Groundwater dynamics are often overlooked within historical climatology because of their complexity and the influence of multiple factors. This study presents a groundwater model for Spain, using an existing tree-ring based summer drought reconstruction to estimate the groundwater depth in Castile and Leon (northwestern Spain) over the 1056-2020 CE period. Spanish groundwater volume fluctuations are found to be associated with quasi-decadal variations in the North Atlantic and Pacific Oceans. The reconstructed annual groundwater depth shows significant oscillations around a mean value of 123 m. Changes in groundwater depths include a wet medieval period ( similar to 1056-1200 CE), recurring megadroughts during parts of the Little Ice Age (similar to 1471-1600 CE), and unprecedentedly large variations during recent decades. Aligning with previous studies for the Iberian Peninsula, our new modelling approach highlights the need to enhance groundwater resilience in anticipation of potentially worsening future drought trends across the Mediterranean.