Il bilancio delle acque sotterranee nella pianura lombarda in relazione agli effetti del cambiamento climatico

The aim of the project is the analysis of the groundwater balance in the Lombardy plain. The project is based on the use of methodologies which take into account the aquifer input and output variations according to climate change. The study area is the Lombardy plain (Northern Italy) which extends from the Ticino river (westward) to the Oglio river (towards the East). Prealpine reliefs border the area to the North and the Po river delimits it to the South. It’s about 6.900 Km2. The surface-water network in the study area is very complex with rivers, canals and fontanili (springs). In this area there is a heterogeneous alluvial aquifer. A regional groundwater flow model is used to analyze the groundwater availability in the study area. The developed model uses 3D geostatistical reconstructions of aquifer parameters (hydraulic conductivity and porosity) based on stratigraphic data collected in TANGRAM (a water wells database). Modflow 2005 is used to implement groundwater flow model and Soil Water Balance code is used to calculate aquifer recharge. Both codes have been created by U.S.G.S.. Main steps of the developed project are: • Data collection; • Conceptual model development; • Mathematical model development and calibration. A wide variety of hydrogeologic data have been collected: georeferenced stratigraphic data (8.492), monthly piezometric levels, daily rivers levels and flows, daily meteo-climatic data, study area land use and soil properties. Conceptual model of the study area is based on a detailed data analysis; it has been focused on the study of the temporal correlations between piezometric level and the fluctuations of groundwater budget input and output. Data confirm that groundwater budget varies along the study area. Furthermore, monthly piezometric levels (2001-2005) from Regional Monitoring Network have been interpolated to rebuild regional water table and groundwater flow. On the other hand, as said before, aquifers geometry and hydraulic conductivity distribution have been extracted from 3D geostatistical reconstruction of aquifer characteristics. Main aspects of a mathematical model are: • Grid assessment; • Boundary condition settings; • Hydraulic conductivity distribution. Study area is discretized into 207 rows, 333 columns and 107 changeable thickness layers. Cells are 500 x 500 m. Boundary conditions, from West to East, are the Ticino, Adda and Oglio rivers to act as head dependent flow boundary type (Cauchy); to the South, the Po river is used as specified head boundary type (Dirichlet). Hydraulic conductivity distribution reflects the heterogeneity of the analyzed system. Soil Water Balance (SWB) code has been used to calculate spatial and temporal variations in groundwater recharge. SWB allows to estimate recharge by use of land cover and soil properties in combination with climatological data. Monthly piezometric levels from Regional Monitoring Network have been used as calibration targets. Model results highlight that groundwater flow models are a good method to analyze groundwater budget in Lombardy Plain. At the moment, the model reproduces groundwater natural system well. It results that a detailed evaluation of groundwater recharge is more important to describe groundwater budget than a detailed reconstruction of aquifer heterogeneity. On the other hand, it’s necessary to collect more data regarding withdrawals to analyze more deeply anthropic influences on the system.