Experimental and modeling study of the soil-atmosphere interaction and unsaturated water flow to estimate the recharge of a phreatic aquifer

The aquifer system, which is the resource of water for the city of Milano (Italy), is a multilayered aquifer, characterized by sandy and sandy-gravel units, connected by discontinuous aquitards. The recharge area of the phreatic aquifer is close to the prealpine area, some tens of kilometers north of the city; nevertheless the local recharge given by rain infiltration through the soil is not negligible and contributes to the mass balance of the aquifer. A field campaign was carried out for 16 months in the suburban area of the Lambro Park to evaluate the local recharge due to the rainfall infiltration through the unsaturated zone; the water table in this area lies about 15 m below the ground surface. Standard meteorological data (atmospheric pressure, rainfall, humidity, wind velocity and air temperature) were collected, along with incident and net radiation, to evaluate potential and actual evaporation from the bare soil using three methods (Bowen ratio, Penman, and Hamon equations). Simultaneously, the volumetric soil water content at different depths (down to 76 cm) was measured with time domain reflectrometry probes. A finite-difference model, which solves the one-dimensional Richards equation in transient conditions, was developed to simulate the flow through the unsaturated zone, to evaluate the characteristic time of recharge and the mass balance. The water flow rate through the ground surface was assigned as the upper boundary condition and was evaluated from the mass and energy balance at the atmosphere–soil interface with different approaches based on the meteorological data and the actual soil water content. The lower boundary condition is given by the saturation condition at the water table. Characteristic retention curves were estimated in laboratory using Richards apparatus. In order to calibrate the model, the numerical results were compared with experimental data for two different periods: A dry period (July 2001) and a wet period (October 2001). Finally, the calibrated model was used to simulate the infiltration and the flow through the unsaturated zone for a 15-year-long period (1988–2003) and to estimate the total phreatic aquifer recharge. Moreover, the time lag between a variation of the infiltration/exfiltration rate and the corresponding variation of the recharge rate was evaluated; the values of the delay time of recharge are longer than those computed with models that approximate the transmission zone as a unique cell.