HR: 0830h
AN: C41B-0958    [PDF]
TI: The Effect of Basin Scale on Diurnal Streamflow Timing in Snowfed Catchments
AU: * Lundquist, J D
EM: jlundquist@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, MC-0213 9500 Gilman Drive, La Jolla, CA 92093-0213 United States
AU: Dettinger, M D
EM: mdettinger@ucsd.edu
AF: United States Geological Survey, SIO, UCSD, MC-0224 9500 Gilman Drive, La Jolla, CA 92093-0224 United States
AB: 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 km$^{2}$, 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. Correlations between diurnal timing and daily mean discharges help to identify which of these three factors dominates at a given basin scale. In basins with areas less than 30 km$^{2}$, snow properties that determine the travel times through the snowpack dominate 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 diurnal timing, in basins larger than 30 km$^{2}$ but with main channel lengths less than 50 km 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. Diurnal timing in these basins is particularly susceptible to change in years when spring storms bring new snow to the basin and reset the geographic distributions of (and travel distances from) snowmelt. The drop in discharge due to colder temperatures is accompanied by earlier daily peak flows because the new snow has a shorter travel distance. Travel times in large basins with main channel lengths longer than 60 km are governed by stream velocities, with peak flows that arrive earlier as mean discharges and channel velocities increase. 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.
DE: 1800 HYDROLOGY
DE: 1860 Runoff and streamflow
DE: 1863 Snow and ice (1827)
SC: Cryosphere [C]
MN: 2003 Fall Meeting