Reply to comment on ‘Characterization of surface and ground water δ18O seasonal variation and its use for estimating groundwater residence times’ by R. E. Criss and W. E. Winston

Abstract

It is the nature of the scientific method to use a variety of models, procedures, techniques and data to quantify and analyse natural phenomena. Recently, in Reddy et al. (2006), we published a study in which υ18O values for precipitation, surfaceand ground-water samples from the Shingobee River Headwaters Area (SRHA) were analysed using an amplitude-attenuation (convolution integral) approach for estimating mean residence times (MRTs). This approach has been used in many small watershed studies (e.g. Stewart and McDonnell, 1991; DeWalle et al., 1997; Burns and McDonnell, 1998; McGuire et al., 2002; Rodgers et al., 2005; McGuire and McDonnell, 2006). In a comment, Criss and Winston (2006) disagree with our method and present an alternative approach for interpreting υ18O values in the SRHA. We feel that this comparison with our methodology is unfair. Criss and Winston (2006) focus their attention on the weaknesses in our method and the strengths of theirs, while overlooking our detailed evaluation of model predictions with other independent watershed hydrologic properties. The comment by Criss and Winston (2006) makes several points, some of which we agree are valid criticisms of our paper. In this response, we will address their concerns regarding (1) our interpretation of the current state of the literature, (2) the data we used in the analysis, and (3) the relative merits of the two modelling approaches. In so doing, we will update the dataset with precipitation υ18O data from 1990 to 2004, and compare the two approaches to provide a fair basis for understanding the strengths and weaknesses of both methods. Additionally, we will discuss some technical issues that arise for scientists in choosing between simple and complex models.