HYDROLOGICAL PROCESSES Hydrol. Process. 15, 3409–3422 (2001) DOI: 10.1002/hyp.1043 Spectral albedo/reflectance of littered forest snow during the melt season Rae A. Melloh, 1 * Janet P. Hardy, 1 Robert E. Davis 1 and Peggy B. Robinson 2 1 US Army Cold Regions Research and Engineering Laboratory, Engineering Research and Development Center, Hanover, NH, USA 2 Geographic Data Technology Inc., Lebanon, NH, USA Abstract: Despite the importance of litter on forest floor albedo and brightness, previous studies have not documented forest floor albedo or litter cover in any detail. Our objective was to describe the seasonal influence of litter on spectral albedos and nadir reflectances of a forest snowpack in a mixed-hardwood stand in the Sleepers River Research Watershed (SRRW) in Danville, Vermont (37 ° 39 0 N, 119 ° 2 0 W). Experimental measurements in a nearby open area at the Snow Research Station of the SRRW nearly duplicated the spectral trend observed in the forest. Spectral albedo and nadir reflectance measurements in the visible and near infrared (350–2500 nm) transitioned from a gently curved shape through the visible range (for finer-grained, lightly littered snow) to one having a peak in the red/near-infrared (near 760 nm) as the snowmelt season progressed (for coarser-grained, more heavily littered snow). The snowpack became optically thin as surface litter reached high percentages. A point-in-time digital photographic survey of the late-lying snowpacks of three forest stands and the open showed that median litter cover percentages in the coniferous, deciduous, mixed-forest, and an open area were 17Ð5, 6Ð1, 1Ð2, and 0Ð04 respectively. A Kruskal–Wallis ANOVA on ranks and pairwise comparisons using Dunn’s test indicated that the litter covers of the three forest stands were significantly different with >95% confidence. The snowpack was relatively shallow (<1 m), as is typical for this area of Vermont. From a remote-sensing standpoint, and since shallow snow and increased grain size also lower the visible albedo, we can expect that snowpack litter will cause decreased albedo earlier in the snowmelt season, at deeper snow depths, and will tend to shift the maximum albedo peak to the red/NIR range as the melt season progresses. Published in 2001 by John Wiley & Sons, Ltd. KEY WORDS spectral albedo; spectral reflectance; littered snow; forest snowpack; Sleepers River Research Watershed INTRODUCTION Forest snow albedo is important in determining snowmelt rates and influencing seasonal climate in forested regions, particularly for less dense forests that permit direct shortwave radiation through canopy gaps. The effective albedo of a snow-covered forest floor depends on snow properties, especially snow grain size, but also on forest debris lying on and within the snowpack. Forest debris has a lower albedo than snow and heats more readily in response to absorption of solar radiation. The debris then heats the snow through conduction and longwave radiation, and this increases snowmelt. Thick layers of debris can insulate the snow and reduce melt rates (Adhikary et al., 1997); however, forest litter on and within snow is typically thin and discontinuous. Davis et al., (1997) showed that solar radiation dominates the energy exchange near the forest floor in boreal conifer canopies of <60% cover. At higher canopy densities longwave radiation becomes important, in which case the net radiation at the snow surface becomes increasingly insensitive to litter cover. Albedos from above a forest canopy can vary dramatically when a snow cover is present beneath the canopy for tree coverage <70% (Ni and Woodcock, 1997); therefore, the reduced effective albedo of littered snow can affect inversion * Correspondence to: R. A. Melloh, CRREL–ERDC, Environmental Research Branch, 72 Lyme Road, Hanover, NH 03755, USA. E-mail: rmelloh@crrel.usace.army.mil Received 10 May 2001 This article is a US Government work and is in the public domain in the USA. Accepted 7 September 2001