Large area in the Himalayas is covered in snow. Snow in the Himalayas is source of water in North India. Seasonal snow cover is a natural reservoir that temporarily stores water and feeds rivers during the summer months. In addition, due to high albedo, snow is also an important component of earth ’ s radiation budget and hence influences climate. Snow cover is very sensitive to climatic variations and is an indicator of climate change (Kulkarni and Rathore 2003). Therefore, monitoring of snow cover is necessary to assess water availability and climate change (Kulkarni et al. 2006). Modelling snow melt run-off will aid in the management of water resources, development of hydropower generation and flood forecasting (Gupta et al. 2005).
Measurement of snow cover area often poses challenges due to inaccessibility. In the field, point measurements of snowfall using gauges have several limitations and the information acquired is insufficient. Hence, remote sensing technique is used extensively to monitor spatial distribution of snow cover. Monitoring of snow cover using satellite images started in 1960 (Singer and Popham 1963). Since then, it has been carried out using various sensors such as IRS WiFS, AWiFS, LISS-II and LISS III, Terra ASTER, Landsat MSS and TM, National Oceanic and
Atmospheric Administration (NOAA) MODIS. Snow cover can be easily identified and assessed using remote sensing data as it has distinct reflectance characteristics. It has high reflectance in optical region but much lower in SWIR. This characteristic is used to develop Normalised difference Snow Index (NDSI) to map snow cover in the Himalaya (Kulkarni et al. 2006).
In the Himalayas, average snow cover area from 2000 to 2011 varied from * 0.3 to 0.03 million km2. Mean depletion of 0.151, 0.034 and 0.003 million km2 of seasonal snow area was observed in the Indus, the Ganga and the Brahmaputra, respectively, from 2000 to 2011 (Fig. 13) (Singh et al. 2014). The average volume of snow stored in the Indus, the Ganga and the Brahmaputra basin is estimated at 54.5, 9.3 and 14.5 billion cubic meters, respectively. Snow volume was estimated using climatological snowfall data from the ESSO-Indian Meteorological Department and snow cover area from Singh et al. (2014). Snow fall data from Dharmsala, Manali, Shimla and Srinagar stations in the Indus, Joshimath and Mukteshwar stations in the Ganga, and Gangtok and Pasighat stations in the Brahmaputra basin were considered.
Estimating overall trend in the snow cover for the entire Himalayan region is difficult due to the large variability in the snow cover. This variation is in spatial and temporal domain, making it difficult to assess long-term changes in snow cover. In addition, reliable estimates of seasonal snow cover for the Himalayan region are
Fig. 13 Monthly March (accumulation) and September (ablation) snow cover of the period 20002011 for three major basins in Himalayan region. a Snow cover map of March (2011) for the Indus Basin; b Snow cover map of September (2011) for the Indus Basin; c Snow cover variation of all three basins for March; d Snow cover variation of all three basins for September. Source Singh et al. (2014) available from year 2002, i.e. after MODIS data. Therefore, decadal trend in snow cover change has shown little variability. Even though no trend in mean areal extent for the entire Himalayan region was observed, declining trend was observed in the Bhutan and other parts of the Himalaya (Figure 14) (Kulkarni and Rathore 2003; Gurung et al. 2011; Shekhar et al. 2009; Kulkarni 2010; Panwar and Singh 2014). Snow cover area in Bhutan declined by -3.27 ± 1.28 % from 2002 to 2010. Further, in many low-altitude basins such as Ravi, snow ablation was observed even in mid-winter (Kulkarni and Rathore 2003; Kulkarni 2010). Therefore, to assess the changes in the seasonal snow cover, different parameters such as basins located in lower altitude should be studied.
A study was carried out at the Ravi and Bhaga basins using AWiFS data of Resourcesat satellite. The investigation was carried out from 2004 to 2007 at an interval of 5 or 10 days (Kulkarni 2010). A low-altitude basin such as the Ravi has shown a different trend of snow melting in summer and winter months than a high altitude basin such as the Bhaga. In Ravi basin, snow accumulation and ablation were continuous processes throughout winter. Even in the middle of the winter, the snow area was reduced from 90 to 55 %. In the Bhaga basin, snow melting was observed in the early part of the winter, i.e. in the month of December, and no significant melting was observed between January and April as shown in Figs. 15 and 16, indicating different snow ablation patterns in various parts of Himalayas. In addition, snow accumulation and ablation pattern using monthly snow cover distribution in the Alaknanda, Bhagirathi and Yamuna subbasins was studied from 2004 to 2012 using AWiFS data as shown in Fig. 17. Further analysis of the data has also shown small statistically insignificant increase in snow cover (Rathore et al. 2015).
Fig. 14 Decadal trend in snow cover of Bhutan from 2002 to 2010. The solid line represents mean annual snow cover area, and the dashed line represents linear trend for the decade. Source Gurung etal. 2011)
Fig. 15 Snow cover depletion curve for Ravi river basin for year 2004, 2005 and 2006. Source Kulkarni et al. (2010)
Fig. 16 Snow cover depletion curve for Bhaga river basin for year 2004, 2005 and 2006. Source Kulkarni et al. (2010)
Fig. 17 Monthly average snow cover from 2004 to 2012 with ±1r. Source Rathore et al. (2015)