Effects of Drainage Projects on Surface Runoff from Small Depressional Watersheds in the North Central Region

Abstract

Surface runoff from small watersheds characterized by numerous depressions was studied statistically and by use of a special purpose watershed model. The statistical analyses illustrated the possible magnitude of the storage effect exhibited by lakes, marshes and other depressions. Because of data limitations statistical techniques could not be used to examine the effects on flood runoff of draining these same areas. The model, described in the Bulletin, represents the process of snowmelt, infiltration, soil moisture storage, evapotranspiration, subsurface and surface runoff for four different land drainage conditions, with or without channel development. Application of the model to two small watersheds in Jackson County, Minnesota indicated that drainage development increases annual runoff, storm runoff and peak discharge. The physical characteristics of the main water course in the watershed was the major factor influencing peak discharge at the watershed outlet. Examination of annual flood flows on the Minnesota River at Mankato suggests that downstream effects of drainage development on large watersheds are much less than indicated by this study on small watersheds. Downstream effects and flooding within a watershed are discussed in general terms in the Bulletin.