Global warming due to anthropogenic greenhouse gas emission is amplified in high-latitude [1
] and -altitude regions [2
], which significantly changes permafrost distribution, and thus affects hydrological processes and conditions [3
]. The changes in the hydrological regime will affect the availability of water resources and the timing and magnitude of floods and low flows [5
], which will play a profound implication on local ecological economy development and water resources management.
During winter, surface runoff is negligible due to freezing conditions, and river discharge with no direct liquid supply of rainfall but snowfall, is assumed to mainly come from the relatively deep groundwater flowing through the unfrozen parts of the ground [9
]. Under climate warming, the groundwater is expected to become more and more important due to permafrost degradation, which enhances liquid water infiltration and supports deep flow paths [9
]. The increased winter discharge or cold-season minimum discharge has been detected in the Arctic and sub-Arctic rivers, such as in Yukon River [12
], Northern Eurasian rivers [13
], Northwest Territories, Canada [11
], Lena River [8
], and entire pan-Arctic [15
] for various time records. This increased trend is speculated to have a close relationship to permafrost dynamics. Model simulations have paid special attentions to the interaction between groundwater and surface water in permafrost regions. Bense et al. [16
] suggested that a large increase in groundwater discharge to streams for the next few centuries will likely occur in response to permafrost degradation due to climate warming. Ge et al. [17
] demonstrated that a three-fold thickening of the active layer will lead to a three-fold increase in groundwater discharge. Wellman et al. [18
] demonstrated that permafrost thaw could accelerate rates of groundwater flow in the active layer above permafrost. Evans et al. [19
] suggested that an increase in mean annual surface temperature of 2 °C could cause a three-fold increase in groundwater contribution to discharge.
Permafrost degradation has been shown to smooth out the seasonal distribution of discharge at catchment scale, which significantly affects groundwater dynamics and resources volumes [8
]. Permafrost extent over a region plays a key role in the distribution of surface-subsurface water interactions [7
]. Compared with non-permafrost regions, permafrost catchments have higher peak flow and lower base flow [20
]. In the permafrost regions, catchments with higher permafrost coverage could have lower groundwater storage capacity, and thus a lower winter base flow and a higher summer peak flow [20
]. Ye et al. [8
] describe a relationship between the ratio of monthly maximum discharge to minimum discharge (Qmax
) and permafrost coverage in the Lena basin, where the hydrological regime is significantly influenced by permafrost extent when it exceeds 40% of the catchment area. Besides, the regions with the higher permafrost coverage correspond to the lower groundwater storage capacity, which may become weaker due to permafrost degradation [10
]. Long-term trends in recession flow, as a proxy for permafrost degradation, have been analyzed in Northern Sweden [10
], Yukon River basin [29
] and Northwest Territories, Canada [31
]. These changes in the recession flow properties are consistent with the observation of permafrost degradation.
Located in the hinterland of the Qinghai-Tibetan Plateau (QTP), the source regions of the Yangtze and Yellow rivers are one of the most important permafrost distribution regions in China. Widespread permafrost degradation has been extensively reported on the QTP [33
]. Acted as the “water tower” of China, the source regions of the Yangtze and Yellow rivers provide substantial water supply for the adjacent lowlands. Understanding the effects of permafrost degradation on the hydrological regime in the area is very urgent for local water resources management and has substantial social, economic and ecological significance.
In this study, for the first time, we estimate the effects of permafrost degradation on the hydrological regime in the source regions of the Yangtze and Yellow rivers via four hydrological variables, namely, winter discharge, winter discharge ratio (proportion of winter discharge contribution to total annual flow), recession coefficient and the ratio of Qmax/Qmin. The primary objectives of the present study are to (1) detect hydrological regime changes of the 10 catchments in the study area and (2) attempt to analyze the effects of permafrost degradation on the hydrological regime by assessing the relationship between hydrological variables and the coverage of permafrost. The result of this study will improve our knowledge of cold region hydrology and its change due to climate impact in permafrost regions.
For 10 catchments in the study area, the correlation analysis between winter discharge ratio and permafrost coverage implied that catchments with higher permafrost coverage could have lower winter discharge ratio in the permafrost regions (Figure 2
a), which was consistent with the common results that dominated the current literature [11
]. However, catchment 9 had higher winter discharge ratio (56%) but with higher permafrost coverage (99%), which may be attributed to the great storage effects of widespread lakes and wetlands in the drainage area. According to the statistics in the late 1980s [49
], there are about 5300 lakes in the source region of Yellow River, and about 80% of them are located in catchment 9, including the two largest fresh water lakes in the source regions (Eling and Zhaling). Summer precipitation in this catchment mainly contributes to groundwater instead of direct surface water due to the widespread lakes and wetlands [35
], which makes an important supply for the following winter discharge. A positive correlation between the average recession coefficient and permafrost coverage, the average ratio of Qmax
and permafrost coverage was also detected, which implied that permafrost degradation could allow more water storage to support winter discharge and smooth out the seasonal distribution of discharge at catchment scale to some extent. Similarly, catchment 9 appeared to be non-consistent with the above trend, the great storage effects of widespread lakes and wetlands may be responsible for it as well.
Under the warming climate, the hydrological regime of the source regions of the Yangtze and Yellow rivers is expected to change significantly due to permafrost degradation. However, using the same statistical methods as Walvoord and Striegl [12
], St. Jacques and Sauchyn [11
], we reported a much lower proportion (0%) showing increased winter discharge for the source regions of the Yangtze and Yellow rivers than for the Yukon River Basin (90%) and the Northwest Territories, Canada (87%) at the p
≤ 0.1 level. Besides, the other three hydrological variables, namely winter discharge ratio, recession coefficient and the ratio of Qmax
also did not change significantly (Table 3
In general, summer precipitation greatly contributes to discharge in the areas underlain by permafrost, and catchments without permafrost have high base flow contributions [7
]. A deeper active layer allows additional water storage and pathways for transferring water, resulting in the dependence of winter discharge on the timing and amount of summer precipitation [50
]. In the source region of Yellow River, the correlation analysis between monthly discharge and precipitation suggested that the monthly discharge was often the result of a combined effect of the precipitation in the current and previous months [35
]. At the boundary of continuous and discontinuous permafrost of the lower Yenisei River, a common increase in the time-lag between precipitation events and stream discharge was detected between 1970s and 1980s [50
]. In this study, a common increase in the time-lag between summer precipitation and the following cold-season monthly discharge for the two periods (before 1985 and after 1985) was also detected (Figure 3
), which implied that permafrost degradation may affect the redistribution of summer precipitation towards the following winter discharge via increasing the soil storage capacity and delaying the release of water into streams. For the source regions of the Yangtze and Yellow rivers, a decline in summer precipitation is noticeable in the majority of the stations as shown in Figure 4
, which is consistent with other researchers [35
]. In order to validate the above results (as shown in Figure 3
and Figure 4
), we turn to precipitation (July–September), DEM (SRTM3), latitude, and longitude data of 36 meteorological stations in and around the source regions (Figure S1
). A linear multiple regression analysis between precipitation, DEM, latitude, and longitude data of 36 meteorological stations was carried out for each year (all passed the statistical tests), and the precipitation data can be interpolated for the whole source regions for each year by using linear multiple regression equations (Figure S2
). Catchment boundaries were used to extract the average precipitation for each catchment. The common increase in the time-lag between summer precipitation and the following winter discharge for the two periods (before 1985 and after 1985) was detected (Figure S3
), and also a decline in summer precipitation was noticeable for most of the 10 catchments as shown in Figure S4
Although permafrost hydrological simulations have suggested that a large increase in groundwater discharge to streams will likely occur in response to permafrost degradation due to the warming climate, they also emphasize the essential hypothesis that sufficient surface water could infiltrate to replenish the shallow groundwater system. Otherwise, there will be a substantial lowering of the water table in the recharge area of the catchments [16
]. In this study, the decreased summer precipitation may result in less liquid water infiltration to supply the groundwater, which could weaken the effects of permafrost degradation on hydrological regime, and result in non-significant change trends for the hydrological variables. This may provide an important reference for permafrost hydrological simulations under permafrost degradation. Besides, unlike the Arctic and sub-Arctic regions, the relatively low ground ice content in the source regions of the Yangtze and Yellow rivers may result in less melting ground ice, and further weaken the hydrological response of permafrost degradation.
This study analyzed the effects of permafrost degradation on the hydrological regime via four hydrological variables for 10 unregulated catchments in the source regions of the Yangtze and Yellow rivers. The relationship between hydrological variables and the coverage of permafrost implies that catchments with high permafrost coverage are expected to have an increased winter discharge ratio, a decreased recession coefficient and a decreased ratio of Qmax/Qmin due to permafrost degradation. However, catchment 9 seems inconsistent with the above rules, the great storage effects of widespread lakes and wetlands may be responsible, which could contribute to more groundwater instead of direct surface discharge and make an important supply for the following winter discharge.
The correlation analysis between summer precipitation and the following winter discharge indicates that permafrost degradation may affect the redistribution of summer precipitation towards the following winter discharge via increasing the soil storage capacity and delaying the release of water into streams. However, unlike the Arctic and sub-Arctic regions, almost no significant changes for hydrological variables were detected over the individual periods of records for each catchment. Decreased summer precipitation seems to reduce water infiltration to supply the groundwater, which could weaken the effects of permafrost degradation on the hydrological regime, and result in non-significant change trends of the hydrological variables.
Model simulations have suggested that a large increase in groundwater discharge to streams will likely occur in response to permafrost degradation due to the warming climate in the ideal scenario. However, this study found that the storage effects of lakes and wetlands and the changes of summer precipitation patterns should be considered in future permafrost hydrological simulations.