Effects of Land Use on the Diffuse Load of Phosphorus and Nitrogen

The diffuse load of phosphorus and nitrogen was studied in 23 small drainage basins in different parts of Finland between 1965 and 1974. Total phosphorus and total nitrogen concentrations were analyzed monthly and runoff measured continuously. Mean concentrations and loads for each basin are presented. The mean concentrations of phosphorus varied from 8.3 to 98 μg 1⁻¹and of nitrogen from 190 to 2400 μg1⁻¹. The dependence of concentrations on runoff was in most basins rather slight, but the highest concentrations were usually observed during the spring floods. The mean loads of phosphorus varied from 4.0 to 44 kgs km⁻² a⁻¹ and of nitrogen from 79 to 740 kgs km⁻²a^−1.. The dependence of concentrations and loads on basin characteristics was studied by linear regression analysis. The percentage of cultivated land was found to be the most descriptive variable of the characteristics of the basin. The percentage of cultivated land also reflects the loading caused by sparse population and by domestic animal population because of the strong intercorrelations between these variables.