Precipitation recycling

The water cycle regulates and reflects natural variability in climate at the regional and global scales. Large-scale human activities that involve changes in land cover, such as tropical deforestation, are likely to modify climate through changes in the water cycle. In order to understand, and hopefully be able to predict, the extent of these potential global and regional changes, we need first to understand how the water cycle works. In the past, most of the research in hydrology focused on the land branch of the water cycle, with little attention given to the atmospheric branch. The study of precipitation recycling, which is defined as the contribution of local evaporation to local precipitation, aims at understanding hydrologic processes in the atmospheric branch of the water cycle. Simply stated, any study on precipitation recycling is about how the atmospheric branch of the water cycle works, namely, what happens to water vapor molecules after they evaporate from the surface, and where will they precipitate? Estimation of precipitation recycling over any large basin, such as the Mississippi or the Amazon, is a necessary step before developing a quantitative description of the regional water cycle. This paper reviews the research on the concept of precipitation recycling and emphasizes the basic role of this process in defining the different components of the atmospheric branch in any regional water cycle. To illustrate the assumptions and limitations involved in estimation of precipitation recycling, we present and discuss a general formula for estimation of precipitation recycling. The recent estimates of annual precipitation recycling ratio from different regions are reviewed and compared. Finally, the dependence of precipitation recycling over any region on the spatial scale is discussed and illustrated by the example of the Amazon basin.