Application of different methods for the estimation of natural groundwater recharge: the Muravera case study

Evaluation of natural recharge is essential for a sustainable management of groundwater resources, especially in coastal areas where saltwater intrusion phenomena can arise. Direct recharge from precipitation represents the main source to phreatic aquifer; however, in complex hydrogeological systems, further sources as lateral recharge or surface water discharge into the groundwater systems needs to be evaluated for an accurate quantification of available resources. In this study, several methods are used to estimate natural recharge of the Muravera alluvial aquifer, in south-eastern Sardinia (Italy), where ongoing seawater intrusion problems have led to a critical deterioration of the groundwater resource with severe environmental and socio-economic impacts. Direct recharge from precipitation is evaluated through the application of two different methods, namely the I) Inverse Water Balance (IWB), and the II) Soil Water Balance (SWB). The IWB approach is based on spatial distribution of simple climatic dataset (precipitation and air temperature) and subsequent evaluation of the infiltration term through the definition of Potential Infiltration Indexes (CIP). The SWB approach, based on a modified Thornthwaite-Mather method, is implemented within a computer code developed by the USGS (Westenbroek et alii 2010); the code calculates spatial and temporal variations of groundwater direct recharge by integrating tabular daily climatological data with gridded datasets containing information about 1) hydrologic soil group, (2) land-use/land-cover, (3) available soil-water capacity, and (4) surface-water flow direction. To evaluate the occurrence of supplementary alimentation sources, the Water Table Fluctuation (WTF) method is also applied. The methodology, implemented within the ESPERE Macro Excel developed by the BRGM, requires continuous groundwater level measurement, and it assumes that piezometric level rises in unconfined aquifers are directly related to aquifer recharge. Advantages of this approach include its simplicity and an insensitivity to the mechanism by which water moves through the unsaturated zone (disadvantage of the SWB). Results of the IWB and SWB methods illustrate that the average direct recharge from precipitation, referred to the same decade 2009-2018, are in good agreements and vary between 2.86 and 4.43 Mm3/year, respectively. Results of the WTF, applied to the available 2019-2020 monitoring data, indicates that a supplementary 3 Mm3/year recharge occurs, mostly from the aquifer interactions with the Flumendosa river.