Future climatic water balance perspectives under climate change scenarios – a Portuguese case study

This study aims to determine the potential evolution of the climatic water balance in the Viso-Queridas Aquifer system, Portugal, under various climate change scenarios, to identify critical patterns and vulnerabilities, providing a spatial assessment of potential changes in water availability over time, which combines point-based climatic water balance with geostatistical techniques.

The Viso-Queridas Aquifer System is located in Coimbra (Portugal). The aquifer is moderately productive, primarily porous, and composed of detrital materials with highly variable textures and a lenticular structure about 200 m thick. Clay layers separate the various aquifer units, giving the aquifer a multilayered character. Due to the variability in granulometric composition, the hydraulic characteristics can vary significantly from one location to another. The aquifer is expected to be bounded at the top by a free surface; however, as depth increases, the multilayered structure quickly introduces confined/semi-confined conditions.

Historical precipitation and temperature data (1971-2000) were obtained from the WorldClim portal, along with future climate projections based on Shared Socioeconomic Pathways: SSP2-45: “Middle of the Road” (intermediate emission: CO2 emissions around current levels until 2050, then falling but not reaching net zero); SSP3-70: “A Rocky Road” (high emissions: CO2 emissions double by 2100) and SSP5-85: “Taking the Highway” (very high emissions: CO2 emissions triple by 2075).

The Thornthwaite equation was used to estimate potential evapotranspiration, enabling the computation of climatic water balances for the historical period and two future timeframes: 2041–2060 (centered on 2050) and 2081–2100 (centered on 2090), at a spatial resolution of 30 arc-seconds. Sequential Gaussian Simulation (SGS) was used to map the spatial distribution of the climatic water balance and its associated uncertainty, while G-cluster analysis was conducted to identify significant spatial clusters.

Analysis focused on August (dry season) and December (wet season) revealed key patterns in the water balance evolution. Critical areas expanding significantly in the eastern part of the study region led to severe deficits (negative values) being most prevalent in August 2090. The already vulnerable area is being affected more and more, which highlights the growing pressure on water resources during the dry season. In contrast, December exhibited positive water balances due to higher precipitation and reduced evapotranspiration; however, critical areas in this month shifted towards the south and southeast, underscoring the persistent vulnerability of the eastern region.

The most pronounced spatial changes were observed especially between 2050 and 2090, where stable zones progressively transitioned to negative balances, revealing the stark contrast between August, and December.

This study highlights how the Viso-Queridas Aquifer system may be increasingly impacted by climate change, with significant seasonal and spatial disparities in climatic water balance. The findings stress the urgency of implementing adaptive water management strategies focused on the most vulnerable areas particularly the eastern regions during summer and the southern areas during winter. These insights aim to assist policymakers in developing sustainable and resilient approaches to safeguard groundwater resources in Portugal, ensuring their availability for future generations.