УДК 551.324.63
Past and prospective changes in the state of Central Asian glaciers
© 2011 г. V.G. Konovalov
Institute of Geography Russian Academy of Sciences, Moscow vladgeo@gmail.com
Статья принята к печати 14 ноября 2010 г.
Central Asia, glacier fluctuations, glaciers runoff, modelling, spatial distribution, total melting.
Колебания оледенения, ледниковый сток, моделирование, пространственное распределение, суммарное таяние,
Центральная Азия.
Estimations have been made on areas of glaciers in 1961, 1980, 1991, 2000 and 2020 years in the main tributaries of the Amudarya and Syrdarya river basins, which constitute the major portion of the glacierized area of the Aral Sea Basin in Central Asia. During the time interval 1961-2000 the area of glaciers in these basins decreased by 2860 km2, or 21.8%. By 2020 the area of glaciers in these basins will be 9435 km2, if the current rate of shrinkage continues. Significant shrinkage of the area of Central Asian glacierization corresponds well with similar information for glaciers in other Asian and European regions. Both statistical parameters (mean value, coefficient of variation) and spatial and temporal distribution of glaciers runoff were determined in high flow and low flow water years. Long-term series of glaciers runoff were computed by the author's model REGMOD, which is briefly described here.
Introduction
Comprehensive analysis and forecasting of the long term regime of glaciation on the scale of large river basins or mountain areas is one of the most important problems of glaciology. Geometrical parameters of glaciers and their hydrological output due to melting of snow and ice at those scales are the most informative and widely used in theoretical and applied research. However, in contrast to what is known about historical ranges of other common hydrological and meteorological parameters, the quality and quantity of the information on geometrical and output parameters is far behind. A very limited number of regional temporal cuts (say, one to four) available for description of the state of glaciers is obviously not enough for valuable and statistically substantiated research on the relationship between fluctuations of climate and regime of glaciers.
My attempts to find published information on the areal extent of continental glaciation in Asia have resulted in the following. Dolgushin [7] and Dyurgerov [21] estimated the area of as 119,730 km2 and 120,680 km2, respectively. However, Ohmura's [29] estimation is 185,204 km2 for the year 1989. There is also a large discrepancy between different sources for the areas of glaciers in other parts of the Asian continent. For example, according to Ohmura [29] the number of glaciers in China is 46,377 and their area is 56,481 km2. However, in contrast, on the Website [http:// wdcdgg.westgis.ac.cn/DATABASE/Glacier/Glacier.asp] of Chinese Glaciers Inventory the number of glaciers and area were given at first as 51,250 and 69,069 km2, respectively, but later as 46,394 and 60,027 km2. In addition, data on individual glaciers in the China Inventory of Glaciers varies over the long time for which data are given: from
1928 to 1986. Joe and Sandeep [23] give number and areas of glaciers in Nepal and Bhutan as 3577 and 6640.6 km2, respectively, whereas Ohmura [29] gives 226 and 7500 km2. Data given in several publications about areas of glaciers in India and Pakistan [23, 28, 29] and others also has significant discrepancies. Other similar examples in current regional information about the states and areal fluctuation of glaciers could be cited.
The total area of glaciers in river basins is important information, but not the only parameter for characterizing their regimes. Other considerations on major scientific and applied importance are the following: volumes (or water resource potentials), annual mass balance, glacier runoff, glacier coefficients (or different relations between glacier topography parameters), and other altitudinal and area characteristics. Using these characteristics, it is possible to separate glaciers into natural groups or aggregations, although the groupings are continuously changing in space and time. Comparative analysis of the spatial and temporal variability in the areas and runoff of glaciers must be based on the use of data for total populations of separate glaciers, or on representative samples from the populations.
Runoff from glaciers in a large river basin is integrated result of ablation processes for hundreds or thousands of individual glaciers. Long-term variability of annual, seasonal, and monthly values of glacier runoff is the subject of great interest for applied glaciology and hydrology. The present paper gives estimations of long-term changes in the surface area and runoff regime of glaciers in the Aral Sea Basin, based on the results of an inventory program and on remote sensing surveys of these glaciers, and modelling of hydrological regime of groups of glaciers.
Fluctuation of areal extents of glaciers
Homogeneity of initial data. To insure homogeneity of data and correct estimations of changes in glacier area at different temporal «cuts» the following methods and standardizations are of key importance: well-understood, consistent criteria for the identification and delineation of glaciers boundaries, and uniform standards for the selection of statistical ensembles of objects and description of their topography. In much past work, initial data used for monitoring the states of glaciers have not met these criteria. For example, the boundaries of morphological parts of glaciers in the ex-USSR Glaciers Inventory [9] were determined by recognition on aerial photos at 1:24,000 scale. However, in later inventorying of the same objects [17] much lower resolution data of space-photo surveys at 1:200,000 scale were used. Therefore, the reliability of assigning boundaries of glaciers was less in the second case. Monitoring the state of glaciers in the upstream portions of the Amudarya river basin was continued [26] by means of images obtained in 2000-2002 from satellites LANDAST 7 (scanner ETM+) and TERRA (scanner ASTER). Spatial resolution of those images varies from 15 to 30 m. Thus, due to inhomogeneity of initial data there is a lack of control on the error in the estimations of change in the area of glaciers. In particular, use of different data sources and methods for determination the area of bare ice on glaciers within the Northern slope of Zailiskiy Alatau for 1955-1990 caused a scattering of 27.6% to 32.0% in the results [16]. Another example of noticeable differences between field and remote sensing determinations of the lengths of Austrian glaciers was reported in [22]. In this case images from the IKONOS satellite with spatial resolution of four meters were used.
Reliability of image-based assignment of boundaries to glaciers may strongly dependent on the regime of summer snowfalls. Summer snow masks the true boundaries of glaciers and can lead to incorrect interpretation of annual or long-term mass balance. It can lead to ambiguity in making comparisons of glacier area determined in dry in wet ablation seasons. The problem of summer snowfalls is not lessened even when snowfalls come at the same times in a certain region, because of efficient sounding from satellites may have a random character. Brief analysis of quality and homogeneity of data on change in size of glaciers shows that regional estimations of temporal fluctuations in glaciers area objectively include unavoidable and qualitatively undefined errors. Inaccuracy in determination of glaciers area can strongly influence on the estimation of water resources, mass balance, and volume of glaciers runoff. These errors can be diminished by enlarging of size of statistical samples of glaciers.
Glaciers area in the Aral Sea Basin. The area of glaciers in the Area Sea Basin region has changed dramatically during the last century. Information on certain aspects of past and future states of glaciers is presented in Table 1. It was obtained after compiling morphometric data for glaciers [8, 9, 17, 26], processing remote sensing images from satellites LANDAST 7 and TERRA, and by applying calculation methods [1, 12, 14].
Rather significant shrinkage of the areas of glaciers in the Aral Sea Basin during 1961-2000 corresponds well with estimations of glacier fluctuations in other mountain regions. Namely:
1) as reported in [3] the area and volume of glaciers in Zailiiskiy Range (Kazakhstan) during the period 19551990 diminished by 29.2% and 32.3%, respectively;
Table 1. Long-term change of glacier area in Central Asian watersheds
Basin/Region Fgb km2 dFgl, km2 dFg,, %
1961 1980 1991 2000 2020* 1961-2000 1961-2000
Western Tienshan 171 147 133 120 107 -51 -29.8
Matcha river 506 438 398 358 318 -148 -29.3
Syrdarya river (1) 548 450 408 367 326 -181 -33.0
Syrdarya river (2) 304 205 164 147 130 -157 -51.6
Vakhsh river 3779 3538 3413 3243 3073 -536 -14.2
Panj Right Tributaries 3548 2905 2780 2389 1998 -1159 -32.7
Panj Left Tributaries 4270 3956 3799 3642 3484 -629 -14.7
Panj river at all 7818 6861 6579 6031 5482 -1787 -22.9
Amudarya upstream 11597 10399 9992 9273 8555 -2324 -20.0
All basins 13126 11638 11095 10265 9435 -2860 -21.8
Notes: Syrdarya river (1) - left tributaries of the main river from Aksu mouth and below; Syrdarya river (2) - left tributaries of the main river between Karadarya and Aksu mouths; Fgj is total area of glaciers for the fixed year; dFgl in km2 or in per cents is change of Fgl for the fixed time interval; figures in bold type are related to the main tributaries of Amudarya river; * - area of glaciers calculated for the year 2020 at the mean rate of summer air temperature (ATs/At), which is 0.007 °C/year. This value was obtained by measurements of summer air temperature Ts during the period 1981-2000 at the Tien-Shan meteorological station located 3614 m above sea level, an
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