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Gregory J. McCabe and Martyn P. Clark

1. Introduction Snowmelt runoff (SMR) is an important source of water for much of the western United States ( McCabe and Wolock 1999 ; Stewart et al. 2004 ), and for snowmelt-dominated basins in the western United States spring/summer runoff can account for from 50% to 80% of total annual runoff ( Serreze et al. 1999 ; Stewart et al. 2004 ). One of the expected hydrologic effects of global warming is a shift in the timing of SMR to earlier in the year ( Gleick 1987 ; McCabe and Ayers 1988

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S.-Y. Simon Wang, Yen-Heng Lin, Robert R. Gillies, and Kirsti Hakala

increased household usage of water in addition to agricultural water use. In the meantime, groundwater storage in the Central Valley has declined by almost 60 million acre feet since the 1960s ( Faunt 2009 ). Climatic factors have affected groundwater in the Central Valley as well. The effects of global warming at the regional scale include a hotter and drier climate ( Dai 2013 ) and earlier snowmelt ( Westerling et al. 2006 ), both of which can aggravate drought conditions. A companion study that

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Christopher R. Hain, Wade T. Crow, Martha C. Anderson, and M. Tugrul Yilmaz

except local precipitation, areas with large irrigation and/or groundwater-based water inputs should demonstrate a negative bias in LE Noah relative to LE ALEXI . Likewise, these same areas should also be associated with relatively low VAR ALEXI (since effects of interannual precipitation variability will be largely muted by irrigation and/or groundwater extraction). Conversely, areas in which bottom-up Noah simulations neglect the impact of tile drainage (i.e., anthropogenic sinks) should be

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Declan Conway, Aurelie Persechino, Sandra Ardoin-Bardin, Hamisai Hamandawana, Claudine Dieulin, and Gil Mahé

-Saharan Africa, robust identification and attribution of hydrological change is, therefore, severely limited by conflicting behavior across basins/regions, low signal-to-noise ratio, sometimes weak rainfall–runoff relationships, and limited assessment of the magnitude and potential effects of LUCC or other anthropogenic influences. An important area deserving further research that we have not looked at is the role of evaporation in rainfall–runoff analyses. d. Future climate change The high levels of

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T. Das, H. G. Hidalgo, D. W. Pierce, T. P. Barnett, M. D. Dettinger, D. R. Cayan, C. Bonfils, G. Bala, and A. Mirin

anthropogenic effects is characterized using precipitation and temperature data from an 850-yr preindustrial control simulation of the NCAR/Department of Energy (DOE) Community Climate System Model, version 3 (CCSM3; Collins et al. 2006 ). The simulation was performed at Lawrence Livermore National Laboratory and used the finite volume (FV) dynamical methods for the atmospheric transport (CCSM3-FV; Bala et al. 2008a , b ). The horizontal spatial resolution of the atmospheric model was 1° × 1.25° with 26

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Benjamin F. Zaitchik, Joseph A. Santanello, Sujay V. Kumar, and Christa D. Peters-Lidard

they specifically focus on conditions where albedo does not have a significant impact on R net . Nevertheless, the distinction is important from the perspective of mesoscale model development and interpretation. Insomuch as both R net , including albedo effects, and energy partitioning are potentially important to soil moisture–precipitation feedbacks, modeling studies of land–atmosphere feedbacks must be clear regarding which mechanisms the model is structurally capable of simulating and how

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Clifford Mass, Adam Skalenakis, and Michael Warner

portions of the western United States and have proposed that anthropogenic global warming could be the cause. The existence of such increases would have substantial implications for public safety, management of rivers for fisheries and power generation, and other significant societal issues. As described in the next section, several papers have examined the potential for changing precipitation intensity under anthropogenic global warming, with a subset studying recent trends in heavy precipitation and

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Gregory J. McCabe, David M. Wolock, and Melissa Valentin

in the western United States, Abatzoglou (2011) indicated that changes in atmospheric circulation [quantified by changes in the Pacific North American (PNA) index] accelerated the decline of S projected because of anthropogenic warming. Abatzoglou (2011) reported that a tendency for PNA values to be above average during recent decades resulted in decreased fractions of winter P that falls as S and increases in snowmelt (MELT). Another useful index of the hydrologic effects of climate

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Mutlu Ozdogan, Matthew Rodell, Hiroko Kato Beaudoing, and David L. Toll

-averaged latent heat flux were small, irrigation caused locally significant increases (up to 60%) in evapotranspiration that reduced surface temperatures, and hence decreased sensible heat flux. More recently, Tang et al. (2007) investigated the effects of natural and anthropogenic heterogeneity (including irrigation) on a hydrological simulation using a distributed biosphere hydrological model system. The results suggest that irrigation leads to increased evapotranspiration, decreased runoff, increased

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Christopher P. Konrad

1. Introduction Globally droughts pose significant social and ecological threats in part because of limited water availability in rivers and streams ( Harding et al. 1995 ; Heim 2002 ; Lake 2003 ; Golladay et al. 2004 ; Bond et al. 2008 ; van Dijk et al. 2013 ; van Lanen et al. 2016 ). Reduced streamflow during drought in the western United States has wide ranging effects, which can impact society and ecosystems as a result of their severity and spatial extent ( Cayan et al. 2010

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