| Peak flows at different return frequencies from 17 small Minnesota River Basin watersheds were investigated to determine the relative influence of climatic, physiographic (soil drainage class, cropland area, etc.) and morphological (channel length, stream density, lake area, etc.) factors. Normalized peak flow data were partitioned into two contrasting sub-regional groups: Flat watersheds with extensive artificial drainage and steep watersheds with less artificial drainage. Empirical modeling procedures, such as regression, parametric and stochastic analysis were used to delineate the impacts of climate, soil drainage, land use, and stream channel characteristics. Results from this investigation, based on a limited number of watersheds, indicate that: (1) Larger normalized peak flows were much more likely in steep watersheds than in flat watersheds; (2) Annual rainfall amounts increased significantly after 1979, but peak flows averaged over all watersheds increased faster than the increases in precipitation; (3) The year 1979 is significant because after 1979 the installation of subsurface drain tile increased due to advances in technology and the wetter climate; (4) In steep watersheds, there was an overbearing influence of climatic variables on spatial variability in peak flows; (5) In flat watersheds, the area of cropland (rather than climatic variables) explained much of the spatial variability in peak flows; (6) After 1979, a small number of steep watersheds experienced a 206% increase in normalized peak flows at the 100 year return frequency, in comparison with flows before 1979; (7) Flat watersheds, in contrast, experienced only a 34% increase in peak flows for a 100 year return frequency after 1979, in comparison with flows before 1979.; The differences in hydrologic response for steep and flat watersheds before and after 1979 suggest that the intensification of subsurface tile drain installation in flat watersheds has helped attenuate the influences of an increasingly wetter climate on peak flows. In contrast, peak flows in a small number of steep watersheds increased unabated with the wetter climate. This study indicates that increases in peak flows due to climate change are much more important in the steeper, less intensively drained watersheds rather than in the flatter, more intensively drained watersheds. |
