Effects of agricultural drainage on streamflow in the Middle Thames River, Ontario, 1949–1980

The effects of surface and subsurface agricultural drainage on streamflow in the Middle Thames River have been studied through the analysis of changes in the volumetric response, changes in the seasonal distribution of streamflow volumes, and changes in the time distribution of runoff response of the Middle Thames in the period 1949–1980.The results indicated that only small changes in streamflow behaviour occurred in the Middle Thames for the period studied. There is evidence of a reduction of less than 25% in the time-to-peak of storm hydrographs on the watershed, but with little change in peak flow rate or centroid-to-centroid lag time. This is consistent with increased channel velocities in hydraulically improved municipal drains, coupled with an increased proportion of storm flow being diverted from overland runoff to rapid subsurface runoff through subsurface pipe drainage. There is no evidence of appreciable changes in volumes of runoff for individual storms or as annual total streamflow or changes in seasonal distribution of streamflow. Key words: agricultural drainage, streamflow, unit hydrograph, storm runoff.