Combined Use of Watershed Models to Assess the Apportionment of Point and Non Point Load Sources to Surface Waters

Abstract

The sustainable management of watersheds is critical since it dealswith many aspects of the anthropogenic use of natural resources.The quality of water resources is a central issue for the managementof watersheds. Over the last few years it has grown worldwide theneed to implement tools for a knowledge-based management of the environmentalresources and processes. Particularly there is the need to evaluatethe response of the system to different pressures and different managementalternatives. This is normally done through a Scenario Analysis whichin many cases implies the use of modeling tools and the creationof a G.I.S. archive of the available knowledge. Understanding thesource apportionment of pollutant loads driven through the watershedto a coastal lagoon or evaluating the socio-economical consequencesof a new environmental policy or the effect of a natural or not naturalcatastrophe are all issues that can be studied through a sound scenarioanalysis. On this perspective, diffuse pollutions effect on surfaceand groundwaters is often difficult to quantify. Diffuse pollutantsenter the environment through a multitude of pathways at differenttemporal scales. During rainfall events most of the diffuse pollutantload follows the surface runoff pathways and, at the net of the plantuptake, reaches the aquifers. A fraction of this load follows thesub-surface runoff pathways and may possibly reach the surface watersafter a certain time lag. Very rarely the sub-surface pollution eventscan be directly correlated to a specific rainfall event. In thisstudy some case studies are presented where river and watershed modellingtools are combined with factor analysis techniques to identify theeffect of the sub-surface runoff on the water quality of specificstream reaches. Through this approach the sub-surface runoff wasproven to be a significant source of diffuse pollution even in dryweather conditions.