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Title: Effects of Changing Winter Precipitation Phase on Streamflow Magnitude in Mountainous Regions
Abstract: Snow and snowmelt produced streamflow (Q) are the dominant sources of water for a large
portion of the global population. In higher elevation regions in the Western United States this is
especially true. Precipitation is highly variable and seasonal in this region of the world.
Generally, most of the precipitation occurs in the winter months and comes as snow in the
higher elevations. However, due to climate change, precipitation and temperature patterns are
changing. It has been observed in many areas across the Western United States that the
fraction of precipitation that is falling as snow (S f ) is declining because of a warming climate. In
addition, it has largely been accepted that warmers temperatures are also causing snow to melt
earlier in the season. These changes are causing unforeseen changes to the snowmelt-driven Q
relationship. Predications on how a warming climate will affect Q response are varied and often
conflicting. In some studies, a reduction in S f and earlier melt off result in less runoff generation
and in other studies a reduction in in S f result in more runoff generation. The relationship that
changing temperature and precipitation patterns have on the magnitude of runoff is still not
well understood. To better understand the relationship between precipitation, precipitation
phase, and the resulting Q, this study uses meteorological data and hydrological data from
multiple sub-basins within the Dry Creek (DCEW) near Boise, Idaho to identify years in each
basin where most of the precipitation fell as either snow or rain. A comparison of the resulting
streamflow’s, Q, was done by calculating runoff ratios (Ro) for each year and sub-basin to
determine if Q is changing with a change in precipitation phase. In addition to finding the trend
between precipitation phase and the resulting Ro, trend analysis was done on various other
explanatory variables such as soil moisture, SWE, S f , snow persistence (Sp), and other
meteorological variables to determine why Q magnitude may be changing. An initial hypothesis
was that sub-basins with more precipitation falling as rain will show a decrease in Ro and that
soil moisture and Sp are controlling variables. It is imperative that we understand how this
change will affect Q magnitude and timing to better predict water availability in snow
dominated regions that will be primarily affected by changing precipitation patterns.
Advisor: Jim McNamara
Committee: Alejandro Flores, Anna Bergstrom