HYDROLOGIC AND WATER-QUALITY IMPACTS OF AGRICULTURAL DRAINAGE

While some of the world's most productive agriculture is on artificial ly drained soils, drainage is increasingly perceived as a major contri butor to detrimental off-site environmental impacts. However, the envi ronmental impacts of artificial or improved agricultural drainage cann ot be simply and clearly stated. The mechanisms governing the hydrolog y and loss of pollutants from artificially drained soils are complex a nd vary with conditions prior to drainage improvements and other facto rs: land use, management practices, soils, site conditions, and climat e. 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 environm ental impacts. Although research results are not totally consistent, a great majority of studies indicate that, compared to natural conditio ns, 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 d rained croplands are usually small compared to cropland on naturally w ell-drained uplands. Increasing drainage intensity on lands already in agricultural production may have positive, as well as negative, impac ts on hydrology and water quality. For example, increasing the intensi ty of subsurface drainage generally reduces loss of phosphorus and org anic nitrogen, whereas it increases loss of nitrate-nitrogen and solub le salts. Conversely, increasing surface drainage intensity tends to i ncrease phosphorus loss and reduce nitrate-nitrogen outflows. Improved drainage is required on many irrigated, arid lands to prevent the ris e of the water table, waterlogging, and salinity buildup in the soil. Although salt accumulation in receiving waters is the most prevalent p roblem affecting downstream users, the effect of irrigation and improv ed drainage on loss of trace elements to the environment has had the g reatest impact in the U.S. These detrimental effects often can be avoi ded by identifying a reliable drainage outlet prior to construction of irrigation projects. Research has shown that management strategies ca n be used to minimize pollutant loads from drained lands. These strate gies range from the water table management practices of controlled dra inage and subirrigation, to cultural and structural measures. For exam ple, 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 managem ent systems must be designed and managed to consider both agricultural and environmental objectives. While significant advances in our knowl edge of environmental impacts and methods for managing these systems h ave improved in the last 20 years, there is much yet to be learned abo ut the complex mechanisms governing losses of pollutants from drained soils.