چكيده لاتين :
Introduction
The central of Iran has warm and dried climate at present so that in some district of the region, precipitation is less than 50 mm which this amount is less than annual potential evapotranspiration. Nevertheless, there are some landforms at present that show more humidity and colder climate in the past. The main goal of the research is assessment of climate condition in Ebrahimabad’ Plain Basin in the past and calculating differences temperature and precipitation between past and present time. Ebrahimabad’s Plain Basin has located in the North-west of Mehriz city in the slope of Shirkuh Mountain along 31° 18´ to 31° 30´ of northern latitudes and 54° 14´ to 54°25´ of eastern longitudes. The range of elevation in the region is between 1600 meters in the output point of the basin and 4100 meters in the peaks of mountains. The area of the region is about 656 square Km. Primary studding show some landform which has created by glaciers such as cirques, U form valleys, and erratic stones. In the researches, we have tried to reconstruct past climate condition according to the landforms as indicators.
Methodology
For reconstructing of the past climate condition in the research, we used of topographical maps (1/50000), geological map (1/100000), aerial photographs (1/55000), and satellite images, Google Earth and DTM with resolution 20 meters, and also some software like GIS, Excel and GPS system. In addition, for studding present climate and its changes in ratio to the past, we used climate data (includes temperature and precipitation) of 12 stations of rain gauge, 6 station of climatology, and 4 station of synoptic for12 years in the region and its surrounding. At first, using topographical maps we have defined the basin. Then, according to the form of contours, aerial photographs, satellite images as well as field studying, we marked glacier cirques. For drawing the maps of present isotherm, we create the correlation between altitudes and annual temperature (for 12 years) amongst surrounding stations the region, which was resulted the linear equation: Then, using altitude points extracted from DEM and above equation, we provided isotherm and isohyets maps for present time. As ever, for drawing past isotherm and isohyets maps, at first we calculated zero C. line (snowline) for the past with Wright and Porter methods. Afterwards, we obtained an equation according to the correlation between altitude and temperature, and according to it, we evaluated adiabatic lapse rate. Using permanent snowline altitude and adiabatic lapse rate it would be draw past isotherm map. Therefore, according to line correlation between temperature and precipitation, it was created the equation y= -23.685T + 509.09. On the equation basis, it was drawn past isohyets map. In the Wright method, we used the average of 60% of cirques. In the Porter method, we calculated the snowline of the region according to accumulation area ratio, cirque-floor altitude, and altitude ratios.
Results and discussion
In according to climate data we have drawn isotherm and isohyets maps for present time. Isotherm map show that temperature changes from 17.4° C. in the output point of the basin to 5.4° C. in the high elevations. Adiabatic lapse rate according to correlation between altitudes and temperature is about 0.52. But, for reconstructing of past temperature, we used of the Tw= ( Hs- H)*.65/100 equation which in it Tw is Pleistocene temperature, Hs is Snowline altitude, and H is the elevation. In fact, with statistical analysis and also Wright and Porter methods, the snowline has been lain between 2150-2250 altitudes in Pleistocene. Besides, the ice and water equilibrium line has been lay in 1560 meters equivalent to 4.8° C. On the snowline basis as well as adiabatic lapse rate for 0.52 C., we have calculated differences between past and present temperatures in the region that is about 14,4° C. .Therefore, we drew isotherm map for the past. The map show that the minimum temperature has been about -10 in the high elevations and maximum about 4.1 for Ebrahimabad plain. Also, with drawing present isohyet map according correlation between precipitation and elevation, we showed that maximum precipitation is about 360 mm. In the high elevation and the minimum is about 96 mm. in the output point of the basin. Then, with drawing past isohyet map and comparing it with present isohyet map, we observed that precipitation in the past has been about 2.5 times compared with now. The map show that the maximum precipitation (740 mm.) has been on the maximum elevation and the minimum has been 410 mm. in the Ebrahimabad’s plain.
Conclusion
Results show great changes in climate in the region so that with field studding we observed many glacier landforms in the region such as a few cirques, U form valleys, stone traces as well as erratic stones. In according to these landforms as indicators for calculating climate changes, we tried to reconstruct climate condition in the past. Thus, we drew isotherm and isohyet maps for past and present time and compared them together using GIS. Results show that the temperature has been colder than now about 14° C. And precipitation has been 2.5 times compared with now. Besides, the area of the glaciers has been spread to about 2200 meters (permanent snowline). But, glacier’s tongue has been descended to about 1600 meters inside the plain, and there has been melt; because temperature in the altitude increased to 4.1° C. the remained erratic in the region affirm the hypothesis.
