8. Simulated streamflow trends
Studying the mechanisms of runoff regime transformation changes in the Arctic basins in current and future climate conditions is important research task. In general, river streamflow in Northern Eurasia and North America is increasing (Shiklomanov & Lammers, 2013). Most of the rivers exhibit the increase of winter base flow (Makarieva et al. 2019a; Spence et al., 2011; Tananaev et al., 2016) and there are different hypothesis about the factors of such changes. The analysis of monthly streamflow data in the basins of the Yana and Indigirka Rivers (1936-2015) has shown the presence of statistically significant (p <0.05) positive trends in May and autumn period (Makarieva et al., 2019a). The values of trends for the Suntar River are the following: 6.8 mm or 103 % in May, 9.9 mm or 49 % in September, 3.3 mm or 70% in October and 0.43 mm or 52% in November (Makarieva et al., 2019a). There is a decrease in precipitation in winter (minus 8 to 13 mm) and the absence of significant changes in other seasons. The rise of annual air temperature by +2,0°C led to an increase in the amount of liquid precipitation and streamflow in September by about 12 mm. The revealed dependences of monthly streamflow on the amount of liquid precipitation in September for small catchments, as well as the single period (1991-1996), when the changes of these hydrometeorological characteristics are simultaneously observed, indicate that the phase state of precipitation can be among main factors affecting the increase of low flows of studied rivers.
According the Mann-Kendall and Spearman rank-correlation tests (Kendall, 1975; Mann, 1945) and Pettitt’s test (Pettitt, 1979) significant positive trends of simulated streamflow was in May (1.3 mm or 118 %), September (10.2 mm or 38.1%), in October (1.3 mm or 33.3%) and November (0.35 mm or 35.9%) (Nesterova et al., 2019). Thus, the simulated trends do coincide with the observed trends of stream flow values (Table 7). The share of liquid precipitation increased by an average of 10 % or 13.6 mm, which corresponds to the observed value of 12 mm. The change point in the autumn season coincides with the observed data and refers to the period 1993-1996 (Fig. 8).