The importance of observations on fluxes to constrain ground water model calibration

The aquifer system in the alluvial basin bordered by Adda, Po and Oglio rivers (Northern Italy) is characterised by a dual flow regime. In shallow sediments, which constitute a phreatic aquifer with high conductivity, great fluxes are driven by the interaction between ground water and the network of surface water, by the infiltration of rain and irrigation water, and by the fluxes drained from depression springs and river valley terraces. The underlying semiconfined aquifers are characterised by minor fluxes driven by water abstraction from wells of the public Water Works. Since most of the ground water flow occurs in the phreatic aquifer, an equivalent single layer 2D steady state flow model has been calibrated. The identification of the transmissivity field at the scale of the model has been obtained by solving an inverse problem with the comparison model method which requires an initial configuration, i.e., reference head, initial transmissivity field, source terms. Most of the head and source data are related to the phreatic aquifer, but most of the estimates of transmissivity are obtained with field tests conducted in deep wells pumping from the semiconfined aquifers, so that this kind of prior information cannot be used directly for model calibration. The inverse problem is underdetermined and a unique solution is not available. Furthermore information on surface hydrology is poor. Therefore many tests with different hypotheses about the initial configuration have been performed and some of them have been selected and used to initialise the automatic inversion procedure.