Hydroclimate of the Upper Colorado River Basin and the western United States, The

2014 Summer.Includes bibliographical references.Understanding water budget variability of the Upper Colorado River Basin (UCRB) is critical, as changes can have major impacts on the region's vulnerable water resources. Using in situ, reanalysis, and satellite-derived datasets, surface and atmospheric water budgets of the UCRB are analyzed. All datasets capture the seasonal cycle for each water budget component. Most products capture the interannual variability, although there are some discrepancies with atmospheric divergence estimates. Variability and magnitude among storage volume change products also vary widely. With regards to the surface budget, the strongest connections exist between precipitation, evapotranspiration (ET), and soil moisture, while snow water equivalent is best correlated with runoff. Using the most ideal datasets for each component, the atmospheric water budget balances with 73 mm leftover. Increasing the best estimate of ET by 5% leads to a better long-term balance between surface storage changes, runoff, and atmospheric convergence. It also brings long-term atmospheric storage changes to a better balance of 13 mm. A statistical analysis and case study are performed to better understand the variability and predictability of the UCRB's hydroclimate. Results show significant correlations (at the 90% confidence level) between UCRB temperature and precipitation, and El Nino - Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) during the fall. However, correlations are typically not greater than 0.4. ENSO and PDO are associated with the second mode of variability in a Principal Component analysis, while the first mode of variability (57% of variance for precipitation and 74% of variance for temperature) displays a high year-to-year variability. A case study of a wet and a dry year (with similar ENSO/PDO conditions) shows that a few large accumulation events is what drives the seasonal variability. These large accumulation events are more dependent on a variety of more local synoptic conditions (e.g., location of low pressure, direction and speed of local winds, amount of moisture available). An analysis of ten winters shows that there are generally less than five large accumulating events in drier winters, with closer to ten in wetter winters. Overall, the statistics and case study show that a consistently accurate seasonal forecast for the UCRB is not achievable at this time. Expanding the ideal datasets selected over the UCRB, an analysis of the errors in atmospheric and surface water budgets is performed for every individual HUC4 basin over the western U.S. Surface water budgets show overall much smaller residual errors than the atmospheric water budgets over the region. Visually analyzing the balances and imbalances, we see that several different areas around the Continental Divide and the Great Basin balance well at the surface, but not as well in the atmosphere; around Arizona, most basins don't balance at either the surface or atmosphere; many of the Pacific coastal basins and basins in the northern Rocky mountains balance well at the surface and in the atmosphere. These balances/imbalances, climate variability, land cover, and topography are combined to delineate five hydroclimate zones. Seasonal and interannual variability is analyzed for each zone. The Pacific Coast zone shows strong agreement amongst the seasonal cycles of all the water budget components, while most of the other zones show an offset in peaks between components during the winter and summer