How Does Stormwater Runoff from Urban Forests Affect Groundwater Recharge?
Urban Forests and Stormwater Management Plans
How does what we plant on the land influence the movement of water? As water conservation becomes an increasingly significant planetary concern, cities, landowners and farmers are becoming aware of how our choices in ground covers affect groundwater hydrology, or the movement of water on, under, and into the land. To gather information that will help cities around the world manage their urban forests and refine their water use and stormwater management plans, researchers are asking questions like these:
What kind of trees contribute more to ground flooding in heavy storms?
What kinds of trees hold water in their branches and leaves to allow more rain to evaporate off rather than funnel down the trunk to the ground?
In Kamloops' McArthur Island Park, many of the trees are wrapped with corrugated plastic troughs of pipe, secured with what looks like bathroom tile grout, leading the rainwater into a holding tote at the base of the tree.
In the bright, warm evenings in this small city at the edge of the Thomson River, people walk over to look at the installation, curious. I spoke with researcher Kyle Bondarchuk one evening as he went about his work measuring the rain water that had accumulated in the totes at the base of the trees during that day's storm.
Stormwater Runoff in Urban Watersheds Contributes to Flooding
The project is researching how different species of trees funnel rainwater differently. In Kamloops, a semi-arid climate, recharging groundwater is an important factor in reducing water usage and diminishing the city’s ecological footprint. Sometimes in periods of heavy rain, low-lying city areas flood, and spring flooding is usual along the riverbanks and floodplains, as meltwater from the Rocky Mountains makes its way downstream. During the rest of the season, the climate is arid, making water conservation a key factor in the city's stormwater management plan.
The research is a collaboration between Thompson Rivers University, the City of Kamloops, and a number of other private, public, and non-profit sector partners. Under the supervision of Dr Darryl Carlyle-Moses of the Department of Geography, it currently provides the research focus for M.Sc. student Julie Schooling.
How the Urban Forest Impacts Groundwater Recharge
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Storm Water Management and Groundwater Hydrology
This research has important implications for land use everywhere, for it focuses on the attributes, rather than the species of the trees. The canopy size and shape, branch angles, trunk size and texture, leaf size and shape, and attachment angle of leaf petioles to branch, are some of the elements that influence how much water the tree funnels down to the ground during a precipitation event.
An example shows how this works. Two trees in the study are an American Beech, which funnels a lot of water, and a Horsechestnut, which funnels less.
The Beech has a small angle of branching, whereas the chestnut branches are almost perpendicular to the trunk. As the next series of photographs shows, the leaves of the Beech angle up like tiny saucers because the petioles are stiff and hold the leaves at an angle above the branches, catching the rainwater and directing it in toward the centre of the tree and concentrating it down the trunk. On the other hand, the Chestnut leaves and petioles are soft and drooping, inclining to drip the water off the ends of the branches, dispersing the water or allowing it to evaporate.
The results of this study may yield important information for city and agricultural land use planning to diminish flooding, recharge ground water, and reduce erosion, and save overburdening stormwater management systems.
In some cities, like Mexico City and Los Angeles that draw their water from aquifers below the city, emergencies loom as ground water depletes. On natural ground cover sites, local species of trees with the characteristics of American Beech might be planted to funnel water into the earth and help recharge the ground water. In cities that frequently have heavy precipitation and flooding, species with characteristics that disperse the rain water, holding it in their canopy until it evaporates, may help reduce flooding. Further research in this study will indicate how choices of tree species in urban forests may affect inputs to the terrestrial water cycle with implications for surface runoff and groundwater hydrology.
The research continues in Kamloops for two years, allowing study of the precipitation of all four seasons.
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