falls on impervious surfaces. Under these conditions, water runs off the land more rapidly, causing increased flows and, if the ratio of rainfall to pavement is high enough, flooding. The opposite side of the coin – development leading to abnormally low streamflows – can be equally problematic. In paved areas, water does not reach the soil to be stored, seeping out into rivers and streams over time. Consequently, less water is available to fill streams during dry times of year. The overall result of urbanization and pavement, therefore, is an increase in both flooding and drought.
With the summer months now upon us, look for streamflows to reach their lowest levels of the year. But remember – it isn’t just the passing shower or thunderstorm that determines how much water is in our streams at this time of year. Thirsty forests also play an important role.