Standardized piezometric indicator to improve the evaluation of the aquifer state. Doñana case study (SW Spain)

The Doñana aquifer system (SW Spain) is a coastal alluvial detrital formation composed of sands and gravels of fluvial, deltaic and marine origin. This aquifer is the basis for the sustainability of the water resources of the Doñana Natural Space, which includes the Doñana National Park.

From 1991 the water administration initiated a monitoring program of piezometric levels of the Doñana area assuming its direct control, which culminated in 1995 with the establishment of the official control network. In 2021 groundwater monitoring was operational in 261 points distributed throughout the territory of Doñana.

To assess the state of the aquifers around Doñana, the water administration uses a quantitative groundwater status index that is calculated from monthly piezometric level readings and requires that the length of the series be long enough to include wet and dry periods. This index has values bounded between 1 and 0. It is based on the comparison of data from the same month of the year. The driest month is used as the reference month.

The use of this index to estimate the quantitative status of the Doñana area, however, is criticized by scientists and environmental associations for its lack of statistical basis. There are other definitions of a standardized piezometric indicator based on statistical criteria, like those used for the calculation of the standardized drought index. This standardized piezometric indicator is already used in other countries such as France. The present work proposes to use the standardized piezometric indicator to assess the state of Doñana aquifers and compare the results with the index currently used by the water administration.