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8. What will tile drainage do to downstream flow and flooding?
Surface drainage increases peak surface runoff flowrates. Tile drainage typically reduces both volume and peak of surface runoff from agricultural fields. Studies show that the overall volume of water lost (surface runoff and tile flow combined) from a tile drained field may increase slightly (5-15%), compared to fields with surface drainage only. Flooding is generally dependent on peak surface runoff rate. Because tile drainage tends to decrease peak runoff rates, this suggests that tile drainage should not increase (and may decrease) the incidence of flooding. Flooding in general, is a watershed-scale phenomenon, and the effects of tile drainage at this scale are not as well understood as the field scale.
University of Minnesota Extension Service
Where land is used for agricultural production, improved drainage has been
found to reduce runoff, peak outflow rates, and sediment losses, compared to
undrained agricultural land.
Brown, Larry C. and Zucker, Leslie A., Agricultural Drainage, Water Quality
Impacts and Subsurface Drainage Studies in the Midwest, The Ohio State
University Extension Bulletin 871, 1998
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