Streamflow changes over Siberian Yenisei River Basin

Abstract

This study analyzes long-term (1935–99) monthly discharge data for the major sub-basins within the Yenisei River watershed in order to document significant streamflow changes induced by reservoir regulations and by natural variations/changes. The results show that both the unregulated upper basin and major lower streams of the watershed experienced streamflow decreases in the early melt period and discharge increases in the late melt season. These changes in snowmelt runoff pattern suggest a delay in snowcover melt in the Yenisei basin perhaps associated with cooling trends during the snowmelt months over central Siberia. This study also demonstrates that the reservoir regulation has significantly altered the monthly discharge regimes in northeast and the upper portions of the Yenisei basin. Constructions of four large dams in the northeast Yensiei regions reduced the summer peak flows in the Angara valley by 15–30% and increased the winter low flows by 5–30%. Operations of two large reservoirs in the upper Yenisei regions enhanced the winter flows by 45–85% and reduced the summer flows by 10–50%. These alterations lead to a streamflow regime change toward less seasonal variation over the eastern and lower Yenisei basin. Because of reservoir regulations, discharge records collected at the Yenisei basin outlet do not always represent natural changes and variations, they tend to underestimate the natural streamflow trends in summer and overestimate the trends in winter and fall seasons. Cold season discharge increase over the Yenisei river is not natural-caused, but mainly the effect of reservoir regulations in the Yenisei basin.

Introduction

Fresh water discharge from northern-flowing rivers in the polar regions plays an important role in regulating the thermohaline circulation of the world's oceans (Aagaard and Carmack, 1989). Studies show that both the amount and the timing of freshwater inflow to the ocean systems are important to ocean circulation, salinity, and sea ice dynamics (Aagaard and Carmack, 1989, Macdonald, 2000, Peterson et al., 2002). Climate over arctic regions has experienced significant changes during the past few decades. For instance, climate changes over Siberian regions include considerable winter warming (Chapman and Walsh, 1993, Serreze et al., 2000, Michaels et al., 2000), winter and fall precipitation increase (Wang and Cho, 1997), winter snow depth increase (Ye et al., 1998), and ground temperature rising and permafrost thawing (Pavlov, 1994). Climate models predict 1–4 °C surface air temperature increase in the 21st century over the earth, with even greater increase in the Arctic regions (Dai et al., 2001a, Dai et al., 2001b). This warming trend will impact the structure, function, and stability of both terrestrial and aquatic ecosystems and alter the land–ocean interaction in the Arctic (Weller, 1998).

Efforts have been reported to investigate and understand the response of large northern river systems to climate change and variation (Vörösmarty et al., 2001, Maguson et al., 2000, Yang et al., 2002, Yang et al., 2003, Ye et al., 2003, Louie et al., 2002, Proshutinsky et al., 1999). Recent studies find that most northern rivers, including the largest arctic rivers in Siberia, show an increasing runoff trend, especially in winter and spring seasons, over the last several decades (Grabs et al., 2000, Zhang et al., 2001, Lammers et al., 2001, Nijssen et al., 2001a, Nijssen et al., 2001b, Yang et al., 2002, Serreze et al., 2002). The causes for these changes are not all clear. It has been suggested that spring discharge increase in Siberian regions is primarily due to an early snowmelt associated with climate warming during snowmelt period (Nijssen et al., 2001a, Nijssen et al., 2001b, Yang et al., 2002, Yang et al., 2003, Serreze et al., 2002), and changes in winter streamflow are perhaps associated with reduction in permafrost area extent and an increase in active layer thickness under a warming climatic condition (Yang et al., 2002, Serreze et al., 2002).

It is important to emphasize that, in addition to climate-induced river streamflow changes and variations, human activities, such as the construction of large reservoirs, inter-basin water diversions, and water withdrawal for urban, industrial and agricultural needs, will also impact river discharge changes over space and time (Miah, 2002, Ye et al., 2003, Vörösmarty et al., 1997, Revenga et al., 1998, Dynesius and Nilsson, 1994). Mainly due to low population and slow economic development in the high latitude regions, human impacts have been considered to be minor in the arctic river basins in comparison with mid to low latitude regions (Shiklomanov, 1995, Vörösmarty et al., 1997, Shiklomanov et al., 2000, Lammers et al., 2001 Lammers et al., 2001). Shiklomanov et al. (1997) shows that the total water consumption in the Yenisei basin with the largest anthropogenic impact over Siberia is about 0.8–1.4% of total river runoff measured at mouth in 1995. The magnitude of this influence is unlikely to produce noticeable effects on discharge into the Arctic Ocean (Shiklomanov et al., 2000). Ye et al. (2003) recently quantified the effect of reservoir regulations in the Lena basin and found that, because of a large dam in west Lena river, peak discharge in the Vului valley has been reduced by 10–80% in the summer season and low flow has been increased by 7–120 times during the cold months. These alterations, plus a remarkable streamflow increase in May and a decrease in June over the Lower Lena basin, lead to a streamflow regime shift toward early peak flow at the Lena basin outlet (Yang et al., 2002, Ye et al., 2003).

To better define the seasonal discharge regimes and their changes, human activities, especially reservoir regulations in the high latitude regions, deserve more attention. This study will systematically analyze long-term monthly and yearly discharge records for the major sub-basins of the Yenisei river watershed. The emphases of this work are to document significant streamflow changes induced by large reservoirs and by natural variations, and to quantify the impacts of observed changes on regional hydrologic regimes. We also discuss the key processes of interaction and feedback between climate, permafrost and river systems of the northern regions. The results of this study will be useful to ongoing national and international efforts of assessing recent changes in the hydro-climatology of the pan-arctic landmass and the terrestrial ecosystems (Vörösmarty et al., 2001). They will also improve our understanding of hydrologic response to climate change and variation in the high latitude regions.