HYDROLOGICAL PROCESSES Hydrol. Process. 20, 741–751 (2006) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.6111 Changes in late-winter snowpack depth, water equivalent, and density in Maine, 1926–2004 Glenn A. Hodgkins* and Robert W. Dudley USGS Maine Water Science Center, 196 Whitten Road, Augusta, ME 04330, USA Abstract: Twenty-three snow-course sites in and near Maine, USA, with records spanning at least 50 years through to 2004 were tested for changes over time in snowpack depth, water equivalent, and density in March and April. Of the 23 sites, 18 had a significant decrease (Mann-Kendall test, p< 0Ð1) in snowpack depth or a significant increase in snowpack density over time. Data from four sites in the mountains of western Maine–northern New Hampshire with mostly complete records from 1926 to 2004 indicate that average snowpack depths have decreased by about 16% and densities have increased by about 11%. Average snowpack depths and water equivalents in western Maine–northern New Hampshire peaked in the 1950s and 1960s, and densities peaked in the most recent decade. Previous studies in western North America also found a water-equivalent peak in the third quarter of the 20th century. Published in 2006 by John Wiley & Sons, Ltd. KEY WORDS snowpack; trends; Maine INTRODUCTION Snowpack data (including the areal snow-cover extent and the snowpack depth and water equivalent) have been used extensively as indicators of climate change in North America. They are important indicators for multiple reasons. Changes in snow-cover extent affect hemispheric and regional heat balances through changes in albedo and outgoing longwave radiation (Groisman et al., 1994; Leathers et al., 1995). Changes in snowpack depth can also affect heat balances because of the energy required to melt snow and evaporate water (Zhang et al., 2004). Changes in snowpack water equivalent (also referred to as the ‘equivalent water content’, i.e. the amount of water contained in the snowpack if it were melted) can impact regional water supply (e.g. Dettinger and Cayan, 1995) and the timing and magnitude of snowmelt runoff, which is often an important factor in regional flooding. As an example, snowmelt runoff added materially to the widespread March 1936 river flooding in the northeastern USA (Grover, 1937). Snowpack in Maine is obviously affected by the local winter climate. The winter climatic gradient in Maine generally parallels the Atlantic Coast, with the warmest weather near the coast and the coldest weather in the western mountains (part of the Appalachian Mountains are located in northern New Hampshire and western Maine), and in far northern Maine (see Figures 1 and 2 for maps of Maine). The median seasonal maximum depth of the snowpack varies from about 500 mm along the coast to more than 800 mm in the western mountains and in northern Maine (Cember and Wilks, 1993). The average water equivalent on or near 1 March ranges from 80 to 130 mm along the coast to 180 to 230 mm in the western mountains and in northern Maine (Loiselle and Hodgkins, 2002). Almost all of the snow-course sites analysed by Loiselle and Hodgkins (2002) are at elevations less than 600 m and thus do not represent the full range of average water equivalent in Maine, since many mountains have elevations higher than 600 m. The average date of * Correspondence to: Glenn A. Hodgkins, USGS Maine Water Science Center, 196 Whitten Road, Augusta, ME 04330, USA. E-mail: gahodgki@usgs.gov Received 14 June 2005 This article is a US Government work and is in the public domain in the USA. Accepted 7 October 2005