Calculating watershed acreage
To understand how water moves around your property, you have to define the watershed area. This information can determine conservation practices and pond capacity.
Radio interview source: Wayne Gobovich, National Agricultural Engineer, NRCS
Stand on a low point and look uphill. The area where rain falls and then gathers where you're standing is considered the watershed. An assessment of this area helps you design a pond.
Wayne Bogovich is a national agricultural engineer with the Natural Resources Conservation Service, or NRCS. He says calculating watershed acreage tells you how much runoff to expect, how big the pond should be, and how much extra storage you would need before the water goes over the emergency spillway. You might also determine if runoff could be polluted.
Bogovich says there are a couple of ways to make the calculations.
"The simplest technique is the way we used to do it 50-years ago, which is just take a U.S. Geological Survey topographic map," he says. "You can actually draw the watershed on the map based on the contour lines. Another way of doing it, you would use GIS, which is Geographic Information Systems, which are electronic maps. And in some of these systems, you can actually put a point where the pond is, and it will actually calculate on its own and draw it for you."
Pond construction isn't the only use of this calculation. Conservation practices such as terraces and waterways are designed using the same technique.
"You need the slope of that watershed, the size of the watershed, you need the vegetative cover on the watershed," says Bogovich. "And what that does is, from that information, you can actually calculate how much water is coming down the hill, and how much water you need to deal with. Because if you don't size those things properly, they're going to run over the dikes and they're not going to perform as they're designed."
Unless you have the right technical background, Bogovich recommends getting help from your local NRCS office to make the watershed calculations.
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