... Often in hydrology, we talk about the hydrology of a watershed. You can estimate the size by treating the lake as a the closest geometric figure, such as a circle, rectangle or triangle, or you can try to directly count the square footage of the lake by using graph paper on a scaled area map. To calculate how … It may be (Contact the local Soil Conservation service for watershed acreage.) Calculating the size, or area, of a lake can be done a couple of different ways. The Capture Ratio is the ratio of the pervious area receiving the runoff (such as a lawn area) to the impervious area that generates the runoff. At this point you have delineated the watershed of the wetland being evaluated. Optional: Calculate the amount of rain that falls on the watershed by finding out the average rainfall and multiplying the value by the watershed area. Related Surface Area Calculator | Volume Calculator. A watershed is an area or region that is drained by a common river system. ft. of lawn area then the Capture Ratio would be 3,000 / 5,000 or 60%. For example, if 5,000 sq. Eventually it will connect with the circle from which you started. calculate area. StreamStats Application. Learn More Measure the roof to get the watershed area, the area from which water will run off. area; B, the basin lag factor, which is the basin length divided by the square root of the channel slope; C, the basin length in miles; D, the main channel slope in feet per mile; E, the total impervious area, in percent of the basin area; and F, the basin where: A = total watershed area (mile 2). ft. of roof area is directed onto 3,000 sq. Write down those dimensions. 2. The delineation appears as a solid line around the watercourse. (Show students the Figure 1 watershed, or a similar image). Draw the general shape of the pond on paper (graph paper works best). 1, 2. This will give you a rectan-gle on which to base the calcula-tion of area (See Figure 2 below). the watershed and down the opposite side of the watercourse. 3. Multiply length (peak to eave) by width to determine square footage. Note logical places for downspouts, like building corners. Draw a rectangle on the pond shape that would approximate the area of the pond if some water was eliminated and placed onto an equal amount of land. Generally, surface water runoff from rain falling anywhere in this area Note places where two roofs form a valley, which will affect runoff because water will come from two sides. The area is based on the “footprint” of the roof, which can be calculated by finding the area of the building and adding the area of the roof’s overhang. The image below shows how differences in roof slope do not change this building’s catchment area. Area is a quantity that describes the size or extent of a two-dimensional figure or shape in a plane. 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