Global Lagrangian Tracking of Continental Precipitation Recycling, Footprints, and Cascades

Abstract

Abstract This study addresses how moisture in continental precipitation is recycled globally from a moisture-tracking perspective. Using a state-of-the-art three-dimensional Lagrangian model and optimized water accounting diagnostics, we complete a 40-yr (1971–2010) tracking of moisture from continental precipitation in the present-day climate. Climatologically, we conclude that 62% of continental precipitation stems from evapotranspiration through Lagrangian tracking—a measure is known as the global “continental precipitation recycling (CPR)” ratio. The result bridges the long-standing gap between the explicit (i.e., moisture trajectory–based) and implicit (i.e., water budget–based) estimates of the global CPR ratio in the literature. On the 1° grid scale, nonlocal terrestrial sources dominate precipitation in almost 70% of the land areas, most prominent in the continental interior. The CPR ratio consistently exhibits a contrasting seasonality between the mid-to-high latitudes and monsoon regions worldwide, from which two kinds of moisture source–regulated hydroclimate are generalized. Given the importance of CPR, we identify terrestrial source hotspots for continental precipitation that deserve careful governance. Using the backward “WaterSip” and a newly proposed forward “WaterDrip” diagnostics, we conceptualize and quantify two important types of cascading moisture recycling (CMR), whereby the moisture from an earlier source precipitates over an intermediate source and then re-evaporates to sustain precipitation farther downwind. The watershed-scale CMR metrics uncover the interdependence among the regional water cycles. Findings here advance the scientific understanding of the atmospheric water cycle as seen through Lagrangian tracking, and offer insights into transboundary watershed management and land-use planning to improve freshwater sustainability. Significance Statement Water is crucial for human civilization. There has been a century-long discussion on the moisture sources of continental precipitation. Using Lagrangian tracking, we show that 62% of continental precipitation stems from evapotranspiration, closing the gap with the budget-based estimate. Terrestrial sources dominate precipitation in 70% of Earth’s surface, especially in the interior of South America, Africa, and Eurasia. Global monsoon regions and the mid-to-high latitudes share a completely different source-regulated hydroclimate. Terrestrial source hotspots for continental precipitation that deserve conservation are identified. Two types of processes that formulate a cascade of regional water cycles are proposed and evaluated. Findings here advance the understanding of the origin of continental precipitation, offering insights into water and land management for freshwater sustainability.