Snowcover along elevation gradients in the Upper Merced and Tuolumne River basin of the Sierra Nevada of California from MODIS and blended ground data Robert Rice a, Roger Bales a, Thomas H. Painter b, Jeff Dozier c a Sierra Nevada Research Institute, UC Merced b National Snow and Ice Data Center, CU Boulder c Bren School, UC Santa Barbara MODIS false color composite of Tuolumne, Merced and adjacent basins.
Upper Tuolumne basin, May 2005. MODIS snowcover product. Snowcover patterns: 2004 & 2005. Snow water equivalent (SWE) analysis. Critical elevation bands: measurement & change.
Tuolumne- 4,184 km 2 Merced- 2,846 km 2 elevation, m 3987 19
MODSCAG MODIS Snow Covered Area and Grain size April 10, 2004 Based on MEMSCAG (Multiple Endmember Snow-Covered Area and Grain Size) Painter et al., 2003, RSE
Filtering and smoothing of MODIS daily snow cover and grain size Objective: produce time series that is spatially and temporally complete and consistent Contributors to noisy daily values Data dropouts and sensor noise Cloud cover Sensor viewing angle (nadir to 65 ) Smearing pixels up to 2.5 in cross-track direction Off-angle views of snow under trees see less snow than nadir views Other Subpixel cloud cover Topographic variability within pixel Etc
Example of cloudy day
Effect of viewing angle (Tuolumne & Merced)
Noisy variability caused by look angle, small clouds, vegetation, topography detail Vegetation causes differences in view angle 8
Need to interpolate and smooth to fill the space-time cube Raw snow cover Interpolated snow cover
Approach Wiener 2D filter to eliminate noise (set noisy values to NaN). Cloud/snow discrimination based on grain size and location (set cloudy values to NaN). Interpolate in time dimension using smoothing splines,, weighted by cos 2 of view angle. Then interpolate in spatial dimension with quickhull algorithm.
Tuolumne 2004 January-July July
Tuolumne 2004 October-2005 July
Tuolumne Upper Tuolumne (2,420 km 2 above 1500-m), snow depth & snow water equivalent (SWE) are measured daily at 7 snow pillows & monthly at 17 snow courses. Distributed SWE estimates developed from a range-wide interpolation. Merced In the Upper Merced (1,755 km 2 above 1500-m), snow depth & snow water equivalent (SWE) are only measured daily at only 3 snow pillows & monthly at 2 additional snow courses.
Melt season SCA: 1.0 0.8 Tuolumne 2004 elev band, m 3750 Tuolumne 2004: 2004: less snow (83% of normal) ground became snow free relatively early SCA, fraction 0.6 0.4 0.2 3450 3150 2850 2550 2250 1950 1650 Each higher elevation band requires ~1 mo longer to become snow free 0.0 3/1/04 4/1/04 5/1/04 6/1/04 7/1/04 8/1/04 9/1/04 10/1/04 Merced 2005 Merced 2005: more snow (163%) ground became snow free ~1 mo later than in 2004
Tuolumne -2004 interpolated, blended SWE Little elevation difference in interpolated SWE at beginning of snowmelt. SWE (m) 1.0 0.8 0.6 0.4 0.2 elevation band, m 3750 3450 3150 2850 2550 2250 0.0 Jan Feb Mar Apr May Jun Jul 2004 1.0 Average SWE across an elevation band, accounting for fractional SCA, shows distinct gradient with elevation. SWE x SCA (m) 0.8 0.6 0.4 0.2 0.0 Jan Feb Mar Apr May Jun Jul 2004
Merced -2005 interpolated, blended SWE Little elevational difference in interpolated SWE at beginning of snowmelt Average SWE across an elevation band, accounting for fractional SCA, shows distinct gradient with elevation
Snowmelt by elevation band based on interpolated, masked SWE: June/July 2005 Progressive contributions from higher elevation bands w/ time. No to little contributions form lowest or highest elevations. Volumne snowmelt, 10 7 m 3 30 25 20 15 10 5 contributions to snowmelt by elev band, m Tuolumne 0 6/15/05 6/20/05 6/25/05 6/30/05 7/5/05 7/10/05 7/15/05 Merced 3750 3450 3150 2850 2250 2550 3450 3150 2850 2550 2250
Alternate estimate of SWE & snowmelt from SCA time series & snowmelt analysis Determine potential snowmelt per elevation band based on energy balance or degree day calculation. Potential snowmelt quantity applied to all areas with snowcover, using fractional SCA per pixel. That is, if an area has snowcover it is assumed to contribute melt at a rate equal to the potential snowmelt times SCA. Amount of snowmelt calculated up to day when SCA is depleted equals beginning SWE.
Tuolumne Degree day factor estimated from snow pillow sites SWE, cm 120 100 80 60 40 Kibbie Ridge Paradise MDW Horse MDW Tuolumne MDW Slide CNY Dana Meadow No systematic variation of degree day factor with elevation. Strong seasonal change in degree day factor. Degree day factor 20 0.5 0.4 0.3 0.2 2004 snow pillow data 0 Jan Feb Mar Apr May Jun Jul Gin Horse Kibbie Paradise Slide Tuolumne Tioga 0.1 0.0 3/1/04 4/1/04 5/1/04 6/1/04 7/1/04
Merced Degree day factor estimated from snow pillow sites No systematic variation of degree day factor with elevation. Strong seasonal change in degree day factor.
60 Contributions to snowmelt by elevation band, 2004 3750 2004 snowmelt based on degree day calculation Volumne snowmelt, 10 7 m 3 50 40 30 20 10 Tuolumne 1950 0 3/1/04 4/1/04 5/1/04 6/1/04 7/1/04 8/1/04 9/1/04 10/1/04 Merced 3450 3450 3150 2850 2550 2250 3750 3150 2850 2550 2250 1950
Tuolumne-2004 Comparing snowmelt based on interpolated SWE vs. degree day calculation Rapid depletion based on interpolation of snow pillow data. Large contributions from below 2100 m by interpolation method. Interpolation over-estimates below 3000 m & underestimates above 3000 m. Volumne snowmelt, 10 7 m 3 Volumne snowmelt, 10 7 m 3 60 Based on SCA depletion and degree day calculation 50 3450 40 3150 30 2850 20 2550 10 2250 1950 0 3/1/04 4/1/04 5/1/04 6/1/04 7/1/04 60 50 40 30 20 10 Based on interpolated SWE 3450 3150 2850 2550 2250 1950 0 3/1/04 4/1/04 5/1/04 6/1/04 7/1/04
Merced -2005 Comparing snowmelt based on interpolated SWE vs. Degree day calculation Lower elevations depleted prior to this June period Interpolated method also shows more rapid depletion at mid elevations Less total melt for interpolated method because snow pillow sites melted out early 3450 3150 2850 2550 2250 3450 3150 2850 2550 2250
One source of error is vegetation Note gap between accumulation season SCA & canopy fraction below ~2400 m elevation
Fraction of snowmelt contributed by various elevation bands 0.6 March - Septemeber 2004 0.5 0.4 Snowmelt Catchment area Fraction 0.3 0.2 0.1 0.0 1650 1950 2250 2550 2850 3150 3450 3750 Elevation band, m Tuolumne Merced
0.6 0.5 0.4 Snowmelt Catchment area March 2004 Tuolumne fraction 0.3 0.2 March snowmelt 0.1 0.0 1650 1950 2250 2550 2850 3150 3450 3750 Elevation band, m Merced Main contributions 2004: 1800-3000 m 2005: 1800-2700 m Colder at higher elevations
0.6 April 2004 Fraction 0.5 0.4 0.3 Snowmelt Catchment area Tuolumne April snowmelt 0.2 0.1 0.0 1650 1950 2250 2550 2850 3150 3450 3750 Elevation band, m Main contributions: 2004: 2100-3000 m 2005: 1800-2700 m Merced
0.6 Fraction 0.5 0.4 0.3 Snowmelt Catchment area May 2004 Tuolumne May snowmelt 0.2 0.1 0.0 1650 1950 2250 2550 2850 3150 3450 3750 Elevation band, m Main contributions: 2004: 2400-3300 m 2005: 2100-3300 m Merced
0.6 0.5 0.4 Snowmelt Catchment area June 2004 Tuolumne Fraction 0.3 0.2 June snowmelt 0.1 0.0 1650 1950 2250 2550 2850 3150 3450 3750 Elevation, m Merced Main contributions: 2004: 2700-3600 m 2005: 2400-3300 m
0.6 July 2004 Fraction 0.5 0.4 0.3 Snowmelt Catchment area Tuolumne July snowmelt 0.2 0.1 0.0 1650 1950 2250 2550 2850 3150 3450 3750 Elevation, m Main contributions: 2700-3600 m Merced
0.6 August 2004 Fraction 0.5 0.4 0.3 Snowmelt Catchment area Tuolumne August snowmelt 0.2 0.1 0.0 1650 1950 2250 2550 2850 3150 3450 3750 Elevation, m Main contributions: 2004: 3000-3900 3900 m 2005: 3000-3600 3600 m Merced
Conclusions: snow in Upper Tuolumne & Merced (2004 & 2005) 36%-of Tuolumne (34%-Merced) snowmelt from above 3000 m. Note: highest snow pillow is 2918 m. Main source of Aug-Sep snowmelt. Also important in July (2004 & 2005) & June (2004). 13%- of Tuolumne (5%-Merced) snowmelt from below 2100 m. Note: lowest snow pillow is 2100 m. Important mainly in March (2004 & 2005) & April (2005). 50%- of Tuolumne (60%-Merced) snowmelt from 2100-3000 m. SCA depletion occurred over 2 mo at 2100-2400 2400 m, 4 mo at 2700-3000 m. SCA depletion required 1 mo longer at each higher elevation. All snow pillows melt out much faster, 1.5-2 2 mo. SCA depletion maps provide a better quantitative basis for estimating basin-scale scale SWE than do snow pillows. Vegetation may cause under-estimation estimation of 20-50% at elevations below 2400 m & 5% overall
Acknowledgement Funded by NASA Grant NNG04GC52A REASoN CAN Multi-resolution snow products for the hydrologic sciences. Xiande Meng (UCM), Peter Slaughter (UCSB).