Different runoff patterns determined by stable isotopes and multi-time runoff responses to precipitation in a seasonal frost area: A case study in the Songhua River basin, northeast China

Runoff patterns are crucial to determine the hydrological response to climate change, especially in a seasonal frost area. In this study, multi-time runoff responses to meteoric precipitation for the period from July 2014 to June 2016 and the period from 1955 to 2010 were obtained to identify different runoff patterns in the Songhua River basin, northeast China, based on six stations. Two distinctly different runoff responses are exhibited: a periodic one in response to precipitation in the Nen River and a constant one in the Second Songhua River under different scales. Stable isotopes in the plain with diverse characteristics also supported these runoff patterns. What is more, gradual runoff relatively less sensitive to precipitation in the Second Songhua Rive was attributed to upstream dam constructions. Furthermore, the Second Songhua River contributes more water to the main stream during January to March at the seasonal scale and in the 2000s at the annual scale, with low precipitation during those periods. This study could have implications for the water management in the Songhua River basin.

Chiew ; Novotny & Stefan ). In this case, a quick response in which the maximum runoff peak coincided with the rainfall peak was observed (Changnon & Kunkel ; Onda et al. ), whereas in other cases, the variability of runoff was reduced due to check dams (Batalla et al. ; Abbasi et al. ), indicating less sensitivity to precipitation.
Unraveling runoff patterns and their spatial-temporal variations is critical for the prediction and management of surface water resources (Birsan et al. ; Wu et al. ). The aims of this study are: (1) to display different runoff patterns of the Songhua River with precipitation-runoff relationships in different time scales (daily, monthly, and annual) and isotopic evidence; and (2) to quantify the contributions of the two sources to the runoff of the main stream. This work will provide important insights into future sustainable water resource management and planning in the study area.

STUDY AREA
This study was conducted in the Songhua River catchments in The two rivers join the Songhua River at Sanchahe and flow northeastward before joining the Heilongjiang River at the outlet of the river basin ( Figure 1).
The Songhua River basin has a mean annual temperature of 3-5 C with significant monthly temperature difference. Mean temperatures are below À20 C in January and 25 C in July. The mean annual precipitation in the river basin during the period 1955-2010 was 525.6 mm, more than 75% of which occurred during the rainy season from June to September (Liu et al. ). In comparison, precipitation is very low from December to February and represents only 5% of the total precipitation. Furthermore, spatial distribution of precipitation across the river basin is heterogeneous. The mean annual precipitation in the southeastern part of the river basin is about 700-800 mm, while that in the western part is only 400 mm.

METHODS
The double-mass curve plotted between precipitation and runoff is commonly used to reveal the streamflow change. To calculate the contributions of total runoff from the two different sources, a two-component mixing model was used. The two-component separation of a runoff hydrograph into runoff from Nen River and runoff from the Second Songhua River can be described as: where, Q N and Q SS are runoff from the Nen River and the Second Songhua River, respectively, and q N=S is the proportion of the Nen River runoff in the total runoff from the Nen River and the Second Songhua River.
The variation coefficient of runoff was employed to analyze fluctuation of daily or annual runoff based on: where Q(t) is the daily or annual runoff for day/year (t), and Q is the average of Q(t). When the value of C v approaches zero, the runoff tends to be constant throughout the period.

Daily precipitation-runoff characteristics
There are four stations in the main stream of the Songhua River. Figure  Therefore, the runoff mechanism must be different for the two tributaries.

Monthly precipitation-runoff characteristics
To further understand the intra-annual variation in runoff, monthly mean runoff was calculated at six stations

DISCUSSION
The runoff responses to monthly precipitation To ascertain the controlling factors of runoff change, the monthly cumulative precipitation and cumulative monthly runoff are presented in Figure 5. There is a good relationship between cumulative precipitation and  To further confirm the effect of precipitation on the runoff change in the stations, the monthly precipitation amount and monthly runoff are plotted in Figure 6. At Fuyu station in the Second Songhua River, the monthly runoff is constant when the monthly precipitation amount Meanwhile, a steady trend was observed at Fuyu station in the Second Songhua River, which indicates that the influence of precipitation on runoff is relatively weak compared to that of other stations.

The runoff responses to annual precipitation
There is an obvious relationship between annual precipitation and runoff at Dalai, Harbin, and Jiamusi stations ( Figure 8), indicating the controlling effect of precipitation on runoff changes at these stations in the annual scale.
However, the cumulative anomaly of annual runoff at Fuyu station in the Second Songhua River markedly differed from that of other stations both in the Nen River and the main stream (Figure 9(a)), which displayed a gentle trend.
There is an obvious jump at Dalai, Harbin, and Jiamusi stations during the year of 1998 (Figure 9(a)). The same 'turning year' was also identified in the studies of Wang

Contributions of tributaries to the main stream
If the total flow of the Songhua River is composed of water from the two tributary rivers, the proportion originating from the Nen River during each day can be calculated ( Figure 11). During the period from January to March, these values are lower than 0.5, which indicates that during this period, the Second Songhua River contributed more water to the total runoff in the main stream. During the rest of the year, the Nen River appears to play a more important role in providing water to the mean stream ( Figure 11). We assumed that the base flow is equal to the flow of the catchment during the lowest discharge period, observed from January until March, the months with the