Cross-Calibration of Two Independent Groundwater Balance Models and Evaluation of Unknown Terms: The Case of the Shallow Aquifer of “Tavoliere di Puglia” (South Italy)

Groundwater represents an essential water resource for human purposes, mainly in those areas characterised by a scarcity of surface water and dry climate. Consequently, tools for assessing the groundwater balance are fundamental for its suitable management. The conventional groundwater balance equation, which considers all the natural and human-induced terms of the balance, such as rainfall, withdrawals, irrigation, etc., sometimes lacks of some important terms. One of the terms of the balance that is most difficult assess is the volume of water exchanged with other neighbouring water bodies (subsurface inflow/outflow). In this case, the estimation must be considered as a poor approximation. In this paper, a novel methodology is proposed that is capable of significantly increasing the accuracy of the groundwater balance when subsurface inflows and outflows are unknown. The improvement is accomplished by comparing two corresponding time series of annual groundwater balances assessed by means of different balance models. The first time series is evaluated by means of the conventional balance equation and the second one by directly estimating the groundwater volumes by means of geostatistical methods. Both these models are supposed to lack specific, even though different, information. Their comparison through simple statistical tools allows them to be calibrated and to recover missing average information. A study case is presented considering the inflow/outflow term and the specific yield as missing information for the conventional and the geostatistical approaches, respectively. The study area is the shallow porous aquifer of the Tavoliere di Puglia (South Italy).