Anne Sawyer, M.S.

Snowpack Depletion Modeling Using Fast All-season Soil Strength (FASST) and SnowModel in a High-elevation, High Relief Catchment in the Central Rocky Mountains

M.S. (Watershed Science) 2007 Colorado State University, Fort Collins, CO, USA 80523-1472
B.S. (Geology) 2001 Carleton College, Northfield, Minnesota, USA 55057

Sawyer, A.E., 2007. Snowpack Depletion Modeling Using Fast All-season Soil Strength (FASST) and SnowModel in a High-elevation, High Relief Catchment in the Central Rocky Mountains. Unpublished M.S. thesis, Watershed Science, Colorado State University, Fort Collins, Colorado, USA, 99pp.


In the western United States, snowmelt from mountain basins has historically provided 70-90% of annual runoff and the winter snowpack acts as a reservoir to store water for spring and summer soil moisture and stream recharge. Modeling the timing and magnitude of snowpack depletion and runoff in mountainous basins is an essential tool for forecasting water supply for irrigation, drinking and industrial uses. Modeled point estimates of snow depth depletion at two forested, sub-alpine sites (using Fast All-Season Soil STrength (FASST) and SnowModel) were compared to observed seasonal snow depths from an acoustic snow depth sensor. Meteorological forcing data for each model were collected at both sites between March and June of 2003 and included air temperature, relative humidity, air pressure, wind speed and direction, incoming and outgoing shortwave radiation and upwelling and downwelling longwave radiation. Precipitation was measured using precipitation gauges near each site.

SnowModel was also used to simulate distributed snow cover depletion and runoff in a mountain catchment, St. Louis Creek (82.5 km2), at varying spatial resolutions of Hydrologic Response Units (HRUs). HRUs were created based on physiographic characteristics of the basin including elevation, slope, aspect and vegetation cover. The number of HRUs in five simulations ranged from one (basin average) to 3726. Snow-covered area (SCA) and basin-average snow water equivalent (SWE) depletion curves were generated for each simulation. Depletion curves were compared to modeled and observed St. Louis Creek discharge. Diversions above the basin outlet necessitated the reconstruction of 2003 St. Louis Creek discharge using statistical relationships between discharge from St. Louis Creek and two smaller gauged streams within the basin using pre-diversion discharge data (1943-1955).

Both FASST and SnowModel successfully simulated one-dimensional snow depth depletion at both sites when compared to observed snow depth using standard statistical metrics for evaluation. SnowModel produced realistic SCA and SWE depletion curves for St. Louis Creek basin, and the finest spatial resolution simulation best represented the spatial variability within the basin and produced the most realistic results. However, as anticipated, the timing and magnitude of runoff was incorrect due to a lack of a runoff routing module within SnowModel.

Advisor: Steven Fassnacht
Co-Advisor: Kelly Elder (USFS RMRS)
Susan Frankenstein (USACE-CRREL)
Greg Butters (Soil and Crop Sciences)

Last update: SRF, 2016-06-15