Snow-cover and Land Surface Temprature investigation, related to the Elevation as a Topographic Factor in the Central Alborz Mountain*

The area covered by snow (SC) and land surface temperature (LST) and their fluctuations in different altitudes of a mountain unit are important in climatic, hydrological and water and ecological resources management. In this study, the relationship between SC and LST in this mountainous unit was examined in monthly, seasonal and annual intervals. For this purpose, Terra and Aqua Satellite image data which are carrying Modis sensor, used in temporal range of 2003-2018.In all time periods studied, a clear relationship between elevation and SC, was observed in the Central Alborz highlands. The relationship between these two environmental indicators are direct, although the rate of change varies on different altitudes. Two specific height thresholds were observed in Central Alborz, the first threshold being at an altitude of 1000 meters and the other at 4000 meters. So that the SC rises to a height of 1000 meters with increasing altitude. After an altitude of 4,000 meters, the slope changes again and starts to decrease. LST variations are the opposite of SC. In general, increasing the height leads to a decrease in LST, but, up to 1000m is an exception to this rule, and increasing the height will increase the LST.

The area covered by snow (SC) and land surface temperature (LST) and their fluctuations in different altitudes of a mountain unit are important in climatic, hydrological and water and ecological resources management. Snow cover and land surface tempratue distribuations on different elevational class would be important from the view point of environmental systems and ecosystems observations and management. One of the major mountainous unit in Iran, which is supplying many human population, is the Central Alborz mountain, located in the northern boundary of Iran.

In this study, the relationship between SC and LST in this mountainous unit was examined in monthly, seasonal and annual intervals. For this purpose, Terra and Aqua Satellite image data with spatial resolusion of 50m which are carrying Modis sensor, used in temporal range of 2003-2018. Digital Surface Model released by the Japan Aerospace Exploration Agency (JAXA) deployed the Advanced Land Observing Satellite (ALOS) between January 2006 and May 2011, used in this research. This data have spatial resolusion of 1 arc secound (~30m) and a vertical RMSE of 4.4 m. and now is one of the most accurate dataset with global coverage and free of charge.

In all time periods studied, a clear relationship between elevation and SC, was observed in the central Alborz highlands. The relationship between these two environmental indicators are direct, although the rate of change varies on different altitudes. Two specific height thresholds were observed in Central Alborz, the first threshold being at an altitude of 1000 meters and the other at 4000 meters. So that the SC rises to a height of 1000 meters with increasing altitude. After an altitude of 4,000 meters, the slope changes again and starts to decrease. LST changes are the opposite of SC changes, in general, increasing the height leads to a decrease in LST of course, up to 1000 meters is an exception to this rule, and increasing the height will increase the LST. This is due to the cooling effect of the Caspian Sea and high humidity at altitudes below 1000 meters and also decreasing vegatation coveabdr density up to 1000m, which mainly includes the northern slopes of Alborz. Forests, forest-steppes and grasslands, are absorbing the sunrays energy and consume it in the process of photosynthesis, and so they prevents, the land surface temperature to be increased. In the highland of central Alborz (the elevation up to almost 1000m) lower humidity and vegetation cover in addition to the rocky surfaces, leads to the higher LSTs. From an altitude of 1000 meters and above, the general trend of increasing altitude leads to a decrease in LST in Central Alborz. Another environmental indicator was defined in this study, which was called the Equilibrium Line Altitude of Land surface Temperature and Snow Cover (ELALS). ELALS is a height at which LST and SC reach equilibrium. The annual average of this environmental index is in the hight of 2,800 meters during the study period in the Central Alborz highlands. The minimum level of ELALS in winter is 1740 meters above sea level. This environmental index tends to reach low altitudes in cold seasons and months and tends to higher altitudes in warmer periods of the year.

Finally, this environmental index can be used in geomorphological studies of glaciers, climates, water resources, hydrological management of basins and ecological studies of mountainous landscapes.