A comparison of different methods of source apportionment of nutrients to river basins

Abstract

Seven different methods of source apportionment were applied in an analysis of the nutrient emissions from point and diffuse sources of 14 river basins in Northeastern Germany. The aim was to compare the results relating to the uncertainties of the methods and to test whether the difference between the emissions and load is caused by systematical errors or by retention and loss processes in the river systems. Beside the simplest method, where the diffuse emission is the difference between the observed transport and the point source inventory, other immission methods including a nutrient retention term are applied. The emission methods used also estimate the diffuse nutrient inputs by different ways. For all methods the relative standard deviation of the estimated diffuse emission was calculated for each of the river basins. The mean standard deviation of the original data sets for all emission methods and the immission approaches with the retention term was 22% for nitrogen and 48% for phosphorus. By variation of additionally introduced coefficients this standard deviation can be reduced to 18% and 24% for nitrogen and phosphorus, respectively. Tests regarding possible systematical under- and/or overestimation of the measured load and the diffuse emissions, respectively, show indications of such a behavior for phosphorus, only. Nevertheless, a large difference between the nutrient emissions and the load remains, which can only be explained by intensive retention processes.