Without the ecosystem services or benefits provided by Earth’s aquatic systems—aquifers, lakes/reservoirs, rivers, large marine ecosystems (LMEs), and open ocean— human survival and wellbeing will be seriously threatened.  Yet, these vital systems continue to be degraded by multiple and complex human-induced and natural stressors that impair their ability to sustainably provide ecosystem services, placing human communities around the world at risk.   Most of the Earth’s surface is covered by aquatic systems that are transboundary, that is, extending across or beyond national boundaries. Since the early 1990s, the Global Environment Facility (GEF) has invested over one billion dollars in bringing countries together to address the major environmental issues in transboundary water systems.  

Despite these efforts, management of transboundary waters is constrained by the lack of a systematic, global comparative baseline assessment of their changing conditions in response to human induced and natural stresses.

The Full-size Project of the Transboundary Waters Assessment Programme, which is funded principally by GEF with cofinancing from other sources, arose out of the need for:

• A global baseline assessment of the status and changing condition of transboundary water systems resulting from human and natural causes, which will allow the GEF, policy makers and international organizations to set science-based priorities for financial resource allocation; and
• The institutional arrangements for conducting periodic future assessments of transboundary water systems to allow the GEF and others to track the results of their interventions. 

The current project, which runs from April 2013 to March 2015, builds on the previous phase during which assessment methodologies were developed for the five types of transboundary water systems: Transboundary aquifers, Transboundary lake basins, Transboundary river basins, LMEs and Open Ocean.

Within the full TWAP project, the Intergovernmental Oceanographic Commission is leading agency for the LMEs and Open Ocean projects.

The global coastal ocean is divided into 66 Large Marine Ecosystems, which encompass waters from river basins and estuaries to the seaward boundaries of continental shelves and margins of coastal currents and water masses.

Earth is an ‘ocean’ planet: 70% of its surface is covered by ocean, and about 50% covered by ocean areas beyond national jurisdiction: the common heritage of mankind. The Open Ocean is the largest of the planet’s transboundary water spaces.

The International Hydrological Programme (UNESCO-IHP) is the lead agency for this project within TWAP. The relative fragility of the hydrological cycles on Small Island Developing States means that the evaluation, planning and development of freshwater resources has to be approached with special care to work within these hydro-environmental limits.