Effect of Agricultural Drainage on Water Quality in Humid Portion of Pacific Northwest

Agricultural drainage first began in Washington State around 1830 and in Oregon around 1840. The first efforts to drain soils for agricultural use began by constructing shallow ditches spaced 18–20 m apart to remove excess surface waters from croplands. Farming of poorly drained soils was done by these systems of beds until the 1930s. These beds interfered with the use of modern machinery and in the 1930s the establishment of beds was diminishing in favor of subsurface tile systems. Between 1930 and 1980 there was a substantial increase in subsurface-tile-drainage activity, which peaked during the 1960s. Since 1985, installations of subsurface and surface drainage have gradually diminished, and constuction of drainage systems has been limited to replacement of failed or deteriorated systems. By reducing saturated soil conditions through subsurface drainage, surface-water quality has benefited by reductions of nutrients and pesticides associated with surface runoff. With the increase of agricultural (dairy) waste applications to cropland, additional research is needed to quantify the impacts of subsurface drainage on ground- and surface-water quality.