There is growing consensus that increased greenhouse gas (GHG) concentrations in the global atmosphere are causing long-term changes to the Earth's climate [Christensen et al., 2007]. The combination of rising GHG forcings, ongoing natural-climate variability, and uncertainty in climate model projections make future climates more uncertain for water resource managers [Brekke et al., 2008]. Additionally, the fact that hydrologic processes, such as runoff, recharge, and evapotranspiration (ET), all covary in time and space, and are correlated to each other, makes it difficult to analyze cause and effects for any one hydrologic process without an integrated framework to model all these processes simultaneously. In environments where summertime streamflow and groundwater discharge is critical for water resources and biological demands, an accurate understanding of the causality of historical and future hydrologic change during these periods is especially important.
 The mechanisms causing observed historical and projected hydrologic change in high-elevation catchments is poorly understood, especially regarding surface water/groundwater interactions (SW/GW). For example, streamflow records across the Western United States indicate predominantly decreasing summertime flow [Kim and Jain, 2010], and 25th percentile annual flows [Luce and Holden, 2009] where groundwater discharge is a major component of the total streamflow. These opposing trends could be viewed as paradoxical, given that several studies suggest that increased annual precipitation will equate to increased annual groundwater recharge, and therefore high summertime flow [Jyrkama and Sykes, 2007; Allen et al., 2010]. Many hydrologic modeling studies support observed decreases in summertime flow, asserting that earlier snowmelt and runoff is the primary cause [Hamlet and Lettenmaier, 1999; Wilby and Dettinger, 2000; Dettinger et al., 2004; Scibek et al., 2007; Mantua et al., 2010; Maurer et al., 2010]. Although these modeling studies provide an explanation of decreasing summertime flow, shifts in snowmelt and runoff timing alone are not complete explanations. Additional clarification on the causality of decreasing summertime flow, and ties to changes in hydrologic timing are needed to assess historical and future trends [Luce and Holden, 2009]. A thorough understanding of the linkage between changes in snowmelt timing and SW/GW interactions will help address an important question in hydroclimate research, that is, how do changes in snowmelt and streamflow timing impact groundwater resources and groundwater-derived surface water resources?
 Recent findings show significant shifts in the timing of snowmelt and observed streamflow in several watersheds in the Sierra Nevada [Coats, 2010], and vulnerability of groundwater to changing climate in the region [Singleton and Moran, 2010]. The purpose of this work is to develop a process-based explanation for decreasing summertime flows that have been reported by previous investigators by using an integrated modeling framework to analyze changing SW/GW interactions. We show that decreased summertime flow is likely part of a broader hydrologic change that is occurring due to earlier onset of the snowmelt pulse and the resulting earlier seasonal drainage in these watersheds. Six different climate model projections are used to force the hydrologic model and demonstrate that projections of earlier snowmelt recession results in decreased summertime flow over a wide range in projected precipitation amounts, including both decreasing and increasing long-term precipitation trends. The use of multiple climate projections are important for providing greater evidence for our explanation of why summertime flows are decreasing because the period of record for these watersheds is short, and thus the climate projections provide greater credence to the statistical significance of decadal or longer trends in the historical streamflow data.
 To simulate the effects of earlier snowmelt runoff on watershed drainage and SW/GW interactions, we rely on the integrated SW/GW interactions model, GSFLOW. Both observed historical data, as well as climate model projections for the 21st century are used to evaluate the significance and implications of decreased summertime flow in the Sierra Nevada. Projections of future hydrologic conditions complement the historical simulations by allowing for a longer simulation period to discern persistent shifts in hydrologic conditions. Models are constructed for three snow-dominated watersheds of the eastern Sierra Nevada tributary to Lake Tahoe and Truckee Meadows hydrographic areas of California and Nevada (Figure 1). The study area is of special interest with regard to water resources because it is representative of many low-permeability bedrock snow-dominated mountainous regions of the Western United States that provide primary water supplies to nearby developed watersheds. The study area is representative of the greater Sierra Nevada because topography, geology, climate, and hydrology are similar over much of the upland regions, where precipitation is the greatest. Important characteristics that are shared among the upland (i.e., >2000 m) watersheds of the Sierra Nevada are the large topographic relief and relatively impermeable shallow bedrock that accentuate the dominance of shallow groundwater-flow paths in the regional system. Because the alluvial aquifers are small and have limited storage, the alluvial aquifers are likely to be more sensitive to climate fluctuations than large valley aquifers. There is additional interest in the drainage processes within the Incline and Third Creek watersheds because these watersheds transport sediment and nutrients to Lake Tahoe, which is nationally recognized for its clarity and recreational value.
1.1. Modeling Background
 Due to model limitations and computing constraints, simulating climate change effects on groundwater hydrology typically has been done with compartmentalized models, in which SW/GW interactions are decoupled or neglected [Vaccaro, 1992; Middelkoop et al., 2001; Scibek et al., 2007; Jyrkama and Sykes, 2007; Tague and Grant, 2009; Allen et al., 2010]. In these studies, if the unsaturated zone is explicitly considered, it is represented as a soil column through which water flows independently of the underlying water table. These models calculate recharge independently of dynamic groundwater levels and SW/GW interactions. Furthermore, the important interplay between snowmelt-derived streamflow and SW/GW interactions are not simulated in a coupled manner, which we will show is a key process that must be considered to evaluate climate-change impacts on summertime flow in snow-dominated regions. In short, the effects of climate on the interactions between SW/GW and resulting summertime flow are not fully understood due to various compartmental model limitations and assumptions [Scibek et al., 2007].
 Recently, with the development of sophisticated computer codes, several studies have applied integrated models to simulate climate change effects on water resources [Maxwell and Kollet, 2008; Ferguson and Maxwell, 2010; Sulis et al., 2011]. These models have provided greater insight into climate change effects on watershed hydrologic processes due to their ability to more realistically simulate feedback between hydrologic processes that occur above and below land surface. Here, we add to these past works by calibrating over a longer period to evaluate the model's ability to simulate low-frequency variations in summertime flow that are associated with groundwater storage, considering climate projections from six climate models and two GHG scenarios, and projecting hydrologic conditions over the next century to assess the combined effects of low-frequency weather cycles and future climate change. Natural climate variability will be an important component of future climate conditions, and a good representation of these historical cycles allows for more realistic projections of water availability and the severity of climate extremes. Researchers have observed both interdecadal and intradecadal periodicities in precipitation and streamflow [Hanson et al., 2006; Perry, 2006], and groundwater levels [Hanson et al., 2006; Laque-Espinar et al., 2007]. These low-frequency signals have been linked to Quasi-Biannual Oscillation (QBO), El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), tidal, and solar cycles [Barco et al., 2010; Burroughs, 2003]. Accurately predicting historical low-frequency responses is central to predicting future low-frequency responses in groundwater storage, discharge to streams and springs, and water-dependent biota. Integrated models that are calibrated to historical interactions of SW/GW over wet and dry periods, and are forced with future climate data over many decades, are better suited to assess how climate change might affect water resources, and in particular, groundwater resources.
1.2. Model Description
 GSFLOW was used to simulate all near-surface and groundwater hydrologic processes within three watersheds of the eastern Sierra Nevada (Figures 1 and 2). GSFLOW simultaneously accounts for climatic conditions, runoff across the land surface, variably saturated subsurface flow and storage, plus connections among terrestrial systems, streams, lakes, wetlands, and groundwater. Runoff and interflow (shallow subsurface flow) cascade to receiving streams or lakes, while including effects of saturation-excess runoff caused by shallow water table conditions. GSFLOW and its precursors have been applied in several basins across the United States to simulate SW/GW interactions [e.g.,Hunt et al., 2008; Markstrom et al., 2008; Niswonger et al., 2008; Doherty and Hunt, 2009; Koch et al., 2011].
 GSFLOW is the integration of the Precipitation Runoff Modeling System (PRMS) and the Modular Groundwater Flow model (MODFLOW). Integration of PRMS and MODFLOW was facilitated by an implicit iterative coupling approach using the Newton linearization method [Niswonger et al., 2011]. Markstrom et al.  and Niswonger et al. provide a complete description of GSFLOW and its theory, and only a broad description is provided herein. PRMS is a modular deterministic, distributed-parameter, physical-process watershed model used to simulate precipitation, climate, and land use on watershed response [Leavesley et al., 1983]. PRMS simulates snowpack processes using a distributed two-layered system that is maintained and modified on both a water equivalent basis and as a dynamic heat reservoir. PRMS simulates snowmelt- and rain-generated runoff in a fully distributed sense, where runoff can cascade among four neighboring surface grid cells, reinfiltrate, or flow to a stream. The soil zone is represented by coupled continuity equations with storages that represent different components of soil porosity (i.e., dead-end verses kinematic and macropore porosity), conceptualized in PRMS as the preferential, gravity, and capillary reservoirs. Water in the soil zone can percolate into the deeper unsaturated zone (MODFLOW), flow horizontally to a receiving grid cell or stream, or evapotranspire to the atmosphere. In areas where the water table is above the base of the soil zone, groundwater can seep into the soil zone. Additionally, groundwater discharge occurs to the surface in areas where groundwater heads are above land surface.
 ET is derived from the vegetation canopy and land surface (sublimation from the snowpack and evaporation off of land surface), within the soil zone, and the deeper unsaturated and saturated zones. Evaporation also can be simulated from surface water, such as from the surfaces of lakes and streams. ET is simulated as a function of the potential (PET), water storage in the vegetation canopy and in the soil zone. Beneath the soil zone, ET is a function of the PET that is not satisfied from the soil zone, root available water content in the deeper unsaturated zone, and water table elevation in the deeper saturated zone. If the water table elevation is above the root depth (i.e., extinction depth) and the PET is not met by the soil and unsaturated zones, then ET is removed directly from groundwater using the formulation developed in the MODFLOW ET Package [McDonald and Harbaugh, 1988]. There are three options in GSFLOW for calculating PET. These formulas are empirical and rely on climate data including, air temperature, solar radiation, and elevation. For this work, the Jensen and Haise solar radiation-temperature empirical formulation for calculating PET was used.Markstrom et al. provide further details, including the distribution of climate data on the landscape and calculations of energy-budget components.
 Flow beneath the base of the soil zone is simulated by MODFLOW, including vertical unsaturated flow, groundwater flow, and with a wide variety of boundary conditions that represent streams, lakes, groundwater development, and many other hydrologic processes. Vertical unsaturated flow is simulated by MODFLOW using the Unsaturated-Zone Flow (UZF1) Package [Niswonger et al., 2006], in which unsaturated flow is simulated using the kinematic-wave equation. The relation between the unsaturated hydraulic conductivity and water content in the unsaturated zone is defined on the basis of the Brooks-Corey function [Brooks and Corey, 1966]. The version of MODFLOW used in this application of GSFLOW is called MODFLOW-NWT, which is a Newton formulation of MODFLOW-2005 that provides capabilities to simulate drying and wetting of groundwater cells [Harbaugh, 2005; Bedekar et al., 2011; Niswonger et al., 2011]. MODFLOW simulates three-dimensional (3-D) confined and unconfined groundwater flow using the conservative form of the continuity equation that is discretized using block-centered finite differences; groundwater head is calculated at the cell center, and flows are calculated at the interface between cells [Harbaugh, 2005]. Following the approach of MODFLOW for solving the 3-D unconfined groundwater-flow equation, the water table is resolved at the subgrid scale that allows a coarse vertical discretization of the subsurface without degradation of the unconfined solution. Similarly, unsaturated flow is simulated using the method of characteristics solution of the kinematic-wave equation that is not dependent on grid-cell thickness [Smith, 1983; Niswonger and Prudic, 2004; Niswonger et al., 2006]. Thus, vertical discretization of GSFLOW models is guided by geologic information rather than constraints associated with numerical stability and accuracy. However, the equations used in GSFLOW are more approximate than full 3-D Richards' equation, which results in some error that must be balanced against errors in parameterization. All surface water in GSFLOW, other than overland runoff, is simulated by MODFLOW packages, including the modified lake (LAK7) and streamflow routing (SFR2) packages [Merritt and Konikow, 2000; Niswonger and Prudic, 2005]. Readers are referred to Markstrom et al.  for details regarding SW/GW interactions, including groundwater interactions with overland flow and lakes.