POPEYE: A river-load oriented model to evaluate the efficiency of environmental policy measures for reducing phosphorus losses

Watershed losses of phosphorus (P) have been a topic of concern for water resource managers over recent decades. To evaluate environmental policies or before implementing mitigation options at the watershed (catchment area) scale, stakeholders often need to analyze the patterns of point and diffuse sources of phosphorus. This information is often not easy to obtain in field conditions. Several statistical modeling approaches have been developed in recent years to respond to this basic operational demand. Point and diffuse sources are often evaluated from power functions established between phosphorus concentration and water discharge. Such models do not explicitly account for in-stream processes which control P concentrations in the hydrographic network and differentiate the P export dynamics of the various forms and inputs of P. To identify the phosphorus sources and evaluate their change in response to environmental policies, we developed a simple and loaded-oriented model (POPEYE – PhOsPhorus, Evaluation of the efficiencY of Environmental policy measures) that computes retention, settling and re-suspension rates of fine and coarse P fractions and their relation to P concentration of bed sediments. The model is calibrated to a long-term database (25 years), and describes the weekly water and chemical fluxes of a tributary of Lac Léman (Lake Geneva, Venoge river watershed, 240 km2). It adequately predicts observed values of fine and particulate phosphorus and reflects the gradual decrease of point and diffuse inputs over the studied period due to agricultural and sewage treatment policies implemented for the control of lake pollution.