Modifying and evaluating a P index for Denmark

The European Union recently adopted the Water Framework Directive aiming at protecting and enhancing the status of aquatic ecosystems. The Water Framework Directive mandates EU member states to develop river basin management plans for each river basin. Consequently, water district managers need tools for identifying critical source areas within a catchment in order to target cost-effective remedial measures. The P Index could be well suited for screening large areas because of its low demand for input data and manpower compared to mechanistic, process based models. The P Index needs to be modified to local or regional conditions in order to incorporate all potential P loss pathways. In Denmark tile drains and leaching are considered major P loss pathways. We modified the Pennsylvania P Index to reflect Danish conditions. The Pennsylvania P Index and the modified Danish P Index were applied on a 1000 km(2) catchment. In order to test the P Indices we examined their capability to correctly rank measured annual diffuse total P losses from twelve sub-catchments within the 1000 km(2) catchment. Both P Indices, calculated on a sub-catchment basis, correlated with total P losses. The Danish P Index, however, performed distinctly better than the Pennsylvania P Index and explained 85% of the variation in measured diffuse total P losses.