Point and diffuse nutrient emissions and loads in the transboundary Danube River Basin. - I. A modelling approach

Nutrient emissions by point and diffuse sources were estimated for the Danube River Basin on the basis of a harmonized and integrated database at the sub-catchment level by application of the MONERIS model. The estimation of nutrient emissions was carried out for the period 1998-2000 for 388 sub-catchments as well as the thirteen countries each responsible for more than 1 % of the total area of the Danube River Basin. A total of 756 kt a -1 nitrogen (N) and 68 kt a -1 phosphorus (P) are emitted by the different pathways. 77 % of N-emissions and 53 % of P-emissions were due to diffuse sources. The dominant pathway is groundwater for N-emissions and point source discharges for P-emissions with 48% and 47 %, respectively. The regionalized analysis facilitates the identification of spatial hot spots for the individual emission pathways. This analysis allows one to deduce specific measures for the reduction of nutrient emissions in the river system of the Danube and the Black Sea. Background nutrient emissions were calculated to approx. 6.5 kt a -1 P and 61 kt a -1 N. Consequently, the human impact is about 10 times higher than the background for P and 12 times higher for N, and 54 and 35% of the total N and P-emissions were due to agricultural activities. Urban settlements are responsible for 50% (P) and 22% (N) of the total emissions. The total nutrient loads of the Danube into the Black Sea were estimated to 22.5 kt a -1 for total P, 396 kt a -1 for dissolved inorganic N, and 468 kt a -1 for total N. The comparison of calculated and observed loads shows that the mean load deviation for the investigated sub-catchments of the Danube River Basin was 21 % for dissolved inorganic N (93 stations) and 32 % for P (65 stations).