Application of SWAT model on three watersheds within the Venice Lagoon Watershed (Italy) : source apportionment and scenario analysis

Aim of this study was the analysis of the source apportionment of three river basins within the Venice Lagoon Watershed (VLW), in the North-Eastern part of Italy, which on the whole cover about 700 km2 (i.e. about 35% of the VLW surface area). The three watersheds are characterised by a very intensive agriculture and landfarming systems. Moreover, in this area the groundwater recharge to surface waters significantly contributes to the total load discharged into the Venice Lagoon. SWAT model application allowed to describe the hydrologic and agricultural characteristics of the watersheds and to assess the source apportionment in terms of point and diffuse sources. Furthermore, SWAT model allowed the evaluation of the benefit due to the application of agri-environmental measures through the simulation of a “better-business” agricultural scenario versus the “business-as-usual” scenario. Out of a total annual nutrient load of about 2200 tNy-1 and of about 140 tPy-1, the dry weather diffuse sources (i.e. groundwater/spring recharge and tributary/irrigation channels coming from bordering watersheds) resulted to be the most important (65% in case of nitrogen and 35% in case of phosphorus). SWAT outputs indicated a runoff load contribution of about 20% of the total nitrogen load and of about 30% of the total phosphorus load; agricultural runoff resulted to be about 2/3 of this runoff load. The simulation of the Better Business scenario indicated a possible reduction in the agricultural runoff loads of about 50% in case of nitrogen and of about 15% in case of phosphorus, which corresponds to a decrease in the total annual load of about 5-7%.