4.2.2. SCOD

Figure 6 shows that the SCOD was delayed from north to south, which meant an obvious increase with latitude. At high latitudes, the SCOD were mainly concentrated within 60–90 d, that is, in November of the current year, and in the northern part of the Daxingan Mountains, the SCOD were at the end of September or early October of the current year. In low-latitude areas, the SCOD were concentrated within 90–120 d, that is, December of the current year. The SCOD were later in some areas of the southern Songnen Plain, and the latest was in January of the next year. The annual average SCOD in Northeast China were mainly concentrated within 60–100 d, from November to the middle of January of the next year, accounting for 84.78% of the total area. Among them, the peak was 70–80 d, which was approximately the middle of November of the current year, with a proportional area of 30.22% (Figure 7).

**Figure 6.** Spatial distribution of annual average SCOD in Northeast China from HY2001 to HY2017.

**Figure 7.** Histogram of annual average SCOD from HY2001 to HY2017 in Northeast China.

Figure 8 shows the significance tests and the trend of SCOD from HY2001 to HY2017. The results of the Theil–Sen trend and Mann–Kendall significance test were consistent. The area in which the SCOD showed an early trend accounted for 31.93%, while the delayed trend accounted for 32.80%. The trends of significant advance and delay were relatively small, only 0.24% and 0.12%, respectively (Figure 8a). Overall, the area with an increase in Figure 8a corresponded to an SCOD trend > 0 d/a in Figure 8b, and that the area with a decrease corresponded to an SCOD trend < 0 d/a.

**Figure 8.** Trend of SCOD in Northeast China. (**a**) Significance test and (**b**) trend of SCOD from HY2001 to HY2017.
