What Makes Rain-on-Snow Events Hazardous: Field Study at Ward Valley, Lake Tahoe Basin

Rain-on-snow events tend to be more hazardous than snow-free-rainfall-runoff or snowmelt events in Western States. The field observations in Ward Creek watershed, Tahoe Basin, showed that the snowmelt induced by energy flux from raindrops scarcely contributes to the hillslope runoff during the major rain-on-snow event of May 7, 2000, due to the cold weather. Spring high flows in the Sierra Nevada may be mainly due to the high soilwater content in the top soil kept by continuous snowmelt water supply. It was also found that the overland flow or longitudinal flow within the snowpack may still happen even over unfrozen and unsaturated topsoil on a relatively mild hillslope (16 %). The overland/in-snow flow may be due to the difference in hydraulic conductivities of the snow and the top-soil. This overland flow within the snowpack may form up to 10 percent of the peak flood discharge at the field hillslope scale. It may be hypothesized that the overland/in-snow flow on the unsaturated top soil may be a common phenomenon. The high flood peak of the rain-on-snow events may be magnified by this insnow fast runoff mechanism as well as the snowmelt by raindrop energy transfer. To test this observation-based hypothesis, further studies on the runoff process within the snowpack are desirable.