**2. Material and methods**

#### **2.1 Study area**

The Central Alborz mountainous range extends from 49º 5´- 53º 5´ longitude to 35º 5´- 37º 2´ latitude. Its area is about 40,000 km2 and covers 64 sub-basins. The lowest, highest and average altitude of the basin is 48 m, 5671 m and 1870 m, respectively. The minimum, maximum and average slope is 5%, 25.4% and 23.56%, respectively. The main slope aspect of this region is directing towards north and south. Climatically, the region is classified to three classes as temperate in north, cold in center, and semi-arid in south. Geological structures mostly consist of mild outcrops that are inconsistent with general trend of eastwest. In western part of Alborz, the structures have northwest-southeast trend, but in eastern part, the structures have northeast-southwest trend. These two inconsistent trends cross each other in Central Alborz.

#### **2.2 Data**

#### **2.2.1 MODIS Data, TERRA satellite**

MODIS encompasses noticeable number of spectral and thermal bands with narrow width, high radiometric resolution, proper width and collecting time, powerful and accurate calibration, and diverse land resolution (MODIS Home page) (http://modis.gsfc.nasa.gov).

In many cases, MODIS provides satellite snow-gauging requirements and therefore these data were used. In this research, images were provided from website (https://wist.echo.nasa.gov/wist) according to Table 1.



The imagery used in this research include MOD02 and MOD09. The MOD02 imagery include 36 bands while MOD09 include 6 bands. In MOD09GA imagery, atmospheric corrections have been done based on 6sv model, as one of the best models in atmospheric corrections with minimum error, suitable for measuring snow surface and detecting cloud from snow (Vermote and Kotchenova, 2008). In MOD09 imagery, corrections have been implemented in way that atmospheric diffusion and reflection were minimum. Since data with higher wavelength are being less influenced by aerosols, suspended particles and nonselective diffusion phenomenon, thermal bands of MOD02 imagery were used.

### **2.2.2 Digital Elevation Model**

358 Remote Sensing – Applications

2003). In this process, based on land type, geographical position and available data, Cloud Mask algorithm uses 14 bands amongst 36 bands of MODIS to test 18 spectral and spatial features (Hall and Riggs, 2002). However, this procedure was modified by Hall and Riggs (2002) who presented a new version of Cloud Mask algorithm (Liberal). This algorithm can analyse the pixels located under thin and transparent clouds (Zhou et al., 2005; Ault et. al., 2006). This procedure identifies the darkness and if it faces to such darkness, it means that sun angle is higher than 85º. This algorithm is called Liberal Cloud Mask algorithm. In fact, Liberal Cloud Mask algorithm functions as subset of spectral tests of old Cloud Mask algorithm

The Central Alborz mountainous range extends from 49º 5´- 53º 5´ longitude to 35º 5´- 37º 2´ latitude. Its area is about 40,000 km2 and covers 64 sub-basins. The lowest, highest and average altitude of the basin is 48 m, 5671 m and 1870 m, respectively. The minimum, maximum and average slope is 5%, 25.4% and 23.56%, respectively. The main slope aspect of this region is directing towards north and south. Climatically, the region is classified to three classes as temperate in north, cold in center, and semi-arid in south. Geological structures mostly consist of mild outcrops that are inconsistent with general trend of eastwest. In western part of Alborz, the structures have northwest-southeast trend, but in eastern part, the structures have northeast-southwest trend. These two inconsistent trends

MODIS encompasses noticeable number of spectral and thermal bands with narrow width, high radiometric resolution, proper width and collecting time, powerful and accurate calibration, and diverse land resolution (MODIS Home page) (http://modis.gsfc.nasa.gov). In many cases, MODIS provides satellite snow-gauging requirements and therefore these data were used. In this research, images were provided from website

Year 2006 2006 2006 2006 2006 2006

Month February February march march march march Day 21 25 4 8 13 15

The imagery used in this research include MOD02 and MOD09. The MOD02 imagery include 36 bands while MOD09 include 6 bands. In MOD09GA imagery, atmospheric corrections have been done based on 6sv model, as one of the best models in atmospheric corrections with minimum error, suitable for measuring snow surface and detecting cloud from snow (Vermote and Kotchenova, 2008). In MOD09 imagery, corrections have been

(MOD35) and uses 7 bands of MODIS and set 4 criteria (Hall and Riggs, 2002, 2004).

**2. Material and methods** 

cross each other in Central Alborz.

**2.2.1 MODIS Data, TERRA satellite** 

(https://wist.echo.nasa.gov/wist) according to Table 1.

Table 1. Temporal table applied images from MODIS sensor data

**2.1 Study area** 

**2.2 Data** 

Digital Elevation Model (DEM) obtained from SRTM Shuttle was used. These data that was in format of GeoTiff had Lambert image system. For transformation of this data to UTM coordinates system, PCI Geomatica software was used.
