Peak flow transitions from snowmelt to rainfall dominance along the Colorado Front Range: Spatial patterns and temporal trends

For a given quantity of precipitation, its form (snow or rain) will affect the magnitude and timing of streamflow, with consequences for both water resource and flood management. Using 20 streamflow gaging stations along a gradient of snow persistence and elevation, we identify annual peak flow sources (snowmelt, rainfall, mixed) from a gridded snow accumulation and melt model. We then relate these peak flow sources to annual average January-June snow persistence (SP), evaluate whether peak flow sources have changed over time, and examine how peak flow magnitude and timing differ with flow source. Results show that watersheds with SP<0.3 (low snow, mean elevation <2000 m) have experienced only rainfall-runoff annual peak flows in the period of record, and watersheds with SP>0.7 (persistent snow, mean elevation >3100 m) have mostly snowmelt-runoff peak flows, with mixed sources between these thresholds. At all elevations, rainfall may produce the annual peak flow, but the likelihood of rainfall-runoff peaks declines with increasing SP. Regional Kendall trend tests show that the contributions of snowmelt to peak flows and total annual inputs have declined in the mixed zone but not in the snowmelt-dominated zone, where SP is still high enough for peak flows to remain snowmelt-dominated. While rainfall runoff has produced some of the highest unit area peak flows in the region, on average snowmelt runoff produces higher unit discharge and more attenuated hydrographs than rainfall runoff. This example illustrates how peak flow sources are most sensitive to loss of snow in elevations where snow transitions from persistent to intermittent, whereas high elevations still have long enough snow persistence that they have remained snowmelt-dominated.