Reply to comment by J. Szilagyi on “Power law catchment‐scale recessions arising from heterogeneous linear small‐scale dynamics”

[1] We are grateful to Szilagyi [2009] for giving us an opportunity to clarify and reiterate the main points of our paper [Harman et al., 2009]. We demonstrated in that paper that within-catchment (between-hillslope) heterogeneity of hydraulic properties can give rise to, and so explain, nonlinear recession slope curves observed at the Panola experimental catchment. We did this analytically, by simplifying the drainage responses of all individual hillslopes that constitute a catchment in terms of linear reservoir theory. We then showed that the increase, seen in Panola, in the apparent nonlinearity of whole-catchment responses with an increase of catchment area could arise from the broader range of hillslope responses (i.e., mean residence times) likely to be sampled in progressively larger catchments. These results are not disputed by Szilagyi [2009]. [2] Szilagyi’s [2009] main claim is that nonlinearity of recession slope curves could arise from hillslope hydraulics alone. We do not dispute this either. In fact, this is already well known in the literature, as highlighted in our paper; see section 4.2 of Harman et al. [2009] for a discussion. Indeed, this well-known fact is what led to the second main point of our paper, which is that if the nonlinearity of the recession slope curve can arise from hydraulics alone, and also, independently, be due to the effects of between-hillslope heterogeneity as we had demonstrated, what is the true, or dominant, cause for the observed nonlinearity? How do we distinguish between the relative contributions of each of these causes? What does this mean for the estimation of hillslope hydraulic properties from recession slope curves through inverse procedures?