| Summary: | Where does precipitation come from? It is not easy to answer this question because of the complex and energy-intensive processes that bring moisture to a certain location and cause moisture to precipitate highly heterogeneously in space and variable over time. Part of the precipitation comes from so-called “moisture recycling”, which is moisture from land evaporation that returns to the land surface as precipitation. It is widely accepted that land-atmosphere interactions play a crucial role in the global climate, but the importance of moisture recycling specifically had, before the research presented in this thesis, not yet been fully quantified. It is, however, important to do so as the magnitude of moisture recycling can be used as an indicator for the susceptibility of our water resources to local and remote land-use change. The main research question of this thesis is: “How important is land evaporation in the hydrological cycle over continents?” Chapter 2 presents the offline Eulerian numerical atmospheric moisture tracking model WAM-2layers (Water Accounting Model-2layers), which is being used throughout the thesis. The underlying principle of this model is simply the water balance. WAM-2layers can be used to track tagged moisture on both the regional and global scale, and both forward and backward in time. The focus of this thesis is the moisture recycling over continents and therefore a near global grid is used, which includes all continents except Antarctica. The ERA-Interim reanalysis, from which evaporation, precipitation, humidity and wind speed is used, is the main data source for input to the tracking model. WAM-2layers provides a fast computation of large scale atmospheric moisture tracking while the two layers ensure that problems such as wind shear are still adequately dealt with. Chapter 3 presents new definitions for continental moisture recycling. The continental precipitation recycling ratio identifies regions that are dependent on upwind evaporation and the continental evaporation ...
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