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i p I I , ; J .~j ,,' :~~ " :;-i ~~j :~ l~ ;~;~ ;.... .>'";~ . :.} " 2. Air photos of the Doodad Watershed show that 22 large and 40 small gullies occur in the watershed. You randomly select one large and two small gullies to examine in the field. The large gully is 900 feet long and ~ eight feet deep. It is cut on a rangeland hillside. The soil exposed in c:o the banks of the gully is a loam. ' The nick point is10 feet wide with ~ clumps of soil at the base that still have grass on their surfaces. Deep ~ rills cut the face of the nick point and about three inches of grass overhang the edge. The banks of the gully are bare with some grass overhanging the top edge. Only a few rills are observed. Two small gullies are observed a few hundred feet away on each side of the large gully. Both of these gullies are similar.. One is 220 feet long and one is 300 feet long. Both are three feet deep and four' feet wide. The nick point and the banks are bare with some rills. About one inch of vegetation hangs over their edge. What is the total amount of gully erosion in the Doodad Watershed? Use direct volume method: (eroding area) (lateral recession rate) (density) r' i _ - e os on 2000 pounds/tDn 10 ft x 8 ft x 0.4 ft/vr x 90 pcf . 1.4 or 1 tons/yr erosion 2000 pounds/ton 1800 ft x 8 ft x 0.06 ft/vr x 90 pcf . 2000 pounds/ ton 39 tons/yr erosion 40 tqns/yr erosion from one large gully 1048 ft x 3 ft x 0.06 ft/yr x 90 pcf . 8.5 tons/yr erosion 2000 pounds/ton 8.5 tons/yr/2 gullies = 4 tons/yr erosion from one,small gully 22 large gullies x 40 40 small gullies x 4 tons/yr = 880 tons/yr tons/yr. 160 tons/yr 1040 tons/yr erosion erosion gully erosion 24