EPISODIC PRECIPITATION AND DISCHARGE EVENTS AND THEIR INFLUENCE ON LOSSES OF PHOSPHORUS AND NITROGEN FROM TILEDRAINED ARABLE FIELDS

Monitoring over a 2-3 year period revealed that a large proportion of the yearly nutrient losses through tile drainage can occur in connecti on with a few runoff events. 45% of the total phosphorus transport fro m a clay-soil experimental field at Vara (58 degrees N 13 degrees E) d uring 1992-94 took place during two such episodes. The first episode o ccurred in January 1993 and the second, more pronounced one occurred i n January 1994. Both started with intense rain on newly frozen soil. A grass cover strongly reduced the losses compared with soil with a cat ch crop and bare soil. In another experimental field south of Stockhol m (59 degrees N 17 degrees E) with clay containing gyttja, autumn epis odes with intense rain on dry soil led to pronounced nutrient losses. Five episodes during a 2-year period accounted for nearly half of the total phosphorus transport. Total phosphorus losses from this site wer e greater than from the Vara site (0.3-0.4 compared to 0.1-0.2 kg/ha.y ear). Although losses of nitrogen during these episodes were less pron ounced than those of phosphorus, they amounted to 24% of the total los ses.