Moisture Recycling Research Articles

Variation in stable isotope ratios of monthly rainfall in the Douala and Yaounde cities, Cameroon: local meteoric lines and relationship to regional precipitation cycle

Hydrogen (D) and oxygen (18O) stable isotopes in precipitation are useful tools in groundwater recharge and climatological investigations. This study investigated the isotopes in rainfall during the 2013 and 2014 hydrological years in the Douala and Yaounde urban cities. The objectives were to generate local meteoric water lines (LMWLs), define the spatial–temporal variations of the isotopes in rainwater and their relationship to the regional precipitation cycle, and determine the factors controlling the isotopic variation. The LWMLs in Douala and Yaounde were δD = 7.92δ18O + 12.99 and δD = 8.35δ18O + 15.29, respectively. The slopes indicate isotopic equilibrium conditions during rain formation and negligible evaporation effect during rainfall. Precipitation showed similar wide ranges in δ18O values from −5.26 to −0.75 ‰ in Douala and −5.8 to +1.81 ‰ in Yaounde suggesting a common moisture source from the Atlantic Ocean. Enriched weighted mean δ18O (wδ18O) values during the low pre- and post-monsoon showers coincided with low convective activity across the entire region. Enriched isotopic signatures also marked the West African monsoon transition phase during each hydrological year. Abrupt wδ18O depletion after the transition coincided with the monsoon onset in the region. Peak periods of monsoonal rainfall, associated with high convective activities, were characterised by the most depleted wδ18O values. Controls on isotopic variations are the amount effect and moisture recycling. The stable isotope data provide a tool for groundwater recharge studies while the isotopic correlation with regional rainfall cycle demonstrate their use as markers of moisture circulation and detecting climatic changes in precipitation.

Paired stable isotopologues in precipitation and vapor: A case study of the amount effect within western tropical Pacific storms

AbstractUnderstanding controls on the stable isotopic composition of precipitation and vapor in the West Pacific Warm Pool is vital for accurate representation of convective processes in models and correct interpretation of isotope‐based paleoclimate proxies, yet a lack of direct observational evidence precludes the utility of these isotopic tracers. Results from a measurement campaign at Manus Island, Papua New Guinea from 28 April to 8 May 2013 demonstrate variability in the stable isotopic composition (δD and δ18O) of precipitation and vapor in individual precipitation events and over a 10 day period. Isotope ratios in water vapor and precipitation progressively increased throughout the period of measurement, coincident with a transition from high to low regional convective activity. Vapor isotope ratios approached equilibrium with seawater during the quiescent period and likely reflected downwind advection of distilled vapor and re‐evaporation of rainfall during the period of regional convection. On a 5 min timescale across individual storms, isotope ratios in precipitation were strongly correlated with isotope ratios in surface vapor. However, individual precipitation isotope ratios were not strongly correlated with surface meteorological data, including precipitation rate, in all storms. Yet across all events, precipitation deuterium excess was negatively correlated with surface temperature, sea level pressure, and cloud base height and positively correlated with precipitation rate and relative humidity. Paired surface precipitation and vapor isotope ratios indicate condensation at boundary layer temperatures. The ratio of these paired values decreased with increasing precipitation rate during some precipitation events, suggesting rain re‐evaporation and precipitation in equilibrium with an isotopically distinct upper level moisture source. Results from the short campaign support the interpretation that isotope ratios in precipitation and vapor in the western tropical Pacific are indicators of regional convective intensity at the timescale of days to weeks. However, a nonstationary relationship between rain rate and stable isotope ratios in precipitation during individual convective events suggests that condensation, rain evaporation, moisture recycling, and regional moisture convergence do not always yield an amount effect relationship on intraevent timescales.

Contribution of recycled moisture to precipitation in oases of arid central Asia: A stable isotope approach

AbstractTerrestrial moisture contributed by surface evaporation and transpiration, also known as recycled moisture, plays an important role in hydrological processes especially across arid central Asia. The stable hydrogen and oxygen isotopes can be used for water budget analysis to calculate the contribution of recycled moisture to precipitation between two locations along the moisture flow. Based on a three‐component isotopic mixing model, the moisture recycling in oasis stations of arid central Asia during summer months is assessed. At large oases of Urumqi, the proportional contribution of recycled moisture to local precipitation is approximately 16.2%, and the mean proportions of surface evaporation and transpiration are 5.9% ± 1.5% and 10.3% ± 2.2%, respectively. At small oases like Shihezi and Caijiahu the contribution of recycled moisture is less than 5%, and the proportion of surface evaporation is much less than that of transpiration. The vegetative cover in arid central Asia is generally sparse, but the evapotranspiration contribution to precipitation cannot be ignored at the widely distributed oases. The oasis effect shows great variability depending on locations and water availability for evapotranspiration.

Revealing Invisible Water: Moisture Recycling as an Ecosystem Service.

An ecosystem service is a benefit derived by humanity that can be traced back to an ecological process. Although ecosystem services related to surface water have been thoroughly described, the relationship between atmospheric water and ecosystem services has been mostly neglected, and perhaps misunderstood. Recent advances in land-atmosphere modeling have revealed the importance of terrestrial ecosystems for moisture recycling. In this paper, we analyze the extent to which vegetation sustains the supply of atmospheric moisture and precipitation for downwind beneficiaries, globally. We simulate land-surface evaporation with a global hydrology model and track changes to moisture recycling using an atmospheric moisture budget model, and we define vegetation-regulated moisture recycling as the difference in moisture recycling between current vegetation and a hypothetical desert world. Our results show that nearly a fifth of annual average precipitation falling on land is from vegetation-regulated moisture recycling, but the global variability is large, with many places receiving nearly half their precipitation from this ecosystem service. The largest potential impacts for changes to this ecosystem service are land-use changes across temperate regions in North America and Russia. Likewise, in semi-arid regions reliant on rainfed agricultural production, land-use change that even modestly reduces evaporation and subsequent precipitation, could significantly affect human well-being. We also present a regional case study in the Mato Grosso region of Brazil, where we identify the specific moisture recycling ecosystem services associated with the vegetation in Mato Grosso. We find that Mato Grosso vegetation regulates some internal precipitation, with a diffuse region of benefit downwind, primarily to the south and east, including the La Plata River basin and the megacities of Sao Paulo and Rio de Janeiro. We synthesize our global and regional results into a generalized framework for describing moisture recycling as an ecosystem service. We conclude that future work ought to disentangle whether and how this vegetation-regulated moisture recycling interacts with other ecosystem services, so that trade-offs can be assessed in a comprehensive and sustainable manner.

A multi-scale analysis of moisture supply associated with precipitation on Isla del Coco, Costa Rica

<p>Wrapped by the Pacific waters and the mist of shipwrecks and pirates stories, one of the rainiest Eastern Pacific islands protects a biodiversity treasure: Isla del Coco. This study presents the analysis of moisture sources linked with contributions to precipitation in the area. The diurnal cycle of precipitation on the island was reviewed from GPS station data previously evaluated using available meteorological data from field campaigns held on the island in 2011 and 2012. Near-surface salinity patterns were also analyzed along with sea surface temperature, evaporation, Ongoing Longwave Radiation (OLR) as well as latent and sensible heat fluxes. Moisture contributions to precipitation on the island are supplied by evaporative sources, and moisture recycling is important. Regional precipitation is a continuous supply of moisture for the atmosphere whereas transport from evaporative sources is seasonally constrained. The analysis of the diurnal cycle of moisture supply suggests that contributions from available moisture linked with precipitation recycling show a slightly delayed response to deep convection in the region. The diurnal cycle of contributions to precipitation from evaporative moisture sources, based on the modeling component of the work, is consistent with the diurnal cycle of precipitation. The trajectory analysis remarks the role of the low level winds, the Intertropical Convergence Zone (ITCZ) position and the stability conditions to modulate the supply of moisture. The moisture contributions from the sources present a different sensitivity to El Niño-Southern Oscillation (ENSO). Contributions from precipitation recycling showed a large variability linked to ENSO as increasing contributions were determined to be related with El Niño during boreal summer and autumn months. The variability of the contributions from a North-east evaporative source is modulated by the response of the Caribbean Low Level Jet (CLLJ; Amador, 1998; Amador, 2008) to ENSO. The South-western evaporative source showed sensitivity to El Niño, as transport was found to decrease (increase) from November to March (May to July), meanwhile the response to La Niña was small. Good agreement amongst the ENSO response of the fields and the known dynamics of the region was found.</p><div> </div>

On the projected increase of Sahel rainfall during the late rainy season

Thirteen CMIP5 models are used to analyse changes in climate over the West African monsoon region between the near future (2031–2070 under the RCP4.5 emission scenario), and a control period (1960–1999 under the historical emission scenario), with a focus on the late rainy season. The monsoon circulation is projected to strengthen and to shift northward leading to more rainfall during the Sahelian season. The results show an increase of the Rainfall amounts in September–October and a delay in the monsoon withdrawal. The increased moisture that fuels the rainfall anomalies is associated with an increase in moisture flux convergence and with local moisture recycling. The moisture transport dominates the water budget change in September while the local recycling is prominent in October. The delay in monsoon withdrawal, although expected from the increase in rainfall in September–October, is not strongly correlated with the size of the monthly anomalies.

Surface-Atmosphere Moisture Interactions in the Frozen Ground Regions of Eurasia.

Climate models simulate an intensifying Arctic hydrologic cycle in response to climatic warming, however the role of surface-atmosphere interactions from degrading frozen ground is unclear in these projections. Using Modern-Era Retrospective Analysis for Research and Applications (MERRA) data in high-latitude Eurasia, we examine long-term variability in surface-atmosphere coupling as represented by the statistical relationship between surface evaporative fraction (EF) and afternoon precipitation. Changes in EF, precipitation, and their statistical association are then related to underlying permafrost type and snow cover. Results indicate significant positive trends in July EF in the Central Siberian Plateau, corresponding to significant increases in afternoon precipitation. The positive trends are only significant over continuous permafrost, with non-significant or negative EF and precipitation trends over isolated, sporadic, and discontinuous permafrost areas. Concurrently, increasing EF and subsequent precipitation are found to coincide with significant trends in May and June snowmelt, which potentially provides the moisture source for the observed enhanced latent heating and moisture recycling in the region. As climate change causes continuous permafrost to transition to discontinuous, discontinuous to sporadic, sporadic to isolated, and isolated permafrost disappears, this will also alter patterns of atmospheric convection, moisture recycling, and hence the hydrologic cycle in high-latitude land areas.

Water stable isotopes in the Yarlungzangbo headwater region and its vicinity of the southwestern Tibetan Plateau

Various types of water samples across the Yarlungzangbo headwater region and its vicinity of the southwestern Tibetan Plateau were collected in summer 2012 for isotopic analysis to augment our specific understanding of regional hydroclimate. In the upper Yarlungzangbo system, contribution of more isotopically depleted tributary influx rather than evaporation is identified as the major hydrological process, and this causes generally progressive depletion in main flow isotopes downstream/eastward. On the regional scale (from the middle Yarlungzangbo basin to the western vicinity of the Yarlungzangbo headwaters), one of the clearest findings is the synchronous increases in river/stream δ18O and d-excess (d-excess= δD – 8δ18O) from east to west. From a geographical and climatological perspective, these can be explained by increasing contributions of summer-time continental/local recycled vs. monsoonal moisture or winter-spring vs. summer monsoon precipitation in the surface runoff generation westward. The Yarlungzangbo headwaters are at a critical transition between the Indian monsoon system in the east and the westerly one in the west as revealed by the intermediate d-excess and δ18O values, while the Himalaya region farther west (i.e. Pulan region) is predominated by the continental recycled moisture as revealed by the highest d-excess and δ18O values. Furthermore, based upon existing isotopic and meteorological data, a predicted local evaporation line for the Yarlungzangbo headwater region and its vicinity is proposed, which can further help to assess how local hydroclimate affect water stable isotopes. Results of this study provide a baseline for isotopic hydroclimate research in the southwestern Tibetan Plateau.

Resilient rainfed technologies for drought mitigation and sustainable food security

Even though drought is one of the most common features affecting rainfed agriculture, it is necessary to consider it as an extreme climatological event that requires different types of alleviating strategies for overcoming it. The risk involved in successful cultivation of crops depends on the nature of drought (chronic and contingent), its probable duration, and frequency of occurrence within the season. These aberrations are expected to further increase in future. A significant fall in food production is often noticed with increase in intensity or extension in duration of drought prevalence. Drought affects not only the food production at farm level but also the national economy and overall food security. Location-specific rainfed technologies are available to cope with different drought situations. Much of the research done in rainfed agriculture in India relates to conservation of soil & rainwater and to drought proofing. The key technologies for drought mitigation are in situ moisture conservation, rainwater harvesting and recycling, resilient crops and cropping systems including contingency crop plans, foliar sprays, and integrated farming systems. However, drought preparedness and real time implementation of contingency measures at field level needs well structured institutional support for farmers with strong government policy and convergence among various institutions. Ministry of Agriculture, Government of India, needs to facilitate the convergence process of various government schemes such as MGNREGA, RKVY, Mega Seed Project, NFSM, NHM, IWMP, Soil health schemes etc. for drought preparedness. National Mission for Sustainable Agriculture (NMSA), one among the missions under the Prime Minister National Action Plan for Climate Change (NAPCC) may take a lead role in implementation of contingency, by inclusion of this activity in State Action Plans (SAP) with a dedicated Nodal Institution /officers and budget provision.

Evolutions of water stable isotopes and the contributions of cryosphere to the alpine river on the Tibetan Plateau

The significant changes of cryosphere on the Tibetan Plateau (TP) are an important cause of concern for the huge population in Asia which depends on the glacier-permafrost feed rivers. Up to now, little is known about the response of runoff to the cryospheric changes under a warming climate. Taking an example of a typical alpine river (hereafter named KLR River) in Kunlun Mountains Pass on the TP to investigate the hydrological processes and quantify the potential recharge sources using stable isotopes and electrical conductivities. The results showed significantly temporal variations in isotopes of precipitation and river water. Deviations in the precipitation isotopes indicated that precipitation in Kunlun Mountains Pass during summer months mainly originated from local recycled moisture and slightly influenced by monsoon. The KLR river was separated into three hydrological components which include precipitation, meltwater of glacier and meltwater of subsurface ground ice. On average, the meltwater of glacier accounted for 56.64 % of total flow, whereas the meltwater of subsurface ground ice contributed 37.44 % and only 5.92 % was supplied by precipitation. Evidently, the contributions of glacier and thawing permafrost are up to as high as 94 %. The cryosphere played a crucial role on the hydrological cycle on the TP which would be important to understand the water resource distribution in permafrost regions on the TP under the global warming.

Summer climate of Madagascar and monsoon pulsing of its vortex

This study analyzes the climate of Madagascar (12°–26°S, 43°–50°E) and its relation to the Indian Ocean during austral summer (Dec–Mar). Moisture converges onto a standing easterly wave and floods are prevalent in late summer. All-island daytime land temperatures exceed 38 °C in October and are ~4 °C above sea temperatures during summer. Analysis of thermally induced diurnal convection and circulation revealed inflow during the afternoon recirculated from the southeastern mountains and the warm Mozambique Channel. Summer rainfall follows latent and sensible heat flux during the first half of the day, and gains a surplus by evening via thunderstorms over the western plains. At the inter-annual time-scale, 2.3 years oscillations in all-island rainfall appear linked with the stratospheric quasi-biennial oscillation and corresponding 80 Dobson Unit ozone fluctuations during flood events. Wet spells at frequencies from 11–27 days derive from locally-formed tropical cyclones and NW-cloud bands. Flood case studies exhibit moisture recycling in the confluence zone between the sub-tropical anticyclone and the lee-side vortex. Hovmoller analysis of daily rainfall reinforces the concept of local generation and pulsing by cross-equatorial (Indian winter) monsoon flow rather than zonal atmospheric waves. Since the surface water budget is critical to agriculture in Madagascar, this study represents a further step to understand its meso-scale summer climate.

Isotopic and chemical composition of precipitation in Riyadh, Saudi Arabia

Only limited data on the isotopic and chemical composition of Riyadh rain are currently available. In this study, we complement these data by analyzing integral samples covering 28 precipitation events between 2009 and 2013. Results of stable isotope analyses are used to establish a Local Meteoric Water Line: δ2H=5.22(±0.38)·δ18O+14.8(±0.9) ‰. Moisture source-related isotopic fingerprints are masked by the continental effect, the altitude effect, sub-cloud evaporation, and moisture recycling. The study of one event for intra-storm variability revealed strong isotopic depletion due to rainout and Rayleigh distillation processes, thus highlighting the general need for integral samples. Tritium analyses of grab samples from 12 events yielded concentrations between 2.8 and 6.4tritium units (TU), which are close to the natural background of a few TU.Major ion concentrations and ratios indicate that solutes are predominantly derived from atmospheric dust originating from limestone outcrops and sabkha deposits. The latter play a role with respect to the elevated Cl−and Na+ contents, but are probably also responsible for the SO42−and a part of the Ca2+ found in Riyadh rain. Observed intra- and inter-storm variabilities of major ion levels necessitate the collection of integral samples and the calculation of precipitation-weighted means, respectively.The obtained isotopic signatures and the precipitation-weighted mean Cl−concentration (9.5mgL−1) may be useful in groundwater assessments, e.g., for the identification of modern recharge and quantification thereof by means of the Chloride Mass Balance method.

Lagrangian tracing of Sahelian Sudan moisture sources

AbstractThe Sahelian Sudan is an arid to semiarid region that depends on the seasonal rainfall as the main source of water, but its rainfall has large interannual variability. Such dry regions usually have their main moisture sources elsewhere; thus, the rainfall variability is directly related to the moisture transport. This study seeks to identify source regions of water vapor for Sahelian Sudan during the monsoon period, from July to September. We have used the Lagrangian trajectory model FLEXPART driven by ERA‐Interim reanalysis for the time period 1998 to 2008. The results show that most of the air masses that reach this region during the monsoon period have their major origins over the Arabian Peninsula, Central Africa, or are associated with the tropical easterly jet. Flow associated with Intertropical Convergence Zone contributes almost half of the total precipitated water; most of it comes from Central Africa. This suggests that moisture recycling is the major contributor, compared to Oceanic sources. The flows from the northeast (Arabian Peninsula and north Asia) and east (Horn of Africa and north Indian Ocean) contribute about one third of the precipitated water. The rest of precipitated water comes from the Mediterranean, subtropical Atlantic, and western Sahel, all with smaller contribution. Our results also indicate that different subregions of Sahelian Sudan have different moisture sources. Such result needs to be taken into account in seasonal forecasting practices.

Use of stable water isotopes to identify hydrological processes of meteoric water in montane catchments

AbstractThis study analyzes the stable isotopic compositions of hydrogen and oxygen (δ2H, δ18O) in montane meteoric waters including precipitation and stream water of central Taiwan to identify hydrological processes in montane catchments. Results of precipitation demonstrate that monsoon and altitude effects are two principal processes affecting δ and deuterium excess (dE) values of inland precipitation in central Taiwan. Furthermore, slope and intercept values of summer and winter local meteoric water line are modified by secondary evaporation effects such as moisture recycling and raindrop evaporation. Additionally, stream water's results indicate that differences in δ values among stream waters reflect isotopic altitude effect whereby lower values are more evident in stream water originating from high‐elevation catchments than low‐elevation catchments. Comparison of the isotopic results between precipitation and stream water indicates that summer precipitation containing recycled moisture is the most important water source for the studied stream waters and indicates that catchment effect and base flow contribution are the two major hydrological processes affecting mountain stream hydrology. The hydrological processes identified by the isotopic study re‐stress the important role of forests in mountain hydrology. Copyright © 2015 John Wiley & Sons, Ltd.

Scaling of extreme rainfall areas at a planetary scale.

Event magnitude and area scaling relationships for rainfall over different regions of the world have been presented in the literature for relatively short durations and over relatively small areas. In this paper, we present the first ever results on a global analysis of the scaling characteristics of extreme rainfall areas for durations ranging from 1 to 30 days. Broken power law models are fit in each case. The past work has been focused largely on the time and space scales associated with local and regional convection. The work presented here suggests that power law scaling may also apply to planetary scale phenomenon, such as frontal and monsoonal systems, and their interaction with local moisture recycling. Such features may have persistence over large areas corresponding to extreme rain and regional flood events. As a result, they lead to considerable hazard exposure. A caveat is that methods used for empirical power law identification have difficulties with edge effects due to finite domains. This leads to problems with robust model identification and interpretability of the underlying relationships. We use recent algorithms that aim to address some of these issues in a principled way. Theoretical research that could explain why such results may emerge across the world, as analyzed for the first time in this paper, is needed.

A 12-year high-resolution climatology of atmospheric water transport over the Tibetan Plateau

Abstract. The Tibetan Plateau (TP) plays a key role in the water cycle of high Asia and its downstream regions. The respective influence of the Indian and East Asian summer monsoon on TP precipitation and regional water resources, together with the detection of moisture transport pathways and source regions are the subject of recent research. In this study, we present a 12-year high-resolution climatology of the atmospheric water transport (AWT) over and towards the TP using a new data set, the High Asia Refined analysis (HAR), which better represents the complex topography of the TP and surrounding high mountain ranges than coarse-resolution data sets. We focus on spatiotemporal patterns, vertical distribution and transport through the TP boundaries. The results show that the mid-latitude westerlies have a higher share in summertime AWT over the TP than assumed so far. Water vapour (WV) transport constitutes the main part, whereby transport of water as cloud particles (CP) also plays a role in winter in the Karakoram and western Himalayan regions. High mountain valleys in the Himalayas facilitate AWT from the south, whereas the high mountain regions inhibit AWT to a large extent and limit the influence of the Indian summer monsoon. No transport from the East Asian monsoon to the TP could be detected. Our results show that 36.8 ± 6.3% of the atmospheric moisture needed for precipitation comes from outside the TP, while the remaining 63.2% is provided by local moisture recycling.

Isotopic and hydrochemical composition of runoff in the Urumqi River, Tianshan Mountains, China

Natural tracers can be used in arid regions to gain an understanding of streamflow generation and seasonal streamflow sources; their application can support water resources management, water quality studies, and assessments of the impacts of climate change. In this study, we identified hydrological processes in different seasons based on the chemical and isotopic compositions of the Urumqi River. Stable isotope (18O and D) ratios and major ion concentrations were measured on a monthly basis at six sites along the Urumqi River from December 2011 to October 2012. Most waters (groundwater, precipitation, glacier, meltwater, and river water) of the Urumqi River basin are the Ca–HCO3 water type. In the slow flow period, the ionic composition of the river water reflects weathering and dissolution of dolomite and gypsum. Water–rock interactions control the hydrochemical processes during the slow flow period, while the hydrochemical characteristics of river water in the quick flow period are controlled by precipitation, glacier water, and groundwater. The seasonal variation in river δ18O and δD values at different sites was similar to that of water entering the headwaters of the Urumqi River basin. The isotopic composition of precipitation was evaluated to obtain information on the effects of subcloud evaporation and moisture recycling on the formation and isotopic composition of precipitation in arid climatic conditions. We identified five different periods in streamflow using water chemistry and isotopic composition.

PV water pumping for carbon sequestration in dry land agriculture

This paper suggests a novel model for analysing carbon sequestration activities in dry land agriculture considering the water-food-energy-climate nexus. The paper is based on our on-going studies on photovoltaic water pumping (PVWP) systems for irrigation of grasslands in China. Two carbon sequestration projects are analysed in terms of their water productivity and carbon sequestration potential. It is concluded that the economic water productivity, i.e. how much water that is needed to produce an amount of grass, of grassland restoration is low and that there is a need to include several of the other co-benefits to justify the use of water for climate change mitigation. The co-benefits are illustrated in a nexus model including (1) climate change mitigation, (2) water availability, (3) downstream water impact, (4) energy security, (5) food security and (6) moisture recycling. We argue for a broad approach when analysing water for carbon sequestration. The model includes energy security and food security together with local and global water concerns. This makes analyses of dry land carbon sequestration activities more relevant and accurate. Without the nexus approach, the co-benefits of grassland restoration tend to be diminished.

On the importance of cascading moisture recycling in South America

Abstract. Continental moisture recycling is a crucial process of the South American climate system. In particular, evapotranspiration from the Amazon basin contributes substantially to precipitation regionally as well as over other remote regions such as the La Plata basin. Here we present an in-depth analysis of South American moisture recycling mechanisms. In particular, we quantify the importance of cascading moisture recycling (CMR), which describes moisture transport between two locations on the continent that involves re-evaporation cycles along the way. Using an Eulerian atmospheric moisture tracking model forced by a combination of several historical climate data sets, we were able to construct a complex network of moisture recycling for South America. Our results show that CMR contributes about 9–10% to the total precipitation over South America and 17–18% over the La Plata basin. CMR increases the fraction of total precipitation over the La Plata basin that originates from the Amazon basin from 18–23 to 24–29% during the wet season. We also show that the south-western part of the Amazon basin is not only a direct source of rainfall over the La Plata basin, but also a key intermediary region that distributes moisture originating from the entire Amazon basin towards the La Plata basin during the wet season. Our results suggest that land use change in this region might have a stronger impact on downwind rainfall than previously thought. Using complex network analysis techniques, we find the eastern side of the sub-tropical Andes to be a key region where CMR pathways are channeled. This study offers a better understanding of the interactions between the vegetation and the atmosphere on the water cycle, which is needed in a context of land use and climate change in South America.

Contrasting roles of interception and transpiration in the hydrological cycle – Part 2: Moisture recycling

Abstract. 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.