Date of this Version
Szilagyi, J. (2009), Comment on ‘‘Power law catchment-scale recessions arising from heterogeneous linear small-scale dynamics’’ by C. J. Harman, M. Sivapalan, and P. Kumar, Water Resour. Res., 45, W12601, doi:10.1029/2009WR008321.
It is demonstrated that a near-linear subsurface runoff response from a short and relatively steep slope segment and a nonlinear response at the watershed scale may primarily arise from geometry rather than from an assumed linear nature of the subsurface runoff response from the hillslope, as Harman et al.  employed for the Panola Mountain Research (PMR) catchment in Georgia. The authors caution in their paper that hydraulic theory (exemplified by the study of Brutsaert and Nieber ) cannot generally account for the heterogeneity in the watershed scale and therefore should be used with certain reservation when employing it for catchment-scale parameter estimation. They base this on observations [Clark et al., 2009] that the PMR watershed in Georgia displays a near-linear (which Harman et al. accept to be linear) subsurface flow response at a short segment (~50 m) of the upper part of a hillslope, while the same response becomes increasingly nonlinear with scale. The authors employ linear reservoirs in parallel, with prescribed distributions (e.g., bounded power law (BPL)) of the storage coefficient and in the limit; when the number of reservoirs approaches infinity, they obtain the required degree of nonlinearity in their summed outflow. With the application of the BPL distribution, the shape of which changes significantly with scale, they can achieve the observed full range and temporally changing nature of the exponent of the recession flow equation. This way, they argue that the nonlinear subsurface flow response from the PMR watershed may emerge from a combination of linear responses similar to what is observed at the short upslope segment.