In the Una transboundary aquifer, shared between Bosnia and Herzegovina and Croatia, groundwater emerges from major karst springs that supply communities, sustain rivers, and support ecosystems across borders. For most people, these springs are the visible face of the water system. What remains hidden is the vast network of underground pathways that feed them.

For municipalities, this hidden system is not an abstract concept: the City of Bihać's public water utility for example, relies primarily on the Klokot and Privilica karst springs for drinking water supply. The condition of these springs therefore matters not only for ecosystems, but also for the thousands of people who depend on them every day.

This hidden groundwater body becomes particularly vital during periods of drought. As droughts and extreme weather events become more frequent globally, aquifers continue to store and release water, helping springs flow, supporting ecosystems, and providing a critical reserve for communities that depend on reliable water supplies.

Across the Dinaric region, this natural buffering capacity makes groundwater one of the most important - and least visible - allies in building resilience to drought and environmental change.

The Dinaric Karst Aquifer System is one of the world’s largest transboundary karst groundwater systems. Rain and snowmelt infiltrate fractured limestone and travel underground through a highly irregular network of fissures and conduits before re-emerging at the surface - at springs, rivers and wetlands.

Along the Una river, groundwater links protected habitats, tourism sites, and drinking-water sources. In these settings, what happens in upstream recharge areas can matter downstream - even when the connection is invisible from the surface.

In the Klokot and Privilica catchment, water can disappear into sinkholes and re-emerge kilometres away, shaping ecosystems that may look separate above ground.

Groundwater also sustains groundwater-dependent ecosystems — springs, wetlands, river corridors, and subterranean habitats - home to rare and highly localised species. Changes in groundwater quality or quantity can therefore affect life both above and below ground.

When discussions turn to drought, attention often focuses on affected rivers, reservoirs and water use. Yet resilience starts much earlier -in the landscapes where groundwater is replenished.

Healthy recharge areas and rock fracturing on the surface and underground allow rainfall to infiltrate into the ground, replenishing aquifers and helping maintain water availability during dry periods. When these areas are degraded by pollution, unsustainable land use or poorly planned development, the ability of groundwater systems to store and release clean drinking water can be weakened. Climate change can intensify these pressures quantitatively by altering rainfall patterns and increasing the frequency and severity of droughts.

Karst aquifers are particularly sensitive. Water can move rapidly through fractures and underground conduits, often with limited self-purification. As a result, contaminants introduced at the surface may travel quickly through the system, affecting springs, ecosystems and water supplies.

Protecting groundwater therefore means protecting the wider landscapes that sustain it.

Groundwater does not recognize national borders.

Across the Dinaric region, groundwater flow paths connect Albania and Montenegro, Bosnia and Herzegovina and Montenegro, and Bosnia and Herzegovina and Croatia through an underground system that extends beyond political boundaries. Water entering the ground in one area may emerge kilometres away, carrying both benefits and risks across borders.

This interconnectedness creates a shared responsibility. Pressures such as inadequate sanitation, illegal dumping, poorly managed waste, increasing tourism demands, and unsustainable land-use practices can affect groundwater quality and ecosystem health beyond the places where they originate.

Protecting groundwater therefore requires more than local action. It requires cooperation, shared knowledge, and coordinated approaches that reflect how these systems actually function.

Protecting springs such as Klokot and Privilica in the Una aquifer requires understanding not only where groundwater emerges, but also how it moves through the wider aquifer system - and how quickly pressures can travel through karst. The DIKTAS II project supports countries in implementing the Strategic Action Programme for the Dinaric Karst Aquifer System and strengthening the governance needed to protect this shared resource.

“People usually notice groundwater only when it is no longer there. But the reliability of a spring during a dry summer depends on decisions made across the whole catchment - sometimes many kilometres away from where the water emerges.” - Boban Jolovic, DIKTAS II project manager.

By bringing together hydrological, ecological and governance perspectives, DIKTAS II contributes to a more integrated understanding of groundwater - not simply as a source of water, but as a foundation for ecosystem health, climate resilience and sustainable development.

As communities across the world adapt to a changing climate, protecting this hidden lifeline will be essential for maintaining healthy ecosystems, reliable water resources and resilient landscapes.

Words by Pomeline Decottignies (UNESCO), Francesca Bampa (UNESCO), Boban Jolovic (UNESCO), and Sinisa Sesum (UNESCO)

DIKTAS II is a Global Environment Facility (GEF) International Waters project implemented by UNDP and executed by UNESCO. It supports transboundary cooperation to improve groundwater governance and promote sustainable management of the Dinaric Karst Aquifer System across Albania, Bosnia and Herzegovina, Croatia and Montenegro.