Investigating Regulatory Low Flows in the Face of Climate Uncertainty

Including potential future climate change variability in water resources planning has become standard practice. A recent report by U.S. EPA cites the need to include climate change considerations in NPDES permitting process. A technical approach is proposed here for addressing this need. The method involves directly incorporating down-scaled global climate model (GCM) output in the calculation of regulatory low flows used in the permitting process. GCM projections of future precipitation variability are translated into low flows in streams at targeted locations through empirical regression models. A stochastic modeling tool provides for probabilistic sampling from a range of GCM projections across multiple climate models and multiple greenhouse gas emission scenarios. Calculated low flows are translated into hypothetical allowable point source discharge concentrations for a conservative solute. It is hoped that results of this type of approach will better assist regulator decision making and understanding of the risks associated with climate change uncertainty in water quality permitting.