عنوان مقاله [English]
The most important parameter in determining the magnitude of the impact of climate change on glaciers is ELA. It has been widely used to infer current and pas climatic conditions. In general, ELA depend on the accumulation of snow during the winter and melting during summer. Determining the equilibrium line altitude is one of the important environmental thresholds in glacial studies. This glacial threshold is used for temperature reconstruction as well as determining the performance of glacial basins and adjacent glaciers, and it is widely used in studies of glacial geomorphology. In the present study, the lie’s and regression methods have been used to determine the equilibrium line altitude of climatic temperature-precipitation. This paper uses relationships that can perform theoretical calculations for altitude of instantaneous glaciations (AIG), glacier built-up sensitivity (GBS) and temperature-precipitation equilibrium line altitude (TP-ELA). These are calculated based on the records of the average seasonal temperature, melting and winter precipitation from meteorological stations, that has been calculated with adiabatic lapse rate and combined with the method of changing precipitation, height basis and topography. In this regard, the rugged units of the margins Isfahan-Sirjan holes were selected for calculations.
Materials and methods
There are several methods for estimating the equilibrium, some of which are related to past glacial environments and some of the related to current climatic conditions. Most methods used to estimate the current equilibrium line are based on the correlation between temperature and altitude. But in this research, lie’s method is used to estimate three component; AIG, GBS and TP-ELA which were finally calculated using the mentioned components, glaciation-sensitive altitude, and the height of the temperature-precipitation equilibrium line. To calculated these three components, the statistics and information of 45 selected stations with an average period 30 years were selected.
Component AIG glaciation sensitivity was calculated by using the following equation:
AIG= H_Station+ (h ×100)
The following equation was used to calculated GBS based on Lie’s research
GBS= [t_0- ∆t × ((H- H_Station)/100)]- [ln [(p_0 × (1+ ∆p)^((H- H_Station)/100))/0.915]/0.339] × 100/∆t(GBS ≠ <0)
To calculated CTP-ELA; the value obtained from the above equation must be added to the height of the earth’s topography.
〖CTP〗_ELA=H+GBS (GBS ≠ <0)
Results and discussion
According to the relationships presented by lie’s et al (2003) and based on the data of meteorological stations int the study area, the glacial built-up sensitivity (GBS), climatic temperature-precipitation equilibrium line altitude (CTP-ELA) and altitude of instantanious glacial were calculated in the elevation units of central Iran and three component maps were drawn using interpolation mehods.
To study the results of using Lie’s method and the results of temperature and altitude regression method, the whole study area was divided into 5 rugged units; Ghom, Kahak, Karkas, Marshenan, heights of Yazd and units of Kerman, and the altitude of the current equilibrium line and snow boundary was calculated.
Based on correlation between temperature and altitude, the values obtained for (Karkas, Marshenan, Shirkooh, Kheibar, Jupar, Lalezar, Hazar, Polvar, Bidkhan, Ghom and Kahak) are 4835m 4180, 4347, 4428, 4936, meters respectively, that none of these rugged units can currently be frozen based on these data and stations. But according to the Lie’s method, the Altitude equilibrium line of current temperature-precipitation int study area for selected rugged units is 4843, 4854, 4991, 5238, 5335 and 4868 meters for (Karkas, Marshenan – Shirkooh, Kheibar – Jupar, Lalezar, Hazar, Polvar and Bidkhan, Ghom, Kahak) respectively. Infact, it should be said that the altitude equilirium line in both methods in higher than the current altitude and don’t have favorable climatic conditions for glaciations. Therefore, it can be concluded that the most important factor in determining the altitude equilibrium line in this part of Iran is the change in latitude so that, with decreasing the latitude in central Iran, the amount of all three environmental statistics were increased.
According to lie’s method, the altitude of the current temperature-precipitation glacial equilibrium line is 5070 meters and the instantaneous height of the glacier is 4922 meters. However all of these elevation units are located in central Iran and have less moisture nutrition than the northern and western heights of Iran. However, differences in the magnitude of these values indicate a significant effect of latitude. In general, the northern half of the study area had more favorable conditions for maintaining glacial conditions and feeding glacial circuses throughout the year, climatically, it has a lower glacial equilibrium line altitude than the middle and southern parts. The reason for this can be considered in increasing the angle of solar radiation in the southern regions and the location of the southern rugged. Units of the Sanandaj-Sirjan zone. In addition, the southern heights will be affected by the local heat of these areas due to the proximity to thermal and super-thermal centers of Lut and low holes.