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 agricultur al use began by constructing shallow ditches spaced 18-20m apart to re move excess surface waters from croplands. Farming of poorly drained s oils was done by these systems of beds until the 1930s. These beds int erfered with the use of modern machinery and in the 1930s the establis hment of beds was diminishing in favor of subsurface tile systems. Bet ween 1930 and 1980 there was a substantial increase in subsurface-tile -drainage activity, which peaked during the 1960s. Since 1985, install ations of subsurface and surface drainage have gradually diminished, a nd 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 re ductions of nutrients and pesticides associated with surface runoff. W ith the increase of agricultural (dairy) waste applications to croplan d, additional research is needed to quantify the impacts of subsurface drainage on ground- and surface-water quality.