عنوان مقاله [English]
The study of climate change and its trend have usually been complicated as problems for specific scientists in these subjects. Obviously, climate has been changed during the history of earth many times, and their evidence are landforms in many regions of the world, where we can readily observe now. The landforms could not be justified with the present climate. In fact, any climate creates specific geomorphic systems. Therefore, the existence of different geomorphic systems in different regions of the world shows that climate has changed frequently over time. In the KhezrAbad basin of Yazd Province we can observe landforms that have been created in different periods of climate of different temperature and precipitation conditions. Thus, with studying of present landforms in the basin, it has been tried to reconstruct past climate. The climate could justify such landforms reasonably. KhezrAbad Basin is located in the south-west Yazd city in the slope of Shirkuh Mountain along 31° 48´ 7" to 32° of northern latitudes and 53° 49´ 42" to 54° of eastern longitudes.
Materials and Methods
For studding the condition of past climate in the research, we have used topographical maps (1/50000), geological map (1/100000), aerial photographs (1/55000), and satellite images. In addition, to study present climate and its changes relative to the past, we have used climate data including temperature and precipitation of 6 stations for 9 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 study, we marked glacier cirques. For drawing the maps of present isotherm, we created the correlation between altitudes and annual temperature (for 9 years) amongst surrounding stations of the region to obtain the linear equation. Then, using the elevation 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, 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, which replaced once 1000 meters instead of “h” and next time 2000 meters. Therefore, with subtraction of these two numbers, we evaluated adiabatic lapse rate. Using permanent snowline altitude and adiabatic lapse rate it would result in past isotherm map. In the next stage, according to line correlation between temperature and precipitation, we got the equation: P= -15.803T+355.16. On the equation basis, it helps draw 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
Investigation about the region at present shows some landforms such as a number of glacier cirque, a U form origin valley and a few secondary valleys as well as a great fan that could not be created with present climatic processes. Therefore, these landforms indicate a big climatic change in temperature that was lower with higher precipitation. With statistical analysis and also Wright and Porter methods, the snowline has been found between 2100-2200 altitudes in Pleistocene. On the snowline basis as well as adiabatic lapse rate for 0.65 and 0.8 C., we have calculated differences between past and present temperatures in the region orderly 12.92 and 13.4° C. In addition, studies showed that the rate of precipitation in the Pleistocene increased to about 176 millimeter more than present average. Thus, the ice and water equilibrium line has been in 1560 meters equivalent to 4.8° C.
With more precise study, the rate of precipitation in Pleistocene has been about 3.38 million cubic meters above 1200 meters height. This volume is about 1.88 as much more than present. The most precipitation volume has been calculated for the altitudes between 2200-2300 meters, and the least for 3000 meters.
The results of this research in the KhezrAbad Basin show some glacial evidence of past climates. The evidence can be observed as 15 small and big cirques, a glacier broad valley, much volume of moraines such as lateral moraines, medial moraines, and terminal moraines, and also tillites. On the basis of these witnesses, the permanent snowline has been on the height of 2200 meters, but the ice and water equilibrium line on 1560 meters. In fact, glacier’s tongue has been descended more than 600 meters and there has been melted ice; because temperature in the altitude increased to 5° C. The remained erratic in the region affirm the hypothesis. Therefore, with comparison of isotherm and isohyets maps of the basin for present and past times, we estimated about 13°C anomalies. Moreover, research findings show the reduction of 176.52 millimeter of precipitation in comparison with the quaternary. Precise calculation confirms that the volume of precipitation in Pleistocene period has been 3.38 million cubic meters above 2200 m. The volume is about 1.88 as much more than present time.