Snow Delineation using MODIS in Indus Basin

Abstract

Remote sensing data in visible and shortwave infrared (SWIR) are very useful for delineation of snow cover. Data from several sensors, e.g. IRS LISS-I to IV, WiFS, AWiFS, Landsat MSS, TM and ETM+, MODIS, AVHRR etc., of fine to coarse resolution are available in the atmospheric windows. There is a tradeoff between resolution and repeativity of data. Cloud coverage is again a problem in having high repeativity. In view of this, MODIS 8-day composite data are selected for monitoring of snow cover in Indus basin for six dates from March to August in the year 2007. Snow was delineated using Normalized Difference Snow Index (NDSI) and Red band. NDSI was used in delineating snow and red band is used to discriminate snow and water. Thresholds (same as Snowmap algorithm of NSIDC) 0.4 and 0.11 are selected for NDSI and red band respectively. Temperature causes melt of snow, which adds to runoff over extended period of time. Since air temperature decreases with altitude with appropriate lapse rate applicable, it is pertinent to do elevation zone wise lumping in empirical snow melt runoff models. Elevation zones are obtained from topographic maps and digital elevation models (DEM). DEM at resolutions from 30 m to 1 km are now readily available in internet. For large areas it is useful to choose medium resolution DEM. For this study, SRTM 250 m DEM is selected. The DEM was used to delineate Indus basin. The basin boundary was modified to match historical boundary in the upper reaches. Using the DEM, 1000 m zones are delineated. DEM is also used to plot hypsometric curve and determine mean and median elevations for zones. Snow cover statistics is computed for elevation zone and snow depletion curves are plotted. Elevation in the basin ranges from 948 to 8572 m. Most of the area lies between 3000 to 6000 m. Snow cover area varies from 14909 to 112520 sq. km. In Zone 8 (7000- 8000 m), maximum snow cover reaches by Mid April, where as in other zones snow cover peaks before March. In Zone 8, depletion starts by July 1st week. In Zone 7, depletion starts by June 1st week. In these zones, snow cover is above nearly 70%. In Zones 6, 5, 4, 3, the snow cover peaks to 80, 60, 60 and 5% respectively. In Zones 5 and 4, most of the snow cover depletes by July and May 1st week. The snow statistics will be useful input to hydrological models.