Resonance of Mediterranean groundwater recharges with long-term Atlantic climate patterns

Challenging the conventional notion that groundwater relies solely on local hydrology, this study reveals the potential for the AMO resonance to supersede the direct impact of seasonal rainfall on the water cycle.
Published in Earth & Environment
Resonance of Mediterranean groundwater recharges  with long-term Atlantic climate patterns
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What links the indicators of the water cycle to groundwater recharge?

Recognising the importance of groundwater is critical for effectively advancing the Sustainable Development Goals (SDGs). Previous research suggests a potential link between anomalies in groundwater recharge and the depletion of underground water storage1. However, hydrological factors also play a role in the complex interactions that shape groundwater recharge patterns and storage dynamics2. The objective is to improve our understanding of water supply indicators in river basins. These indicators are valuable not only for the scientific community, as they also provide guidance for landscape conservation planning3 and assist governments in making evidence-based decisions4.

Within the hydrological cycle, factors influencing groundwater change extend to storm, soil moisture, snowmelt and deeper recharge. These changes can be affected by atmospheric patterns and the memory in the Earth’s climate variations operating on different time-scales (Fig. 1).

 

Fig. 1. Overview of the Earth’s water cycle that supports groundwater resources. This map offers insights into the water cycle indicators (MSSI, PDSI and SSI) used in the groundwater prediction model. The Earth's climate system exhibits memory to over interannual to decadal time intervals, primarily in the slowly shifting heat content of the ocean in response to rapid atmospheric variability. This oceanic process serves as the framework for large-scale atmospheric circulation patterns5. At the same time, climate fluctuations in the water cycle strongly influence many components of the Earth system, including precipitation, water supply, ecosystems and the Earth’s groundwater. Image adapted from: https://www.freepik.com/free-vector/natural-landscape-with-river_801905.htm#page=9&query=landscape%20water%20cycle&position=27&from_view=search&track=ais&uuid=1d5a10a6-4e12-4d1f-8cfa-b9f5f5e6c389.

 

Our article presents the findings of a millennium-long study (801-2020 CE) that reconstructs groundwater recharge (GWR, mm yr-1) in the Tiber River Basin (TRB). We grounded our research on water cycle indicators, like the monthly storm severity index (MSSI), the Palmer drought severity index (PDSI) and the snow severity index (SSI), which can be traced back in history. Leveraging this rich historical knowledge, we modelled the GWR for the TRB. Given its considerable hydrological importance within the Italian peninsula, the TRB serves as an ideal case study to gain insights into water resources in the region. Our study of the TRB offers critical perspectives on hydrological processes, groundwater dynamics, and the intricate interplay between surface water and groundwater. This understanding is essential for ensuring sustainable water availability and stability throughout the Italian peninsula, which, due to its central location within the Mediterranean Sea, represents a microcosm of the broader Mediterranean region.

 

How has the climate-groundwater drought relationship evolved over time in the Mediterranean region? 

Groundwater availability has consistently been a natural occurrence in the TRB throughout its history. In our assessment, we looked at the groundwater drought severity (GWDS), commonly described using a deficit index6. This index indicates the beginning of a GWDS event when it drops below a specified threshold (e.g., the 10th percentile) and ends when it rises again7.

In Italy, the GWDS is generally affected by prolonged droughts, as observed during the Medieval Climate Anomaly (MCA) and more recently. For example, historical records from 1131 CE describe extreme heat, rivers drying up and famine8. The maximum GWR occurred in 860 and 1240 CE, with 814 and 849 mm yr-1, respectively. Interestingly, evidence of drier climatic conditions is reflected in isotopic data from Sicily around 10501150 CE9. Examining the corresponding evolution of the Atlantic Multidecadal Oscillation (AMO) during the MCA period, it is evident that the AMO index consistently remained above zero. In particular, during the phases of greatest drought, it hovered around +0.3 between 950 and 1100 CE (Fig. 2).

 

Fig. 2. Coevolution of the Atlantic Multidecadal Oscillation (AMO, orange line; source: Mann et al.11) and groundwater drought severity (10th percentile of GWR, blue line). Background image adapted from: https://www.freepik.com/free-vector/empty-park-landscape-scene-with-riverside_13831357.htm#page=5&query=groundwater%20landscape&position=31&from_view=search&track=ais&uuid=0cfe98d4-e11d-4a3a-a1fc-9df658a7f6e3.

 

The AMO refers to a pattern of multidecadal North Atlantic sea-surface temperature anomalies, exhibiting warming/cooling cycles (AMO>0/AMO<0) with a periodicity of ~60 years10. During the Little Ice Age (LIA), between about 1300 and 1850 CE, periods of restricted GWDS were associated with a negative mode of the AMO.

Since 1500, the lowest GWR levels, specifically the 10th percentile, have been steadily declining, and this declined continued until the end of the time-series in 2020 (Fig. 2). This trend aligns with the evolution of the water level of Lake Trasimeno in central Italy. Between 1400 CE and the early 18th century, the lake showed stability with minimal oscillations, followed by a gradual decline until the mid-18th century. From then on, the lake level has fallen drastically until the present day. After the end of the LIA, the intensity of GWDS increased, with the percentage of years below the 10th percentile reaching 16% (Fig. 2).

During this period, Lake Trasimeno experienced oscillations, and in the late 1950s, a severe water crisis occurred, leading to the lake’s maximum depth dropping to less than 3 m. The changing climate and the presence of a new outlet, which prevents water storage above the outlet threshold, have contributed to the lake’s mean level consistently remaining low12. This drying trend is particularly evident in the lower end of the GWR (10th percentile level), which has experienced a notable decline since the 1980s, continuing to the present day.

 

Implications of groundwater modelling for the SDGs

The research has practical applications, reaching various sectors and promoting informed and sustainable practices in water resource management. Indeed, this study emphasises the critical synergy between historical hydrological information and advanced modelling approaches, creating a bridge between the past, present and future hydroclimatic conditions. The developed model, by successfully recreating past recharge patterns, has become a valuable tool for water resource managers, urban planners and policymakers. It plays a pivotal role in tracking progress towards the SDGs, contributing to the pursuit of a sustainable future. By combining historical hydrological insights and contemporary computational capabilities, this approach promotes decision-making by providing a comprehensive understanding of complex hydro-meteorological interactions, climatic oscillations and groundwater recharge.

The practical outcomes of this research have tangible benefits for stakeholders involved in water resource management. Water utilities can optimise infrastructure spending, forecast water availability accurately and develop robust drought management programmes by gaining enhanced insights into historical changes in recharge. Urban planners can use this knowledge to formulate sustainable growth strategies, aligning with the goal of creating inclusive, safe and resilient cities, while considering anticipated changes in groundwater availability. In addition, the agricultural sector can improve irrigation operations for greater efficiency and reduced environmental impact, benefiting from the refined understanding provided by our research.

 

References

  1. Fatichi, S., Peleg, N., Mastrotheodoros, T., Pappas, C. & Manoli, G. An ecohydrological journey of 4500 years reveals a stable but threatened precipitation–groundwater recharge relation around Jerusalem. Adv. 7, eabe6303 (2021).
  2. Letz, O. et al. The impact of geomorphology on groundwater recharge in a semi-arid mountainous area. Hydrol. 603, 127029 (2021).
  3. Long, D. et al. South-to-North water diversion stabilizing Beijing’s groundwater levels. Commun. 11, 3665 (2020).
  4. Diodato, N. et al. Climatic fingerprint of spring discharge depletion in the southern Italian Apennines from 1601 to 2020 CE. Res. Commun. 4, 125011 (2022).
  5. Brune S, Baehr J., Preserving the coupled atmosphere-ocean feedback in initializations of decadal climate predictions. WIREs Clim Change 11: e637. (2020).
  6. Tallaksen, L. M. & Van Lanen, H. A. Hydrological drought: processes and estimation methods for streamflow and groundwater, volume 48 (Elsevier, 2004).
  7. Hanel, M. et al. Revisiting the recent European droughts from a long-term perspective. Rep. 8, 9499 (2018).
  8. Lichtenthal, P. Manuale di geografia fisica. Seconda edizione originale rifusa e considerevolmente accresciuta (Giuseppe Cioffi, 1854). (in Italian)
  9. Sadori, L. et al. Climate, environment and society in southern Italy during the last 2000 years. A review of the environmental, historical and archaeological evidence. Sci. Rev. 136, 173-188 (2016)
  10. Knight, J. R., Folland, C. K. & Scaife, A. A. Climate impacts of the Atlantic Multidecadal Oscillation. Res. Lett. 33, L17706 (2006).
  11. Mann, M. E. et al. Global signatures and dynamical origins of the Little Ice Age and Medieval Climate Anomaly. Science 326, 1256-1260 (2009).
  12. Burzigotti, R., Dragoni, W., Evangelisti, C., Gervasi, The role of Lake Trasimeno (central Italy) in the history of hydrology and water management (International Water History Association, 2003).

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