Nutrient load modelling during floods in intermittent rivers: An operational approach

In the Mediterranean area, lagoons and coastal waters are often under the influence of intermittent rivers that contribute the majority of pollutant loads during flood events. Reliable tools for evaluating pollutant loads from land sources are needed for managing the water quality of the receiving waters, but available water quality models are not suitable for simulating these flash flood events. An operational tool, D-PoL (Diffuse- Pollution Load), to estimate the pollutant loads transferred during rainfall events by small Mediterranean rivers is described. The tool is based on a rainfall-load semi-distributed approach and was developed with the aim of simulating temporal variations in fluxes of dissolved pollutants. The model simulates pollutographs at the outlet of the catchment during rainfall events: the rainfall triggers the processes. The model can be described as a rainfall-load model. Three parameters describe the processes involved: the initial stock of pollutants on the hillslopes, the production parameter (which is related to the lag time of the catchment) and the routing parameter (which is related to the lag time of a basic river reach). First, sensitivity analysis demonstrated that each of the parameters controlled one key-feature of the pollutograph. The initial stock of pollutants influenced the amplitude of the pollutograph, the production parameter controlled the recession period, and the routing parameter influenced the rising phase. Second, the model was calibrated for dissolved inorganic nitrogen and dissolved phosphorus on a 20-flood data set. The sets of optimal parameters were fitted to empirical relationships for both elements. On the 20-flood calibration data set, the simulated total loads with adjusted parameters compared well with the observed parameters, except for very small events. Finally, the D-PoL model was checked against a new set of data, a 10-flood validation data set. This final validation of the model showed that the dynamics of the pollutograph were not perfectly reproduced, but that simulated total loads agreed with the observed loads.