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This chapter examines the physical impl;lcts to stream systems in the state, as , well as possible legal and administrative Implications of de minimis standards for injury based upon an annual depletion$ standard. In this report, de minimis means a small amount of depletion in a calendar year that could be allowed without requiring the owner of a well causing the depletion, to augment or replace the water depleted or removed from the stream and aquifer system. The analyses included a review of types o~ water uses authorized by the Legislature where small amounts of un-replaced def1lletions to tributary water sources were deemed acceptable such as small capacity wells used for residential and livestock purposes and water stored in small reservoirs (livestock water tanks). The State Engineer recommends that, prior to changing the current policy, the Division of Water Resources solicit and eValuate public comment on a de minimis standard for depletion from the pumping of ~ Denver Basin "not non-tributary" aquifer based on an annual pumping volume of 3 aere-feet per year. Chapter 9 - The Effect of Four PercEjnt Replacement and Two Percent Relinquishment Requirements on Future Water Supplies, Existing Water Rights and the Need for Replacement of Post Pumping Depletion Resulting from Withdrawal of Denver Basin Groundwater (Technical Addendlilm No.2) Chapter 9 summarizes Technical Addendum No.2, which is a study conducted by staff of the State Engineer's Office to evalu~te the requirements of SB-85-5 to replace 4 percent of the pumping volumes for "riot non-tributary" aquifers and 2 percent relinquishment for non-tributary aquifers. To evaluate the effect of the current repl~cement (4%) and relinquishment (2%) requirements on future water supplies, water rights and need for replacement of post pumping depletions resulting from wi,thdrawal of Denver Basin ground water, the following is required: ' a) updating and improving of existing gro~nd water models, b) use of the model to evaluate current :depletions and predict long term future depletions to the discharge areas of (he South Platte River Basin drainages, and ' c) evaluation of the model results and e~ect on existing water rights, future water supplies, and Denver Basin Aquifers. ' The model developed for this study uses: the USGS MODFLOW computer code. This code is well accepted in the engineering community and is considered the best groundwater modeling code currently available. In computer modeling, the assumptions and data used to calibrate the model are the most important factors. Factors such as historical pumping, recharge to the aquifer from precipitation, and the - 10 -' I ,I ,I I I t I I I f I I j I I I I I I