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11 <br />I~ <br />greater mean slope, greater overland flow, and less root density on the <br />regraded sites as reasons for the larger sediment yield. Not surpris- <br />ingly, they found sediment yield much greater where overland flow was <br />much higher. The authors stress that their data should not be extra- <br />polated to larger basins. They point out that in semiarid climates both <br />the discharge per unit area and the sediment delivery ratio - the ratio <br />of volume of sediment moved out of a basin to estimated volume of sedi- <br />ment produced in the basin (Schumm, 1977, p. 71) - decrease with an <br />increase in drainage area. <br />~~ In another study of erosion and infiltration in the Northern Great <br />Plains, Gilley et a1. (1977) applied simulated rainfall to plots on <br />native rangeland and regraded nontopsoiled and topsoiled land in western <br />• North Dakota. Some of the nontopsoiled regraded land was cultivated <br />with a rototiller to a depth of 5 to 8 cm (2 to 3.1 in). The rainfall <br />application of 6.4 cm/hr (2.5 in/hr) simulated natural rainfall intensity <br />and was applied in three phases: a one hour run at existing conditions, <br />a wet run 24 hours later, and finally straw was placed on the nontop- <br />soiled and topsoiled regraded land at a rate of 4500 kg/ha (4005 lbs/acre} <br />fora 30 minute run immediately after the wet run. The sediment pro- <br />duction from the native rangeland was 200 kg/ha (.178 lbs/acre), almost <br />all from the wet run, and the overland flow was only 127, of the applied <br />rainfall. The spoil loss from the nontopsoiled regraded land averaged <br />15000 kg/ha (13350 lbs/acre) and 21000 kg/ha (18700 lbs/acre) from the <br />cultivated and noncultivated spoils, respectively, with overland flow <br />averaging 66% of the rain applied on the cultivated spoils and 74% of <br />• the rain applied on the noncultivated spoils. Straw mulch reduced soil <br />