1988-04-11_PERMIT FILE - C1981008A (9)

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Figure 7-1, there may be some discharge from the Dakota coal on the southern side of Tuttle Draw, but the overburden aquifer is the major source of discharge (inflow) to the surface water baseflow (outflow) within the vicinity of Nucla and proposed Nucla East Mine. Overburden Cross Sections A and C (see Exhibit 7-2) combined contribute .306 cfs of discharge to Tuttle Draw. Cross Section B contributes .59 cfs of discharge to Calamity Draw. Based on the potentiometric surface an equal amount of discharge is also released to Calamity Draw on its south side (opposite Cross Section B). Assuming this is the case, the overburden releases (discharges) 1.18 cfs of water to the baseflow in Calamity Draw along that cross section. The flow difference from the upper to the lower surface water sites on the Tuttle Draw and Calamity Draw based on instantaneous surface water baseflow values from Table 7-3A range between 0.250 and 0.366 cfs and 1.14 and 1.9 cfs, respectively. In conclusion, the overburden aquifer is the primary source of discharge (inflow) which maintains the surface water baseflow (outflow) of Tuttle and Calamity Draws. The coal and underburden aquifers discharge much less water than the overburden with most of that discharge being released downstream of the lower surface water monitoring sites on Tuttle and Calamity Draws (SW-N3 and SW-Nl03), respectively. Evapotranspiration. Evapotranspiration (ET) is a major component of discharge at the Nucla and Nucla East mining areas. To determine the amount of ET, the Baseline Vegetation Studies Area Map (Exhibit 10-2, Tab 10) was used. The total study area encompasses 618 acres in and adjacent to the proposed Nucla East Mine permit area. Of the total acreage included in the study area 13 percent (83 acres) are swales (wet standing water with hydrophytic vegetation), 25.6 percent (159 acres) is irrigated pasture, 21.6 percent (133 acres) is in hay production, 16.6 percent (102.4 acres) is under annual crop production, 1.8 percent (11 acres) consists of orchards, 1 .0 percent (6 acres) consists of ponds and the remaining 20.4 percent (124 acres) is sagebrush grasslands, disturbed areas, dwellings or roads. The percentages of each vegetation type are generally representative of adjacent areas as well . Most of the ET or consumptive use rates used were taken from the Soil Conservation Service (SCS) Colorado Irrigation Guide which is a field office technical guide. The consumptive use rates used in the guide were calculated by a computer program that uses the modified Blaney-Criddle method (Table 7-313). The consumptive use rates for the vegetation types not included on Table 7-313 were calculated by other methods which are footnoted and referenced on Table 7-4. In determining the ET rate for the study area it is assumed that the irrigation practices evenly distribute 7-17 Revised 04/11/88