THE EXPORT OF PARTICULATE MATTER, PARTICULATE PHOSPHORUS AND DISSOLVED PHOSPHORUS FROM 2 AGRICULTURAL RIVER BASINS - IMPLICATIONS ON ESTIMATING THE NONPOINT PHOSPHORUS LOAD

The transport of particulate matter (PM), particulate phosphorus (PP) and dissolved phosphorus (DP) in two agricultural river basins was intensively measured during 1 year (October 1986-September 1987). Seasonal and stage-dependent rating relationships between the concentration of particulate inorganic (PIM) and organic matter (POM) and discharge were calculated for the Lyngbygaards river whereas only single rating relationships were established for the Aarhus river, in which a high proportion of the catchment area drains to lakes (48%). The export coefficients for PIM were rather similar for the two catchments (228 and 271 kg ha-1 yr-1) when only including the part of the river basins actively involved in sediment delivery and in the range previously reported for lowland rivers in Canada and U.K. Significant relationships between PIM and POM and the associated phosphorus fractions were calculated. During low-flow periods the P-content of PIM were high (1.5 and 1.2%) decreasing to 0.5% during high flow. On the contrary, the P-content of POM was constantly around 1% except for a period in late summer in the Lyngbygaards river where the P-content decreased to 0.2-0.3% during the first autumn storms following weed cutting. A very high P-enrichment in PM compared with average soil P-content was documented (7-14) indicating the selective delivery and routing of PM in lowland rivers. In both rivers the concentration of dissolved inorganic P (DIP) were inversely related to discharge demonstrating the dilution of P from point sources at increasing flow. However, in the Lyngbygaards river significantly higher concentration levels of DIP were recognized at periods of storm run-off possibly indicating the importance of DIP delivery with surface run-off and P-releases during resuspension of PM from the river bottom. The non-point P-load was nearly equally high (1.15 and 0.90 kg ha-1 yr-1) in the two agricultural basins when omitting the part of the Aarhus river basin draining to the deep Stilling/Solbjerg lake which retains appreciable amounts of P. Equally high non-point P-loads are reported for agricultural basins in Finland and Norway whereas appreciable lower non-point P-loads are normally reported for agricultural areas in Sweden and Denmark.