Hydrologic and Thermal Regimes of Coarse Blocky Materials and Imbalance in Seasonal Snowpack Contribution to the Total River Runoff in Tien Shan Mountains, Central Asia

Central Asia is a water-stressed area where projected climate change could further decrease stream flow and groundwater recharge (IPCC, 2007). General circulation models suggest that the increase in summer diurnal temperatures over Central Asia is likely to be higher relative to that in other regions (IPCC, 2007). Therefore, we expect a further degradation of glaciers and alpine permafrost and decrease in snow cover. Under continued atmospheric warming the decrease in snowfall will lead to a decline in snow melt contribution to river runoff. Increased glacier melting will compensate this process for some period of time. But eventually, a further decrease in glacial area would lead to a decline in the contribution of glacier melting to the river runoff. Under continuing warming and permafrost degradation in Central Asia, the ground ice could increase future water supply, and the melt waters from permafrost could become an increasingly important source of fresh water in this region in the near future. Mountain permafrost and associated periglacial landforms contain large quantities of stored fresh water in the form of ice. The moraines, rock glaciers and other coarse blocky materials have especially high ice content (30-70% by volume). Recent observations indicate a warming of permafrost in many mountain regions with the resulting degradation of ice-rich permafrost. Permafrost temperature has increased by 0.5 to 1.5 deg C in Tien Shan Mountains, Central Asia during the last 35 years. At the same time, the average active-layer thickness increased by 23% in comparison to the early 1970s. Runoff from the active layer contributes a significant amount of water during the summer time, when snowmelt has finished and the ground ice melt starts and intensifies. The thickness of the active layer is one of the dominant factors controlling the subsurface flow conditions. Air temperature, precipitation and ground structure are other components influencing water flow. Blocky materials and other coarse debris, such as taluses, accumulate a significant volume of water in the form of perennial and seasonal ice. We conducted observations on seasonal ground ice accumulation and ablation inside of coarse debris, runoff, precipitation, evaporation, and temperature dynamics in blocky materials of various genesis. The mean annual temperatures inside coarse debris are typically 3-4 deg C below the mean annual air temperatures. In such conditions, ice-rich permafrost may develop within coarse debris even in areas with mean annual air temperatures above 0 deg C. We have found that the highest rate of ice accumulation inside of coarse debris occurred during the spring time when air temperature during the daylight cross 0 deg C threshold and melted and condensed water penetrates into coarse debris, then freezes and cements the debris. The result is an ice-block mass with ice content as high as 35-50% by volume. Over a warm season, the melting seasonal ice in the upper portion of coarse debris is an important factor in seasonal redistribution of the total river runoff.