Groundwater is a major source of water in the western US. However, there are limited recharge estimates in this region due to the complexity of recharge processes and the challenge of direct observations. Land surface Models (LSMs) could be a valuable tool for estimating current recharge and projecting changes due to future climate change. In this study, simulations of three LSMs (Noah, Mosaic and VIC) obtained from the North American Land Data Assimilation System (NLDAS-2) are used to estimate potential recharge in the western US. Modeled recharge was compared with published recharge estimates for several aquifers in the region. Annual recharge to precipitation ratios across the study basins varied from 0.01% to 15% for Mosaic, 3.2% to 42% for Noah, and 6.7% to 31.8% for VIC simulations. Mosaic consistently underestimates recharge across all basins. Noah captures recharge reasonably well in wetter basins, but overestimates it in drier basins. VIC slightly overestimates recharge in drier basins and slightly underestimates it for wetter basins. While the average annual recharge values vary among the models, the models were consistent in identifying high and low recharge areas in the region. Models agree in seasonality of recharge occurring dominantly during the spring across the region. Overall, our results highlight that LSMs have the potential to capture the spatial and temporal patterns as well as seasonality of recharge at large scales. Therefore, LSMs (specifically VIC and Noah) can be used as a tool for estimating future recharge in data limited regions.

Widespread droughts can have a significant impact on western United States streamflow, but the causes of these events are not fully understood. This dissertation examines streamflow from multiple western US basins and establishes the robust, leading modes of variability in interannual streamflow throughout the past century. I show that approximately 50% of this variability is associated with spatially widespread streamflow anomalies that are statistically independent from streamflow's response to the El Niño-Southern Oscillation (ENSO). The ENSO-teleconnection accounts for approximately 25% of the interannual variability in streamflow, across this network. These atmospheric circulation anomalies associated with the most spatially widespread variability are associated with the Aleutian low and the persistent coastal atmospheric ridge in the Pacific Northwest. I use a watershed segmentation algorithm to explicitly track the position and intensity of these features and compare their variability to the multi-basin streamflow variability. Results show that latitudinal shifts in the coastal atmospheric ridge are more strongly associated with streamflow's north-south dipole response to ENSO variability while more spatially widespread anomalies in streamflow most strongly relate to seasonal changes in the coastal ridge intensity. This likely reflects persistent coastal ridge blocking of cool-season precipitation into western US river basins. I utilize the 35 model runs of the Community Earth System Model Large Ensemble (CESMLE) to determine whether the model ensemble simulates the anomalously strong coastal ridges and extreme widespread wintertime precipitation anomalies found in the observation record. Though there is considerable bias in the CESMLE, the CESMLE runs simulate extremely widespread dry precipitation anomalies with a frequency of approximately one extreme event per century during the historical simulations (1920 - 2005). These extremely widespread dry events correspond significantly with anomalously intense coastal atmospheric ridges. The results from these three papers connect widespread interannual streamflow anomalies in the western US - and especially extremely widespread streamflow droughts - with semi-permanent atmospheric ridge anomalies near the coastal Pacific Northwest. This is important to western US water managers because these widespread events appear to have been a robust feature of the past century. The semi-permanent atmospheric features associated with these widespread dry streamflow anomalies are projected to change position significantly in the next century as a response to global climate change. This may change widespread streamflow anomaly characteristic in the western US, though my results do not show evidence of these changes within the instrument record of last century.

The impacts of transported and locally-produced pollutants on western US air quality during summer 2008 are studied using the multi-scale Sulfur Transport and Deposition Modeling system. Transported background (TBG) is an indicator of the influences from extra-regional emissions or the lower stratosphere. The magnitude of TBG is expected to increase as the emissions from international sources grow. This trend is especially important in the context of US air quality standards, which tend to become more stringent to protect human health and ecosystems. Forward sensitivity simulations in which the model boundary conditions and emissions are perturbed show that TBG strongly and extensively affect western US surface ozone (more than half of the total), compared to other contributors to background ozone (North American, NA, biomass burning, BB and biogenic emissions), and the impacts differ among various geographical regions and land types. The stratospheric ozone impacts are weak. The TBG ozone contributes most to western US ozone among all TBG species, and TBG peroxyacetyl nitrate is the most important species among ozone precursors. Compared to monthly mean 8-hour daily maximum ozone, the secondary standard metric "W126 monthly index" shows larger responses to TBG perturbations and stronger non-linearity to the size of perturbations. Overall the model-estimated TBG impacts negatively correlate to the vertical resolution and positively correlate to the horizontal resolution. The estimated TBG impacts weakly depend on the magnitude of uncertainties in the US anthropogenic emissions. The transport/subsidence processes that link airmasses aloft with the surface pollution level are analyzed by trajectories, time-lag correlation and adjoint sensitivity analyses. Various types of observations are used to identify source regions and transport processes, and to improve model prediction using the four-dimensional variational data assimilation during a long-range transport episode. The sectoral and geographical contributions to summertime US black carbon (BC) distributions are studied. NA emissions heavily (>70%) affect the BC levels from the surface to 5 km, while non-NA plumes compose more than half of the BC above 5 km. NA and non-NA BB, NA transportation and non-NA residential emissions are the major contributing sectors. Aircraft measurements during the California phase of the Arctic Research of the Composition of the Troposphere from Aircraft andSatellites (ARCTAS-CARB) field campaign show that BC/(organic matter + nitrate + sulfate) mass ratios fairly well represent BC's warming potential over southern California, which can be approximated by BC/(organic matter + sulfate) and BC/sulfate for plumes affected and unaffected by fires, respectively. The responses of BC/(organic matter + sulfate) and BC/sulfate to removing each emission sector indicate that mitigating NA transportation emissions has the highest potential for regional air quality and climate co-benefits. Contributions from NA BB and extra-regional emissions differ for summer and spring (April 2008).

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