When we think about a country's water reserves, we almost always think of dams. However, it is beneath our feet that an important part of the capacity to withstand drought periods and reduced surface water availability lies. Water infiltrated into the soil and geological formations is stored and released over time, allowing aquifers to buffer precipitation variability and ensure supply when other sources run scarce. But this reserve is not unlimited, nor does it automatically replenish when rain returns.
An aquifer is a stock of natural capital whose evolution depends on the balance between recharge and extraction. If we persistently withdraw more water than the system replenishes, piezometric levels decline and the stock diminishes. In coastal aquifers, excessive pressure can also promote saltwater intrusion, degrading a resource whose recovery is slow and costly. Water used today ceases to be available to meet future needs, generating externalities that are hardly reflected in the price paid for water.
Recent years show that this tension is already real. In the Algarve, reduced reserves have led to restrictions on groundwater extraction. In Grândola, water availability has entered the discussion about the intensity of coastal tourism development. In Almada, recent supply difficulties and the strengthening of extraction have once again highlighted the role of groundwater in meeting demand peaks. Agriculture, tourism, urbanisation and consumption compete for the same water, generating benefits but also distinct pressures on its future availability.
In a context of greater climate uncertainty, there is also an option value in keeping water in the aquifer. Preserving reserves enables responses to droughts, demand shocks or future needs not yet known. Managing aquifers does not mean preventing their use, but deciding how much to withdraw today without compromising tomorrow's options. Water that remains in the aquifer is not water without value—it is natural capital, security and adaptive capacity whose value increases precisely as water becomes scarcer.
