Investigating groundwater response to meteorological and agricultural drought under increased water demand: insights from a Mediterranean coastal aquifer using numerical modeling

Meteorological drought is a natural phenomenon caused mainly by a prolonged precipitation deficiency, that may propagate to the surface and groundwater systems leading to the manifestation of hydrological drought events. The impacts of drought are often less visible in the subsurface due to sparse observational records while the response of groundwater to weather variability depends on antecedent groundwater levels and hydraulic and storage properties of the aquifer system. Although groundwater is often the only resilient water resource in arid and semi-arid areas, a notable decline in groundwater levels can be difficult to manage.

There is increasing evidence that coastal groundwater, which serves as the main water source for various needs (urban water supply, agriculture, etc.), is at even greater risk in semi-arid areas where the quality and quantity of fresh water stored in aquifers is threatened by seawater intrusion. It is important to note that, in these islands, periods of low recharge coincide with peak water consumption, which in turn leads to overexploitation of the aquifers to meet the increased water demands.

To that end, the present study focuses on the assessment of the complex relationship between drought conditions and coastal groundwater, emphasizing on its multidimensional nature which involves the consideration of several factors, such as pumping regimes, land use, water demands, subsurface heterogeneity, geomorphology of the study area and hydraulic connection to the sea. The principal goal is to identify critical features through a comprehensive modeling approach using distributed numerical modelling and easily accessible data and tools, providing the means for informed water management, especially in ungauged coastal aquifers.

The study analysed the case of a coastal aquifer located in the Greek island Kalymnos in the Aegean Sea for a period of 73 years (1950-2022). The primary source of groundwater in the study area is a calcareous unconfined coastal aquifer. A transient three-dimensional variable-density flow and salt transport numerical model was developed using SEAWAT code. Time-varying recharge input, was simulated with the ZOODRM model, a distributed recharge model. The pumping regimes were calculated based on both urban and agricultural water demands. Three drought indices for various timescales were employed for assessing drought evolution throughout the study period. That is, the Reconnaissance Drought Index (RDI) indicating the meteorological conditions, the Effective RDI (eRDI) and the Agricultural Standardized Precipitation Index (aSPI). The last two were utilised for identifying the agricultural drought conditions. The MH-data software was used for managing the meteorological input data (precipitation and potential evapotranspiration) that were obtained from the ERA5-Land database and the DrinC software was used for the drought analysis.

The outcomes of the study identified significant correlations between the freshwater volume and the drought indices, indicating the response of the aquifer to meteorological and agricultural drought. The time-varying pumping and recharge, along with the corresponding meteorological and agricultural drought conditions, also provide insights on water availability and potential water depletion during drought episodes. The proposed workflow may serve as an effective and cost-efficient strategy that may be utilized in areas with limited field data.