MODIFYING AND EVALUATING A P INDEX FOR DENMARK HANS ESTRUP ANDERSEN ∗ and BRIAN KRONVANG National Environmental Research Institute, Department of Freshwater Ecology, Vejlsoevej 25, DK-8600 Silkeborg, Denmark ( ∗ author for correspondence, e-mail: hea@dmu.dk) (Received 5 September 2005; accepted 27 February 2006) Abstract. 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. Keywords: phosphorus, P index, water framework directive 1. Introduction European rivers, lakes, reservoirs and coastal waters suffer from excess inputs of nitrogen (N) and phosphorus (P) (eutrophication) derived from both point sources and non-point sources (European Environment Agency, 1999). Despite the achieve- ment in many European countries of large reductions in P discharges from point sources, the water quality of rivers, lakes and estuaries nevertheless continues to de- teriorate due to diffuse P losses (Sharpley et al., 2000; Kronvang et al., 2002). This was one of the reasons for the European Union to recently adopt the Water Frame- work Directive aimed at protecting and enhancing the status of aquatic ecosystems (European Parliament, 2000). The Water Framework Directive mandates EU mem- ber states to develop management plans for each river basin. Consequently, water district managers need tools for assessing diffuse P losses at the catchment scale. The aim of our study was to initiate the development of a P loss assessment tool suitable for Danish conditions, and which water district managers can easily adopt. An assessment of P losses from agricultural land is, however, not straightforward. Water, Air, and Soil Pollution (2006) 174: 341–353 DOI: 10.1007/s11270-006-9123-0 C  Springer 2006