2 Study Area
The Irtysh River, flowing in East Kazakhstan, is the main water resource of the country, with an average annual flow of 33.8 km3, and the most affected by floods (Kazhydromet, 2006). Its upper catchment (196,000 km2, Bayandinova et al., 2017; Fig.1) encompasses five sub-basins, namely Uba (9,917 km2), Bukhtarma (13,663 km2), Narym (1,852 km2), Kurchum (4,975 km2) and Kara Ertis (143,847 km2) from northwest to southeast. Kara Ertis is the largest basin and the Kazakh name of the Irtysh river before the confluence into Lake Zaisan. The river originates in the southwestern Altay Mountains at 2500 m a.s.l and flows northwest through the uplands of northern China, where it is joined by streams originating in western Mongolia. The major tributaries of the upper river course originate on the western side of the Altay Mountains, and their basins are part of the East Kazakhstan administrative region. Narym, Kurchum (length: 218 km) and Bukhtarma (336 km) flow into the Bukhtarma reservoir; downstream of the city of Öskemen, the Irtysh is joined by the Uba River (Bayandinova et al., 2017). It then passes by Semey and Pavlodar (all with a population of 300,000 or above) in northern Kazakhstan and enters Russia, joining the Ob and eventually flowing into the Kara Sea. Water intake occurs in China and through a cascade of large water reservoirs namely the Ust-Kamenogorsk, Bukhtarma and Shulbinsk in Kazakhstan, (Huang, 2014), used for irrigation and hydroelectric power production; in fact, the transboundary nature of the river, shared between Kazakhstan, China, Mongolia and Russia, has led to conflicts over water usage (Hrkal et al., 2006; Alimkulov et al., 2017).
The climate of the study area is significantly affected by the presence of the Altay Mountains, capturing atmospheric moisture transported by the Westerlies and resulting in precipitation totals up to 1500-2000 mm yr-1 above 600 m a.s.l at the Irtysh headwaters (Zhang et al., 2018) and semi-arid rain-shadow conditions on the leeward Mongolian slopes (Huang, 2014; Klinge et al., 2003; Malygina et al., 2017). Temperature ranges are extreme, between +41°C in summer and -47 °C in winter (Malygina et al., 2017). In contrast with high precipitation in Upper Irtysh, the lower river basin is very dry, with annual rainfall not exceeding 250 mm at Pavlodar in North Kazakhstan, 500 km downstream of Öskemen (Huang, 2014).