Assessment of the impact of climate change to alluvial aquifer: an uncertainty analysis

In this paper, an uncertainty analysis of the processing chain from General Circulation Models scenarios to impact on an alluvial aquifer is presented. To this goal, we examined the forecast climate change impact on the Umbra Valley aquifer (central Italy), in which the human pressures in terms of groundwater usage are significant. In order to understand which part of the modelling processing chain mainly contributes to the final uncertainty we considered the combined effects of: 1) three different General Circulation Models; 2) two different downscaling procedures; 3) the stochastic generation of daily T and P time series; 4) the parameterization of the Thornthwaite-Mather soil water balance model, adopted to estimate the effective infiltration to the aquifer; 5) the uncertainty arising from the procedure adopted to calibrate/validate a groundwater flow model developed for the study area using the MODFLOW2005 code. The final results show that the uncertainty due to the groundwater flow model calibration/validation is comparable to that arising from the whole processing chain from the GCM choice to the effective infiltration estimates. Moreover, while the uncertainty on the effective infiltration estimate appears to be centered around the mean of the scenarios, the uncertainty on the piezometric heads presents an asymmetry clearly related to the non-linearity of the aquifer system response to external drivers: this issue should be carefully taken into account when the impact of climate change on groundwater is assessed.