Gravimetric Determination of Storage Coefficient and Storage Change of Groundwater in an Uncontrolled and Unconfined Aquifer

Adequate management of water resources in aquifer systems implies knowledge of the different parameters of hydrological balance. However, only limited data exist for many aquifers, which lead to the production of hydrological models with high levels of uncertainty. This research estimates spatially and temporally the groundwater storage change and the storage coefficient in unconfined aquifers and under uncontrolled conditions by means of gravimetric data and validates the estimates by two procedures based on piezometric data. The application of the three procedures was carried out in an aquifer located in the Etla Valley, Oaxaca, Mexico. Piezometric data and gravitational field measurements from October 1996 and May 2018 were employed. Groundwater storage change was calculated via piezometry, residual Bouguer anomaly and gravimetry. The piezometric and gravimetric procedures estimated similar spatial distributions of groundwater recharge and discharge areas but different reductions in groundwater storage volume. The gravimetric method estimated a loss that was 50% lower than that produced by the procedures based on piezometry. The storage coefficient varied spatially in a range between − 0.22 and 0.30, assuming a theoretical relationship between gravimetric and piezometric variations based on the Bouguer slab model.