Contents lists available at ScienceDirect Water Security journal homepage: www.elsevier.com/locate/wasec Scalable solutions to freshwater scarcity: Advancing theories of change to incentivise sustainable water use D. Garrick a, ⁎ ,T.Iseman b ,G.Gilson a , N. Brozovic c , E. O'Donnell d , N. Matthews e , F. Miralles-Wilhelm b,f ,C.Wight a ,W.Young g a University of Oxford, United Kingdom b The Nature Conservancy, United States c Daugherty Water for Food Global Institute at the University of Nebraska, United States d University of Melbourne, Australia e Global Resilience Partnership, United States f University of Maryland, United States g World Bank, United States ARTICLEINFO Keywords: Incentives Sustainable water use Water markets Water scarcity Theories of change ABSTRACT Increasing competition for freshwater underscores the need for scalable solutions to manage and mitigate the impactsoffreshwaterscarcity.Appropriaterulesandincentivescanfacilitateashifttomoresustainablepatterns ofwateruse,includingwaterreallocationtorestoreriversandaquifersinregionsalreadyexperiencingexcessive waterextractions.Despitetheirpotential,watermarketsandotherincentive-basedapproachestomanagewater have struggled to scale up beyond pilot initiatives due to political resistance, fnancing shortfalls and data defcits. Recent advances in our understanding of incentives for sustainable water use can help to overcome persistentbarriersthathavehinderedpasteforts.Weprovideareviewoftwowell-establishedyetdisconnected felds of research and practice, and establish an agenda at their intersection: (i) incentive-based approaches to water allocation and management and (ii) theory-of-change approaches to strategic development and impact evaluation. Doing so allows us to situate incentive-based approaches to water allocation in a broader context, identifying the potential synergies and frictions with wider processes of institutional reform and infrastructure development.Anexplicitfocusontheory-of-changeapproachescanidentifythekeyassumptionsandknowledge gapshinderingprogress,suchasdrivers,enablingconditionsandsequencingissuesfordiferentapproaches,and the key factors constraining adoption in diferent contexts. We identify three areas where interdisciplinary re- searchcansupportthedevelopment,implementationandevaluationoftheoriesofchangeforwatermarketsand other incentive-based water management: (1) identifying where and when to develop diferent types of in- centives for sustainable water use, (2) designing and testing incentives as part of a broader package of in- stitutional reforms and infrastructure investments and (3) systematically evaluating the impact of incentives usingdiversedatasources(fromsatellitestothe feld),multiplemethodsandmultiplecriteria.Aglobalnetwork of water researchers and practitioners, supported by common frameworks and observatories, can enable sys- tematiclearningfromexperimentstostrengthenincentivesforsustainablewateruseandscaleuptheiradoption. 1. The challenge of sustainable water use 1.1. Spread of freshwater scarcity Overhalfoftheglobalpopulationexperiencesseverewaterscarcity for at least one month of the year [1]. Urbanisation, rising incomes, changing diets, and growing populations are intensifying competition forwater,whilstclimatechangeisdecreasingthereliabilityofavailable suppliesandinfrastructureinmanyareas.ExperiencesfromCapeTown to California are emblematic of these global trends. By 2050, im- balancesbetweensupplyanddemandareprojectedtoinvolveanurban surface-water defcit of up to 6.75 million m 3 [2] with approximately fourbillionpeopleexpectedtoliveinseverelystressedriverbasins [3]. Thesepressuresarestrainingriversandaquifers,leadingtohotspotsof competition for water between cities, agriculture, hydropower and ecosystems. https://doi.org/10.1016/j.wasec.2019.100055 Received 7 June 2019; Received in revised form 23 November 2019; Accepted 26 November 2019 ⁎ Corresponding author at: School of Geography and the Environment, University of Oxford, South Parks Road, Oxford OX1 3QY, United Kingdom. E-mail address: dustin.garrick@ouce.ox.ac.uk (D. Garrick). Water Security 9 (2020) 100055 2468-3124/ © 2019 Published by Elsevier B.V. T