39 Snow cover and its melt dominate regional climate and water resources in many of the 40 world's mountainous regions. Snowmelt timing and magnitude in mountains are controlled 41 predominantly by absorption of solar radiation and the distribution of snow water equivalent 42 (SWE), and yet both of these are very poorly known even in the best-instrumented mountain 43 regions of the globe. Here we describe and present results from the Airborne Snow 44 Observatory (ASO), a coupled imaging spectrometer and scanning lidar, combined with 45 distributed snow modeling, developed for the measurement of snow spectral albedo/broadband 46 albedo and snow depth/SWE. Snow density is simulated over the domain to convert snow 47 depth to SWE. The result presented in this paper is the first operational application of remotely 48 sensed snow albedo and depth/SWE to quantify the volume of water stored in the seasonal 49 snow cover. The weekly values of SWE volume provided by the ASO program represent a 50 critical increase in the information available to hydrologic scientists and resource managers in 51 mountain regions. 52 53 3 54 55Introduction 56Snow cover and its melt dominate sources in many of the world's mountainous regions, and 57 in adjacent areas dependent on river flows originating from mountain basins. In the western 58 United States, snowmelt runoff dominates the surface water hydrology, providing more than 59 75% of the total freshwater (Bales et al., 2006). However, the region faces significant water 60 resource challenges due to the intersection of increasing demand from population growth and 61 changes in runoff volume and timing due to climate change (Christensen et al., 2004; 62 Christensen and Lettenmaier, 2007). 63Observations indicate an ongoing reduction in the seasonal duration of mountain snowpacks 64 Mote et al., 2005;Hamlet et al., 2007;Clow, 2010), a trend likely to 65 continue under unimpeded warming associated with climate change (Christensen and 66 Lettenmaier, 2007;Deems et al., 2013b). Moreover, increasing temperatures in desert systems 67 will increase dust loading to mountain snow cover (Munson et al., 2011), thus reducing the 68 snow cover albedo and accelerating snowmelt runoff (Painter et al., 2007;Painter et al., 2010; 69 Skiles et al., 2012). 70The two most critical properties for understanding timing and magnitude of snowmelt runoff 71 are the spatial distributions of snow albedo and snow water equivalent (SWE) (Blöschl, 1991; 72 Kirnbauer et al., 1994). Despite the importance of these properties in controlling volume and 73 timing of runoff, the mountain snowpack remains poorly quantified around the globe (Bales et 74 al., 2006) (Barnett et al., 2005, leaving runoff and climate models poorly constrained and our 75 physical understanding of mountain snowmelt driven systems incomplete. 76
4In the western US, we have relatively sparse measurements of SWE, mostly at lower and 77 middle elevations and only a few per basin (Bales et al., 2006). These measurements are used as 78 in...
