Spatial distribution of snow water equivalent across the central and southern Sierra Nevada

1. Spatial distribution of snow water equivalent across the central and southern Sierra Nevada C33E-0592 Roger Bales, Robert Rice, Xiande Meng Sierra Nevada Research Institute, University of California, Merced Scope Comparison of reconstructed snowmelt with point and modeled data The aim of this work is to develop spatial estimates of snow accumulation and melt over time in the Sierra Nevada, for multiple hydrologic and ecological research applications. Daily snowmelt, gridded at 500-m spatial resolution, was reconstructed for 2001-2009 using canopy-corrected fractional snow covered area (fSCA) from MODIS and a temperature-index snowmelt calculation. The MODIS fractional SCA was based on the MODSCAG (MODIS Snow Covered Area and Grain size/albedo) model, and provides a daily estimate of SCA across complex terrain that avoids some of the inherent biases in a binary SCA product. The reconstructed snowmelt values were then compared with available point precipitation and SWE measurements, and with a gridded precipitation product. Results are shown for wet (2006), dry (2007) and average (2008) years. SCA corrections As there are no spatial ground-truth measurements for validation of the amount and spatial variability of the resulting product, snowmelt amounts were compared with point precipitation measurements, point (peak seasonal) SWE measurements and an interpolated/gridded precipitation product (PRISM). Changes in gridded snowmelt were compared with melt recorded at snow pillows and with discharge recorded at stream gauges. Kings River basin Bishop Creek basin uncorrected corrected uncorrected corrected wet 12 Feb 2008 10 Mar 2008 10 Apr 2008 Comparison with snow-pillow measurements dry Sample fSCA images for central and southern Sierra Nevada, 2008 ave At peak accumulation the gridded cumulative snowmelt values were generally within 25% of SWE measured at snow pillows in the same 500-m grid cell, but SWE at each snow pillow melted out 2-6 weeks before snow disappeared from the corresponding grid cell. 11 May 2008 11 Jun 2008 7Jul 2008 Comparison with station precipitation measurements Empirical fSCAcorrections of up to 40% were made in dense forests of the western and 30% in the eastern Sierra Nevada to account for the satellite only detecting snow in viewable gaps in the canopy. Corrections were done using available vegetation data and an analysis of MODSCAG fSCA values for individual pixels (see poster C33E-0589). Summary of reconstructed snowmelt by elevation band Back-reconstructed snowmelt Bishop Comparison is made with raw data, with no partitioning the precipitation into rain versus snow. However, at the sites shown, precipitation was largely snow. At higher elevations both station measurements and interpolated /gridded precipitation were generally within 50% of snowmelt reconstructed for the same location, with greater uncertainty near the rain-snow transition elevation. Kings Bishop Creek Kings River Comparison with PRISM data along a NE-SW transect For ease of viewing, fSCA values are lumped in to 4 bins. Areas with no black/white shading have no detectable fSCA. Kings River basin and Bishop Creek basins are outlined. Bishop Annual and monthly lapse rates Kings Kings basin Owens basin Values are from reconstructed snowmelt estimates, at left Snowmelt was estimated using daily degree-day values and daily degree-day values based on monthly lapse rates, and daily degree-day coefficients (below). Results suggest that highest SWE was in the 3150-3750 range, with the 3750 elevation being higher on the east side of the range. Back-calculatedcumulative snowmelt does not equal SWE on the ground at any given date, as the timing of precipitation is not accounted for in the calculation. Summary of findings The reconstructed snowmelt data, which provide a gridded index of peak snow water equivalent, are a potentially more-accurate spatial estimate of precipitation, snow accumulation and snowmelt in mountain areas dominated by snow versus rain than are station data or gridded/modeled products that are based largely on lower-elevation stations. Degree-day coefficients used to estimate snowmelt Kings Owens Degree days for the temperature-index model were calculated using monthly average lapse rates, with values varying by ±25% over the seasonal snowmelt period. Plots show monthly and annual average temperatures and lapse rates for 17 stations in the Kings basin (W. Sierra) and 17 stations in the Owens basin (E. Sierra). Annual average marked by × and fit by heavy dashed line (slope -6.8oC per 1000 m). Slopes of the individual monthly fitted lines are given in the legends. Slopes were generally higher and more variable on the east side of the range. PRISM estimates for the west side fail to show the elevational differences apparent in the reconstructed snowmelt estimates. PRISM estimates on the east side do show a similar elevational dependence as found in the snowmelt estimates, but are significantly higher. Acknowledgements Elevation-dependent degree-day coefficients were estimated from snowmelt rates observed at snow-pillow sites distributed over the region, with values generally increasing over the snowmelt period. This work was supported in part by NASA grant NNG04GC52A and NSF grant BES-0610112. fSCA products were developed by T. Painter and J. Dozier.