A new view on the hydrological cycle over continents

Abstract

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 recycling ratio identifies the importance of evaporation to sustain downwind precipitation. Global maps showing the spatial distribution of two ratios are presented and together they provide a new way to describe continental scale moisture feedback within the hydrological cycle. It is estimated that on average 40% of all terrestrial precipitation is derived from continental sources and 57% of all terrestrial evaporation returns as precipitation to continents. Mountain ranges can play an important role in continental moisture recycling by either “blocking” moisture from entering the continent or by “capturing” the moisture from the atmosphere to enhance recycling. Overall, this chapter demonstrates the important role of global wind patterns, topography and land cover in continental moisture recycling patterns and the distribution of global water resources. Chapter 4 presents a novel approach to quantify the spatial and temporal scale of moisture recycling, independent of the size and shape of the region under study. As such, this approach overcomes the previously existing problem of scale- and shape dependency of regional moisture recycling ratios. It is shown that in the tropics or in mountainous terrain the local length scale of recycling can be as low as 500 to 2000 km. In temperate climates the length scale is typically between 3000 to 5000km whereas it amounts to more than 7000km in desert areas. The local time scale of recycling ranges from 3 to 20 days, with the exception of deserts, where it is much longer. Analysis of both the length and times scales identifies several hot spots of high local moisture recycling, in particular, in and around mountainous areas. It is also found that local moisture recycling plays a much more important role in summer than in winter. Chapter 5 present a new image of that global hydrological cycle over land, which, in contrast to traditional images of the hydrological cycle includes a quantification of moisture recycling, partitioned evaporation and the lifetime of all these processes separately. It is demonstrated that evaporated interception is more likely to return as precipitation on land than transpired moisture. On average, direct evaporation (essentially interception) is found to have an atmospheric residence time of 8 days, while transpiration typically resides 9 days in the atmosphere. Interception recycling has a much shorter local length scale than transpiration recycling, thus interception generally precipitates closer to its evaporative source than transpiration, which is particularly pronounced outside the tropics. The results suggest that the effect of land-use change on moisture recycling is very different during wet and dry seasons, and also during summer and winter, indicating that seasonality is important to consider when analysing effects of land-use change. During the wet season, increased or decreased interception could amplify or attenuate the local moisture recycling signal, but land-use change needs to be drastic to influence the evaporative fluxes in a way that this signal would have continental scale influence. During the dry season, land-use change (in particular deforestation), could lead to reduced transpiration, hence reduced moisture recycling, and therefore a drier dry season. Chapter 6 describe the concept of atmospheric watersheds. Precipitationsheds show how precipitation depends on upwind evaporation and evaporationsheds show how evaporation sustains precipitation downwind. The biggest sources and sinks are generally found close to the region of interest. However, for West Africa it is shown that, outside the rainy season, more distant sources, of in particular transpiration, are very important for the hydrological cycle as well. As such, this chapter illustrates how land-use change in one region alters evaporation and moisture recycling, and hence, influences precipitation, in a geographically separate region. It is concluded that land evaporation plays a major role in the hydrological cycle over continents as on average more than half of it returns as precipitation over land. Strong local moisture feedback is generally found in very wet regions, or in regions where it is enhanced by topography. Interception and transpiration are found to have contrasting roles in the hydrological cycle. While interception mainly works as an intensifier of the local hydrological cycle during wet spells, transpiration remains active during dry spells and is transported over much larger distances downwind where it can act as a significant source of moisture. The concepts of the precipitationshed and evaporationshed can be effectively used as tools to study the moisture recycling effect of land-use changes in specific regions of interest.