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RULE 2 PERMITS <br />Where S is the maximum potential difference between P, Precipitation, and Q, Runoff, in inches at the <br />time of a storm's beginning. <br />Estimating composite Curve Numbers of 67 and 63 for the premining and postmining conditions <br />respectively results in maximum potential differences of 4.93 inches for the premining condition and 5.87 <br />inches for the postmining or reclaimed condition. Utilizing these relationships results in an estimated <br />mean annual runoff or 0.35" and 0.24" for the premining and reclaimed postmining conditions, <br />respectively. These relationships are tabulated below. <br />Item Pre -Mining Post -Mining <br />Curve Number CN <br />Runoff, Q, in inches <br />Potential difference, S, in inches <br />Initial abstraction, la, in inches <br />67 <br />63 <br />0.35 <br />0.24 <br />4.93 <br />5.87 <br />0.99 <br />1.17 <br />During the life of the mine, it is anticipated that a total of 2298 acres will be eventually disturbed in <br />Streeter Gulch and other areas tributary to Goodspring Creek. Another 1512 acres will be disturbed in <br />areas which are included in the Taylor Creek -Wilson Creek drainage basins. Mean annual runoff from <br />these reclaimed areas are estimated to be as follows: <br />Drainage Basin <br />Premining <br />Postmining <br />Difference <br />Good Spring Creek <br />79 ac. ft. <br />55 ac. ft. <br />-24 ac. ft. <br />Taylor Creek <br />40 ac. ft. <br />28 ac. ft. <br />-12 ac. ft <br />The hydrologic consequences of mining on the various drainage basins downstream of the Colowyo Mine <br />site are summarized below: <br />Drainage & Premining Postmining Difference Post/Pre <br />Location AF AF AF % <br />Good Spring Creek <br />at Axial <br />1,479 <br />1,455 <br />-24 <br />98.38 <br />Taylor Creek <br />at Mouth <br />110 <br />98 <br />-12 <br />89.09 <br />Wilson Creek <br />Near Axial <br />1,372 <br />1,360 <br />-12 <br />99.13 <br />Milk Creek <br />at Mouth <br />20,600 <br />20,564 <br />-36 <br />99.83 <br />Yampa River <br />at Maybell <br />1,120,000 <br />1,119,964 <br />-36 <br />100.00 <br />Runoff in the permit area, which is diverted or detained, will continue to reach the historic drainages. A <br />portion of the water rights held by Colowyo on Goodspring Creek may be consumed in accordance with <br />the augmentation plan discussed in Exhibit 7. This possible consumption will possibly affect the <br />agricultural practices on a minor portion of the Goodspring Creek Valley Floor. However, since the <br />Colowyo Coal Company owns the property to be withdrawn from irrigation, and these lands are not an <br />integral part of any ranching or farming operation, the effect will be minimal. Runoff detained in the <br />detention ponds will be released from time to time in accordance with the NPDES permit. These ponds <br />will only interrupt flow resulting from precipitation falling directly on the mine and associated areas. As <br />Rule 2 Permits 2.04.7-22 Revision Date: 4/7/17 <br />Revision No.: RN -07 <br />