Physics‐based hydrologic‐response simulation: Seeing through the fog of equifinality

Equifinality The need to characterize the distributed nature of near-surface hydrologic response has been stressed recently by Burt (2005), Bloschl (2006), and Sidle (2006). Serendipitously, 31 classic papers related to the processes governing streamflow generation have recently been reprinted in the Benchmark Papers in Hydrology series (Beven, 2006a). The limitations and benefits of comprehensive physics-based hydrologic-response simulation were discussed in two earlier commentaries (Loague and VanderKwaak, 2004; Loague et al., 2006). Herein, the topic of equifinality, a perceived Achilles’ heel for physics-based simulation, is addressed. In the most general sense, equifinality is the case where different conditions lead to similar effects. Beven (2006b) thoroughly reviews the equifinality problem for hydrology and related disciplines (also see Haines-Young and Petch, 1983; Culling, 1987; Aronica et al., 1998; Hankin and Beven, 1998; Schulz et al., 1999; Beven, 2000, 2002a,b; Brazier et al., 2000; Beven and Freer, 2001; Hreiche et al., 2002; Binley and Beven, 2003). It is worth noting that the equifinality problem is not limited to the earth sciences. For example, equifinality is well-known in biology (Rogers, 2000), management (Doty et al., 1993; Gresov and Drazin, 1997), and psychology (Gottlieb, 2001; McClure et al., 2001). In the context of hydrology and deterministic-conceptual simulation, equifinality frequently refers to more than one parameter set providing an equally good (or poor) representation of the integrated response. Discussing the issue of equifinality, Savenije (2001) wrote as follows: Although we can consider equifinality as a nuisance since it implies that looking for more understanding through detailed distributed modelling is a dead-end track, it also offers an opening to the revival of larger-scale hydrological laws. Commenting on the statement by Savenije (2001), Loague and VanderKwaak (2004) wrote: Obviously, one should consider the intended use of a distributed model before characterizing the approach as a dead-end track (i.e. large

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