4.1. Impacts of Climate Warming on High Flow Timings
Our results from the paired-year approach indicated that both study watersheds exhibited earlier timings of high flows in the warming years compared with those in the reference years. The average timings of high flows in the warming years were significantly (
p < 0.05) advanced by 21 and 25 days in the UHR and GR watersheds, respectively. The observed earlier high flow timings were consistent with many findings in other high-latitude and -altitude cold regions. For instance, Morán-Tejeda, et al. [
14] found that spring peaks due to snowmelt had been shifting earlier within the hydrological year in 27 mountain rivers in Spain from 1976 to 2008, and the significant increasing trend of spring temperature was the main co-variable responsible for the observed changes in the streamflow timing. Using a temperature-based snow module coupling with a grid-based distributed hydrological model, Bell et al. [
11] found that annual maxima tend to occur earlier in the water year associated with the large reductions in the ensemble mean of the number of lying snow days in the northerly regions of Britain in future. The similar results were also found in the north-central USA [
20], in New England [
47], in western North America [
12,
48] and in north-east Scotland [
49].
Although earlier streamflow timing has been found across the world in the context of climate warming, the mechanisms vary in snow and rain dominant regions. In snow-dominant regions, river flow is dominated by snowmelt water, and climate warming leads to earlier snowmelt that in turn results in an advance in the timing of peak spring runoff and the increased fractions of annual flow occurring earlier in the water year [
10,
12]. In order to detect the changes in snowmelt timing, the Julian date, on which 7-day moving average daily mean air temperature starts to rise above zero, was considered as snowmelt timing in two study watersheds. The average snowmelt timing in warming period was 4.1 and 5.4 days earlier than those in the reference period in the UHR and GR watersheds, respectively. In the study watersheds, annual peak runoff generally occurs in summer and coincides with peak rainfall [
22]. Also, annual peak runoff and high flows may occur in spring associated with the occurrence of ROS events. As shown in
Figure 7, spring snowmelt/rain high flow occurred more frequently in the warming period with nine and seven water years in the UHR and GR watersheds, while in only two and one water years in the reference period. According to the earlier snowmelt timing, the significant increasing trends in spring rain, spring rain ratio and air temperature (
Table 2), and their significant correlations with high flow timings (
Table 3 and
Table 4), it is safe to conclude that the observed earlier high flow timings were at least partly because the occurrence of earlier spring snowmelt/rain-generated high flows became more frequent, which was attributed to more precipitation falling as rain and earlier snowmelt timing in spring in the warming years, and consequently resulting in more and earlier ROS events. The snowmelt in a ROS event provides an additional input of water for runoff beyond rain precipitation alone [
23,
50], which can result in earlier spring high flows.
Despite more spring snowmelt/rain generated high flows occurring in the warming period, summer/fall rain-generated high flows were still dominant in each water year over the entire study period (
Figure 7). After removing the high flows occurring in spring, the average summer/fall rain generated high flow timings in selected warming years were 9 and 8 days earlier than those in the reference period in the UHR and GR watersheds, respectively. Such advances in average summer/fall rain-generated high flow timings were much smaller than those (21 and 25 days in the UHR and GR watersheds) including spring high flow, which indicated that the more frequent spring high flows played a critical role in advancing average high flow timing in warming years. Although summer/fall rain-generated high flow is mainly controlled by heavy rain regimes [
51], antecedent soil moisture condition is the other important factor influencing high flow timings [
52,
53]. Wet soil moisture condition can lead to a short lagged time between peak rainfall and peak discharge [
54]. Thus, climate warming-induced earlier snowmelt may result in the advanced wet soil moisture condition, which consequently lead to the earlier saturation excess overland flow and earlier high flows in wet season within the water year in study watersheds. This suggests it is essential to consider the direct effects of climate warming on high flow regimes including earlier snowmelt and more spring rain, and the indirect effects of climate warming including changes in antecedent soil moisture conditions for understanding the mechanism of climate warming-induced earlier high flow timings in the rain-dominated cold region.
4.2. Impacts of Climate Warming on High Flow Magnitude
The impacts of climate change on flood risk have attracted great attention in the context of climate warming [
18,
55]. Our results indicated that both study watersheds showed the lower magnitude of high flows in the selected warming years than those in the reference years with the significant (
p < 0.05) reduction in the warmer GR watershed. This means that the flood risk was reduced by the observed warming in study watersheds, which is consistent with many findings in other high latitude and altitude cold regions. For example, Molini, Katul and Porporato [
16] found that peak discharge is limited by snow melting dynamics in a warm regime, and is reduced by decreased winter snow accumulation. Hamlet and Lettenmaier [
56] found that the relative cold river basins where snow processes dominate the annual hydrologic cycle showed reductions in flood risk due to climate warming induced reductions in spring snowpack in the western U.S. in the 20th century. The reductions in the 100-year return period flood were also found in parts of far north-east Europe, which was closely related to reduced snow accumulation-induced decreasing spring runoff peak [
57].
By contrast, there were also studies concluding that global warming increases flood risk globally [
18,
58], especially in the high-altitude regions [
55]. For instance, Allamano et al. [
17] analyzed peak discharge time-series recorded in 27 gauging stations in the Swiss Alps and found a significant increase of flood peaks during the last century, which was attributed to the effects of recorded increases of temperature and precipitation intensity. Also, a global modeling study demonstrated a large increase in flood frequency in south-east Asia, peninsular India, eastern Africa and the northern half of the Andes, with small uncertainty in the direction of change for the end of this century with a warmer climate [
18]. Such increased flood peaks and flood frequency were closely related to the frequent extreme precipitation events in the context of global warming [
18,
33,
59].
In rain-dominant basins, floods are mostly associated with storms in the wet season [
56]. In study watersheds, although high flow magnitude was significantly (
p < 0.05) related to the rainfall regime including summer and annual precipitation (
Table 3), they were also significantly (
p < 0.05) related to
Tmax and
Ta6, which exhibited significantly (
p < 0.05) positive trends in the UHR and GR watersheds, respectively, over the study period. This indicated that the reduction of high flow magnitude may be attributed to the combined effects of changes in precipitation characteristics and climate warming. Although the summer and annual precipitation amount were similar in the selected paired years, the precipitation characteristics including intensity, frequency and duration may significantly affect the high flow magnitude [
60,
61]. However, there was an opposite trend between high flow magnitude and extreme precipitation (heavy precipitation) in the study watersheds. For example, in UHR watershed, the average magnitude of heavy precipitation (maximum 5% of daily precipitation in the water year) and extreme precipitation (annual maximum daily precipitation) increased by 4.7% and 28.3%, respectively, in warming years. We also found that the average magnitude of spring high flows was much lower than that of summer/fall high flows, with the reductions being 21.6% and 31.3% in the UHR and GR watersheds, respectively, in the warming period. This suggested that climate warming-induced more frequent spring high flow also contributed to the reduction of average high flow magnitude. The impacts of other precipitation characteristics, such as frequency and duration, on high flow magnitude need more process-based researches to investigate.
In addition to precipitation characteristics, antecedent soil moisture also influences high flow magnitude [
52,
53]. For instant, Ryberg, et al. [
20] found that the odds of summer/fall peaks occurring have increased across the north-central USA, when controlling for antecedent wet and dry conditions and geographical differences by using different models. In study watersheds, higher air temperature resulted in earlier snowmelt and more spring high flows in selected warming years, which consequently resulted in a longer low flow period between snowmelt spring high flow and summer peak as spring high flows occurred earlier. Thus, the magnitude of summer/fall rain-generated high flow was expected to be lower in the warming years than that in the reference years because of the lower antecedent soil moisture [
54]. Such changing hydrography can also be seen in the hydrography of the UHR watershed in the warming year 2004 (
Figure 6), in which the annual maximum discharge occurred in early spring, while there was no peak runoff generated in summer after a long low flow period, despite the rainfall peaks in July.
4.3. Implications of High Flow Regime Change for Future Watershed Management
Climate warming-induced earlier snowmelt and consequently advanced streamflow timing have attracted extensive attention in the snow-dominated regions, where water supply is dominated by melting snow packs, and increased fractions of annual flow occurring earlier in spring will lead to an issue of water shortages later in the highest water demand seasons of summer and autumn [
5]. Our results indicated that earlier high flow timings also happened in the warming years in the study rain-dominated watersheds in the cold regions of north-eastern China. Such high flow regime shift was attributed to more frequent high flow events in spring because of the increased spring rain and earlier snowmelt in the warming years. This finding provided useful information for water resource management, especially for flood forecasting. High flows are expected to happen earlier when there was a higher spring rain in the water year. Also, wet season peak flows will be at a relatively low level when more high flow events happen in spring in a warming climate, even though the magnitude of peak rainfall maintains the same level. Such high flow regime shifting has positive effects on flood risk management [
62]. However, it is a challenge for water resource management, because of the earlier high flows lead to a shift in annual flow spring, away from summer and autumn when demand is highest. This may cause a shortage of water for irrigation and domestic water supply, especially for the more southerly region (GR watershed), as the downstream of this region is Nenjiang Plain that is one of the major crop-producing areas in north-eastern China. Thus, it is necessary to consider the construction of large reservoirs or hydropower stations to store early water yield and release at other times of the year for an effective water resource management in the context of future global warming [
63].
4.4. Limitations and Future Research Direction
Although this study successfully detected the response of magnitude and timing of high flows to climate warming in two large rain-dominated watersheds in the cold region of north-eastern China by using a paired-year approach in the last four decades, several limitations may still exist. Firstly, although the spatial heterogeneity in climate was expected to be minimal because the study region is mainly characterized by gentle undulations [
22], the sparse monitoring of precipitation may cause measurement errors given the large size of the study watersheds [
64]. The climate stations are both located at the outlet of the watersheds with lower elevation; and the precipitation at high elevation may be more than that measured at the climate stations, especially for the part of precipitation falling as snowfall [
65]. Second, the study watersheds are located in the southernmost distribution of permafrost that extends from the Arctic region of Eurasia [
66], which experienced rapid warming and thawing due to the significant warming in north-eastern China in the past half a century [
67,
68]. The permafrost thaw can alter the conditions of soil moisture [
69] and the surface hydrologic connectivity [
70], and consequently affect the high flow regimes. The impacts of permafrost thaw on high flow regimes can be considered as the indirect impacts of climate warming in this study. Thirdly, it should be noted that the results could be different due to the different maximum allowable bias of controlling precipitation variables for pairing. According to the current bias (15%), 71% and 85% of all study water years were selected for the magnitude analysis, while the percentages were 87% and 79% for the timing analysis for the UHR and GR watersheds, respectively. If the bias is set to be 10%, the percentages of paired years for both magnitude and timing analysis will be reduced to approximately 50%, and the changes in timing will increase from 21 and 25 days to 22 and 27 days, while the changes in the magnitude will increase from 13.7% and 14.0% to 17.6% and 24.9% for the UHR and GR watersheds, respectively. Although the changing trends of high flow regimes are consistent when the bias changed, the magnitude of changes in high flow regimes increase is associated with the decreasing bias. This means the relatively small allowable bias helps increase the representatives of selected paired years. However, it also reduces the sample size, which may consequently lead to the uncertainty of statistical analysis. This highlights the importance of consideration of the selected maximum allowable bias used in the paired-year approach in future studies. Nevertheless, in this study, we believe the data from selected paired years based on the current maximum allowable bias (15%) were reasonably representative of the trends of the entire study water years because the results were consistent with calculated based on the relative small bias (10%) discussed above as well as those from the Mann–Kendall trend test over the entire study period in the current study. Finally, this study mainly focuses on the impacts of climate warming-induced changes in precipitation regimes on high flow, and still lacks the responses of high flow regime to the changes in precipitation characteristics in the wet season, which need more process-based and modeling studies to investigate.