Relationship between water surplus and piezometric heads in the Upper Tiber aquifer (Central Italy)

The present work shows the results of a hydrogeological study carried out on the alluvial aquifer of the Upper Tiber basin (131 km2), characterized by no-flow boundary along the SW part and by constant head boundary in the NE part (alluvial fans). The analysis of groundwater levels of five piezometers, monitored by ARPA Umbria during 2001-2015 period, shows a different seasonal variability between the right and the left banks of the Tiber River. On the right bank, alluvial deposits with high permeability values outcrop, having a thickness of up to 100 m, which host an unconfined aquifer (Transmissivity, T≈10-1 m2/s): Pistrino, Piosina and Riosecco piezometers show a maximum groundwater monthly fluctuation of less than 3 m. On the left bank (T≈10-2 m2/s), groundwater monthly fluctuations for the Cerbara and San Giustino piezometers reach maximum values of 17 m and 8.7 m respectively. Rainfall and piezometric data series have been studied by means of spectral and cross spectrum techniques. Spectral analysis has been applied in determining the presence and statistical significance of climate cycles by processing data derived from rain gauges and the longest time series of piezometric heads (PH) of Pistrino and Riosecco piezometers. This analysis shows the presence of frequencies with high statistical significance (>99%), such as semiannual, annual, or others related to the warm phase of the ENSO (2.2 to 4.5 years). There are other cycles but with less statistical significance such as NAO or Sunspot. Frequencies below 0.5 years have been observed with high statistical significance. Some of these cycles detected in the piezometric series could be related to the Moon cycles (periodical expansions and compressions related to Earth tides).The cross spectrum analysis has been carried out by the Lomb-Scargle method to establish (at monthly scale) the relationship between rainfall and PH and as well as water surplus (WS) and PH.The analysis shows a delay between the rainfall signals and the response of PH which can range from 10 to 80 days. The delay between WS and PH of Pistrino and Riosecco is about 45 days. In conclusion, multiple regressions, performed over all piezometers, were used to estimate the monthly groundwater level (Ht) considering the groundwater level of the previous month (Ht-1), and WS up to the previous two months (WSt; WSt-1; WSt-2). Good relationships were obtained with Coefficients of Determination (CoD) of about 0.90.Results obtained with multiple regressions agree with those obtained from cross spectrum analysis.