Keywords: surface-groundwater model, diffuse pollution, SWAT, ModFlow, water quality, catchment management.

The availability of water for human use is often compromised by the poor quality of this precious resource. Water potentially available for human use is those part of water resources which is respecting the restrictions existing at national or local level, including; a) water quality restrictions, b) ecological restrictions and c) geopolitical restrictions. The Southern European countries are often affected by problems related to both water quantity (i.e. irrigation) and water quality (i.e. diffuse pollution).

This paper presents the development and application of an integrated simulation procedure named ISSM (Integrated Surface and Subsurface Model) [1]. ISSM is based on the watershed scale model SWAT [2]), linked with the physically based groundwater model MODFLOW 2000 [3], to the Burana-Po de Volano (B-PV) watershed, which is situated in Northern Italy. The catchment is strongly characterized by the high-density of natural rivers and artificial channels. This area is especially important because it drains directly into the Sacca di Goro Lagoon (SG), one of the major areas of shellfish production in Italy. This lagoon has been subjected to eutrophication because of the high nutrient inputs coming from the watershed. The model was applied with the aims to investigate the interactions amongst: a) the agricultural activities in the watershed, b) the water quality in the river and streams network, c) fish farming production in the SG. Moreover, ISSM has been developed and applied with the intention of providing a useful tool to for the local authorities in charge for the water management of the channel network and the coastal lagoon.

ISSM has been applied and calibrated in the period 1991-2001, according to the data available. Water percolation was calibrated based on annual average values from 1992 to 1999. SWAT showed a good capability to estimate the water percolation to the shallow aquifer with an efficiency of around 0.75 in the entire area. The calibration of the groundwater module of ISSM was carried out at the nine calibration wells available within the catchment. The dynamics of the groundwater table is strongly determined by the interaction with the channel network. Due to this interaction, the aquifers trends show maximum values in late spring and summer, while minimum values are present from autumn to early spring. ISSM performs good simulation of the groundwater dynamics, (efficiency ranges between 0.581 and 0.786). Water quantity has been calibrated at the main outlets of the watershed. Predicted and measured flows at the four water pumps have been compared according to the available data. Overall, ISSM is giving a good simulation of the water outflow at the system outlets, (efficiency ranging from 0.621 to 0.577). Generally ISSM correctly estimates high flow during the summer period, while it shows a tendency to underestimate the water outflow outside of the cropping season. Water quality has been calibrated according to the data collected in two measuring station within the area. ISSM performed a good simulation for NO3, NO2 and NH4 while it shows some problem in simulating the phosphorus dynamics.

The development and application of ISSM is useful to analyze and simulate the water contamination in a medium scale watershed. The model provides a useful tool for water managers, which have to take a decision dealing with water quantity and quality at watershed scale. For this reason the development of the ISSM has been carried out taking into account the needs and the use of local authorities working in the B-PV watershed. In this context ISSM has been also linked with a hydrodynamic model adapted for the Sacca di Goro [4]. This linkage allows simulation on different scenarios concerning the application of fertilizer and crop rotation in the watershed on the eutrophication status of the lagoon. Moreover, ISSM is integrated with a multi-objective optimization routine that includes economical or ecological considerations on the water distribution system, agriculture management practices and application of policy constrains which could be then evaluated in terms crop production and changes in the surface and sub-surface water contamination.

1

Galbiati, L., Bouraoui, F., Elorza, F.J., Bidoglio, G., "Modelling of point and diffuse pollution in a Mediterranean watershed", Ecological Modelling, 193, 4-18, 2006. doi:10.1016/j.ecolmodel.2005.07.036

2

Arnold, J.G., and Allen, P.M., "A comprehensive surface-ground water flow model", Journal of Hydrology, 142, 47-69, 1993. doi:10.1016/0022-1694(93)90004-S

3

Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., "MODFLOW-2000, the U.S. Geological Survey modular ground-water model - User guide to modularization concepts and the Ground-Water Flow Process", U.S. Geological Survey, Open-File Report 00-92, 121 pages, 2000.

4

Marinov, D., Zaldivar, J.M., and Norro, A., "Hydrodynamic investigation of Sacca di Goro coastal lagoon (Po River delta, Italian Adriatic Sea shoreline)", EUR 21178 EN, EC, JRC, 2004. doi:10.1016/j.ecolmodel.2005.07.042

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