Changes of water balance components of the lakes in the steppe region under climate changes (A case study of Ogii lake, Central Mongolia)

Уур амьсгалын эмзэг байдалд өртөж буй нууруудын усны тэнцлийн судалгаа (Өгий нуурын жишээн дээр)

Abstract

The Ogii Lake, which is located in the semi-arid steppe region of Mongolia was selected in this study variation of the climate and climate parameters that impacted to vulnerable regime and resources of the lake for the last 30 years around the Ogii Lake and components of the lake water balance including the changes in the water surface area of the lake. Also, climate change vulnerability issues included in the studies. The Mann-Kendall (MK), Innovative Trend Analysis Method (ITAM) and Sen's Slope Estimator Test (SSET) are used for statistical analysis of variation and trends of water and climate parameters in the lake basin. The variation of lake water surface area was determined by the Normalized Difference Water Index (NDWI) and consequently dynamics of the water balance of the lake have analyzed. The last 30 years climate data of Ogii meteorological station from Arkhangai provinces which covers period from 1991 to 2021 have used in these studies. As for the lake water balance data, inflow data of Khogshin Orkhon river and other data from Ogii-Ogii hydrological station including water level and other parameters are used lake water balance. The missing runoff data of the Khogshin Orkhon river for period of 1995-2015 were extended by using method of analogy river using observed data for period of 1991-1994 and 2016-2021. Also, for the water surface area of Ogii Lake, satellite  data of July and August months from Landsat 4 and 5 TM between 1991 and 2012 and Landsat 8 OLI satellite data between 2013 and 2021 were used.  Water level of the lake for period of 1991-2001 were reconstructed by using relationship between water level and lake surface area and water level data of 2002-2021 is observed one. The Mann-Kendall (MK) method shows that air temperature and evaporation from open surface have increasing trend in last 30 years while river runoff has decerasing trend. The results of the studies show that the average annual temperature has increased by 2^oC in the last 30 years around Ogii Lake region. As an impacts of such warming the water surface area of the lake have decreased by 1.4 km² or 5.4 percent and the water volume has decreased by 0.021 km² or 11.5 percent over the last 30 years. At same time, the lake water level has lowered by about 80 cm mainly due increase of evaporation caused by climate warming in the region. As shown results of NDWI index estimation that the water surface area of the Ogii lake has significant variation during the period of 1991-2021. Largest surface area is observed in 1993 by reaching 26.7 км² while lowets area is observed in 2011 with value of 20.2 км² and by 2021 lake surface area extended to 24.8 km ² showing some recovery trend in recent yeras. Long term means surface area of the Ogii lake to be 24.03 км² during period of 1991-2021. As for ratio of main components of the lake water balance, the runoff of Khogshin Orkhon river is about 55% of the total inflow while 20% of the inflow to be precipitation on the lake water surface and remaining percentage is considered as groundwater contribution to the lake as mean of last 30 years.