Distribution of debris thickness and its effect on ice melt at Hailuogou glacier, southeastern Tibetan Plateau, using in situ surveys and ASTER imagery Yong ZHANG, 1,2 Koji FUJITA, 2 Shiyin LIU, 1 Qiao LIU, 3 Takayuki NUIMURA 2 1 State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China E-mail: zhangy@lzb.ac.cn 2 Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan 3 Key Laboratory of Mountain Environment Evolvement and Regulation, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China ABSTRACT. Debris cover is widely present in glacier ablation areas of the Tibetan Plateau, and its spatial distribution greatly affects glacier melt rates. High-resolution in situ measurements of debris thickness on Hailuogou glacier, Mount Gongga, southeastern Tibetan Plateau, show pronounced inhomogeneous debris distribution. An analysis of transverse and longitudinal profiles indicates that the ground-surveyed debris thicknesses and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)-derived thermal resistances of debris layers correlate strongly over the entire ablation area. Across- and along-glacier patterns of ASTER-derived thermal resistance correspond well with spatial patterns of debris thickness, which may reflect large-scale variations in the extent and thickness of the debris cover. The ice melt rate variability over the ablation area simulated by a surface energy-balance model that considered thermal resistance of the debris layer indicates clearly the crucial role of debris and its spatial continuity in modifying the spatial characteristics of melt rates. Because of the inhomogeneous distribution of debris thickness, about 67% of the ablation area on Hailuogou glacier has undergone accelerated melting, whereas about 19% of the ablation area has experienced inhibited melting, and the sub-debris melt rate equals the bare-ice melt rate in only 14% of the ablation area. INTRODUCTION Many of the glaciers in the Tien Shan, Karakoram, Kunlun, Himalaya and Hengduan ranges have extensive mantles of supraglacial debris in their ablation areas (Li and Su, 1996; Benn and others, 2001; Shi and others, 2005; Zhang and others, 2007). Meltwater is an important component of the water cycle in these regions (Yao and others, 2004). The wide distribution of supraglacial debris mantles on these glaciers strongly affects the rate of ice melting, which in turn modifies the spatial pattern of mass balance and glacier response to climate change (Østrem, 1959; McSaveney, 1975; Nakawo and Young, 1981; Mattson and others, 1993; Kayastha and others, 2000; Nicholson and Benn, 2006). An empirical relationship between supraglacial debris thickness and ice melt rate has been established based on experi- mental observations of glaciers (Østrem, 1959; Nakawo and Young, 1981; Mattson and others, 1993; Adhikary and others, 1997; Kayastha and others, 2000) and laboratory experiments (Reznichenko and others, 2010), which reveals that debris thickness principally controls the sub-debris ablation rate. Therefore, estimation of the extent and thickness of debris cover and its effect on ice ablation is crucial in determining glacier runoff and evaluating water resources in affected regions. Because of practical difficulties in surveying on debris- covered glaciers, detailed in situ measurements of debris thickness have only been performed on a few debris-covered glaciers (e.g. Nakawo and others, 1986; Nicholson and Benn, 2006; Mihalcea and others, 2008a,b) and estimating the spatial variation of debris thickness with sufficient accuracy has thus far proven difficult (Rana and others, 1997; Brock and others, 2007; Racoviteanu and others, 2009). Currently, satellite data have the potential to provide information over wide spatial and temporal scales for monitoring glaciers across large and remote glacierized regions (Raup and others, 2007; Racoviteanu and others, 2009). Several approaches to debris-cover mapping have been applied to different debris-covered glaciers (e.g. Bishop and others, 2001; Taschner and Ranzi, 2002; Paul and others, 2004; Racoviteanu and others, 2008). However, there is no single best method for debris-cover mapping that can be applied to large regions without some manual corrections of the resulting outlines; these various methods have not yet been compared and a superior method has thus not yet emerged (Racoviteanu and others, 2009). Some investigations of debris-covered glaciers using satellite data have focused on extracting surface temperature information for numerical models of ice melt beneath debris cover (Rana and others, 1997; Nakawo and Rana, 1999) or deriving supraglacial debris-cover patterns (Mihalcea and others, 2008b). To assess the effect of debris cover on ice ablation, several numerical models have been proposed, generally based on energy balance for the debris layer and driven by meteorological variables and the physical properties of debris covers (e.g. McSaveney, 1975; Nakawo and Young, 1981, 1982; Kayastha and others, 2000; Han and others, 2006; Nicholson and Benn, 2006; Reid and Brock, 2010). The main challenge in applying these models is the requirement for high-quality input parameters related to the extent, thickness and thermal properties of debris covers. Journal of Glaciology, Vol. 57, No. 206, 2011 1147