ALBERT

Description: Recent research has revealed that upwind land-use changes can significantly influence downwind precipitation. The precipitationshed (the upwind ocean and land surface that contributes evaporation to a specific location's precipitation) may provide a boundary for coordination and governance of these upwind–downwind water linkages. We aim to quantify the variability of the precipitationshed boundary to determine whether there are persistent and significant sources of evaporation for a given region's precipitation. We identify the precipitationsheds for three regions (i.e., western Sahel, northern China, and La Plata) by tracking atmospheric moisture with a numerical water transport model (Water Accounting Model-2layers, or WAM-2layers) using gridded fields from both the ERA-Interim (European Reanalysis Interim) and MERRA (Modern-Era Retrospective Analysis for Research and Applications) reanalyses. Precipitationshed variability is examined first by diagnosing the persistence of the evaporation contribution and second with an analysis of the spatial variability of the evaporation contribution. The analysis leads to three key conclusions: (1) a core precipitationshed exists; (2) most of the variance in the precipitationshed is explained by a pulsing of more or less evaporation from the core precipitationshed; and (3) the reanalysis data sets agree reasonably well, although the degree of agreement is regionally dependent. Given that much of the growing-season evaporation arises from within a core precipitationshed that is largely persistent in time, we conclude that the precipitationshed can potentially provide a useful boundary for governing land-use change on downwind precipitation.

Description: The contribution of land evaporation to local and remote precipitation (i.e. moisture recycling) is of significant importance to sustain water resources and ecosystems. But how important are different evaporation components in sustaining precipitation? This is the first paper to present moisture recycling metrics for partitioned evaporation. In the companion paper Wang-Erlandsson et al. (2014) (hereafter Part 1), evaporation was partitioned into vegetation interception, floor interception, soil moisture evaporation and open-water evaporation (constituting the direct, purely physical fluxes, largely dominated by interception), and transpiration (delayed, biophysical flux). Here, we track these components forward as well as backward in time. We also include age tracers to study the atmospheric residence times of these evaporation components. We present a new image of the global hydrological cycle that includes quantification of partitioned evaporation and moisture recycling as well as the atmospheric residence times of all fluxes. We demonstrate that evaporated interception is more likely to return as precipitation on land than transpired water. On average, direct evaporation (essentially interception) is found to have an atmospheric residence time of 8 days, while transpiration typically resides for 9 days in the atmosphere. The process scale over which evaporation recycles is more local for interception compared to transpiration; thus interception generally precipitates closer to its evaporative source than transpiration, which is particularly pronounced outside the tropics. We conclude that interception mainly works as an intensifier of the local hydrological cycle during wet spells and wet seasons. On the other hand, transpiration remains active during dry spells and dry seasons and is transported over much larger distances downwind, where it can act as a significant source of moisture. Thus, as various land-use types can differ considerably in their partitioning between interception and transpiration, our results stress that land-use changes (e.g. forest-to-cropland conversion) do not only affect the magnitude of moisture recycling, but could also influence the moisture recycling patterns and lead to a redistribution of water resources. As such, this research highlights that land-use changes can have complex effects on the atmospheric branch of the hydrological cycle.

Description: The contribution of land evaporation to local and remote precipitation (i.e., moisture recycling) is of significant importance to sustain water resources and ecosystems. But how important are different evaporation components in sustaining precipitation? This is the first paper to present moisture recycling metrics for partitioned evaporation. In the companion paper, Part 1, evaporation was partitioned into vegetation interception, floor interception, soil moisture evaporation and open water evaporation (constituting the direct, purely physical fluxes, largely dominated by interception), and transpiration (delayed, biophysical flux). Here, we track these components forward as well as backward in time. We also include age tracers to study the atmospheric residence times of these evaporation components. As the main result we present a new image of the global hydrological cycle that includes quantification of partitioned evaporation and moisture recycling as well as the atmospheric residence times of all fluxes. We demonstrate that evaporated interception is more likely to return as precipitation on land than transpired water. On average, direct evaporation (essentially interception) is found to have an atmospheric residence time of eight days, while transpiration typically resides nine 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. We conclude that interception mainly works as an intensifier of the local hydrological cycle during wet spells. On the other hand, transpiration remains active during dry spells and is transported over much larger distances downwind where it can act as a significant source of moisture. Thus, as various land-use types can differ considerably in their partitioning between interception and transpiration, our results stress that land-use changes (e.g., forest to cropland conversion) do not only affect the magnitude of moisture recycling, but could also influence the moisture recycling patterns and lead to a redistribution of water resources. As such, this research highlights that land-use changes can have complex effects on the atmospheric branch of the hydrological cycle.

Description: Recent research has revealed that upwind land-use changes can significantly influence downwind precipitation. The precipitationshed (the upwind ocean and land surface that contributes evaporation to a specific location's precipitation) may provide a boundary for coordination and governance of these upwind-downwind water linkages. We aim to quantify the variability of the precipitationshed boundary to determine whether there are persistent and significant sources of evaporation for a given region's precipitation. We identify the precipitationsheds for three regions (i.e. Western Sahel, Northern China, and La Plata) by tracking atmospheric moisture with a numerical water transport model (WAM-2layers) using gridded fields from both the ERA-Interim and MERRA reanalyses. Precipitationshed variability is examined first by diagnosing the persistence of the evaporation contribution and second with an analysis of the spatial variability of the evaporation contribution. The analysis leads to three key conclusions: (1) a core precipitationshed exists; (2) most of the variance in the precipitationshed is explained by a pulsing of more or less evaporation from the core precipitationshed; and, (3) the reanalysis datasets agree reasonably well, although the degree of agreement is regionally dependent. Given that much of the growing season evaporation arises from within a core precipitationshed that is largely persistent in time, we conclude that the precipitationshed can potentially provide a useful boundary for governing land-use change on downwind precipitation.

Description: It is well known that rivers connect upstream and downstream ecosystems within watersheds. Here we describe the concept of precipitationsheds to show how upwind terrestrial evaporation source areas contribute moisture for precipitation in downwind sink regions. We illustrate the importance of upwind land cover in precipitationsheds to sustain precipitation in critically water stressed downwind areas, i.e. dryland agricultural areas. We first identify seven regions where rainfed agriculture is particularly vulnerable to reductions in precipitation, and then map their precipitationsheds. We further develop a framework for qualitatively assessing the vulnerability of precipitation for these seven agricultural regions. We illustrate that the sink regions have varying degrees of vulnerability to changes in upwind evaporation rates depending on the extent of the precipitationshed, source region land use intensity and expected land cover changes in the source region.

Description: It is well known that rivers connect upstream and downstream ecosystems within watersheds. Here we describe the concept of precipitationsheds to show how upwind terrestrial evaporation source areas contribute moisture for precipitation to downwind sink regions. We illustrate the importance of upwind land cover in precipitationsheds to sustain precipitation in critically water stressed downwind areas, specifically dryland agricultural areas. We first identify seven regions where rainfed agriculture is particularly vulnerable to reductions in precipitation, and then map their precipitationsheds. We then develop a framework for qualitatively assessing the vulnerability of precipitation for these seven agricultural regions. We illustrate that the sink regions have varying degrees of vulnerability to changes in upwind evaporation rates depending on the extent of the precipitationshed, source region land use intensity and expected land cover changes in the source region.