Modeling surface water quality with limited data: A calibration approach applied to the Middle Tagus Basin (Spain)

The ecological and chemical status of a river results from the combination the quality of its waters and the magnitude of the flow that circulates in its riverbed. In densely populated regions or in areas with intensive agriculture the understanding of the origin and behaviour of pollutants reaching the river is particularly important in order to improve the ecological status of surface water bodies. This requires the development of water quality models that reproduce the response of the river to different pressures starting from measurements of Waste Water Treatment Plant (WWTP) effluents and estimation of diffuse pollution from agricultural sources, and from observations of flow and concentration of pollutants in the receiving waters. Typically, observations on a regular basis (daily, weekly) of quantity and quality of pressures and receiving waters have been used to callibrate the model coefficients. However, very often the available observations are too sparse to allow such an approach. There is still limited research investigating how to calibrate the models when the available data is not dense enough to confront pressures and observations on similar dates and locations. Thus, this study aims to define how to calibrate the coefficients describing the evolution of the model with sparse observational data. This will be achieved through the exploitation of the statistical properties of the available data. A specific goal function is developed to maximize model performance. The model is applied to Middle Tagus Basin (central Spain), which is an interesting case study for several reasons. Firstly, several surface water bodies in the area have nitrogenous and phophorous concentrations that exceed the limits set according to the European Water Framework Directive (WFD). Secondly, this area receives a large volumen of waste water from the densely polulated metropolitan area of Madrid. Thirdly, urban point pressure is combined with the diffuse pollution from agriculture, without a clear understanding of the relative contribution of each pressure on the final concentrations in the river. Last but not least, the basin is subject to a water transfer (Tagus-Segura water transfer) that detracts an average of 350 hm3 /year from the Tagus headwaters to transfer to the Mediterranean coast, further limiting the Tagus' capacity to dilute the pollution originating in the Madrid region.