CONTROLLED VERSUS CONVENTIONAL DRAINAGE EFFECTS ON WATER-QUALITY

Fertilized cropland is a potential nonpoint source of nitrogen and pho sphorus, which can contribute to the nutrient enrichment of surface wa ter ecosystems. Research has shown that agricultural drainage water ma y contain fertilizer nutrients and pesticides. Nitrogen and phosphorus are present in drainage outflow primarily because of the addition of fertilizer, which results from the change in land use following draina ge improvements instead of from the mere installation of drainage. Yet , public concern for the environment has led to a general criticism of agricultural drainage for degrading water quality. Controlled drainag e has been recognized in some states as a best management practice to reduce the transport and delivery of nitrogen and phosphorus to sensit ive surface waters. This article summarizes and compares water quality impact associated with conventional and controlled drainage practices . Nitrogen and phosphorus reductions of 30% to 50%, resulting from con trolled drainage are reported in several studies. Several researchers believe further reductions are possible by carefully managing fertiliz ation rates and timing to match crop uptake. Further minimization of t he off-site impact of agricultural production will require a better un derstanding of the mechanisms of fate and transport of nutrients and p esticides and their interaction with drainage and other water-manageme nt alternatives such as controlled drainage.