Pathways to the Gauge: the Diurnal Streamflow Cycle as an Indicator of Snowmelt Rates and Transport

Snow water storage and the average rates and paths of meltwater transport within a basin are difficult to monitor directly because snow occurs in patches of nonuniform depth and density and because it melts at varying rates, particularly in mountainous regions. However, another source of information is the daily variability in the discharge in snowfed streams, which provides a unique, indirect measure of these factors. During snowmelt-dominated seasons, the relative amplitude of the diurnal fluctuations, measured as half the difference between the daily peak and subsequent daily minimum, divided by the average discharge during that period is largest when most meltwater reaches the gauge within one day of melting and is smallest when most meltwater arrives as part of the recession curve several days after melting. The timing of the maximum discharge each day also relates to basin drainage rates. The day's peak flow follows the peak radiative forcing most closely when meltwater transport is most rapid. The lag time between melt forcing and discharge increases with longer travel times. Because snowmelt input ceases at night in many rivers, leaving only water contained in storage to drain, the falling limb of the daily hydrograph can be modeled with a recession curve, where flow is approximated as Q(t)=Q₀e^-kt, where Q is discharge, Q₀ is discharge at time zero, k is a recession coefficient, and t is time. Larger values of the recession coefficient, k, correspond to rapid meltwater drainage through the snowpack and basin, while smaller values indicate longer meltwater storage in snow, subsurface, and groundwater reservoirs. Rates of transport are greatest when basin storage is small and the magnitude of discharge is large. Because the snowpack is a dominant water storage compartment in many alpine basins, its depth and extent greatly affect meltwater travel times. Hence, diurnal flow characteristics can indicate the relative size of the snow reservoir over the melt season. These relationships are illustrated in several basins in the Sierra Nevada and Rocky Mountains of the Western United States.