CLIMATE CHANGE IMPACTS ON ICE REGIME OF THE RIVERS IN MONGOLIA

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1 Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice Dunedin, New Zealand, 2nd 6th December 2002 International Association of Hydraulic Engineering and Research CLIMATE CHANGE IMPACTS ON ICE REGIME OF THE RIVERS IN MONGOLIA Batima Punsalmaa 1 and Batnasan Nyamsuren 2 ABSTRACT One distinguishing characteristic of Mongolian rivers is that all of them are covered by thick ice layers of about meter for five or six months a year during and small rivers are even frozen to the bottom. During the last 60 years the annual mean air temperature is increased by 1.56 ºC. Winter temperature is increased by 3.61 ºC and spring/ temperature by ºC. However, the summer temperature has decreased by 0.3 ºC. Particularly, the temperature is increased rapidly in the months of March, May, September and November and summer cooling comes mostly in months of June and July. As a consequence of climate change there is clear changes in ice regime of the rivers in Mongolia. There are about 3 15 days shift in starting and finishing dates of ice phenomena. Accordingly the duration of ice cover on the river is shortened by 5 15 days. About 5 20 days shortening in ice breaking dates. It is also observed that the depth of ice cover on the river is reducing. HYDROLOGY The total water resources in Mongolia amount 599 km 3 : 83.7 % in lakes, 10.5 % in glaciers and 5.8 % in rivers (Myagmarjav and Davaa, 1998). The rivers in Mongolia are of mountainous origin and the major source of water is precipitation. Therefore its upland watershed is small and relatively isolated, stream flows are low with the steep slopes of the stream bed which causing relatively high velocities, and bottom scour removes silt and clay, leaving rocks, gravel, and sand. The water of the rivers is cool and clear. There are four main climatic water regimes observe in the rivers of Mongolia. These are: 1. Winter low flow period which lasts approximately from December to April; 2. Spring runoff period due to snow melting which lasts approximately from April to June; 3. Summer runoff period due to rain, which lasts approximately from June to September 1 Institute of Meterology and Hydrology, Hudaldaany gudamj 5, Ulaanbaatar-46, Mongolia 2 Institute of geography, P.O. Box 664, Ulaanbaatar-24, Mongola

2 4. Warm season low water period, which is usually from after the rainy season to the winter low flow period. However, this includes the short periods between floods due to rain These four regimes can be observed separately or combined depending on the basin characteristics and climatic factor of the particular time. Regime 2 and 3 are usually combined in the rivers flowing from the Mongol-Altain mountain, which is not a case in the rivers flowing from the Hangain mountain most of the time. CLIMATE AND ITS CHANGES IN LAST 60 YEARS Mongolian annual mean temperature ranges from 8.3 ºC to +8.3 ºC. It is 4 ºC in the Altai, Hangai, Hentein and Huvsgul mountainous region and 6.8 ºC in the mountains and big river valley, +2 ºC in the desert-steppe, and +6 ºC in the southern Gobi. The zero iso-line of annual temperature goes along the latitude of 46 ºN in the way as separating the mountainous area from the Gobi-desert area. The permafrost soils are distributed in area with annual mean temperature of 2 ºC. The average temperature of January is 25 ºC in river valley, 15 to 20 ºC in the Gobi, and 12 to 15 ºC in the southern Gobi (Natsagdorj, 2000). The dynamics of the air temperature and precipitation for the last 60 years have been made on the base of observed data from 25 meteorological stations that evenly distributed over the country. During last 60 years the annual mean air temperature is increased by 1.56 ºC. Winter temperature is increased by 3.61 ºC and spring- temperature by ºC. However the summer temperature is decreased by 0.3 ºC. Particularly, the temperature is increased rapidly in the months of March, May, September and November and summer cooling comes mostly in months of June and July. Changes in temperature have also spatial character: winter warming is more pronounced in high mountain and in wide valleys between mountains and less in the steppe and Gobi. There is a little increasing trend in precipitation for last 60 years. The country s average precipitation between 1940 and 1998 has increased by 6 % in which summer precipitation by 11 % but spring precipitation decreased by 17 %. The spring dryness occurs mainly in May. There is not much change in April precipitation and a little increase in May precipitation. ICE REGIME Current condition One distinguishing characteristic of Mongolian rivers is that all of them are covered by thick ice layers of about meter for five or six months a year during. The small rivers are even frozen to the bottom. The monitoring/observation of river ice regime was begun in Ice depth is measured in 10 th, 20 th and last day of a month. The ice fan, young shore ice and slob ice in river are the ice phenomena. The average date of icy phenomena formation is

3 third decade of October. Average duration of ice/boom is about days varies from 4 to 35 days in. The rivers freeze up lasts from the end of October to second decade of November. The spring ice break occurs in second and third decades of April. As an example the ice regime characteristics of three different sites of the Khovd river are given in Table 1. Table 1: The ice regime characteristics of three different sites of the Khovd river Bayannuur Myangad Ulgii Date Starting date of of ice phenomena in of brash in of complete ice cover in of debacle in spring end of ice phenomena in Autumn spring brash Duration/ days Spring debacle Ice cover average X/25 X/29 XI/11 IV/15 IV/ earlest X/06 X/06 X/29 III/11 IV/ latest XI/01 XI/11 XII/01 IV/24 V/ average X/24 X/27 XI/21 III/24 IV/ earlest X/06 X/07 XI/09 III/05 III/ latest XI/05 XI/06 XII/14 IV/26 V/ average X/16 X/22 XI/03 IV/21 V/ earlest X/01 X/04 X/17 III/16 IV/ latest XI/01 XI/08 XII/02 V/10 V/ Changes in dates of ice phenomena As a result of changes in climate in Mongolia there have been found clear changes in dates of Autumn and Spring ice phenomena at the rivers. There are about 3 15 days shift in starting and finishing dates of ice phenomena depending on geographical location of the river basin. Particularly, number of days in delayed start of Autumn ice phenomena is much longer in the rivers in Western part than in Eastern part of Mongolia. Figure 1 and 2 illustrate the differences of changes in dates of Autumn ice phenomena starting date at Khovd, (located in the Western part of the country) and at the Kherlen (located in the eastern part of the country) river. 5-Nov 31-Oct 26-Oct 21-Oct 16-Oct 11-Oct 6-Oct 1-Oct 26-Sep Figure 1: Changes in ice phenomena starting date at the Khovd river

4 15-Nov 5-Nov 26-Oct 16-Oct 6-Oct 26-Sep Figure 2: Changes in ice phenomena starting date at the Kherlen river It is also found that Spring ice phenomena finishing date was started earlier (Figure 3). Accordingly, the duration of ice cover on the river is shortened by 5 15 days (Figure 4). 8-Jun 29-May 19-May 9-May 2 1 Figure 3: Changes in Spring ice phenomena finishing date at Kherlen river Day Figure 4: Changes in duration of ice cover at the Kherlen river One of the important information in river ice monitoring is breaking date. According to the observation records the breaking dates are shortened about 5 20 days. Number of shortened days at the rivers are also different depending on the location. For example the changes in ice breaking date at the Khovd and Kherlen river are shown in Figure 5

5 and 6. It is also observed that the depth of ice cover on the river is reducing. 19-May 9-May Mar 10-Mar Figure 5: Changes in ice breaking date at the Khovd river 24-Apr 1 14-Apr 4-Apr 25-Mar 20-Mar 15-Mar Figure 6: Changes in ice breaking date at the Kherlen river CONCLUSION The total water resources in Mongolia amount 599 km 3 : 83.7 % in lakes, 10.5 % in glaciers and 5.8 % in rivers (Myagmarjav and Davaa, 1998). The rivers in Mongolia are of mountainous origin and the major source of water is precipitation. During last 60 years the annual mean air temperature is increased by 1.56 ºC. Winter temperature is increased by 3.61 ºC and spring- temperature by ºC However, the summer temperature is decreased by 0.3 ºC Particularly, the temperature is increased rapidly in the months of March, May, September and November and summer cooling comes mostly in months of June and July. Consequently there are clear changes in river ice regime at the rivers. The changes in Autumn ice phenomena starting date and Spring ice phenomena finishing dates as well as ice breaking dates are different depending on geographical location of the river basin. Ice cover depth at the river is reducing because of increased temperature of the winter time in the last 60 years. REFERENCES Myagmarjav, B. and Davaa, G. eds. Mongolia Surface Water. Monograph. Ulaanbaatar, Mongolia (1998) 139p. Natsagdor, L. Climate Change. In Climate change and its impacts in Mongolia. P. Batima and D. Dagvadorj, eds., NAMHEM and JEMR, Ulaanbaatar (2000)