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within delineated areas within the geologic formation known as the Northern San <br /> Juan Basin in southwestern Colorado to be nontributary for purposes of his <br /> administration and permitting of wells pursuant to C.R.S. § 37-90-137(7). <br /> The State Engineer's finding is based primarily upon the testimony and evidence <br /> regarding the development, operation and calibration of five single layer finite <br /> difference numerical ground water models (the "NSJB finite difference models") for <br /> the Pictured Cliff, Cliff House, Menefee, Point Lookout, and Dakota Formations <br /> utilizing the U.S.G.S. MODFLOW modeling code. The State Engineer finds that <br /> there is clear and convincing evidence in the record that the NJSB finite difference <br /> models are capable of conservatively delineating nontributary areas within the subject <br /> formations in manner that is consistent with C.R.S. § 37-90-103(10.5) and that <br /> provides a useful tool for the State Engineer's administration and permitting of <br /> ground water wells pursuant to C.R.S. § 37-90-137(7). <br /> First, the State Engineer finds that the use of finite difference models to be an <br /> appropriate method for delineating nontributary areas in the Northern San Juan Basin. <br /> Models utilizing the U.S.G.S. MODFLOW modeling code are a well established tool <br /> for modeling ground water flow. All of the parties to this proceeding conceded that a <br /> appropriately developed MODFLOW numerical model is a useful tool for the State <br /> Engineer's administration and permitting of ground water wells pursuant to C.R.S. <br /> § 37-90-137(7). Moreover, finite difference MODFLOW models are an accepted tool <br /> for Colorado water rights administration purposes, and were used to determine the <br /> location of nontributary areas for the Denver Basin aquifers. Finite difference models <br /> are able to address both confined and unconfined aquifer conditions. Because of <br /> confining layers, the model need not consider cross-formational flow; accordingly, <br /> the use of a single layer model is appropriate. <br /> Second, the State Engineer finds that the numerical NSJB finite difference models <br /> more than adequately represent the geologic and hydrogeologic characteristics of the <br /> pertinent formations for purposes of developing a ground water flow model meeting <br /> the objective of this rulemaking. The undisputed testimony established that the <br /> conceptual model accurately reflects the geologic data available for the Northern San <br /> Juan Basin. <br /> Third, the State Engineer finds there is clear and convincing evidence that the NSJB <br /> finite difference models more than adequately integrate the conceptual model and <br /> data underlying that model into numerical models utilizing the U.S.G.S. MODFLOW <br /> modeling code. The testimony of Jon Ford, P.G.,provided a thorough description of <br /> how the NSJB conceptual model was translated into numerical models. The <br /> numerical models appropriately reflect the geologic and hydrogeologic data <br /> developed for the NSJB conceptual model. The State Engineer finds the inputs to the <br /> models to be appropriate. The storativity and permeability values were based upon a <br /> review of literature and upon site-specific data obtained from wells completed in the <br /> Produced Nontributary Ground Water Rules 2 CCR 402-17, Statement of Basis and Purpose <br /> -27- <br />