Seasonal and
Spatial Patterns in Diurnal Cycles in Streamflow
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the paper:
A remarkable feature of many rivers in the western United States is a regular daily variation in streamflow—a variation that is 10-20% of the mean flow in half of the unimpaired rivers that we have studied in the region (Figure 1, Merced River diurnal cycle and amplitude). These cycles are good indicators of the factors involved in the water balance for each river, thus providing useful information for detecting climate change.
This climate change application arises because there is a substantial difference in the timing, relative magnitude, and variation of diurnal flow between rivers in which water is added diurnally (as from snowmelt), and rivers in which water is removed diurnally (as from evapotranspiration (ET) and infiltration). While snowmelt-dominated rivers achieve their largest diurnal fluctuations during the high flows of spring melt, evapotranspiration/infiltration-dominated rivers exhibit strongest diurnal cycles during summer months, when discharge is low. In large snowmelt-dominated basins, the time of peak discharge shifts to later in the day as the melt season progresses and the snow line retreats to higher elevations. In evapotranspiration/infiltration-dominated rivers, the hour of peak flow occurs consistently in the morning throughout the summer. Snowmelt-dominated rivers have sharp rises and gradual declines in discharge each day, while evapotranspiration/infiltration-dominated rivers have a gradual rise and sharp decline each day.
Several rivers shift from snowmelt to evapotranspiration/infiltration characteristics during late spring or summer. The date of this shift, along with other interannual variations in diurnal cycle characteristics, should prove useful for diagnosing how climate changes are affecting watersheds.
These findings stem from a study of 100 unimpaired rivers with U.S. Geological Survey hourly discharge measurements during 1996-2000 (and over half of the unimpeded rivers had a detectable diurnal cycle). This study emphasizes the climatological value of these hourly stream flow samples, which are ongoing measurements (though multidecadal periods are not yet readily available).