A method for physically based model analysis of conjunctive usein response to potential climate changesR. T. Hanson,1L. E. Flint,2A. L. Flint,2M. D. Dettinger,1,3C. C. Faunt,1Dan Cayan,1,3and Wolfgang Schmid4Received 12 April 2011; revised 28 October 2011; accepted 21 December 2011; published 4 February 2012.[1] Potential climate change effects on aspects of conjunctive management of waterresources can be evaluated by linking climate models with fully integrated groundwater–surface water models. The objective of this study is to develop a modeling system that linksglobal climate models with regional hydrologic models, using the California Central Valleyas a case study. The new method is a supply and demand modeling framework that can beused to simulate and analyze potential climate change and conjunctive use. Supply-constrained and demand-driven linkages in the water system in the Central Valley arerepresented with the linked climate models, precipitation-runoff models, agricultural andnative vegetation water use, and hydrologic flow models to demonstrate the feasibility ofthis method. Simulated precipitation and temperature were used from the GFDL-A2 climatechange scenario through the 21st century to drive a regional water balance mountainhydrologic watershed model (MHWM) for the surrounding watersheds in combination witha regional integrated hydrologic model of the Central Valley (CVHM). Application of thismethod demonstrates the potential transition from predominantly surface water togroundwater supply for agriculture with secondary effects that may limit this transition ofconjunctive use. The particular scenario considered includes intermittent climatic droughtsin the first half of the 21st century followed by severe persistent droughts in the second halfof the 21st century. These climatic droughts do not yield a valley-wide operational droughtbut do cause reduced surface water deliveries and increased groundwater abstractions thatmay cause additional land subsidence, reduced water for riparian habitat, or changes inflows at the Sacramento–San Joaquin River Delta. The method developed here can be usedto explore conjunctive use adaptation options and hydrologic risk assessments in regionalhydrologic systems throughout the world.Citation: Hanson, R. T., L. E. Flint, A. L. Flint, M. D. Dettinger, C. C. Faunt, D. Cayan, and W. Schmid (2012), A method forphysically based model analysis of conjunctive use in response to potential climate changes, Water Resour. Res., 48, W00L08,doi:10.1029/2011WR010774.1. Introduction[2] Climate change is likely to have important influenceson water-resources management options that will be neededto sustain groundwater by conjunctive use strategies [Alleyet al., 1999; Alley, 2001]. In most watersheds, groundwaterresources are really part of a single resource comprising pre-cipitation, surface water, and groundwater resources thatrequire combined simulation and analysis. Influences of cli-mate change may be manifested as changes in streamflow inregions suitable for agriculture, and in the fundamentalinterplay between natural and societal water supplies anddemands. With respect to groundwater, these climate-relatedchanges may include significant variations in recharge,discharge, and groundwater withdrawals in concert with, andindependently from, climatic influences on surface waterresources. Many representations and considerations of theseinfluences may have neglected the variations in near-termpolicy and operational decision making on seasonal to inter-annual time scales, and ignored the effects of climatechanges on long-term policy and capital investment deci-sions on interdecadal time scales [Gleick and Adams,2000;Gleick et al.,2006;Aerts and Droogers,2004;Intergovern-mental Panel on Climate Change (IPCC), 2008; CaliforniaNatural Resources Agency, 2009]. Some effects of climatechange on agriculture have been addressed by previous stud-ies [Frederick et al., 1997; California Department of WaterResources (CADWR), 2005, 2008a; U.S. Climate ChangeScience Program,2008;Lettenmaier et al.,2008;Karhl andRoland-Holst, 2008]. Others have included these featuresbut have not completely represented both components (sur-face water and groundwater) of conjunctive use and,1U.S. Geological Survey, San Diego, California, USA.2U.S. Geological Survey, Sacramento, California, USA.3Climate, Atmospheric Sciences, and Physical Oceanography ResearchDivision, Scripps Institution of Oceanography, University of California,San Diego, La Jolla, California, USA.4Department of Hydrology and Water Resources, University of Arizona,Tucson, Arizona, USA.This paper is not subject to U.S. copyright.Published in 2012 by the American Geophysical UnionW00L08 1of23WATER RESOURCES RESEARCH, VOL. 48, W00L08, doi: 10.1029/2011WR010774, 2012especially, the role or effects of groundwater resources[IPCC,1996;Aerts and Droogers,2004;Gleick et al.,2006; Hanak and Lund,2008;Chung et al., 2009]. Thus, amethod to assess the short- and long-term perspectives isneeded to understand how climate change may effect con-junctive use in a supply and demand framework to assessdevelopment, management, and sustainability of water resour-ces [Alley et al., 1999; Alley, 2001; Alley and Leake,2004;Gurdak et al., 2009; Hanson et al., 2010b].[3] Both climate change and variability along withincreased human demand with potential land use changeswill affect the distributions of supply and demand compo-nents [Vörösmarty et al., 2010 ; Aerts and Droogers, 2004]and sustainable water development [Scanlon et al., 2006]throughout the world’s regional aquifers. Recent studies[Hanson et al., 2002, 2004, 2006, 2009; Gurdak et al.,2007, 2009; Kumar and Duffy, 2009] have identified quasi-periodic cycles in hydrologic time series of precipitation,groundwater, and streamflow that appear to correspond toquasiperiodic climatic forcings such as ENSO, NAMS,PDO, and AMO [Dettinger et al., 1998; Gurdak et al.,2009]. Additional recent studies also have indicated thatclimate change has started to affect the streamflow in re-gional watersheds of North America such as the Sierra Ne-vada and the Rocky Mountains [Stewart et al., 2004, 2005 ;Milly et al., 2005 ; Barnett et al., 2008; Das et al., 2009;Gray and McCabe, 2010], and has affected groundwaterrecharge such as in Sierra Nevada watersheds [Earman andDettinger, 2008; J. L. Huntington and R. G. Niswonger,Role of surface and groundwater interactions on projectedbase