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{}1)2,538 will be steeper. These hlllslopes probably will have greater runoff and ero- sion rates than the original undisturbed hillslopes. As reclamation proceeds and vegetation becomes reestablished, the erosion rate should gradually decrease to amounts comparable to adjacent undisturbed hillslopes. An estimate of the change in sediment yield due to surface mining may be computed by the PSIAC method. As described previously, this method rates the character of a watershed in nine categories (table 3). An estimated sediment yield is determined by comparing the numerical score of the watershed being ev.aluated with the numerical score of watersheds with measured sediment yields. The PSIAC method Is appropriate for estimating the increased sediment yield due the surface mining, because some of the categories listed in table 3, such as soil and vegetation cover, will change. Other categories, such as surface geology and cl imate, will not change. Approximately 6.0 million tons (5.4 million t) of coal were mined in the Yampa River basin during 1976. The amount of coal mined has increased significantly since 1962, and there is some uncertainty as to how rapidly coal mining and utilization will expand In the basin in the near future. Three alternative levels of coal production through 1990 have been assumed (Udis and others, 1977; Steele and others, 1978). The three estimates assume that, In 1990, 80 percent of the coal production will come from surface mines and 20 percent from underground mines. It is projected that surface-mined coal will increase to 8.0 million tons (7.3 million t) per year as a slow- growth estimate, 16 million tons (14.5 million t) per year as a moderate- growth estimate, and 24 million tons (21.8 million t) per year as a rapid- growth estimate. By using a coal-yield ratio of 20,000 tons per acre (44,840 t/ha) of land mined, these projected levels of coal production can be expressed in terms of land area disturbed per year. It Is. further assumed that the disturbed land will be partly reclaimed within 5 years and complete- ly reclaimed in 10 years. On the basis of these projections and assumptions, the area of land affected each year by mining as well as partly reclaimed can be calculated for each of the three levels of production for 1990. For purposes of this analysis, it will be assumed that all Increased surface min- ing will occur In the Yampa River subbasin (U.S. Department of the Interior, 1976) . Estimates of sediment yield land were computed using the PSIAC computed for the unmlned area, with the measured sediment yield. that the calculated sediment yield from recently mined and partly reclaimed method. First, a numerical rating was and the calculated sediment yield .compared The numerical rating then was revised so agreed with the measured sediment yield. Not all of the factors rated by the PSIAC method will change due to surface mining. Bedrock geology and climate, of course, will remain unchanged by surface mining. Other factors, such as runoff and topography, will most likely change slightly; whereas, soil type, ground cover, and land use will be radically changed. The numerical rating of these factors must be adjusted accordingly. The PSIAC method indicates annual sediment yields of 4,000 tons/mi2 (1,400 t/km2) from unreclaimed surface-mined land and 2,000 tons/ml2 (700 t/km2) for partly reclaimed land. 27 ,!..C.