PRECIPITATION RECYCLING

The water cycle regulates and reflects natural variability in climate at the regional and global scales. Large-scale human activities that i nvolve changes in land cover, such as tropical deforestation, are like ly to modify climate through changes in the water cycle. In order to u nderstand, and hopefully be able to predict, the extent of these poten tial global and regional changes, we need first to understand how the water cycle works. In the past, most of the research in hydrology focu sed on the land branch of the water cycle, with little attention given to the atmospheric branch. The study of precipitation recycling, whic h is defined as the contribution of local evaporation to local precipi tation, aims at understanding hydrologic processes in the atmospheric branch of the water cycle. Simply stated, any study on precipitation r ecycling is about how the atmospheric branch of the water cycle works, namely, what happens to water vapor molecules after they evaporate fr om the surface, and where will they precipitate? Estimation of precipi tation recycling over any large basin, such as the Mississippi or the Amazon, is a necessary step before developing a quantitative descripti on of the regional water cycle. This paper reviews the research on the concept of precipitation recycling and emphasizes the basic role of t his process in defining the different components of the atmospheric br anch in any regional water cycle. To illustrate the assumptions and li mitations involved in estimation of precipitation recycling, we presen t and discuss a general formula for estimation of precipitation recycl ing. 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 dis cussed and illustrated by the example of the Amazon basin.