The Transboundary Precipitationshed of a Non-Transboundary Watershed: Emerging Challenges for Water Resources Management

Abstract Terrestrial moisture recycling (TMR) plays a crucial role in the global distribution of water. This process establishes atmospheric connections between watersheds, which are often overlooked in traditional water management that focuses solely on surface hydrology. Located in northwestern South America, the Magdalena Watershed is a non-transboundary watershed that is essential for Colombia’s water and energy security. However, its water availability depends heavily on evapotranspiration from other watersheds and countries. Using a moisture-tracking simulation, we show that this non-transboundary watershed becomes transboundary when analysed through its precipitationshed. We estimate that around 30% of the watershed’s annual precipitation results from Regional Moisture Recycling (RMR), which is moisture originating in terrestrial areas outside the watershed, primarily the Orinoco and Amazon watersheds that span neighbouring countries. A key difference between watersheds and precipitationsheds lies in their physical nature. While watersheds are generally static, precipitationsheds are inherently dynamic. We illustrate this dynamic nature for the Magdalena’s precipitationshed, showing both its seasonal variability and its sensitivity to the El Niño–Southern Oscillation (ENSO). These transboundary and dynamic atmospheric connections challenge the conventional approach to water management that is based solely on static watershed boundaries.