What is a static head measurement? Evaluating structural error and implications for regional groundwater modeling

摘要

Most groundwater modelers avoid using static heads measured from active productionwells because they can introduce a bias into model calibration. However, in the deepconfined Cambrian-OrdovicianSandstone Aquifer System in the Central Midcontinent ofNorth America, dedicated observation wells are sparse and remote from areas of mostconcentrated pumping. As a result, in areas where drawdown is the greatest and modelingis most needed, only static heads from production wells are available for calibration.This paper evaluates two leading sources of discrepancies in using production well data,spatial and temporal structural error (S.E.). A simple Theis solution is used to evaluate thepotential magnitude of spatial S.E. when calibrating a regional MODFLOW model withcoarse cell resolution. Despite theoretical analyses indicating that spatial S.E. could besignificant, statistical analysis of the model results suggests that temporal S.E. is dominant.Long (ranging over decades) or frequent (monthly) head datasets are key in understandingtemporal S.E., to better capture water-levelvariability. In this study, the range instatic head observations impacted estimates of the remaining time a well could extractwater from the aquifer by 0.1 to 16.0 years. This uncertainty in future water supply ishighly relevant to stakeholders and must be assessed in hydrographs depicting risk.