Water discharge and leaching of nitrate

A network of measuring stations was established for monitoring the leaching of nutrients from arable land to surface water and groundwater. The monitoring activity is a part of the National Swedish Environmental Monitoring Programme. Data from the network as discharge, groundwater pressure, concentrations of nitrate and recorded agricultural practicies were used. Mathematical models for water and heat and for nitrogen were used in more penetrating studies of causes of nitrate leaching. The water and heat model generates the necessary studies driving variables for the nitrogen model. The nitrogen model represents transformations of nitrogen in the soil and transport of nitrogen within and from the soil. The water and heat model was modified to enable better partitioning between surface runoff and pipe discharge. Good partitioning between these two kinds of flows is of vital importance if the nitrogen model is to calculate accurate nitrate losses. The result prove that the amout of nitrate in the soil profile and the disharge rate are decisive factors for the amount of nitrate in the soil profile and the discharge rate are decisive factors for the amount of nitrate leached. The amount of available nitrate in the soil is regulated by many factors, among which crop type is one. Annual grops contributed more to the amount of nitrate leached than perennial crops owing to the longer time of nitrogen uptake by the perennials. Ploughing of leys did not increase losses compared to the losses from established ley unless the ploughing was undertaken in the middle of the growing period and followed by repeated cultivations under fallow conditions favourable for mineralization and nitrification. Simultations proved that nitrate content in the soil in the beginnging of september and the litter mineralization during September-March largly explainrd the between-year variation in nitrate leaching under normal discharged conditions. Leaching of nitrate increased above normal and decreased below normal discharge for the same amount of mineral nitrogen available for leaching. A necessary condition for leaching to groundwater was decreasing groundwater pressure with depth. With the water and heat model it was possible to calculate the flow to surface water and groundwater separatly. The reliability in simulated groundwater recharge was concidered by comparison with the difference between stream runoff and drainage discharge