The Precipitationshed – Methods, Concepts, and Applications

Patrick W. Keys

The PrecipitationshedMethods, Concepts, and Applications

Patrick W. Keys

c�Patrick W. Keys, Stockholm 2016

ISBN 978-91-7649-464-6

Printed in Sweden by Holmborgs, Malmö, Stockholm 2016Distributor: Stockholm Resilience Centre, Stockholm University

Papers I, II, and III and corresponding figures are reprinted here with permission from thepublishers.

Cover Image: The Thunderstorm, c�Tez Goodyear (Creative Commons, CC BY-ND 2.0)

Images preceding Papers I, III, IV, and V: c�Patrick W. Keys 2016, All Rights Reserved.

Image preceding Paper II: c�Brokentaco (Creative Commons, CC BY 2.0)

Abstract

Human societies are reliant on the functioning of the hydrologic cycle. Theatmospheric branch of this cycle, often referred to as moisture recycling inthe context of land-to-land exchange, refers to water evaporating, travelingthrough the atmosphere, and falling out as precipitation. Similar to the surfacewater cycle that uses the watershed as the unit of analysis, it is also possibleto consider a ‘watershed of the sky’ for the atmospheric water cycle. Thus, Iexplore the precipitationshed - defined as the upwind surface of the Earth thatprovides evaporation that later falls as precipitation in a specific place. The pri-mary contributions of this dissertation are to (a) introduce the precipitationshedconcept, (b) provide a quantitative basis for the study of the precipitationshed,and (c) demonstrate its use in the fields of hydrometeorology, land-use change,social-ecological systems, ecosystem services, and environmental governance.

In Paper I the concept of the precipitationshed is introduced and explored forthe first time. The quantification of precipitationshed variability is describedin Paper II, and the key finding is that the precipitationsheds for multiple re-gions are persistent in time and space. Moisture recycling is further describedas an ecosystem service in Paper III, to integrate the concept into the ex-isting language of environmental sustainability and management. That is, Iidentify regions where vegetation more strongly regulates the provision of at-mospheric water, as well as the regions that more strongly benefit from thisregulation. In Paper IV, the precipitationshed is further explored through thelens of urban reliance on moisture recycling. Using a novel method, I quan-tify the vulnerability of urban areas to social-ecological changes within theirprecipitationsheds. In Paper V, I argue that successful moisture recyclinggovernance will require flexible, trans-boundary institutions that are capableof operating within complex social-ecological systems. I conclude that, in thefuture, the precipitationshed can be a key tool in addressing the complexity ofsocial-ecological systems.

Keywords: water, atmosphere, precipitationshed, moisture recycling, variabil-ity, ecosystem services, social-ecological systems

Sammanfattning

Våra samhällen är beroende av vattnets kretslopp på planeten. Den atmosfäris-ka delen av detta kretslopp över land kallas för fuktåtercirkulering och refererartill vattnet som avdunstar från land, färdas genom atmosfären som vattenånga,och slutligen faller som nederbörd över land. Analogt med vattendelare somanalysenhet för ytvatten, är det också möjligt att tala om vattendelare för detatmosfäriska vattnet. Dessa kallas för nederbördsdelare och definieras som deuppvindsområden på jorden som utgör avdunstningskällor för en given regi-ons nederbörd. Denna avhandlings främsta bidrag är att (a) introducera kon-ceptet nederbördsdelare, (b) tillhandahålla en kvantitativ grund för studier avnederbördsdelare, och (c) visa hur begreppet kan användas inom ämnesområ-dena hydrometeorologi, markanvändningsförändringar, social-ekologiska sy-stem, ekosystemtjänster och miljöstyrning.

I Papper I introduceras och utforskas begreppet nederbördsdelare. Kvantifi-ering av nederbördsdelares variabilitet beskrivs i Papper II, där en viktig slut-sats är att fuktkällorna för en given plats är beständiga i tid och rum. Papper

III beskriver fuktåtercirkulering som en ekosystemtjänst, i syfte att förankrakonceptet inom miljömässig hållbarhet och miljöstyrning. Papper III kartläg-ger även de områden i världen där vegetationens reglering av fukt i atmosfärenär extra betydelsefull, samt de regioner som drar mest nytta av vegetationensförmåga att reglera avdunstning. Papper IV undersöker nederbördsdelare urett urbant vattenförsörjningsperspektiv. Med hjälp av en ny metod kvantifie-rar jag olika stadsområdens sårbarhet för social-ekologiska förändringar inomderas nederbördsdelare. I Papper V argumenterar jag för att en framgångsrikstyrning av fuktåtercirkulering kommer att kräva flexibla, gränsöverskridan-de institutioner som är i stånd att arbeta inom ett komplext social-ekologisktsystem. Jag drar slutsatsen att nederbördsdelare i framtiden kan bli ett viktigtverktyg för att hantera social-ekologiska systems komplexitet.

Nyckelord: vatten, atmosfär, nederbördsdelare, fuktåtercirkulering, variabili-tet, ekosystemtjänst, social-ekologiska system

This thesis is dedicated to my family.

List of Papers

Papers included in this thesis

The following papers and manuscripts, referred to in the text by their Romannumerals, are included at the end of this thesis.

PAPER I: Analyzing precipitationsheds to understand the vulnerabil-

ity of rainfall dependent regions.

Keys, P.W., van der Ent, R.J., Gordon, L.J., Hoff, H., Nikoli, R.,and Savenije, H.H.G. Biogeosciences (9), 733-746 (2012).DOI: 10.5194/bg-9-733-2012

PAPER II: Variability of moisture recycling using a precipitationshed

framework.

Keys, P.W., Barnes, E.A., van der Ent, R.J. and Gordon, L.J.Hydrology and Earth System Sciences, 18 (10) (2014).DOI: 10.5194/hess-18-3937-2014

PAPER III: Revealing Invisible Water: Moisture Recycling as an Ecosys-

tem Service.

Keys, P.W., Wang-Erlandsson, L., and Gordon, L.J. PLoS One,11 (3) (2016).DOI: 10.1371/journal.pone.0151993

PAPER IV: Megacity precipitationsheds reveal reliance on regional evap-

oration for moisture supply.

Keys, P.W., Wang-Erlandsson, L. and Gordon, L.J. Landscapeand Urban Planning (under review)

PAPER V: Approaching Moisture Recycling Governance.

Keys, P.W., Wang-Erlandsson, L., Galaz, V., Ebbesson, J., andGordon, L.J. manuscript

Relevant co-authored publications not included herein.

i Conditional Relationships between Drought and Civil War in sub-

Saharan Africa.

Bell, C.M. and Keys, P.W. Foreign Policy Analysis. (2016).DOI: 10.1093/fpa/orw002

ii Global root zone storage capacity from satellite-based evaporation.

Wang-Erlandsson, L, Bastiaanssen, W. G. M., Gao, H., Jagermeyr, J.,Senay, G.B., Gordon, L.J., Keys, P.W., van Dijk,A. I. J. M., Guerschman,J.P., and Savenije, H. H. G. Hydrology and Earth System Sciences, 20,1459-1481 (2016). DOI: 10.5194/hess-20-1459-2016

iii Contrasting roles of interception and transpiration in the hydrological

cycle - Part 2: Moisture recycling.

van der Ent, R. J., Wang-Erlandsson, L., Keys, P. W., and Savenije, H. H.G. Earth System Dynamics, 5, 471-481. (2014). DOI: 10.5194/esd-5-471-2014

iv Food Policy and Civil War in Africa’s Drought-Suffering States.

Bell, C.M., Keys, P.W., and Stossmeister, R. in: Conflict-Sensitive Adapta-tion to Climate Change in Africa, eds. Bob, Urmilla and Salome Bronkhorst.Berliner Wissenschafts-Verlag (Berlin Science Publishers), Germany (2014)

v Releasing the pressure: water resource efficiencies and gains for ecosys-

tem services.

Keys, P.W., Barron, J., and Lannerstad, M. United Nations EnvironmentProgramme, ISBN: 978-92-807-3340-5 (2012)

vi Chapter 12: Watershed management through a resilience lens.

Barron, J. and Keys, P.W. in: Integrated Watershed Management in Rain-fed Agriculture. eds. Wani, S.P., Rockström, J., and Sahrawat, K.L. CRCPress. (2011)

vii Precipitation extremes and the impacts of climate change on stormwa-

ter infrastructure in Washington State.

Rosenberg, E.A., Keys, P.W., Booth, D.B., Hartley, D., Burkey, J., Steine-mann, A.C., and Lettenmaier, D.P. Climatic Change, 102 (1-2), 319-349.(2010). DOI: 10.1007/s10584-010-9847-0

Contents

Abstract v

Sammanfattning vii

List of Papers ix

Acknowledgements xiii

1 Introduction 17

1.1 Moisture Recycling as a Socially-Relevant Geophysical Process 171.2 The Precipitationshed is a Management Entry Point . . . . . . 221.3 Summary of this work . . . . . . . . . . . . . . . . . . . . . 23

2 Methods 27

2.1 WAM-2layers . . . . . . . . . . . . . . . . . . . . . . . . . . 272.2 STEAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.3 STEAM+WAM . . . . . . . . . . . . . . . . . . . . . . . . . 29

3 Contributions 31

3.1 Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323.3 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 353.4 Trans-disciplinary Impact . . . . . . . . . . . . . . . . . . . . 36

4 Limitations and Considerations 39

5 Next Steps 41

6 Advice to Future PhD Students 43

7 Concluding Remarks 47

8 References 49

Acknowledgements

This PhD would not have been possible were it not for the trust that my ad-visor Line Gordon granted me as we embarked four years ago on what hasbeen a true test in tele-commuting. Line trusted that with periodic visits tothe Stockholm Resilience Centre, and regular guidance via Skype and email,that we could develop a successful research relationship. I’m grateful for thatopportunity, and have learned a tremendous amount along the way.

To my colleagues Ruud van der Ent and Lan Wang-Erlandsson, thank youfor being critics, advisors, and guides during my PhD. Its surprising to havefound a group of people to whom I only felt gratitude when you told me thatmy methods were incorrect, that my logic was mistaken, or that the underlyingrelevance was questionable. I could not have asked for a better group of peoplewith whom to share this journey.

To the many reviewers of this thesis, and the articles herein, I appreciate yourpatience and encouragement. I’m especially grateful for those of you whooffered feedback on papers that were perhaps out-of-bounds of your own re-search. Have comfort. They were out-of-bounds for me, too.

To my fellow PhD students, thank you for the philosophical debate, the ideal-ism, and the fun. I can’t tell you all how much your intellectual and personalconnections have meant to me, and I hope that as we spread out around theworld that we can continue to be waypoints for one another, academic or oth-erwise.

Thank you to Professor Scott Denning, and his BioCycle research group atColorado State University, who provided an enthusiastic and welcoming aca-demic home-away-from-home.

Finally, to Elizabeth, my fulcrum for the last four years, without whose supportthis PhD would not have been possible - Thank You.

1. Introduction

The global water cycle is the bloodstream of the biosphere, flowing in andthrough every living organism and ecosystem, and connecting the disparateecologies of our planet to one another. The liquid water that is visible in rivers,lakes, and oceans is a critical part of the water cycle - yet only a part. Alongwith the reservoirs of water hidden beneath the Earth, the atmospheric branchof the water cycle contains vast flows of water.

Each year, between 400,000 and 500,000 cubic kilometers of water evapo-rates from the global oceans and flows into the atmosphere (Trenberth et al.,2011). While most of this evaporation returns to oceans as rainfall, between30,000 and 45,000 cubic kilometers of water vapor flows back toward land.Furthermore, between 70,000 and 100,000 cubic kilometers of water evapo-rates from the land into the atmosphere where it combines with oceanic evapo-ration. Then, between 105,000 and 130,000 cubic kilometers of water falls asprecipitation on land, providing the water that will eventually flow off the landinto the rivers, and back to the ocean. This process of evaporation from theocean and land, traveling through the atmosphere as water vapor, and return-ing to the surface as precipitation is often referred to as moisture recycling.

1.1 Moisture Recycling as a Socially-Relevant Geophysi-cal Process

Until very recently, the study of moisture recycling was limited to the fields ofhydrology, hydrometeorology, and atmospheric science. The interest was pri-marily rooted in a traditional viewpoint of understanding the geophysical pro-cess of how much evaporation enters the atmosphere, where it flows as watervapor, and where it falls out as precipitation. Along the way, some researchersbegan referring to moisture recycling as a local process, where evaporation isreturned immediately to the same local area as precipitation (e.g. Lettau et al.1979). Others referred to moisture recycling as a tele-connected process, thatbrought evaporation from one location to another, distant location as precipita-

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landocean ocean

evapora,

on

evapora,

on

evapora,

on

precipita

,on

precipita

,on

precipita

,on

watervaportransport watervaportransport watervaportransport

precipita

,on

evapora,

on

Schematic of Atmospheric Branch of Water Cycle

Figure 1. Two-dimensional schematic of the atmospheric branch of the watercycle, emphasizing terrestrial moisture recycling relationships. Within the or-ange dashed box, precipitation that is primarily of oceanic origin falls onto land,and then the water that evaporates is from the land surface. This water is trans-ported through the atmosphere, and falls out as precipitation again, and is thenre-evaporated on land again.

tion (e.g. Koster et al. 1986). And still other researchers include both of theseperspectives in their definition of moisture recycling (e.g. van der Ent et al.2010). I use this last more inclusive definition throughout this dissertation.

Theories of land-atmosphere interactions, particularly the consequences ofland-use change on precipitation, have been around for centuries. The pop-ular 19th century notion that the ‘rain-follows-the-plow’ developed separatelyamong both North American and Australian farmers (Reisner 1993). Clima-tologists of that time believed the mechanism of land-use change driving moreprecipitation involved, among other things, the stirring up of soil moisturewhich increased local rainfall. Though these are intriguing ideas, this theoryis now understood as mostly wrong, especially since modern climatologicalreconstructions show the Midwestern US was experiencing a wetter than nor-mal period contemporary with the ‘rain-follows-the-plow’ theory. Nonethe-less, land-use change does have very real consequences for the atmosphericbranch of the water cycle, and the ‘rain-follows-the-plow’ theory highlightedan early effort to link land-use change to the atmospheric water cycle, and tofurther social impacts.

In the 1970’s, a theoretical explanation for land-use change impacting pre-cipitation was proposed, particularly in the context of desertification. Charneyet al. (1973) and Otterman (1974) both suggested that as vegetation is removed

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from the land-surface, and brighter soils are exposed to the sky, that this bareground would reflect more energy (rather than absorb it). As a result of thisnet reduction in surface energy, there would be less energy emitted to the at-mosphere to induce convective cloud formation and subsequent precipitation.This theory, which was also empirically demonstrated, remains one of the cor-nerstones of land-atmosphere interactions. Recent work has highlighted therole that oceans also play in governing surface drying (or wetting), especiallyin the Sahel (e.g. Giannini et al., 2003; Giannini, 2016). The early work ledby Charney and Otterman complements this more recent work, by identify-ing local and proximate land-atmosphere interactions, and how they fit withthe more distant and ultimate causes of desertification. Given that the mech-anisms that drive land-atmosphere interactions appear to operate at differentspatial and temporal scales, it may be revealing to compare how atmospheric,hydrologic, and land surface processes overlap in space and time (Figure 2).

Glo

bal

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00 k

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Mes

osca

le

100

km

Loca

l 10

km

Re

gion

al

1,00

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Spat

ial s

cale

Hour Day Season Year Decade

Timescale

Comparing Moisture Recycling to Other Atmospheric and Surface Processes

Climate Change El Niño

and La Niña

Ground Water

Meteorological

Hydrological

Local

Moisture Recycling

Large scale

LEGEND

Small River Basins

Large River Basins

Atmospheric Transport

e.g. Aerosols, CO2

Glaciers

Land Surface

Irrigation or Land-Use Change

SST Variability

Soil Moisture and Vegetation Variability

Mesoscale Convective

systems

Convective Cells

Figure 2. Comparing moisture recycling (orange circle) with the spatial andtemporal scales of meteorological, hydrological and surface processes. This isadapted from Tuinenburg, 2013.

Though there is limited overlap between ‘irrigation and land-use change’ (right-most green circle) and ‘moisture recycling’ (dashed, orange circle), they arestrongly connected via ‘soil moisture and vegetation variability’ (left-mostgreen circle). In other words, land-use changes are likely to occur on time-scales that are mostly disconnected from moisture recycling, but the conse-

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quences of land-use change for moisture recycling manifest at the seasonaland annual scale.

There are several mechanisms by which land-use change can modify mois-ture recycling. Using ‘deforestation for the purpose of cropland expansion’ asan example of land-use change, there are many possible interactions that canlead to changes in rainfall. At the local scale, the interaction between the newcropland and the new forest edge can induce a significant temperature gradientthat can induce moist static instabilities (e.g. Lawrence and Vandecar, 2015),which can lead to local increases in precipitation. These energy gradient ef-fects appear to diminish as spatial and temporal scales increase, with corre-sponding increases in the importance of regional moisture budgets, includingthe magnitude and timing of evaporation to the atmosphere (i.e. moisture re-cycling). Finally, as spatial and temporal scales increase further, regional andglobal drivers start to dominate, such as sea-surface temperatures, interannualclimate oscillations (e.g. El Niño Southern Oscillation), and perhaps even cli-mate change. In my work, I focus primarily on the regional and sub-regionalscale, where moisture recycling effects are most prominent (e.g. van der Entet al., 2013).

In recent decades, the ability to represent the processes of moisture recyclingin computer simulations has improved, and consequently advanced our under-standing tremendously. Some of the early modeling work that explicitly soughtto relate moisture recycling studies with socially-relevant implications werethe studies that aimed to understand the role of moisture recycling and regionalflooding, particularly in the USA (Dirmeyer and Brubaker, 1999; Dominguezet al. 2006). This type of research was expanded to large river basins globally(Bosilovich et al., 2006). These, and other studies, primarily sought to under-stand how the large-scale atmosphere influenced specific human societies viamoisture recycling. Conversely, early empirical work examining the impactthat human societies have on moisture recycling focused on deforestation ofthe Amazon as a potential cause of changes in downwind precipitation (e.g.Salati et al., 1971).

Several studies have explicitly examined the consequences of anthropogenicland-use change on moisture recycling processes, and how these altered pro-cesses further affect precipitation (see Table 1). All these studies use modelsimulations, and can be categorized according to those representing actualchanges, i.e. land-use changes that have already happened, such as irriga-tion of the Central Valley of California, and theoretical changes, i.e. poten-tial changes such as extensive Amazonian deforestation. Likewise, the stud-

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ies generally explore either irrigation scenarios (generally leading to increasedevaporation), or de-vegetation scenarios (generally leading to decreased evap-oration). As a note, I use the word ‘evaporation’ to refer to all evaporativefluxes, including vegetation interception, transpiration, soil interception, soilevaporation, and open water evaporation. My use of evaporation ought to beunderstood as referring to what others refer to as evapotranspiration (Savenije2004).

Table 1. Summary of literature values for land-use changes and the associatedimpact to downwind precipitation. This table is adapted from Paper IV.

!

AUTHOR

Salih&et&al.&(2013)

Lo&and&Famiglietti&(2013)

Tuinenburg&et&al.&(2013)

Spracklen&et&al.&(2015)

Swann&et&al.&(2015)

Badger&and&Dirmeyer&(2015)

Halder&et&al.&(2015)

Keys&et&al.&(2016)

Wei&et&al.&(2013)

Bagley&et&al.&(2012)&

REGION LAND-USE- TYPE-OFCHANGE-(LUC) LUC absolute %

East&Asia Theoretical K&K&K& K11.89%Central&Asia K&K&K& K16.70%

North&America K&K&K& K8.34%South&America K&K&K& K16.90%West&Africa K&K&K& K9.64%Sudan&(Sahel) Deforestation,&replaced&

by&grass&or&desertTheoretical grass:&K1mm/day&to&+0.5&mm/day

desert:&K2.1&mm/day&(or&more)&to&+0.5&mm/day

grass:&K25%&to&+5%desert:&K52.5%&to&+5%

California Irrigation&replacing&grassland&

Observed from:&+2mm/month&to&12mm/month

about&+15%

India,&Ganges&plain

Irrigation Observed from:&K200&mm/yr&in&Eastern&India&to&+200&in&W.&India,&N.&India,&and&

Pakistan

from:&K15%&in&Eastern&India&to&+15&to30%&in&W.&India,&N.&

India,&and&PakistanIndia Observed from&120mm/yr&to&10mm/yr from&+22%&to&+2%

China from&28&mm/yr&to&2&mm/yr from&+5%&to&+0.4%

US from&5&mm/yr&to&0.4&mm/yr from&+1.1%&to&+0.1&%

Sahel from&4.5&mm/yr&to&0.4&mm/yr from&+3%&to&+0.2%

Amazon Deforestation&(replaced&by&variety&of&landKuses&

depending&on&simulation)

Observed&and&Theoretical

K&K&K& from:&K0%&(0%&deforestation)&to&~20%&(with&100%&

deforestation)

Amazon Deforestation&(replaced&by&cropland)

Theoretical from&+1mm/day&Kto&K3mm/day&(+365mm/yr&to&K900mm/yr)but,&most&changes&are&0

from&+17%&to&K25%&(Howver,&most&changes&=&0%)

Amazon Deforestation&(replaced&by&heterogenous&

cropland)

Theoretical from&K8mm/day&to&K2mm/day&in&NW&Amazon&(K1460&mm/yr&

to&K365&mm/yr);&from&K1&mm/day&to&+1&mm/day&in&southern&and&eastern&Amazon&(K365&mm/yr&to&+365mm/yr)

K&K&K

South&Asia Deforestation&(replaced&by&cropland)

Observed from&K16&mm/yr&to&+16mm/yr K&K&K

Amazon&(Mato&Grosso)

Deforestation,&replaced&by&desert

Theoretical from:&K80mm/yr&to&K10mm/yr from:&K6%&to&K1%

Irrigation&replacing&variety&of&different&

landKuses

CHANGE-IN-PRECIPITATION

Difference&between&&natural&vegetation&and&

bare&soil

The wide range in impacts that land-use change can have on rainfall are visiblein Table 1. Though its generally true that increased transpiration (via irriga-tion) leads to more precipitation downwind, there are exceptions, as with thefindings of Tuinenburg et al. (2013), where increased irrigation led to lessprecipitation downwind due to changes in atmospheric circulation. Likewise,though its generally true that decreased vegetation (e.g. via deforestation)leads to less precipitation downwind, the range of this decrease can be verylarge, even for the same study area (see the difference between Badger and

21

Dirmeyer (2015) and Swann et al. (2015) in the Amazon). The key messagehere is that though land-use change appears to have a significant impact ondownwind precipitation, the magnitude (and sometimes the sign) of this im-pact is less certain.

Research that examines the role of land-use change in modifying moisture re-cycling patterns implicitly has social relevance. However, until recently thefield of moisture recycling research lacked a specific spatial heuristic withwhich to structure the discussion of ‘regions that provide moisture (e.g. evap-oration sources) and regions that receive moisture (e.g. precipitation sinks)’.The precipitationshed is a response to this gap.

1.2 The Precipitationshed is a Management Entry Point

As far as people are concerned, the core insight of moisture recycling researchto date is that land-use change in one place may affect precipitation in another.One reason this is so central to social systems is that humanity as a whole isoverwhelmingly reliant on rainfall for its food production. 80% of global foodproduction is produced in rainfed, rather than irrigated, agricultural systems(Molden, 2007). Much of this food production is reliant on rainfall that arisesas evaporation over other land surfaces, so-called terrestrial moisture recycling(e.g. Bagley et al., 2012; Paper I). Furthermore, this rainfed crop productiondoes not include the rainfed woodlands, rangelands, and myriad other uses ofrainfall for human well-being.

As a result of humanity’s reliance on terrestrial moisture recycling, it would beuseful to have a tool that (a) can assist in the identification of areas that providerainfall to a given location, and (b) is flexible enough to incorporate multipledatasets, models, and spatial and temporal scales. The precipitationshed is thistool. The precipitationshed is defined as the upwind surface (both water andland) that contributes evaporation to a given location’s precipitation (Figure 2;also see Paper I). As depicted, evaporation rises up from the surface (thin,wavy arrows pointing up), flows through the atmosphere (large arrows), andfalls as precipitation downwind (stippled arrows pointing down). The parallelconcept to the precipitationshed is the evaporationshed, which is essentially thereverse of the precipitationshed concept: the area that receives water as precip-itation from a given location’s evaporation (see Paper III). The reason that aunit of analysis like the precipitationshed is necessary for engaging the socialdimensions of moisture recycling is that there needs to be a way to identifythe specific social and ecological systems that provide, or receive, moisture.

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Relevant social systems can vary, ranging from small (e.g. agricultural com-munities) to large (e.g. large countries). Ecological systems can also vary,ranging from small (e.g. a small forest patch) to large (the Amazon basin).However, given that humanity is currently modifying more than 50% of theEarth’s land surface (Ellis et al., 2010), it is important to recognize that thesocial and ecological systems on which I focus are tightly coupled, denoted associal-ecological systems, with both fast and slow feedbacks generating unex-pected behavior. Importantly, these social-ecological systems are also complexadaptive systems, which makes it all the more important to recognize the in-herent potential for nonlinear and surprising behavior in the regions that areexperiencing change (Folke et al., 2005).

1.3 Summary of this work

In this dissertation, I investigate: (a) the concept of the precipitationshed, (b)how robust the patterns are in space and time, (c) how the concept may be

Figure 3. Conceptual diagram of a precipitationshed. Originally published inPaper I in Biogeosciences, and reproduced here under the Creative CommonsAttribution 3.0 License.

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integrated into existing discourses of human-environment interactions, and (d)how the concept might connect more broadly with the governance and man-agement of the Earth system. Paper I is the pilot study for this research, andintroduced the concept of the precipitationshed by exploring the vulnerabilityof rainfall-dependent dryland agricultural systems. In that research, I identi-fied the spatial domain of each region’s precipitationshed, and qualitativelyexplored the vulnerability of each precipitationshed to patterns of land-usechange. The key findings of Paper I are as follows:

1. The precipitationshed is a novel concept in hydrology that can providenew insights into understanding how evaporation in an upwind area isrelated to precipitation downwind.

2. There are key hotspots globally where rainfed agriculture in drylands ishighly reliant on precipitation that comes from terrestrial sources.

3. Moisture recycling is vulnerable to the interconnected pressures of land-use change, geopolitical complexity, and population pressure; this is es-pecially true in China.

In Paper II, I answer a key reservation about the utility of the precipitation-shed concept, namely whether or not the precipitationshed has low or highvariability. In general, the spatial extent and shape of precipitationshed bound-aries are persistent in time. Additionally, the primary source of variation forthe precipitationshed is a pulsing of evaporation, rather than a spatially shiftingpattern. These two findings legitimize subsequent exploration, by confirmingthat the source regions providing moisture (i.e. evaporation sources) persis-tently contribute moisture to their sink regions (i.e. precipitation sinks). Thekey findings of Paper II are as follows:

1. Precipitationsheds persist in space and time.

2. The leading pattern of variability is a pulsing (increase or decrease) fromthe core precipitationshed area.

3. Precipitationshed shapes generally agree when using different input datasets.

Paper III extends the concept of terrestrial moisture recycling; wherein I cal-culate the specific fraction of moisture recycling that is regulated by vegeta-tion. I wanted to understand how much of current evaporation is due to thecurrent vegetation (relative to a hypothetical bare-soil condition). In this pa-per, I quantified the global benefits provided by current vegetation for moisturerecycling by comparing the amount of moisture recycling that takes place inthe current world versus a world where all land surfaces have been turned tobare soil surfaces. I defined this vegetation-regulated moisture recycling as an

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ecosystem service, and found that many parts of the world are either importantproviders of this service, or important beneficiaries of this service. Likewise,a case study in the Mato Grosso region of Brazil demonstrates how the con-cept could be operationalized in a more specific location, and provided a pathforward for integrating the concept into future analyses of ecosystem servicetrade-offs. The key findings of Paper III are:

1. Vegetation-regulated Moisture Recycling (VMR) ecosystem services area significant source of rainfall for many regions.

2. Some regions rely on current vegetation for equal to or greater than 50%of annual rainfall.

3. VMR ecosystem services can provide an important seasonal benefit,shortening the dry season.

Several previous studies had suggested that the relative importance of terres-trial moisture recycling increased during dry years. I explored this idea inPaper IV through the lens of urban water supplies, and used a previously iden-tified group of megacities globally that were reliant on surface or groundwaterfor their municipal water. Of the 29 megacities globally, 19 of them were re-liant on terrestrial sources of moisture for a third or more of their precipitation.Likewise, many of the megacities were significantly more reliant on terrestrialsources of moisture recycling during dry years. Using an integrated moisturerecycling vulnerability analysis, 6 megacities were found to be highly vulner-able to potential changes in the precipitationshed. The key insights of Paper

IV are:

1. Of the 29 megacities globally, 19 rely on evaporation from land for morethan 1/3rd of the annual rain entering their watersheds.

2. Many of the 19 megacities are more dependent on land evaporationsources (rather than oceans), during dry years.

3. The vulnerability analysis reveals that 6 megacities are highly vulnerableto changes in moisture recycling.

In Paper V, I examine how to approach the governance of moisture recycling,particularly through the lens of international law and institutions. I focus onthe nation as the unit of analysis (similar to the examination of the import andexport of moisture from Dirmeyer et al. 2009), and thus examine geopoliti-cal interactions, as well as international legal perspectives. I first calculatedthe precipiationsheds of 30 nations, and then developed a typology based onthese moisture recycling patterns. Using the results of the typological classifi-cation, I examine how current legal and institutional perspectives might inform

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moisture recycling governance. The key insights of Paper V are:

1. There is a broad range of geopolitical complexity in the case study na-tions, with the number of nations overlapping the precipitationshed rang-ing from 2 to 13.

2. For the less complex precipitationsheds, legal principles such as “do noharm” and “equitable sharing of resources” may be most appropriatestrategies for governance. However, as the number of actors increasesand the moisture recycling relationships become more complex, otherapproaches may be necessary including: strengthening existing (or cre-ating) transnational collaborations, forming multinational scientific as-sessments, diffusing of norms with the aim to reduce harm, and estab-lishing common but differentiated responsibilities.

3. The complexity of moisture recycling means institutional fit will be dif-ficult to generalize for all precipitationsheds, and that for the most com-plex precipitationsheds, institutions will likely be emergent rather thanprescribed.

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2. Methods

I use two models to explore moisture recycling: the Water Accounting Model(WAM-2layers) (Van der Ent, R. J., 2014), and the Simple Terrestrial Evapora-tion to Atmosphere Model (STEAM) (Wang-Erlandsson et al., 2014). WAM-2layers is a model that is able to keep a detailed account of the atmosphericwater cycle, and STEAM simulates evaporation from the land-surface. Bothof these models require input data, which I detail below.

2.1 WAM-2layers

Research that aims to track the flow of water through the atmosphere can use avariety of methods, including: (a) isotopic tracers, (b) a model that can simu-late the pathway of different parcels of moisture through the atmosphere (i.e.,Lagrangian approach), or (c) an accounting procedure that can keep track ofwhere moisture travels (i.e., Eulerian approach). I use the latter in my research,since the Water Accounting Model 2layers (i.e. WAM-2layers) keeps trackof the vertical flux of evaporation and precipitation, as well as the horizontalfluxes of water vapor around the planet. The WAM-2layers does not simu-late anything, as does a weather or climate model, but rather the model trackswhere water moves around the Earth, based on input data. I used a single-atmospheric-layer version of the WAM in Paper I, and the WAM-2layers (i.e.two atmospheric layers, see Figure 4) in all other papers in this dissertation.The results from the two versions of the model are, in general, comparable, andanalyzed in van der Ent et al. (2013). Hereafter, I will refer to the moisturetracking procedure I used as the WAM-2layers.

Imagine a single piece of land, and the column of atmosphere that exists abovethat land (Figure 4). The WAM-2layers first tracks evaporated water (i.e. wa-ter vapor) that rises up into the lower layer of the atmosphere. Next, this watervapor is mixed with water that arrives into that column from all directions. Atthe same time, the water vapor in the lower layer of the atmosphere is verti-cally mixed with the water vapor in the upper layer of the atmosphere, which isalso mixed with water vapor that arrives from all directions. The precipitation

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5

2

3

1

4

1 = Evaporation rises up from the surface of the Earth as water vapor into the atmosphere. 2 = The evaporation enters the lower layer of the atmosphere, and the water is mixed with water vapor coming from north, south, east, and west. 3 = Water vapor is exchanged between the lower and upper layers of the atmosphere, based on vertical winds. 4 = The water vapor from the lower layer enters the upper layer of the atmosphere and is mixed with water vapor coming from north, south, east, and west. 5 = Precipitation falls on the surface of the Earth as a mixture of the evaporation that entered the atmosphere, as well as the water vapor that had traveled into the upper and lower layers of the atmosphere.

Upper layer

Lower layer

Earth’s surface

Atm

osph

ere

Overview of a Single Column of Atmosphere in the WAM-2layers

Figure 4. Diagram of moisture exchange in a single column of atmosphere forthe WAM-2layers.

that falls out of the bottom of the column of atmosphere is thus a combina-tion of the evaporation that entered that column of air, as well as other watervapor that was mixed into that column of air from outside the column. Thisprocess is completed simultaneously at all locations on the planet, and mois-ture is tracked from where it enters and exits the atmosphere, as evaporationand precipitation, respectively. Thus, in stepping forward in time, it is possibleto identify when and where a specific region’s precipitation entered the atmo-sphere as evaporation.

Since the WAM-2layers does not simulate anything, it requires specific inputdata for evaporation, precipitation, wind, specific humidity, and surface pres-sure. With these data, it is possible to reconstruct the atmospheric branch of thewater cycle, and calculate both precipitationsheds and evaporationsheds. In myresearch I have used a type of data called reanalysis data. These data combineoutput from weather forecast models that is combined with observed data fromsatellites, weather balloons, surface measurements, and other sources. I haveused two reanalysis products, including the European Reanalysis Interim prod-uct (i.e. ERA-Interim) from the European Centre for Medium-Range WeatherForecasting (Dee et al., 2011), and the Modern Era Retrospective Reanaly-

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sis product (i.e. MERRA) from the National Aeronautics and Space Agency(NASA) (Bosilovich et al., 2011). The data have several parameters, includinghow frequently data are available (i.e. the timestep) and the spatial resolu-tion. The WAM-2layers uses precipitation and evaporation data at the 3-hourtimestep, and humidity, surface pressure, and wind data at the 6-hour timestep.I use data that are at the 1.5 x 1.5 degree grid resolution, and span the pe-riod 1979 to 2014. These parameters were slightly different for the MERRAdataset, with the same timestep and data period, but with a 1.0 x 1.25 degreegrid resolution. The ERA-Interim data were used in Papers I thru V, andthe MERRA data were used specifically in my analysis of moisture recyclingvariability in Paper II.

2.2 STEAM

In addition to the WAM-2layers model, some of my research uses the Sim-ple Terrestrial Evaporation to Atmosphere Model (STEAM), which realisti-cally simulates evaporation from the land surface. STEAM simulates evapora-tion based on a variety of input climatological variables (such as precipitation,temperature, humidity, surface wind). STEAM also includes data on soil type(Harmonized World Soil Database), land cover type (using Moderate Resolu-tion Imaging Spectroradiometer satellite data; Friedl et al., 2010), and irrigatedcroplands (from the MIRCA2000 dataset, Portmann et al., 2010).

STEAM intercepts incoming precipitation into five different stocks. First, thevegetation canopy intercepts incoming precipitation. Second, the soil surfaceintercepts the precipitation that makes it through the canopy. Third, water istaken up into the soil and can evaporate from the soil column. Fourth, watercan be taken up into the roots of plants and into plant tissues. Fifth, surpluswater can pool in open bodies of water. STEAM, then, simulates evaporationin the following order: vegetation interception, transpiration, soil interception,soil moisture evaporation, and finally open water evaporation.

2.3 STEAM+WAM

Performing the coupling procedure whereby STEAM and WAM-2layers arelinked enables the exploration of the changes in the atmospheric moisture bud-get that result from land-use change, and its impacts on evaporation. Thecoupling is performed by initially allowing the evaporation from STEAM to

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modify the input evaporation for the WAM-2layers. This, in turn, modifiesthe water vapor in the atmosphere, and eventually the precipitation. Then,for a second run of STEAM, the altered precipitation is used as the input forSTEAM to modify the amount of moisture available for the evaporation parti-tioning. This process is repeated for several iterations, until the changes in themoisture budget converge, such that the difference at every gridcell betweenruns is below 1%.

I use a coupled version of STEAM+WAM in Paper III, and incorporate theoutput from Paper III into Paper IV and Paper V.

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3. Contributions

This dissertation constitutes a major advance in the field of moisture recycling,not least because it develops new conceptual tools that can be used for empir-ically understanding the hydrological cycle, but also because it explores newmethods and applications associated with the precipitationshed. The contribu-tions of my dissertation can be separated into three broad categories: concepts,methods, and applications.

3.1 Concepts

There are three key conceptual contributions made in this dissertation:

• Introduction and description of the precipitationshed

• Moisture recycling as an ecosystem service

• Typology of moisture recycling among nations

First, the introduction of the precipitationshed concept (Paper I), including theinitial definition and description, represents the foundation for this dissertation.Given that the precipitationshed is like a watershed, it transforms the atmo-spheric branch of the water cycle from a purely geophysical phenomenon, to aphenomenon with traceable flows of water, many of which people can change.It bears mentioning that since Paper I was published in 2012, the precipita-tionshed concept has been actively discussed in the land-atmosphere journalliterature (Paper I has 33 citations, and Paper II has 6 citations, accordingto Web of Science as of August 2016). Several articles on moisture recyclingprocesses and anthropogenic land cover change impacts to moisture recyclinghave discussed the precipitationshed concept (Gimeno et al., 2012; Goesslingand Reick, 2013; Mahmood et al., 2013; Savenije et al., 2013, Wheater andGober, 2013). Other researchers studying how land cover change can enhanceprecipitation downwind have acknowledged the utility of the concept for help-ing envision the upwind to downwind connections related to moisture transport(Makarieva et al. 2013; Keenan et al., 2013; Gerten 2013; van Noordwijk etal., 2014). As an extension of the original concept, the core precipitationshed

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(Paper II) provides the conceptual basis for persistent moisture sources re-gions, and the quantitative justification for precipitationshed science.

The second conceptual contribution regards the significant impact that ecosys-tems can and do have on moisture recycling processes, discussed in Paper

III. As such, the conceptual framework of Vegetation-regulated Moisture Re-cycling (VMR) ecosystem services enables the explicit linking of the atmo-spheric water cycle to the biosphere. Though this work has only recently beenpublished, this concept may end up being a significant contribution to the fieldof ecosystem services, since this kind of systemic connection between ecosys-tem functions, specifically in the context of moisture recycling, has hitherto notbeen included in ecosystem service assessments, inventories, and comparisons.

Thirdly, Paper V contains an approach for describing how countries exchangemoisture with one another, allowing for a clearer integration of moisture recy-cling (and the precipitationshed specifically) into existing international gover-nance. This typology of moisture recycling exchange among nations make itpossible to quickly identify the types of governance approaches that may bebest suited for a given region.

3.2 Methods

The three categories of methodological contribution I make in this dissertationare in:

• modeling advances

• moisture recycling science

• precipitationshed analytics

In terms of the modeling, my work has provided an approach for comparingtwo different datasets in the WAM-2layers, and improved the coupling be-tween the STEAM and WAM-2layers models in a land-use change context.

First, I expanded the functionality of the WAM-2layers (Paper II) to usea different driving dataset (i.e. MERRA) from the one it was designed for(i.e. ERA-Interim). This was an important step in my own learning about themodel, as well as for understanding how the WAM-2layers could be used inother research, including global climate model output. This process has alsobeen constructive for subsequent partnerships and collaborations with other in-stitutions that are applying the WAM-2layers in entirely new data contexts.

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Second, I coupled the WAM-2layers to the STEAM model to simulate howland-use change affects moisture recycling. The original single-atmospheric-layer version of the WAM had been used in conjunction with output from an-other land-surface model to simulate how irrigation affects moisture recycling(Nikoli 2011). Van der Ent et al. (2014) used evaporation data generated bySTEAM, and then used WAM-2layers to track this moisture. Finally, the initialcoupling procedure for the STEAM+WAM was first published in Paper III.Thus, my research built off of previous efforts to link land surface modeling tothe WAM-2layers, by further developing scripts for the land-use change cou-pling procedure, as well as protocols for propagating the changes in moisturethrough the model.

The second methodological advance I made was the quantification of Vegetation-regulated Moisture Recycling (VMR) ecosystem services. This quantificationis truly novel in that no other researcher has suggested anything similar to thecalculations in Paper III. Likewise, the definition of VMR ecosystem servicesis independent of either data or model. Thus, other researchers who aim toanalyze VMR ecosystem services may do so with their own data and their ownmoisture tracking procedure, providing that they have multiple land-use sce-narios and can track both evaporation and precipitation on a spatially-explicitgrid.

The third category of methods I developed were various precipitationshed an-alytics, including:

1. Multiple precipitationshed boundary calculations

2. Precipitationshed variability quantification

3. Seasonal comparison approach between dry and wet years

A major component of the precipitationshed is the specification of the bound-ary, which defines a spatially explicit source region of evaporation. In Paper I,the boundary was defined as the surface area that contributed 70% of sink re-gion precipitation. This approach was based on the fact that I wanted to includethe sources that contributed more than two thirds of sink region precipitation.In Paper II, I modified this definition from a fraction of total precipitation,to a depth of contribution. This change was based on a desire to relate theprecipitationshed to actual depths of evaporation that could be theoreticallymeasured as precipitation in the sink region. For example, the 1-millimeter(mm) boundary was used in Paper II, which means that every surface thatcontributed 1mm of evaporation (calculated as the volume of evaporation di-

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vided by the surface area of the 1.5 x 1.5 degree gridcell) was included withinthe precipitationshed boundary. This definition was used in the definition ofthe core precipitationshed referring to the region that contributes this minimumamount during every year of analysis. So, in Paper II, the boundary referredto any gridcell that contributed 1mm of evaporation during every year of anal-ysis. In Paper V, I moved away from the gridcell based boundary definition,and adopted a ‘national contribution’ boundary, where the relevant precipita-tionshed is defined as including the nations that contributed 1% or more of sinknation precipitation. I want to emphasize that the different boundary definitionmethods each have their own strengths and weaknesses, and that the selectionis important but also ought to be tailored to the questions being asked.

Along with the core precipitationshed being included in the boundary defi-nition, it was also a component of the variability analysis in Paper II. Theapproach used to quantify the core precipitationshed, notably the persistentcontribution of evaporation from a given region, was also a means to explorevariability, since it helps identify the frequency of contribution for a given sinkregion’s precipitationshed (for more on this, see Figure 4 in Paper II). In addi-tion to the simple metric of persistence, empirical orthogonal function (EOF)analysis was used to understand the key patterns of variability in the precipi-tationsheds. This was primarily done so that myself and others exploring theprecipitationshed phenomena could be confident that the core precipitation-shed boundaries were corroborated by the statistical variation in the precipita-tionshed. Indeed, the primary reason for variation in evaporation contributionwithin a precipitationshed was due to interannual pulsing of more (or less)evaporation, rather than a shifting from one location to another. Both the per-sistence and EOF analyses provided the justification for continuing precipita-tionshed science (i.e. Papers III, IV, and V).

The third precipitationshed analytic I developed, was the approach for com-paring dry and wet year precipitationsheds (Paper IV). This method identifieswhether there are significant differences between the evaporation contributionthat was coming from land during either dry or wet years. The steps were: (1)identify the years that corresponded to dry, neutral, and wet years in the sinkregion, (2) calculate the average evaporation contribution associated with eachof those categories, and (3) calculate the anomalous evaporation that occurredduring either the dry or wet years. This method provided insights into whichparts of the precipitationshed were particularly important during dry (or wet)rainfall years, which helped clarify whether there was increased vulnerabilityto land-use change during dry or wet years. This analysis fed directly into thequantitative vulnerability analysis, which I discuss in the next section.

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3.3 Applications

Finally, there are three key applications I develop in this dissertation:

• Qualitative and quantitative precipitationshed vulnerability analyses

• Vegetation-regulated Moisture Recycling (VMR) ecosystem service de-scription and quantification

• Analysis of moisture recycling exchange among nations

First, the identification of a precipitationshed provides two key outputs: aspatially-explicit region of evaporation contribution to a specific location; and,a boundary that allows for a finite analysis of various factors, including vul-nerabilities. Using the boundary I develop two kinds of vulnerability indicesin this work, specifically:

1. An index based on a literature review of how vulnerable a given loca-tion’s precipitation is to various pressures (Paper I).

2. An index capturing how a given location’s moisture recycling might beaffected by current and potential stresses (Paper IV).

The qualitative index created for Paper I was initially meant as a proof-of-concept approach for how the precipitationshed might be used. This index,and the inclusion of land-use change impacts to the precipitationshed, has beenhighlighted as one of the primary contributions of Paper I (e.g. Gimeno et al.,2012; Lele et al., 2013; Mahmood et al., 2013; Savenije et al., 2014). Thequantitative index created in Paper IV expands on the original index in Paper

I, and highlights (a) the current (i.e. a priori) vulnerability of water resourcesfor the sink region, (b) the current status of the precipitationshed, and (c) thepotential for land-use change to impact moisture recycling. In this way, the in-dex in Paper IV represents a quantification of moisture recycling vulnerabilityfor a given location. The index was developed for use in the context of urbansurface watersheds, but it could be applied more broadly assuming the specificdriving datasets are available.

The Vegetation-regulated Moisture Recycling (VMR) ecosystem service anal-ysis, from Paper III, that provides a method for both global and local calcu-lations, could be a significant contribution of this dissertation. In recent years,several different authors and research groups have converged on the idea thatthe evaporation arising from the land-surface, particularly from forests, oughtto be considered an ecosystem service in terms of climate regulation and tenta-tively in terms of atmospheric moisture regulation (e.g. van Noordwijk et al.,

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2014; Ellison et al., 2012). My work is, however, the first to provide (a) a cleardefinition of this service, (b) a method for its quantification, (c) a global andregional (i.e. case study) analysis, and (d) a conceptual framework for how thismight integrate into broader ecosystem service assessments. Thus, the work inPaper III could be a clear departure point for moisture recycling to integratewith new disciplines.

Finally, the analysis of moisture recycling exchange among nations, describedconceptually and quantified in Paper V, provides an approach for integratingmoisture recycling science into international law and policy. Specifically, Iprovide a complete method of how to classify a given set of moisture recyclingrelationships among nations (or for a single nation) into a typology, whichis then matched to different approaches for moisture recycling governance.Ultimately, there is much to be done to bridge science and policy regardingmoisture recycling, but Paper V is a key first step on that path.

3.4 Trans-disciplinary Impact

Along with the specific conceptual, methodological, and applied contributionsI make in my dissertation, many disciplines are impacted by my work. Table2 below highlights these impacts. In surface hydrology, the watershed (a.k.a.catchment or basin) is the single common spatial unit that is used for distin-guishing related areas. In moisture recycling there has not been a similar con-cept that the scientific community has used. In general, though, when previouswork has referred to ‘sources of evaporation’ they have used the same ideas,but different language to describe the spatial aspects. The precipitationshed(and the evaporationshed) could be a common platform or heuristic for themoisture recycling community to use. This would be beneficial for many rea-sons including: a common descriptive unit would make it easier to interrelateand compare different research; it would be more straightforward to follow orsearch out research on the topic; and, a scientific body of work could beginbuilding around the precipitationshed.

There could be challenges to the broader use of the precipitationshed, depend-ing on the moisture recycling method being used, such as isotope tracking orLagrangian modeling (e.g. Stohl et al., 1998), since the results of these anal-yses do not necessarily produce the same type of spatial data. However, thereare many examples of similar types of results presented in other work, thatwould lend themselves quite easily to using the precipitationshed framework,notably much of the work by the widely used quasi-isentropic back trajectory

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model (QIBT) that has been used in many other moisture recycling studies(e.g. Dirmeyer and Brubaker, 1999; Bagley et al., 2012; Wei et al., 2013).Furthermore, the number of people that are actively using the WAM-2layersis growing, which suggests there is a broader community that could also po-tentially use the precipitationshed (e.g. Zemp et al., 2014; Zhao et al., 2016;Duerinck et al., 2016).

Perhaps more important than providing a heuristic for the hydrology, atmo-

Table 2. Integrated insights from this dissertation for relevant fields.

DISCIPLINE NEW INSIGHTS

Hydrometeorology, Atmospheric Science, and Hydrology

a) Better understanding of atmospheric moisture source and sink dynamics b) New insights about what drives moisture recycling variabilityc) Refined appreciation of the role of vegetation for moisture recycling regulationd) Connection between large-scale geophysical phenomena and specific impactse) Awareness of a broader research and practitioner community in which there exists an appetite for this science

Water Resources Management

a) A new unit of analysis for exploring water stress & vulnerability (i.e. the precipitationshed)b) An improved sense of the criticality of upwind land-cover and land-use c) Improved sense of dry season (and dry year), vulnerabilities for inflows to reservoirs

Ecosystem Services

a) Clearly defined moisture recycling as an ecosystem service related to vegetation-regulation (expanding on loose definitions related to climate regulation, etc.)b) New (or alternative) representations of how ecosystems provide ecosystem servicesc) Awareness of new, and quantified specific trade-offs that emerge (that were either unknown or ignored in the past

Land-Change Science

a) Identified that the consequences of land-use change expand well beyond surface runoff effects, and extend into changes that can affect downwind rainfallb) Evaporation is not a loss from the system, if the system domain is expandedc) Land-use change impacts not just the magnitude of moisture recycling, but also the timing of moisture recycling

Urban Studies

a) Water resources could be vulnerable to more than watershed characteristics and general climate variability, specifically land-use change in the precipitationshedb) There may be surprising opportunities to influence precipitation within the city and within the watershed via policies aimed at trade, land-use, conservation incentives, etc. that target activities in the precipitationshed

International Law and Politics

a) There exist specific, trackable connections between hitherto separate actorsb) Though regulation of activities, proof of harm, or enforcement of activities may be difficult to establish, other legal means may be possible for justly and legally addressing how region's activities that affect one-anotherc) Institution-building and finding the 'right fit' will be key to successful management

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spheric science, and climate communities, is the bridge that my work can buildbetween social-ecological systems, land-use change, and moisture recycling.Given that the key socially-relevant insight of moisture recycling is that land-use change can modify rain elsewhere, it is critical to provide useful analyticaltools to understand how land-use change interacts with moisture recycling.Similarly, since land-use change is a spatially explicit process, it is useful tohave a consistent, spatially explicit method for evaluating subsequent impactson moisture recycling. The precipitationshed and evaporationshed are thesemethods.

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4. Limitations and Considerations

There are several aspects of the research that warrant a brief explanation interms of limitations and additional considerations. First, I want to emphasizethat the moisture tracking procedure that I use in this dissertation relies oninput data and is integrated into one atmospheric layer in Paper I, and twoatmospheric layers in Paper II thru Paper VI. The fact that the WAM-2layersuses input data means that it is not simulating any emergent features of theatmosphere, i.e. changes in circulation, changes in humidity, or changes in theenergy budget. Other studies that have used fully dynamic and fully coupledland and atmosphere models reveal that for certain parts of the world thesechanges and impacts in the atmosphere matter more than others. For example,in the monsoon regions of south Asia land cover change has a demonstratedimpact on the atmospheric circulation, and subsequently impacts precipitationdownwind (e.g. Tuinenburg et al., 2014). However, other studies have foundthat the impact on circulation from land cover change can be modest in otherregions, such as in the Amazon where changes to the atmospheric circulationfrom deforestation appear to be between 5 and 10% of mean annual winds (e.g.Bagley et al., 2014).

Also, an aspect of the climate system that is not incorporated into my anal-ysis is how changes in land cover might influence the overall energy budgetof the region that is being studied. Evaporation, which can also be understoodas latent heat flux, has a counterpart in sensible heat flux. And as latent heatflux goes down (as a fraction of total heat loss), sensible heat flux increases.These changes in the fraction of heat flux that is either latent or sensible canand do have impacts for how precipitation forms and how the atmosphere cir-culates. And as before, these changes can be more or less important dependingon where you are on the planet, and the spatial extent of the change. All thisbeing said, several comparisons between our work and others’ have found thatthe modeling set-up I use continues to be robust to both qualitative and quan-titative scrutiny (e.g. Bagley et al., 2012; van der Ent et al 2013; Zhao et al.,2016). This could be due to the fact that I tend to look at large spatial scales,and over long periods of time. The data on which I base my moisture recyclinganalyses (i.e. the ERA-Interim and MERRA reanalysis datasets, discussed ear-

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lier in the Methods section) tend to represent the broad features of the climatesystem reasonably well. There are some problems associated with accuraterepresentation of precipitation, and this is most apparent in terms of very smallamounts of precipitation or intense precipitation. However, since my analysesare primarily based on large-scale, mean features of both evaporation and pre-cipitation I am confident my results are robust.

Another important consideration is that the climate system is likely to be stronglyinfluenced by global climate change both in the coming decades, and espe-cially towards the latter half of the 21st century. As a result, in the context ofdiscussions related to management or governance, it is critical that the stake-holders and scientists involved in such questions carefully discern the scopeof their questions, to identify the best models and data to be used. This in-cludes both spatial and temporal scope, but also the granularity and precisionof the processes they wish to represent. I note that there are certain scales thatare simply inappropriate for use with moisture recycling or precipitationshedanalysis, such as the scale of a farmer’s field or even that of a city, since othermeteorological processes will dominate any moisture recycling effects.

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5. Next Steps

There are many directions that this research can go, depending on the dis-cipline. From the perspective of traditional atmospheric and hydrometeoro-logical research, there continue to be dozens of moisture recycling studiesevery year. Thus, there is unlikely to be a shortage of foundational scienceon the understanding of moisture recycling processes in the coming decades.Furthermore, there has been a steady increase during the last decade in high-resolution, coupled land and atmosphere modeling, with a particular focus onmoisture recycling processes (e.g. Bagley et al., 2014; Swann et al., 2015).Likewise the studies have focused on understanding the impacts of variousland cover change scenarios on moisture recycling. A research area that re-mains relatively open is understanding how future climate change will impactmoisture recycling. Since climate change is projected to have significant im-pacts on the very large-scale features of the atmosphere, such as where thestorm tracks are located, this will have fundamental impacts on moisture re-cycling patterns. Understanding how climate change interacts with land-usechanges, and subsequently moisture recycling will be a key area of research.

There remains, a large gap in understanding how land cover change (or climatechange) induces moisture recycling changes that impact the stability of down-wind and tele-coupled societies. That is to say, although there is a great dealof research and activity understanding the bio-geophysical processes, there ismuch less emphasis on the social-ecological consequences that arise from sys-tematic changes in the atmospheric water cycle. In this way, a future direc-tion of this research is to better understand the interaction of changes in mois-ture recycling with social-ecological processes that are the recipients of thesechanges. As I have found in my own work in this dissertation, working acrossdisciplines will be necessary to answer these types of questions.

One pathway forward in this area of research would be to use the STEAM +WAM modeling framework to simulate how land cover change impacts mois-ture recycling processes, and precipitation downwind. Likewise, this analysiscould be complemented by a dynamic land and atmosphere model that cansimulate the interactions between the surface of the earth and the climate sys-

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tem. This comparative multi-method approach would assist in quantifying thevalidity of STEAM+WAM more effectively than has already been done, andcould reveal new insights about which processes are most important in a mois-ture recycling context. With the output of this comparative study, the rangesof precipitation impacts that are likely to result from the prescribed changesin land-use could be identified. The truly novel component of this next stepwould be to integrate these changes and precipitation with a systems modelthat represents the feedbacks and interactions in the recipient social-ecologicalsystems. By using the range of precipitation impacts that result from land-usechange, it may be possible to identify whether or not tipping points emergein the recipient social-ecological systems across the spectrum of precipitationimpacts.

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6. Advice to Future PhD Students

During my PhD, I have gleaned some important insights into conducting re-search on moisture recycling... but also into research in general. Here are someof those insights.

1. Maintain steady communication with your advisor on the goals of yourresearch. It’s easy to get side-tracked, especially early on in your PhD.Your advisor has a sense of where the research ought to go, and how theindividual PhD fits into the broader field.

2. Listen to your advisor, as well as yourself. If you think there’s a promis-ing research path try and pursue it. There are few instances in the futurewhere you’ll have that opportunity. In my own research, Paper II wouldnot have happened without this self-motivation, and ultimately the vari-ability analysis and approach represents a very important justificationfor precipitationshed science as a whole. Once I convinced my advisorof the value of Paper II she was onboard, but it took a little bit of myown self-motivation to get us there.

3. Pursue interesting ideas, and explore new methods, since you neverknow where they’ll end up. The PhD is a rare time where you developenough skills to (a) ask interesting questions, (b) conduct really interest-ing work, and (c) have very few other responsibilities. Thus, take advan-tage of this time and dive deeply into your topic. Likewise, take advan-tage of the opportunity to learn new things. In the context of my ownresearch, that meant learning a lot more about Matlab, interpreting newdatasets, coupling models together, and working in trans-disciplinaryteams. Papers III and IV were simply interesting ideas that I pursued,which both eventually evolved into complete pieces of work.

In addition to these three thoughts, my PhD ended up being very trans-disciplinary.This was somewhat by design, given that from the outset I wanted to write thefinal paper about moisture recycling governance. However, in practice thiswas not a natural path, at least for most PhD students. In terms of my ownbackground (natural science) and moisture recycling (geophysics), I did nothave a familiarity with many of the topics that are important when engaging in

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transdisciplinary research, perhaps most importantly an individual philosophyof science. I credit much of my own advancement in transdisciplinary workto the community at the Stockholm Resilience Centre (SRC), which fostered(at least in me) an appreciation for new ideas and broad thinking. The tips Iwould give to anyone who wants to engage in trans-disciplinary work are thefollowing:

1. Test your ideas often, particularly with people that will probably dis-agree with you. It’s uncomfortable to share things with your peers,and even more so when you might be told that the idea is wrong, ill-conceived, or silly. Deal with it. As a PhD student you must learnhow to develop new ideas, communicate these ideas, and handle legiti-mate (and illegitimate) criticism. At the SRC, the best opportunities forthis are speedtalks, brown-bag lunches, and informal seminars. In thecontext of transdisciplinary work, it is important to get used to feelinguncomfortable.

2. Listen to other’s ideas, especially if their methods or topics are differentfrom your own. This is a direct corollary of the first tip, however it relatesmore to strategically broadening your research worldview. This meansmaking time to attend speedtalks, brown-bag lunches, and informal sem-inars that others are giving, on a wide range of topics. Likewise, bothconferences and visiting other institutions are critical to having your aca-demic worldview challenged.

3. Surround yourself with people who are smarter than you, especially inthe fields you know little about. I heard once that you are the averageof the five people with whom you spend the most time”, but I can’tremember who said it. I think this can be extended to research, and inthe context of trans-disciplinary research it means that you must seekout the people who you want to be like, and then wiggle your way intotheir domain of influence. This might mean strategic advisor selection,joining committees, or asking your advisor to make connections for you.

4. Read. A lot. Of course as a PhD student, you must read a lot of jour-nal literature. This is not what I mean here. I mean that as a trans-disciplinary scientist, you must broaden your horizons by reading abouttopics that are outside your immediate research area, but nonethelessrepresent excellent writing. This can be nonfiction or fiction, print ordigital, news or opinion. Whatever it is you do read, make sure it’s highquality writing.

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5. Find people with whom you disagree and learn why. One of the key rea-sons I was able to get so much from my PhD, was the oft times heateddebates in which myself and my PhD colleagues would engage. Ulti-mately, we ended up agreeing about many things, but the road to thisagreement was paved with lots of introspection, exploration, and discus-sion. I would expect anyone who wishes to attain a Doctor of Philosophyin any discipline to be able to fruitfully engage with those they disagreewith. In transdisciplinary research, a plasticity of mind is essential.

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7. Concluding Remarks

The concept of the precipitationshed has been explored in great detail and inmany different ways within this dissertation. Since the initial publication of theprecipitationshed concept, it has been critiqued and discussed in the academicliterature. Along with that, there has emerged an appetite in the practitionercommunity to have a better understanding of the tools and resources that areavailable for understanding the relationship between changes in land cover andeventual precipitation. Thus, I hope that the study of the precipitationshed,and how social-ecological systems interacting with moisture recycling morebroadly, persists within the academic community and leads to ever improvedunderstanding. Specifically, the scientific community ought to continue to in-vestigate the quantitative nature of the precipitationshed such as the variabilityand seasonal differences, as well as a qualitative features of the precipitation-shed since it is an inherently socially relevant phenomena. Ultimately for thisconcept to be expanded and incorporated into broader hydrological thinking,hydrologists and water resources scientists who are typically concerned withsurface water will need to start connecting their branch of the water cycle tothe atmospheric branch. Finally, this work is critical given that global envi-ronmental and anthropogenic forces are leading to increasingly unpredictablechanges. The precipitationshed is a key contribution to a scientific toolkit formoving towards a sustainable future for humanity in the Anthropocene.

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