Evaluation of Snow Cover Depletion to Support Snowmelt Runoff Prediction for the Cache la Poudre River, Colorado

The Cache la Poudre River in northeastern Colorado is a source of water for many agricultural, municipal, and industrial users. Most runoff in the basin is generated from snowmelt, but snow measurements are sparse, located only at a few high elevation SNOTEL stations and snow courses. Over much of the watershed, no snow measurements are available to support runoff forecasts. For this study we analyzed snow covered area (SCA) depletion characteristics to evaluate whether SCA data could improve snowmelt runoff prediction. Moderate Resolution Imaging Spectroradiometer (MODIS) 8-day snow-cover products were obtained for the Cache la Poudre basin from 2000 to 2006 for March through June of each year. We analyzed snow cover depletion characteristics for spatial subsets of the basin, including sub-basins and elevation bands. Regression analyses compare the 8-day SCA images to 8-day average stream flow at the USGS canyon mouth gauge (the forecasting location). Results from regression analyses show a wide range of relationships between SCA and streamflow (0.032<0.92), mostly as a result of high inter- annual variability in the flow regime. SCA image impairment from cloud cover was generally low but did impact results in some years. For sub-basins, the strongest correlations between SCA and streamflow were for high elevation sub-basins (0.602<0.92), whereas for elevation bands, the strongest correlations were for a mid-elevation band, 2680-3042 m (0.602<0.92). The poorest relationships between SCA and streamflow occurred for low elevation bands, 1591-1953 m and 1954-2315 m, and very high elevation bands, 3406-3768 m and 3769-4131 m. The strong relationship between SCA and discharge at middle elevations suggests that runoff prediction can be improved by monitoring snow cover within these areas. The initial rise in the snowmelt hydrograph correlates well with SCA depletion at middle elevations, whereas the onset of peak flow does not occur until a significant change in snow water equivalent (SWE) is observed at the high elevation SNOTEL sites. These relationships between snow data and runoff will be used to develop and test a new forecasting methodology that integrates both the ground-based and satellite- derived observations.