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Calculation Brief <br />Evaluation of the Sensitivity of Percolation Estimates through <br />Mine Rock Stockpiles on the Simulated Precipitation Rate <br />If the value of HPR is less than this minimum reasonable value, some of the daily <br />precipitation on the wettest day of the year will not be applied in the model. At a HPR <br />value of 1.905 cm or higher, all precipitation is distributed in the first hour of the <br />simulation. <br />Table 1 and Figure 1 summarize the results of the HPR sensitivity analysis. The <br />corresponding UNSAT-H output files are included in Attachment 2. <br />Table 1. Summanj of UNSAT-H modeling results (all results in centimeters). <br />HPR (cm/hr) Annual Percolation (cm) Annual Runoff (cm) <br />0.1 2.09E-02 4.70E-02 <br />1 1.30E-03 8.41E+00 <br />2 8.76E-04 9.78E+00 <br />4 8.76E-04 9.78E+00 <br />The evaluation shows that higher simulated precipitation rates (HPR) result in <br />decreased percolation and increased runoff. UNSAT-H does not simulate runoff <br />implicitly. Runoff is estimated as the precipitation rate that is in excess of the <br />infiltration rate. The UNSAT-H model simulates infiltration in a two-step process. <br />First, infiltration is set equal to the precipitation rate during each time step. Second, if <br />• the surface soil saturates, the solution of that time step is repeated with the surface <br />node saturated. The resulting flux from the surface into the profile is the infiltration <br />rate (Fayer 2000). If the precipitation rate exceeds the infiltration rate, the excess <br />precipitation is assumed to be runoff. The sensitivity analysis shows that at values of <br />HPR greater than approximately 1 cm/hr, the surface soil saturates and the <br />precipitation rate exceeds the infiltration rate. This leads to runoff, and a resulting <br />decrease in estimated annual percolation. <br />6.0 Conclusions <br />An analysis of the sensitivity of the UNSAT-H model to the rate of precipitation was <br />conducted to address a DRMS (2009) comment regarding whether the UNSAT-H <br />model accounted for potential "high-intensity, short-duration precipitation events <br />such as thunderstorms". The precipitation rate in the UNSAT-H model is controlled <br />by a variable called HPR, which is the rate of precipitation expressed in centimeters <br />per hour. The evaluation shows that precipitation rates of greater than approximately <br />1 cm per hour result in saturation of the surface soil, an excess of precipitation in <br />relation to infiltration, and generation of runoff. This decreases the amount of water <br />that is available to percolate through the development rock pile, and decreases the <br />estimated annual percolation as shown in Table 1 and Figure 1. A HPR value of 0.1 <br />cm/ hr. results in negligible runoff, and maximizes the amount of water available to <br />infiltrate the surface of the pile and to potentially percolate through the base of the <br />10 pile. <br /> <br />C:\Nelson_work\Denison\UNSAT_HWPR sensbAtyWan 4 HPR Sensitivity talc brief.doc