Preliminary Estimation of Snow Covers Extents of Astore River Basin in Northern Areas , Pakistan

An assessment of snow accumulation and depletion is necessary for the management of water resources in the region, where a great portion of annual precipitation falls in form of snow. This study was done to estimate the snow covered area of Northern Areas (Astore) in snow accumulation and snow ablation period with the aid of remote sensing (RS) techniques. Satellite image of Moderate Resolution Imaging Spectroradiometer (MODIS) was used to evaluate snow cover and classify the behavior of different image pixels in snow covered, partially covered and snow free pixels respectively. In order to nullify the obstruction offered by clouds only one cloud free image per week was selected for the years of 2008-2009 and data was analyzed using different ArcGIS and ERDAS Imagine tools. Watershed area of Astore river basin was calculated by using Digital Elevation Model. Normalized Differential Snow Index (NDSI) was used to draw snow cover map. Snow covered area was calculated in square kilometer and curves of snow cover extents were made for both years separately. Furthermore correlation curves were made for snow cover fluctuation with varying precipitation, minimum temperature and maximum temperature for years of 2008-2009. The observed correlation coefficient between snow cover, maximum temperature, minimum temperature and precipitation was found to be -0.784 (0.003), -0.767(0.004), 0.561(0.058) respectively for both the years. Moreover multiple linear regression equation was generated for future snow cover prediction under different climatic scenarios. January and February was found to be snow accumulation period and for May, July, June it was snow ablation period for both years.


Introduction
Pakistan has a variety of climatic regime varying from hot deserts to cold snow covered areas.The Northern Areas of Pakistan have cold climate, in winters due to western depression yields snow over Himalaya-Karakorum-Hindukush (HKH).In summers the accumulated snow melts to recharge Himalayan river system.This runoff water is accumulated in water reservoirs for irrigation and hydro power production.Generation of runoff water provide millions of people living in mountainous regions with fresh water (Dahri, Ahmad, Leach, & Ahmad, 2011).
Snow falls in the months of September -February and melts in the moths of March -August in Northern Areas of Pakistan.Frozen snow and glaciated ice in the mountainous Northern regions of Pakistan serves as its reserved hydrological resource.Conventional techniques for the measurement of snow coverextents are challenging and time consuming.More over these areas of Pakistan are difficult to access as they have varying topography and terrain.Nowadays remote sensing (RS) is proved to be a feasible tool to study snow covered areas.
Moderate Resolution Imaging Spectroradiometer (MODIS), aboard terra spacecraft of Earth Observing Systems (EOS) is being very handy for the estimation of Normalized Differential Snow Index (NDSI).Snow cover is highly influenced by changing climate and meteorological parameters.Snow falling and melting both are greatly influenced by daily minimum and maximum temperatures (Rasul, Chudhry, Mahmood, Hyder, & Dahe, 2011).Numerous researches were done on snow cover extent of other districts of Northern Areas but Astore wasremaining neglected.Therefore this study was designed to analyze the impacts of meteorological parameters on the snow cover of Northern Area, Astore.Basic aim of this study was to find out the spatio-temporal distribution of snow cover of Astore river basin for the period of 2008-2009.Additionally to evaluate the effect of meteorological variables on snow cover distribution.Snow cover is the visible indicator of changing climate, snow cover extent and snow melt runoff are some of the major parameters directly related to the climatic changes.Hence this study signifies the effects of changing climatic conditions on the behavior of snow cover extent of Astore river basin.

Study Area
Astore river basin (Figure 1) is located in the high mountains of HKH region with snow peaks exceeding 7000m in height.This area is mostly covered by perennial snow and ice (Akhtar, Ahmad, &Booij, 2008).The geographic location of Astore is 34º, 45' to 35º, 38' N latitude and 74º, 24' to 75º, 14' E longitude.Elevation varies from 1,200m to 7,500m above mean sea level.The total catchment area is 4,214 km 2 with 607 km 2 glaciated area (Nabi, Latif, Habib-ur-Rehman, & Azhar, 2011).Glaciers are mainly nourished by westerly disturbance (Hewitt, Wake, Young, & David, 1989) According to SHIP (Snow and Ice Hydrology Project, 1990) 80-90% of the area gets covered by snow, seasonal melting of snow and glaciers ice both are large contributor of discharge water in summer season.

Data Acquisition
Moderate Resolution Imaging Spectroradiometer (MODIS) snow product was utilized to estimate the snow cover of the study area.Satellite images for snow cover L3 Global 500m Grid (MOD10A2) at 250mfrom MODIS/Terra was acquired (Hall, Riggs, & Salomonson, 1995).Cloud free images (Figure 2) from the period of January, 2008 to December, 2009 were downloaded from http://ladsweb.nascom.nasa.gov/data/search.htm.The MODIS snow cover algorithm works on the principle of high reflectance of snow in visible band 6 and band 4, likewise these two bands are used to calculate NDSI (Lopez, Sirguey, Arnaud, Pouyaud, & Chevallier, 2008).Data for daily precipitation, maximum temperature and minimum temperature over the period of 2008-2009 of Astore station was provided by Pakistan Meteorological Department.

Correlation between Snow Cover and Climatic Variables
Bi-variable correlation analysis of snow cover extent in Astore in relation with the independent climatic variables i.e. maximum and minimum temperature and precipitation described in Table 2.
Y = Snow cover, X 1 = Maximum temperature, X 2 = Minimum temperature, X 3 = Precipitation This analysis indicated that 66% (R 2 = 0.661) of snow cover was dependent on the climatic variables.Regression Equation 2 can be used to predict the future snow cover in the study area under different climatic scenarios.

Conclusion
Remote sensing is a very helpful technique for acquiring information about an object or phenomenon and also for processing the gathered information.Remote areas which are not easily accessible, their monitoring can be done by using RS techniques and GIS application.As conventional methods are not very handy in calculating snow cover extents of Northern Areas (Astore) of Pakistan, therefore satellite data was used.Total 90 temporal MODIS satellite images have been selected for 2008 and 2009 to estimate snow cover of Astore river basin and its correlation with different meteorological parameters.By using different RS techniques snow covered extent was calculated.It is clear from the graphs that with the ascending maximum temperatures and minimum temperatures, snow cover deplete due to melting.Hence, there is a negative correlation exist between snow cover and maximum and minimum temperatures.Precipitation in form of snow has a positive effect on snow cover, as precipitation increases, snow cover extents also increases thus it is presenting a positive correlation.Climatic fluctuations cause's variations in the amount snow melt runoff leading to decrease or increase in snow cover extents.Conducive climatic conditions like low maximum and minimum temperature and high precipitation rate support buildup of snow and vice versa.Consequently it was concluded that meteorological parameter influences the rate of precipitation in form of snow which leads to varying distribution of snow over the Astore river basin in different months of the year.

Figure 1 .
Figure 1.Base map of the study area (Astore river basin) Figu Figure 4. Sno Figur

Table 1
presented the monthly average value of snow cover of Astore river basin and % snow cover change for both the years.was more fluctuation in average monthly snow cover in the year of 2008 in comparison with 2009.Table1.Monthly % change and monthly average in Snow cover extent for years 2008 and 2009

Table 2 .
Correlation coefficients for different parameters

S.No. Variables Snow Cover Max Temperature Min Temperature Precipitation
As the summer season starts the rising temperature causes the snow to melt down.It was clearly shown in the correlation curve Figures6 and 7as temperature increased, snow cover extent decreased.

Table 3 .
Multiple linear regression analysis