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Surface run-off and erosion in relation to soil and plant cover on high grazing lands of central Utah

Stewart, G., Forsling, C.L.
Agronomy journal 1931 v.23 no.10 pp. 815-832
soil erosion, runoff, grasslands, grazing, vegetation, precipitation, Utah
On the elevated grazing lands of central Utah, surface run-off and the sediment carried have been studied since 1915. Two nearby experimental watersheds of approximately equal area, at an elevation of about 10,000 feet, and so located and equipped as to permit the measurement of the precipitation, the run-off water, and the sediment carried, differed widely in their plant cover. One of these with a considerably eroded soil supported from 1915 to 1920, inclusive, a plant cover of 16%. During 1921, 1922, and 1923 the plant cover was increased tO 40% and has been maintained until the present. The other watershed with a little-eroded soil had a well-established plant cover of 40% throughout the period under study. Rain gauges on the two areas showed a slightly greater summer precipitation on the area that was well vegetated from the beginning, and yet the run-off caused by summer rainfall was 5.96, 3.54, and 2.16 times as great for the three periods of 1915-20, 1921-23, and 1925-29, respectively, from the area at first poorly vegetated. The sediment removed was about in the same proportion, viz., 5.42, 2.82, and 2.48, respectively, for the three periods. There was, however, little difference in the sediment carried in each 1,000 cubic feet of surface run-off. Roughly, 3 to 5 times as much total water ran off the area with the poorer plant cover. For the three periods of years above noted, the run-off was 10.33, 8.74, and 5.49% of the total rainfall on the area with the poorer plant cover as compared with 4.10, 2.88, and 1.05% for the continuously well-vegetated area. Approximately two-thirds of the total precipitation came as snow and fully half the total was available for spring run-off from melted snow and from spring rains which came while the snow was melting. Run-off began at the same time on the two watersheds but ceased 20 days earlier on the one that was well-vegetated from the beginning. During the years 1926-29, inclusive, 60.2% of the total water available for spring run-off actually ran off the formerly depleted watershed, but only 17.3% from the well-vegetated one. If the years 1916-18, when one area had a 16% plant cover, are compared to the years 1926-29 when the same area had a new 40% cover, the total sediment eroded decreased 46% and the amount carried in each 1,000 cubic feet of run-off water decreased 57%, due to improving the plant cover. Of the total run-off from the depleted area during 1915-20, 4.6% was due to summer rainfall and 95.4% due to melted snow. The summer rainfall, however, carried 80.1% of the total sediment removed. After the plant cover had improved to 40%, only 1-3% of the total run-off resulted from summer rains. Eroded and non-eroded soils were compared on the newly vegetated watershed and on another area about 2 miles away. The noneroded, dark-colored surface soil of the newly vegetated watershed contained on the average 0.337% nitrogen as compared to 0.258% and 0.088% on the partly eroded and badly eroded areas, respectively. The badly eroded soils supported only annuals or non-palatable perennials, while the partly eroded soils supported palatable perennial plants and the non-eroded soils a good growth of forage grasses in addition to palatable perennials. The other area 2 miles away showed 0.382% nitrogen for uneroded dark-colored soils which supported grasses, 0.202% for the partly eroded soils which supported only poor perennial plants, and 0.043% on the exposed raw clay subsoil of a road cut which produced after 7 years only a few annual knotweeds. Water-soluble phosphorus was not found to be as closely related to the degree of soil erosion or to the kind of plants supported. Phosphorus is probably not a limiting plant nutrient on these soils.