How does drainage influence surface runoff and flooding?
Rainfall provides water for surface runoff and infiltrating the soil. The route that water takes as it flows through the landscape plays a very important role in the amount and rate of total runoff, and this is affected greatly by land use. When natural vegetation is disturbed or converted into field crops and pasture, peak runoff rates at the field edge can increase dramatically. Often these conversions are accompanied by some surface drainage practices. In general, subsurface drainage tends to decrease surface runoff (sometimes one- to three-fold) and decrease peak surface runoff rates when compared to surface-drained or undrained land. The decrease occurs because water flows more slowly through the soil to reach the drainage system (and eventually the outlet) than it would as surface runoff. The later arrival of drainage flow may cause the overall peak outflow (surface + drainage) to decrease. Moreover, when the amount of runoff is reduced, the speed of its flow may also decrease. W
