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I I. I I I I I I I Ie I I I I I I I. I I 4.0 Adequacy of Groundwater Flow Models Economic analysis of the effective life of the Denver Basin aquifers must be done under actual present conditions and our best estimate of expected future conditions. Estimates of the required lift of water to the ground surface should be based on expected actual groundwater levels in the aquifers and in the wells. Estimates of future water levels can be developed by simulating the flow conditions in the aquifer in the future. To be accurate, such a simulation should take into account the physical processes affecting groundwater flow. The processes of greatest importance in this discussion of the existing simulation models of the Denver Basin are: precipitation recharge, leakage between the aquifers, leakage to and from surface water bodies, horizontal flow within the aquifers, and confined and unconfined storage. At present, static groundwater levels in the Denver, Arapahoe, and Laramie-Fox Hills aquifers are above the tops of the aquifers over most of the basin. Groundwater stored under pressure is called artesian or confined storage. It has been estimated that the water in confined storage in the Denver Basin is less than 0.1 percent of the total amount of recoverable storage. After this confined storage has been drained, the groundwater level is at the top of the aquif: - a.. . additional withdrawals result in actual dewatering of the pore spaces in the aquifer. Groun I[,':~r stored in these pore spaces is called unconfined storage and makes up most of the aquifer storc.' . Since use of the confined storage may result in groundwater level changes of hundn:Js of . .;et, this storage can be very important in an analysis of the actual expected groundwater le\ ds. 4.1 State Engineer's Office Denver Basin Groundwater Flow Models The State Engineer's Office groundwater models are described in the report "Denver Basin Ground Water Model Report", by the Colorado Division of Water Resources, November 15, 1985, Technical Appendix C. These models are intended to be used for a very specific purpose: definition of the location of nontributary groundwater in the Denver Basin bedrock aquifers under the conditions of Senate Bill 5. In this modeling effort, the major aquifers in the 5 HRS WATER CONSULTANTS,INC.