Abstract 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.

Citation: Keys PW, Wang-Erlandsson L, Gordon LJ (2016) Revealing Invisible Water: Moisture Recycling as an Ecosystem Service. PLoS ONE 11(3): e0151993. https://doi.org/10.1371/journal.pone.0151993 Editor: Gary Stuart Bilotta, University of Brighton, UNITED KINGDOM Received: January 20, 2016; Accepted: March 7, 2016; Published: March 21, 2016 Copyright: © 2016 Keys et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: All relevant data are available from the TU Delft data repository (DOI: 10.4121/uuid:ed07abf7-8593-4c11-9fbd-6b27634530ef). Funding: The research for this work was supported by The Swedish Research Council Formas, under grant number 1364115. Auxiliary support was provided by Scott Denning and the BioCycle Group at Colorado State University. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

Introduction An ecosystem service is a benefit derived by society that can be traced back to an ecological process [1], and water is often an important part of ecosystem service assessments [2]. Surface waters such as lakes, rivers and wetlands are regularly included in ecosystem service inventories, as providing fish habitat, flood mitigation, and maintaining water quality [3, 4]. However, evaporation from ecosystems, and the subsequent water vapor that flows outward from those ecosystems, has been largely ignored by the ecosystem services community. In most instances where evaporation has been considered, it has been perceived as neutral or detrimental to the generation of ecosystem services, particularly with regard to agricultural production where evaporation is often referred to as a net water loss [5]. However, water that leaves the earth’s surface as evaporation does not disappear, but rather flows through the atmosphere as water vapor, and eventually falls out as precipitation, such as rain or snow [6]. In other words, moisture is recycled from the point of evaporation, through the atmosphere, to downwind locations where it becomes precipitation. Global model simulations have revealed that on average 40% of global annual rainfall comes from upwind, land evaporation, but this number varies significantly, and can be much higher in certain places, e.g. China [7]. Many places that receive moisture from ecosystems upwind are economically dependent on rainfed activities, implying the importance of the regulation of moisture by upwind ecosystems to downwind societies [8, 9]. Importantly, land-use change is altering many of those upwind ecosystems, with significant effects on evaporation [10, 11]. These changes in evaporation in one location can significantly alter the precipitation that eventually falls downwind [12–14]. Furthermore, there is very low spatial variability between the upwind sources and downwind sinks of moisture recycling [15]. As a result, there are important trade-offs that might emerge when considering land-use decisions in one location, and how those decisions impact people and places in another location. In order to handle environmental decisions that require evaluating tradeoffs between multiple options of land-use change, a growing community of scientists and policy-makers employ ecosystem service frameworks [16, 17]. However, since vegetation-regulated moisture recycling has not been integrated into ecosystem service frameworks, it is not included in these evaluations of land-use change trade-offs. Here, we argue that Vegetation-regulated Moisture Recycling (VMR) is a critical ecosystem service that must be quantified and evaluated for its relative importance around the planet. We define VMR as the evaporated water that returns as precipitation downwind that is attributable to vegetation on land. VMR can thus be estimated as the amount of water that is regulated by current (i.e. present-day) vegetation relative to desert vegetation, in terms of both (a) the vegetation-regulated evaporation that enters the atmosphere, and (b) the fraction of precipitation falling downwind that can be attributed to upwind vegetation. We present a method for quantifying where VMR provides a significant ecosystem service. We use this method in an idealized comparison of a global simulation of current vegetation with a global simulation where all terrestrial surfaces are converted to desert vegetation. These global simulations involve both a land-surface hydrology model that simulates evaporation [18], and an atmospheric moisture budget model that tracks where moisture enters the atmosphere as evaporation, where it flows around the planet, and where it eventually falls out as precipitation [19]. Our results depict the importance of VMR in terms of the generation of the ecosystem service (i.e. VMR sources), and in terms of the potential beneficiaries of the ecosystem service (i.e. VMR sinks). Furthermore, we explore how our method could be used in a more practical case study, and synthesize our findings in a generalized framework. The novelty of our work is not in the moisture recycling analysis, nor in the idealized global simulation of current vegetation versus desert vegetation. Rather, the novelty is (a) our integration of moisture recycling and VMR into a quantified ecosystem services approach, and (b) the generalization of these findings into a framework that can be broadly applied using different modeling setups or different sources of data.

Materials and Methods We use a coupled land surface and atmospheric moisture budget model in this research. We employ the Simple Terrestrial Evaporation to Atmosphere Model (hereafter, STEAM) which is a land-surface hydrology model that is specifically designed to realistically partition evaporation fluxes to the atmosphere. STEAM has been described in detail in previous work so we provide only a brief description below, and direct the reader towards the full model documentation [18]. The Water Accounting Model 2layers (hereafter, WAM-2layers) is an atmospheric moisture budget model that tracks water from its evaporative origin on the planet, through the atmosphere, and to its fate as downwind precipitation. The Water Accounting Model (both 1 and 2 layer versions) has been thoroughly described and applied in previous work [7, 8, 15, 20–23], so we provide a summary description below. Finally, the coupling procedure that we employ in our research has been previously explained, so we provide only a brief overview below [19]. STEAM STEAM uses several meteorological and soil variables as inputs to estimate evaporation from five different stocks of accumulated water on the land surface. The driving datasets for STEAM include meteorological data from the ERA-Interim data archive, including evaporation, precipitation, snowfall, snowmelt, temperature, dew-point, wind speed, and short- and long- wave radiation [24]. The data span the time period 1997 to 2014, and were downloaded at the 3 hourly timestep, at a resolution of 1.5° × 1.5° grid resolution. The land surface data come from the Moderate Resolution Imaging Spetroradiometer dataset (MODIS), and are based on the International Geosphere Biosphere Programme land-use classes [25]. The soil data is based on the Harmonized World Soil Database. The evaporation partitioning that STEAM performs is divided into five key steps. First, the vegetation canopy intercepts rainfall, and accumulates and evaporates water. Second, as water drips off the canopy, it is intercepted by the soil surface and evaporates. Third, as water saturates the soil surface, it can be partitioned as moisture in the soil column or taken up by plant roots and transpired. Fourth, excess water can pool as open water bodies and evaporate. Fifth, water can freeze and accumulate until it is melted, and eventually evaporates. The evaporation values that are generated by STEAM have been compared against several existing products, in whole and in part. For example, the total evaporation values (sum of all partitioned fluxes) are within the interquartile range of the global, multi-model synthesis product LandFlux-EVAL. A rigorous and detailed evaluation of STEAM, relative to other evaporation estimates, found that it performs well in estimating total evaporation, individual partitioned fluxes (e.g. vegetation interception), and across different vegetation types [18]. WAM-2layers Given that we want to know where land evaporation later falls as precipitation on land downwind, we must be able to track moisture around the planet. There are many moisture budget or tracking approaches [26–28], and we employ the Water Accounting Model, 2layers (hereafter, WAM-2layers) to perform our global moisture tracking analysis [19]. The WAM-2layers tracks the flow of moisture around the planet using an Eulerian approach. Imagine a column of air above a given location. At each time step, the WAM-2layers keeps track of how much water enters the atmosphere as evaporation, as well as how much water exits the atmosphere as precipitation. In between each time-step, the WAM-2layers calculates how much water moves between each grid cell in each of the four cardinal directions. Thus, as we step forward in time the WAM-2layers provides an accounting of how much moisture enters and exits the atmosphere, as well as where it travels horizontally in the atmosphere, for all locations globally. As the name suggests, the WAM-2layers has two atmospheric layers, meaning there is a layer of the atmosphere close to the surface of the earth, as well a layer higher in the atmosphere. The purpose of this is to capture the varying wind speeds at different altitudes in the atmosphere, i.e. wind shear. The WAM-2layers requires six variables as input for its calculation. In this paper, we use the ERA-Interim data, which was downloaded at the 1.5° × 1.5° grid resolution [24]. These data include: 6-hourly winds (zonal and meridional) and relative humidity; 6-hourly surface pressure; and 3-hourly precipitation and evaporation. The model is run at the 15-minute time step to eliminate numerical errors, and thus the data are discretized to the 15-minute time step, using linear interpolation. The data span the period January 1997 to December 2014. The Water Accounting Model (one and two layer versions) has been used extensively [7, 8, 20–23], including in a favorable comparison with an RCM [21]. Likewise, WAM-2layers has also been used with other driving data (the Modern Era Retrospective Analysis for Research, MERRA) to explore its sensitivity to multiple data sources, as well as to understand moisture recycling variability [15]. Model coupling Coupling STEAM and WAM-2layers requires two key steps. First, STEAM’s simulated evaporation is substituted for the ERA-Interim evaporation in WAM-2layers. Second the changes in moisture input (i.e. evaporation) are propagated and tracked using WAM-2layers, resulting in modified precipitation [19]. Also, given that land-use change will change evaporation, and subsequently the moisture available for precipitation, we must run the STEAM and WAM-2layers for several iterations so that the new moisture budget converges. We define convergence as achieving a less than 1% difference in annual precipitation for every grid-cell between consecutive model runs. Hereafter, we refer to the coupling between STEAM and WAM-2layers as STEAM+WAM. Using STEAM+WAM we run two land-use scenarios: current vegetation (based on MODIS land-cover) and desert vegetation (by changing all land-surfaces globally to the ‘barren’ land class in STEAM, which includes sparse, desert vegetation). In the desert vegetation scenario, all land parameters are changed (e.g. minimum and maximum leaf area index, root depth, albedo, etc.). As a result, all aspects of the evaporation partitioning regime will be changed, including all five partitions (canopy interception, ground interception, soil moisture evaporation, transpiration, and open water evaporation). We run both scenarios for 18 years, using the first three years as “model spin-up”, leaving 15 years for analysis (2000–2014). In this way, we are able to isolate the impact of changes to evaporation on global moisture recycling patterns. Note on the scope of coupling procedure In this paper, we only study the land-use change effects on rainfall through moisture recycling. However, there are also other more complex interactions between land and atmosphere at play, e.g. land-use change effects on the atmosphere’s thermal structure, changes to atmospheric circulation, and interactions with monsoon systems. While reductions in precipitation always follow from reductions in evaporation in moisture recycling, these reductions in precipitation can also follow from increases in evaporation that then drive other types of land-atmosphere interactions (e.g. irrigation changing land-ocean temperature gradients, and altering monsoon onset [29, 30]). These interactions act all at once, but the dominating mechanism depends on the spatial scale of change as well as on the region of change. Studies have shown that local scale (100–1000 km) perturbations are important for the thermal structure, while moisture recycling operates at the regional scale (larger than 1000 km), and atmospheric circulation is modified by changes at the regional to global scale [20, 31, 32]. Obviously, the desert vegetation scenario we simulated is not, nor aims to be, realistic. Rather, the purpose of it is to provide a theoretical baseline for the vegetation-regulated moisture recycling calculation. Finally, the effects of land use change on precipitation are difficult to simulate with fully coupled climate models, due to noise and model uncertainty [33]. Although simplified, the STEAM+WAM coupled simulation isolates the role of changing land-use on changes to evaporation, with subsequent changes to precipitation. Since the circulation, ocean evaporation, and climate aspects are kept identical in the scenarios, we can attribute any changes we see in atmospheric moisture content, or eventual downwind precipitation, to changes in evaporation. Calculation of VMR ecosystem services A key contribution of this work is the method we introduce to quantify vegetation-regulated moisture recycling (VMR) ecosystem services. Given that the service is generated in one place (i.e. sources of VMR) and is potentially realized as a benefit in another place (i.e. sinks of VMR), we provide a detailed overview of our calculation methods. Sources of VMR. First, we identify the evaporation recycling ratio by identifying the fraction of evaporation from a given grid-cell that returns as rainfall downwind; note that we are using the same approach used in previous studies [7, 8]. The evaporation recycling ratio is defined for each grid cell as, (1) where, E c,track is the evaporation from current vegetation that returns to land as precipitation downwind, E c is the total evaporation from current vegetation, and ε c is the terrestrial evaporation recycling ratio for current vegetation. Second, we are able to identify the extent to which vegetation regulates evaporation flows, by subtracting the tracked evaporation from the desert vegetation scenario from the tracked evaporation from the current vegetation scenario, and dividing this by the total evaporation, from current vegetation. This generates a ratio we call vegetation regulation, where for each grid cell, (2) where, E current is the amount of evaporation that is recycled downwind under current vegetation, E desert is the amount of evaporation that is recycled downwind under desert vegetation, and V E is the fraction of evaporation that is vegetation-regulated. To better understand what this ecosystem service means in a tangible sense we can combine the previous information to generate a global distribution of vegetation-regulated evaporation recycling services. Formally, (3) where for each grid cell, E track,current is the amount of evaporation that is recycled downwind under current vegetation, E track,desert is the amount of evaporation that is recycled downwind under desert vegetation, E current is the total evaporation from current vegetation, and VMR E is the ecosystem service associated with evaporation that is both vegetation-regulated and falls as precipitation downwind, as a fraction of total evaporation. Sinks of VMR. Now, we identify the precipitation-recycling ratio by identifying the fraction of precipitation falling in a given grid-cell that originated as evaporation upwind; note that we are using the same approach used in previous studies [7, 8]. The precipitation-recycling ratio is defined for each grid cell as, (4) where, P c,track is the precipitation that returns to land as precipitation downwind, P c is the total precipitation, and ρ c is the terrestrial precipitation recycling ratio. Next, we identify the extent to which vegetation regulates downwind precipitation by subtracting the tracked precipitation from the desert vegetation scenario from the tracked precipitation from the current vegetation scenario, and dividing this by the total precipitation associated with current vegetation. This generates a ratio we call vegetation-regulated precipitation, where for each grid cell, (5) where, P track,current is the amount of precipitation that is generated as upwind land evaporation under current vegetation, P track,desert is the amount of precipitation that is generated as upwind land evaporation under desert vegetation, P current is the total precipitation, and VMR P is the ecosystem service associated with precipitation that is both vegetation-regulated and originates as evaporation on land upwind, as a fraction of total precipitation.

Discussion We have, for the first time, described Vegetation-regulated Moisture Recycling (VMR) as an ecosystem service. We have also demonstrated its applicability at global and regional scales and generalized these findings to a broadly applicable framework. We now explore the implications of our findings, particularly in the context of broader land-use, hydrological, and governance concerns. Overall VMR Patterns At the global scale, the lower importance of VMR in the tropics controverts much popular wisdom about the role of tropical vegetation for sustaining terrestrial moisture recycling [41, 42]. The temperate regions of North and South America, and temperate Eurasia experience the highest rates of VMR (Figs 1c and 2b). Additionally, it is important to consider that our global analysis (Figs 1 and 2) is based on annual average characteristics, so some interannual variability is masked. This is clearly visible in the Mato Grosso case study, where VMR plays a particularly important role during the dry season (Fig 3d). The pronounced gap between the blue line (current vegetation) and red line (desert vegetation) during the dry season (May to September) indicate the importance of Mato Grosso’s current vegetation for sustaining dry-season rainfall. This dry-season supply of moisture further underscores the importance of evaporation processes unique to vegetation, such as transpiration [18]. Land-use change affecting VMR sources The regions with particularly high rates of VMR source evaporation (Fig 1c) emphasize the importance of current vegetation in sustaining downwind precipitation, and that significant land-use change could adversely affect VMR ecosystem services. The intense source areas in Europe are not likely to experience dramatic land-use change, given the maturity of those economies [43], although agricultural land abandonment is an increasingly important phenomena [44]. Large agricultural expansions are similarly unlikely in North America and western Russia, yet both of these regions could potentially experience evaporation reductions given persistent and widespread forest fires that interrupt forest succession, and force ecosystems to transition into lower-evaporation regimes [45, 46]. Likewise, pervasive land degradation (e.g. soil salinization, topsoil erosion) and land-use change in east Africa and south Asia could potentially reduce VMR evaporation [47]. Our case study reveals the importance of Amazonia (particularly Mato Grosso), a region often discussed in relation to moisture recycling [23, 48] for sustaining potential VMR benefits in parts of South America. Though the regional benefits are diffuse, some regions receive between 2 and 6% of total rainfall from this area. Likewise, the temporal dimension (see Fig 3d) highlights not only the overall precipitation delivered by VMR, but also the persistence of moisture delivery into the dry, winter/spring season (May-September). This difference in dry season moisture supply is especially important for fragile systems that are already stretched to the brink, such as the Sistema Cantareira, which brings drinking water to the Sao Paulo metropolitan area [49]. Water scarcity in VMR beneficiary regions Regions with high VMR benefits (Fig 2c) coincide with many areas under current, and projected, water scarcity [22, 50]. Much of sub-Saharan Africa receives between 10–30% of precipitation from VMR, with some areas as high as 40% or more. This dependence on upwind VMR is striking given sub-Saharan Africa’s continued reliance on subsistence, rainfed agriculture. As populations grow and available land shrinks, the vulnerability of sub-Saharan Africa to any reduction in VMR ecosystem services increases substantially [51]. At the other end of the rural to urban gradient, many global megacities receive drinking water from regions that rely 40% or more on VMR [52]. To use our case study region as an example, there are three megacities within the evaporationshed (Buenos Aires, Sao Paulo, and Rio de Janeiro), of which the latter two are currently experiencing an unprecedented water crisis. Despite a VMR dependence of between 20 and 30% (for Sao Paulo and Rio de Janeiro, respectively; Fig 2b), even small changes in precipitation arising from upwind land-use change could have big impacts to the fragility of urban water supplies. Governance challenges for VMR ecosystem services The diffuse nature of VMR leads to potential difficulty in relating biophysical service generation in one place with specific, tangible, and traceable benefits downwind. Nonetheless, existing literature related to diffuse ecosystem services, such as pollination, air pollution, and migratory animal species, could provide useful analogs for policy intervention [53, 54]. In particular, spatial information can facilitate inter-comparison among overlapping ecosystem services, and the trade-offs that arise from pursing different management strategies [55]. Our analysis provides both spatial and temporal information that can be used to compare VMR benefits with other ecosystem services with overlapping spatial or temporal attributes. The governance of terrestrial moisture recycling services is also likely to be challenging, due to the diffuse and spatially extensive nature of the service. Yet, there are multiple existing methods of governance for transboundary phenomena such as trade pacts, resource treaties, and collective ecological priorities [56, 57].

Conclusions Vegetation-regulated Moisture Recycling (VMR) is an ecosystem service. We have demonstrated a method for quantifying this at global and regional scales, where vegetation regulates the atmospheric branch of the water cycle, specifically evaporation and precipitation. We have also provided a broadly applicable framework for incorporating VMR into existing ecosystem services assessments. This is very important given that the use of ecosystem services has become a cornerstone of natural resource management and sustainability efforts. Framing the regulation of atmospheric moisture as an ecosystem service that is both influenced by humans and has the potential to impact downwind human societies, can improve future analyses of ecosystem services related to hydrological change, large-scale land-use change, and land-atmosphere tele-connections.

Acknowledgments The research for this was supported by The Swedish Research Council Formas, under grant number 1364115. Auxiliary support was provided by Scott Denning and the BioCycle Group at Colorado State University. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Author Contributions Conceived and designed the experiments: PK. Performed the experiments: PK. Analyzed the data: PK. Contributed reagents/materials/analysis tools: PK LW. Wrote the paper: PK LW LG.