Influence of debris cover on terminus retreat and mass changes of Chorabari Glacier, Garhwal region, central Himalaya, India D.P. DOBHAL, Manish MEHTA, Deepak SRIVASTAVA Centre for Glaciology, Wadia Institute of Himalayan Geology, Dehra Dun, Uttarakhand, India E-mail: msmehta75@gmail.com ABSTRACT. Recent studies of Himalayan glacier recession indicate that there is wide variability in terminus retreat rate and mass balance in the different sectors of the mountain range, primarily linked to the topography and climate of the region. Variable retreat rates of glacier termini and inadequate supporting field data (e.g. mass balance, ice thickness, velocity, etc.) in the Himalayan glaciers make it difficult to develop a coherent picture of climate change impacts. In this study, the results of a detailed mapping campaign and ground-based measurements of ablation rate, terminus retreat and ice loss are reported for the period 2003–10. In addition, background information from an old glacier map (Survey of India, 1962) was compiled and terminus recession measurements were carried out from 1990 field photographs of Chorabari Glacier, central Himalaya. Ourablation stake network results suggest that the influence of debris cover is significant for Chorabari Glacier mass balance and terminus retreat. The terminus survey finds that the glacier is retreating, but at a lower rate than many other non-debris- covered glaciers in the region. The recession and ablation data (particularly in the upper ablation area at higher altitudes) suggest that the ice volume loss of the glaciers is of greater magnitude than the slow terminus retreat and, if the process continues, the lowermost part of the glacier may reduce to a quasi- stationary position while significant ice loss continues. INTRODUCTION The Himalayan mountain range contains thousands of glaciers of widely varying properties, which are spread over nearly 37 000 km 2 with an east–west range >2000 km (Raina and Srivastava, 2008). This large geographic extent, with complex and extreme topography along with variable climatic conditions, results in an inhomogeneous set of glacial recessions. The primary climatic forcing, moving from west to east, is a decreasing influence of the mid- latitude westerlies and an increasing influence of the Indian summer monsoon (Bookhagen and Burbank, 2010). Thus, the distribution of glaciers in the Himalaya is uneven, with a higher concentration of glaciers in the northwest than in the northeast of the mountain range. In general, glaciers in the region have debris-covered ablation areas with debris thickness ranging from millimetres to tens of centimetres. As a consequence of the complex climate system, glacial geometry, glacier surface properties and geology, the recession rates of the glaciers are variable (Scherler and others, 2011). Various studies, such as mapping of debris-cover extent and its correlation with glacier melting and recession, have been carried out using remote-sensing and field-based data in different parts of the world (Lougeay, 1974; Bishop and others, 2001; Taschner and Ranzi, 2002; Paul and others, 2004; Buchroithner and Bolch, 2007; Stokes and others, 2007; Bolch and others, 2008; Shukla and others, 2009; Kamp and others, 2011; Scherler and others, 2011). Supraglacial debris on glaciers is commonly found to have significant control on the rate of ice ablation (Bozhinsky and others, 1986; Lundstrom and others, 1993). A debris cover influences the terminus dynamics and modifies a glacier’s response to climate change (Scherler and others, 2011). Surface ablation rates are generally increased in the presence of a thin (<5 cm) debris cover, but are significantly reduced when a thick (>5 cm) debris cover is present (Østrem, 1965; Lundstrom and others, 1993; Mattson and others, 1993; Reznichenko and others, 2010; Scherler and others, 2011). A thin and patchy debris cover reduces the albedo and elevates shortwave radiation absorption, whereas ablation rates are strongly reduced further down the glacier due to the insulating effect of thicker debris (Mattson and others, 1993; Jackson and Fountain, 2007; Reznichenko and others, 2010). The local mass balance of debris-covered glaciers is distinctly nonlinear and non- monotonic with elevation. Generally ablation takes place below the equilibrium-line altitude (ELA) and ablation rates increase with decreasing elevation; however, at Chorabari Glacier at lower altitudes, where the debris cover thickens to >5 cm, ablation rate decreases. In the central Himalaya a large number of glaciers are debris-covered, especially in the ablation zone, which can exist over stagnant termini. Over some glacier surfaces, growing meltwater ponds and surface lowering due to internal melting indicate that these glaciers are downwasting on the whole. This paper presents recent results of ablation observations on Chorabari Glacier from 2003 to 2010. Our aim is to quantitatively evaluate the influence of the debris cover on summer ablation and terminus recession and to discuss the effects of debris cover on mass-balance processes. Study site Chorabari Glacier (30846’20.58’’ N; 7982’59.381’’ E) is a medium-sized compound valley-type glacier covering an area of 6.6 km 2 . The glacier is located in the Mandakini River basin of the Alaknanda catchment (a tributary of the River Ganga) (Fig. 1a). Chorabari Glacier has its accumu- lation area below Bhartkhunta peak (6578 m a.s.l.) and Kedarnath peak (6940 m a.s.l.) and flows from north to south between 6400 and 3895 m a.s.l. with an average Journal of Glaciology, Vol. 59, No. 217, 2013 doi: 10.3189/2013JoG12J180 961