Controlled drainage Ð effects on drain out¯ow and water quality Ingrid Wesstro Èm * , Ingmar Messing, Harry Linne Âr, Jan Lindstro Èm Department of Soil Sciences, Swedish University of Agricultural Sciences, P.O. Box 7014, SE-75007 Uppsala, Sweden Accepted 12 April 2000 Abstract A ®eld experimental project was set up in southern Sweden to assess the effects of controlled drainage on hydrology and environment. Controlled drainage makes it possible to vary the drainage intensity with the variation in drainage requirement during season by controlling the height of a riser in the drain outlet and thus to a certain degree control the amount of out¯ow of solutes via the drainage system. During periods with low drainage demand, the riser in the drain outlet can be raised and the groundwater level in ®eld will rise up to the level of the riser before the discharge takes place. Three plots, each with an area of 0.2 ha (40 m50 m) were installed on a loamy sand. One plot was drained by conventional subsurface drainage (CD) and two plots were drained by controlled drainage (CWT). The plots contained four lateral drain tubes, at 10 m spacing and placed at 1 m depth. Each plot was isolated by a double layer of plastic sheeting placed in the back-®lled trenches to a depth of 1.6 m to prevent lateral leakage and subsurface interactions. Measurements of precipitation, drain out¯ow and soil and air temperatures were carried out hourly. Groundwater levels were measured and samples of drain out¯ow were collected twice a month for nitrogen and phosphorous analyses. Mineral nitrogen contents in soil were measured three times a year. Controlled drainage had a signi®cant hydrological and environmental effect during the 2 years of measurement (1996±1998). Compared with CD, the total drain out¯ow from CWT was 79% less in Year 1 and 94% in Year 2. The total reduction in nitrate losses with CWT corresponded to the reduced out¯ow rates. Compared with CD, the total amounts of nitrate in drain out¯ow were 78% less in Year 1 and 94% in Year 2. The highest concentrations of nitratewere measured at the time of the largest out¯ow rates. The phosphorous losses were 58% less for CWT as compared to the CD values in Year 1 and 85% less in Year 2. The reduction in nitrogen content in the soil pro®le during the winter season was 60±70% less in CWT than in CD. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Controlled drainage; Drain out¯ow; Nitrogen leaching; Phosphorous losses; Mineral nitrogen content in soil Agricultural Water Management 47 (2001) 85±100 * Corresponding author. Tel.: 46-18-671183; fax: 46-18-672795. E-mail address: ingrid.wesstrom@mv.slu.se (I. Wesstro Èm). 0378-3774/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0378-3774(00)00104-9