Modeling nutrient loads to the northern Adriatic

•Simulation of nutrient loads was done for entire NA watershed consisting of 17 individual watersheds.•Po River watershed is dominating the nutrient loads to the NA.•Agricultural areas and WWTP were identified as the major sources of nutrients to the NA.•Proper management decisions were defined to reduce the nutrient loads.

SummaryThe northern Adriatic (NA) is one of the most productive areas of the Mediterranean Sea with a watershed area spread over four neighboring countries, Italy, Slovenia, Switzerland and Croatia. Up to date research include mainly the nutrient load estimations from the biggest nutrients sources, like the Po River watershed (Italian part). This research aims to estimate the loads from nearly the entire contributing area, including the Italian, the Slovenian and the Croatian part. The ArcView Generalized Watershed Loading Function (AVGWLF) model was applied to simulate nutrient loads from the NA watershed. The model can simulate runoff, sediment and nutrients (nitrogen and phosphorus) loads from the watershed, by taking dispersed and point sources into account. Calibration and validation of the model were performed on the data from 1999 to 2002 and 2003 to 2007, respectively. Comparison between the simulated and measured loads in the Po River watershed shows a good match and thus high reliability of the model. As expected, the Po River dominated the nutrient loads to the NA. However, the results also point to other contributing areas, e.g. smaller watersheds with high intensity of nutrient loads, causing significant local impacts (e.g. Lagoon of Venice). The model also shows that the major sources of nutrients are not only agricultural but also urban areas without proper wastewater treatment (secondary at present). Proper management of these areas, such as introducing suitable (tertiary) wastewater treatment, or other nutrient management measures shall reduce the nutrient loadings to the NA.Further work is focused to use the model for optimizing the control and management of the activities in the contributing areas, smaller as well as bigger sub-watersheds.