We appreciate the interest in the identifiability and relative parameter error variance reduction statistics that we describe in Doherty and Hunt (2009). Dr. Hill’s comments, referred to here as Hill (2010), include concerns specific to our paper, but also examples and issues that extend beyond our paper to the work of others. As such, they extend to broader methodological issues which underpin our general approach to model calibration and uncertainty analysis. In this regard they are similar to those raised in a recent paper (Hill, 2008a) and in a number of scientific presentations (Hill, 2006, 2007, 2008b, 2009). We consider a discussion of these issues vital to achieving changes in approaches to parameter estimation and uncertainty analysis that we believe will substantially enhance the ability of the modeling community to address societally relevant problems. We hope that discussion of the topics raised in Hill (2010) will increase the understanding of the approach that we are trying to advance, and are therefore pleased to engage in such a discussion. The material covered in Hill (2010), and therefore our response, encompasses the larger fundamental difference in approach between older, traditional methods developed for problems with few parameters, and new methodologies for parameter estimation and uncertainty analysis based on solution of highly parameterized, ill-posed inverse problems that seek to redress the deficiencies of these older methods. Hunt et al. (2007) summarize the underlying issues; they define a ‘‘traditional” approach to model calibration as one which relies on pre-calibration user-intervention to formulate an over-determined (more useful observations than parameters solved for), well-posed (mathematically solvable) inverse problem through pre-emptive parameter simplification. In distributed hydrologic models, traditional parameter simplification is most commonly accomplished using devices such as zones of piecewise constancy, and may be accompanied by the introduction of prior information pertaining to one or a number of elements comprising the simplified parameter set. A nagging concern with these and similar approaches, however, is that the configuration of such zones, and