Jessica Lundquist and Michael Dettinger Scripps Institution of Oceanography and United States Geological Survey

Abstract: The daily phase of diurnal cycles of streamflow is a measure of travel times in a basin. When indexed by the hour of maximum discharge each day, the phase represents the time from peak snow melt in the afternoon to the time most water reaches the gage, which ranges from a few hours to almost a full day later. Hourly discharge measurements from sixty basins in the western United States, with areas ranging from 1 to 10,000 km2, illustrate how diurnal streamflow timing varies with basin size and gives insights into how observed diurnal cycles may be used to infer the progress of snowmelt within a given basin. The travel time increases with longer percolation times through deeper snowpacks, increases with longer travel times overland and along longer stream channels, and decreases with faster instream flow velocities. In basins with areasless than 30 km2, travel time through the snowpack dominates diurnal streamflow timing. In particular, daily peak flows shift to earlier in the day as the snowpack thins and the mean discharge increases. Travel time, and thus, daily peak flows shift to earlier in the day as the snowpack thins and the mean discharge increases. In larger basins, heterogeneous snow properties act to cancel out the snowpack signal. Travel time, and thus diurnal timing, in larger basins is dominated by in-channel flow distances, so that the daily peak is delayed as snowmelt is increasingly drawn from the farther reaches of the basins. These observed patterns of diurnal timing can be reproduced by simple models that couple porous-medium flow through evolving snowpacks and free-surface flow in stream channels.