This reduction in runoff is due to the fact that, though a land use such as agriculture has considerably less built-up and impervious surface than low-density residential land use, the type of drainage used in the fields (e.g., tile drains) and the drainage patterns artificially established due to ploughing patterns actually result in greater amounts of runoff. The straight lines made on the fields during ploughing increase runoff by acting as channels into which water easily flows. 

Low-density residential areas, on the other hand, have large open spaces around them that are covered with grass and trees that effectively act like pastures and forest in terms of reducing runoff. As a result, low density residential areas produce less runoff that an equal area of agricultural land.