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).