While some of the world's most productive agriculture is on artificially drained soils, drainage is increasingly perceived as a major contributor to detrimental off-site environmental impacts. However, the environmental impacts of artificial or improved agricultural drainage cannot be simply and clearly stated. The mechanisms governing the hydrology and loss of pollutants from artificially drained soils are complex and vary with conditions prior to drainage improvements and other factors: land use, management practices, soils, site conditions, and climate. The purpose of this paper is to present a review of research on the hydrologic and water quality effects of agricultural drainage and to discuss design and management strategies that reduce negative environmental impacts. Although research results are not totally consistent, a great majority of studies indicate that, compared to natural conditions, drainage improvements in combination with a change in land use to agriculture increase peak runoff rates, sediment losses, and nutrient losses. Nevertheless, sediment and nutrient losses from artificially drained croplands are usually small compared to cropland on naturally well-drained uplands. Increasing drainage intensity on lands already in agricultural production may have positive, as well as negative, impacts on hydrology and water quality. For example, increasing the intensity of subsurface drainage generally reduces loss of phosphorus and organic nitrogen, whereas it increases loss of nitrate-nitrogen and soluble salts. Conversely, increasing surface drainage intensity tends to increase phosphorus loss and reduce nitrate-nitrogen outflows. Improved drainage is required on many irrigated, arid lands to prevent the rise of the water table, waterlogging, and salinity buildup in the soil. Although salt accumulation in receiving waters is the most prevalent problem affecting downstream users, the effect of irrigation and improved drainage on loss of trace elements to the environment has had the greatest impact in the U.S. These detrimental effects often can be avoided by identifying a reliable drainage outlet prior to construction of irrigation projects. Research has shown that management strategies can be used to minimize pollutant loads from drained lands. These strategies range from the water table management practices of controlled drainage and subirrigation, to cultural and structural measures. For example, controlled drainage has been found to reduce nitrate-nitrogen and phosphorus losses by 45 and 35%, respectively, in North Carolina. It is becoming increasingly clear that drainage and related water management systems must be designed and managed to consider both agricultural and environmental objectives. While significant advances in our knowledge of environmental impacts and methods for managing these systems have improved in the last 20 years, there is much yet to be learned about the complex mechanisms governing losses of pollutants from drained soils.
