4.6 Simulated streamflow trends
Studying the mechanisms of runoff regime changes in Arctic basins under current and future climate conditions is an important research task. In general, river streamflow in Northern Eurasia and North America is increasing (Shiklomanov & Lammers, 2013). Most of the rivers exhibit an increase of winter base flow (Makarieva et al. 2019a; Spence et al., 2011; Tananaev et al., 2016) but there are different hypotheses about the factors influencing 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 the autumn period (Makarieva et al., 2019a).
The observed values of trends for the Suntar River are the following: an increase of 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 of 8 to 13 mm and an absence of significant changes in other seasons. The increase of annual air temperature by +2.0°C has led to an increase in the amount of liquid precipitation in September by about 12 mm. According Makarieva et al. (2019a), more precipitation is falling as rain in the autumn-winter period, leading to higher resultant streamflow. As a result, liquid precipitation is a major factor affecting the increase in low flows in this region.
Positive trends in simulated streamflow match those in observed streamflow increases (Table 7). According to the Mann-Kendall and Spearman rank-correlation tests (Kendall, 1975; Mann, 1945) and Pettitt’s test (Pettitt, 1979) significant positive trends of simulated streamflow occurred in May, September, October and November (Nesterova et al., 2019). The maximum differences between the simulated and observed runoff are in May: 1.3 mm vs. 6.8 mm respectively. The most important is the modeling of autumn changes, because the changes in this period affect the runoff formation in winter low flow. Simulated trends generally do coincide with the observed trends of streamflow values in September (10.2 vs. 9.9 mm), October (1.3 vs. 3.3 mm) and November (0.35 vs. 0.43 mm) respectively. The change point in the autumn season coincides with the observed data and refers to the period 1993-1996 (Fig. 8).