| Diurnal fluctuations in streamflow provide new insights into snowmelt distributions and processes operating on different spatial and temporal scales. Using hourly data from over 100 USGS streamgages, combined with in-depth field measurements and modeling in Yosemite National Park, this thesis addresses four major issues: (1) spatial and seasonal cycles in diurnal cycles in streamflow, (2) the relative influence of synoptic weather patterns versus topography (such as elevation and aspect) on spring snowmelt timing, (3) the effects of basin scale and heterogeneity on diurnal streamflow timing in snow-fed catchments, and (4) interannual variations in diurnal streamflow characteristics.; Diurnal cycles are ubiquitous in snowmelt-driven streamflows and also appear in other seasons and/or other catchments where evapotranspiration and/or infiltration are dominant influences. Snowmelt-dominated rivers are characterized by sharp rises and gradual declines in discharge each day, while evapotranspiration/infiltration-dominated are characterized by a gradual rise and sharp decline each day.; Elevation and aspect affect snowmelt and diurnal cycles on average, but these effects can be overwhelmed by short-term weather and climate. In Spring 2002, flow in streams at all gage elevations rose at the same time. A synchronous onset is preceded by a clear synoptic pattern and can be forecast by physically-based models.; In small basins (<30 km2), the hour of day when streamflow peaks is directly correlated to snow depth. In most larger basins, spatial heterogeneity in snow depth dampens the snow's effect on diurnal cycle timing, and during the first half of the melt season, the hour of peak discharge varies less than +/-1 hr.; Diurnal cycle characteristics, such as shape, relative amplitude, and timing of peak flow, differ between wet and dry years. These year-to-year differences provide insights into variations of the length of snowmelt versus evapotranspiration-dominated seasons, the relative contributions of fast (overland) runoff routes to stream channels versus slow (subsurface) routes, and seasonal evolutions of snowmelt-contributing areas, at the basin scale.; Together, these findings are helping to shape a new understanding of western snowmelt and runoff processes and may provide new tools for characterizing snowmelt and streamflow variations, using an existing, but underutilized, data resource, hourly flow measurements. |
