Flow dynamics in large river basins: Self-similar network structure and scale effects.

摘要

The hydrologic response of a basin is shaped by the non-linear interaction between runoff generation, river-network geomorphology and the hydrodynamics of channel flow. In this research, the processes which contribute to the variance of the network hydrologic response are investigated by coupling the river network organization with hydraulic geometry relations, thereby relaxing the assumption of spatially invariant celerities and hydrodynamic dispersion coefficients. The presence of spatially varying celerities induces a dispersion effect, identified as kinematic dispersion, on the network travel time distribution. Its contribution to the total dispersion is comparable to that due to the heterogeneity of path lengths, that is, geomorphologic dispersion, and significantly larger than the hydrodynamic dispersion. If this contribution is ignored the hydrograph shows a higher peak flow, a shorter time to peak and shorter duration. The relative contributions of the three dispersion mechanisms (i.e, geomorphologic, kinematic and hydrodynamic) as a function of scale, or Strahler order of the basin, are studied. It is found that in basins with self-similar network structure in which Horton's laws of stream lengths and stream areas hold, the dispersion coefficients obtained for equilibrium flow conditions asymptotically obey Horton's law type of relations. Under these conditions, the mean and variance of the instantaneous response function are scale invariant or self-similar. These analytical results are substantiated by the analysis of the Vermilion and the Mackinaw River basins, in central Illinois, the Upper Susquehanna River basin in New York, and the Rogue River basin in Oregon. The geomorphologic characteristics of the drainage networks for the study basins were obtained from DEMs. The river networks were extracted from both 30 m and 3 arc-second DEMs and compared to those obtained from the national hydrographic dataset (NHD). The effect of hillslope dynamics on the different variance-producing mechanisms has been also described and analyzed. It was found that the hillslope dynamics tends to counteract the effect of geomorphologic dispersion, that is, the hillslopes act as equalizers in the variability of channel lengths.