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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 />• <br />sample are included in Attachment 2. Vapor flow was simulated using a tortuosity <br />of 0.66 and a vapor diffusivity of 0.27 centimeter squared per second. The average <br />soil temperature was assumed to be 283 degrees Kelvin. <br />¦ The climate data used in the model includes precipitation data from Uravan, <br />Colorado, which is similar in magnitude to precipitation at the site based on <br />regional evaluations presented by Weir et al. (1983). Because wind speed, wind <br />direction and relative humidity data are not available for the Uravan site, these <br />data were compiled from a nearby weather station located in Montrose Colorado. <br />Average percent cloud cover data were compiled from Grand Junction, Colorado. <br />Solar radiation data for the site were compiled from the National Solar Radiation <br />Database. Other meteorological parameters used in the model include an albedo of <br />0.4, altitude of 1,737 meters, and average annual atmospheric pressure of 852 <br />millibars. <br />¦ A variable flux atmospheric boundary was applied at the surface and a unit <br />gradient boundary was applied at the bottom of the 3 meter profile. Vapor pressure <br />at the surface was computed based on the daily atmospheric humidity specified in <br />the climate input files. The daily temporal distribution of potential evaporation was <br />assumed to be sinusoidal. A pressure head of 1X 10-7 centimeters was used for the <br />initial conditions at all nodes. • <br />¦ The model was run using a 39 year initialization period followed by an average <br />precipitation year (assumed to be 1968). The initialization period is based on <br />sequential average precipitation years to reduce the potential effect of initial <br />conditions regarding the soil moisture at each node on the estimated percolation <br />resulting from the average precipitation year. <br />The variable HPR is the rate that precipitation is applied in the UNSAT-H model <br />expressed in centimeters per hour. The reasonable values for HPR can be evaluated <br />by considering the maximum daily precipitation for the simulated year. This <br />evaluation is based on precipitation data from Uravan, Colorado for the year 1968. <br />The simulated year (1968) is an actual year in the historical precipitation record that <br />has total annual precipitation as close as possible to the long term average. The <br />maximum daily precipitation for 1968 was 0.75 inches (1.905 cm). The minimum <br />reasonable HPR value is calculated as: <br />Maximum Daily Precip. (cm) <br />Minimum Reasonable HPR = 24 hours/day <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 /> <br />CANelson work0enison\UNSAT_H\HPR sensitivity\Sunday HPR Sensitivity talc brief.doc