As an aquitard, permafrost weakens the downward infiltration from precipitation, glacier, and snow-cover melt-water and raises the supra-permafrost water table, influencing the streamflow and soil water content in the active layer. Surface waters and near-surface soil water availability are important for the growth and survival of alpine vegetation (Li et al., 2020a; Cao W. et al., 2021). Meanwhile, the melting of ground ice could release more water to soil, impacting the groundwater and the water cycle at a local scale, and even at a regional and global scale (Walvoord and Kurylyk, 2016; Schuur and Mack, 2018; Yang et al., 2019). Permafrost degradation has significant implications on alpine ecosystems through alterations in soil hydrological regimes (Wrona et al., 2016; Zhao et al., 2020). Coupled with lowering permafrost table and thickening active layer, regional permafrost degradation may result in lowering supra-permafrost water table and declining water-holding capacity of near-surface soils in the aeration zone (Jin X.-Y. et al., 2021; Jin H.-J. et al., 2021; Lv et al., 2022). As a result, the capillary water cannot move upward to the shallower soil layer where the roots of herbaceous and aquatic plants concentrate. Thus, near-surface soil could salinize by the enhanced evaporation, thereby restricting the growth of most vegetation (Wang et al., 2006; Qin et al., 2017; Sun et al., 2020).

Water resources in the SAYR are mainly recharged by precipitation, snow- and ice-melt, and groundwater of various types. Under a warming climate with permafrost degradation, mean annual evaporation increased and surface runoff have decreased (Lan et al., 2013; Wu et al., 2018; Yan et al., 2020). The mean annual runoff volume was approximately 2.02 × 10¹⁰ m³ and decreased by 5.5 × 10⁹ m³ from 1960 to 2007 as recorded at the Tanag and/or Jimai hydrological stations, mainly due to the reduced runoff from mid-June to late October, i.e., the growth season (Wang T. et al., 2018; Wu et al., 2018). However, for the entire SAYR, the annual runoff exhibited a decreasing trend (6.51 mm/10a), while the annual precipitation increased (6.48 mm/10a), suggesting that other factors may influence the change trend of runoff, such as the potential evapotranspiration and more importantly permafrost degradation (Wang T. et al., 2018). The analysis of stable isotopes (δ¹⁸O and δD) in a representative watershed of the HAYR show a contribution ratio of precipitation of the surface runoff at 52.2–53.5% and that of ground ice melting at 13.2–16.7% (Ma et al., 2019; Yang et al., 2019). Water from melted ground ice directly contributes approximately 4.8% to the annual runoff of the Yellow River in of the SAYR, whereas the recharge from melted ground ice cannot completely offset the increase in land surface evapotranspiration under a persistently warming climate, hence resulting in a decrease in surface runoff, but otherwise an increase in groundwater storage (11.4 ± 13.9 × 10⁸ m³/a from 2003 to 2009) (Xiang et al., 2016; Lin et al., 2019). However, little is known about the moving trajectories of water from ice melting, total volume water from the melting ground ice, and the magnitudes of melt-water of this ground ice to contribute to surface and subsurface water.

According to the observations in the past 30 years, the local groundwater table in the Yellow River valley plain near the Madoi county town continuously lowered by 0.8–1.5 m, with a maximum lowering decreased depth of 1.8 m, with an average annual lowering rate of approximately 0.5 m/10a (Cheng and Jin, 2013). In the meantime, this lowering groundwater table may also be associated with good soil drainage due to a high percentage of sandy and gravely constituents of soil texture (Gao et al., 2018; Lv et al., 2022). According to the model results of the percentage reduction of the single-spring runoff modulus, the recharge amount of the supra-permafrost water in the SAYR decreased by 15 × 10⁸ m³ during the 14 years from 1989 to 2003, with an average annual decrease rate of 1.1 × 10⁷ m³/a (Jin et al., 2009; Cheng and Jin, 2013). Together with a slight wetting and an enhanced evapotranspiration, the sharp decrease in recharge to the supra-permafrost layer resulted in a lower level of the supra-permafrost water table, thus causing water tables in the HAYR to continuously decline (Wang T. et al., 2018; Wu et al., 2018).

Although the melting of ground ice may replenish water resources and increase the total groundwater storage—though probably at a very limited magnitude or rate— warming climate, increasing evaporation, and changing land surface processes have reduced runoff generation, resulting in a lower groundwater table (Wu et al., 2018; Lin et al., 2019; Sun et al., 2020). The main source of soil water is precipitation and with the continuous temperature rise, the evaporation of soil water increases, while the annual precipitation has been fluctuating with a minor increase trend of 16 mm/10a (Qin et al., 2017; Xie et al., 2021). As a result, near-surface soil moisture, the key to maintaining the long-term growth of herbaceous plants, is depleting continuously and is on the decline. As a result, the regional water table lowers as well. In arid regions on the Qinghai-Tibet Plateau, this kind of groundwater table for sustaining the long-term growth of herbaceous plants is called the “the burial-depth of ecologically safe groundwater table” (Liang et al., 2007; Wang et al., 2012). In permafrost regions, we can thus rename it as the “burial-depth of ecologically safe supra-permafrost table” (BESST).

Field surveys have found that the deepening permafrost table leads to a lowering BESST (Wang et al., 2006; Yang et al., 2013). When the groundwater table is less than 2 m, soil water content may suffice plant growth and accordingly maintain high vegetation cover (Wang et al., 2012; Jin et al., 2020). When the burial depth of groundwater table is lower than 2 m, most of alpine plants with a shallow root system suffer the drought stress, and only a few types of drought-tolerant plants with well-developed root systems may access the limited available soil water for survival (Jin et al., 2020). Therefore, the vegetation coverage decreases. When the burial depth of the groundwater table is greater than 5 m, herbaceous plants often wither and eventually die from drought and only shrubs with deep roots can be occasionally observed (Wang et al., 2006). This indicates that changes or variations in the permafrost table control the BESST and the unfrozen water content of near-surface soils, thereby affecting the growth of plants (Wang et al., 2012). In cold and arid regions, a burial depth of 2 m of the groundwater table can thus be regarded as the threshold of the ecologically safe groundwater table (Liang et al., 2007; Wang et al., 2012), or the BESST. At a BESST deeper than 2 m and with slow replenishment by surface water infiltration, the capillary water cannot rise to the shallow soil layer, where the roots of most plants are concentratively distributed, so most plants are subjected to long-term drought stress from the growth environment, leading adverse successions and perpetual degradation of alpine grasslands (Liang et al., 2007; Li et al., 2020a; Li et al., 2020b; Jin X.-Y. et al., 2021, Jin H.-J. et al., 2021). However, the mechanisms for this depth control of the alpine vegetation growth remains poorly understood.

As an aquitard, permafrost help enrich soil water above the bottom of the active layer or above the permafrost table in the presence of the supra-permafrost subaerial talik, which increases evaporation and runoff, but reduces downward infiltration, thus favoring the formation of thermokarst lakes and ponds, shallow rivers, and wetlands in lowlands (Walvoord and Kurylyk, 2016; Schuur and Mack, 2018). There are numerous rivers and lakes in the HAYR and wetlands are widely distributed. There are more than 5,300 lakes in the SAYR with a total area of more than 1,270.77 km² (Luo et al., 2020c). These lakes, which are characterized as small and densely distributed, are close to the main streams along the Yellow River, and low-lying flat marshes. According to the remote sensing data, the area and number of lakes greater than 0.01 km² both experienced four stages from 1986 to 2019: reduction (1986–2004), increase (2004–2012), reduction (2012–2017), and increase again (2017–2019) (Luo et al., 2020c). The number of lakes greater than six hectares (>0.06 km²) decreased from 405 to 261 in Madoi County from 1990 to 2000 (Dong et al., 2009). Small lakes were extremely unstable because of their small area, shallow water depths, and high susceptibility to disturbances (Watts et al., 2014; Luo et al., 2020c; Șerban et al., 2020; Șerban et al., 2021). Since the 21st century, especially after 2004, the area and number of lakes have begun to increase, possibly due to the increased water resources replenished by ground ice meltwater (Li et al., 2008). To date, there is an urgent need to develop and improve the methods for mapping the distribution and changes of medium and small water bodies using the remote sensing in permafrost regions. Furthermore, we also need to have in-depth understanding of influences of permafrost degradation on water-heat dynamics of thermokarst lakes (Șerban et al., 2020; Șerban et al., 2021).

The main types of alpine wetlands are lacustrine wetlands, alpine peat bogs, and marshlands. The areal extent of wetlands is about 2,126.33 km², accounting for approximately 10% of the total catchment area of the HAYR (Niu, 2020). The lacustrine wetlands in this region, as represented by those in the peripheries of the Gyaring and Ngöring lakes, are mainly distributed in lowlands, while the marshes are mostly close to rivers and lakes and are on the low-lying shadowy slopes (Li et al., 2016c; Luo et al., 2020a). Under the influence of climate warming, strong evaporation, and permafrost degradation, wetlands are shrinking (Gao et al., 2013; Dong et al., 2009). From 1990 to 2004, wetlands were continuously shrinking at a rate of 2,864.48 km²/a, resulting in a total areal reduction of 40,102.65 km² during this 14-year period, including a decrease of 23,298.09 km² in the first 10 years and a decrease of 16,804.56 km² years in the last 4 years (Li et al., 2009). After 2004, the wetland shrinkage and degradation began to slow down (Li et al., 2009). The area of wetlands near Madoi town decreased by 570 km² from 1990 to 2013, including a decrease of 1,481 km² from 1990 to 2001, and an increase of 911 km² from 2002 to 2013 (Li et al., 2016c). At present, these wetlands are very unstable, with a high rate of conversion from wetlands to other land types (Dong et al., 2009; Wang J. et al., 2020).

Under a warming climate, annual precipitation in the SAYR has fluctuated and increased slightly. Rising temperature has led to an increase in the ALT and land surface evaporation. Every 1 cm increase in the ALT may result in a 1.3 mm decrease in annual runoff and a 0.9 mm increase in annual evapotranspiration in the SAYR (Wang Y. et al., 2018). In addition, the thawing of permafrost resulted in an increase in the downward permeability of soil water, further reducing the surface runoff (Wang Y. et al., 2018; Wu et al., 2018). Under a warming climate, precipitation in the SAYR has fluctuated and increased only slightly. If this situation continues, the water balance in the SAYR will negatively feed back to the warming climate, eventually leading to the shrinkage and degradation of marshes.

Changes in precipitation, runoff, and permafrost degradation are the main influencing factors for wetland hydrothermal dynamics in the SAYR (Wang et al., 2001; Wu et al., 2018; Wu et al., 2021). Persistent climate warming has enhanced land surface evaporation, lowered the burial depth of the permafrost table, and has deepened the active layer and the aeration/vadose zone, resulting in a deeper groundwater table, causing the continuously distributed wetlands to become fragmented or even completely disappear (drained) (Li et al., 2016c; Li G. et al., 2020c). With shrinking wetlands, alpine vegetation coverage declined, in turn, resulting in the bare patches of ground surface (blackened soil spots) in grasslands (Yi et al., 2011). In addition, the increased rodent activity and wind erosion reduced the area of marsh meadows and further increased the area of bare ground patches, leading to the degradation of moist and paludal alpine meadows dominated by Artemisia wellbyi into alpine steppes dominated by constructive species Artemisia absinthium and Stipa capillata. Furthermore, the water conservation capacity may have declined considerably (Wang et al., 2001; Feng et al., 2006; Qin et al., 2017). These hydrothermal processes are interactive, interdependent, and produce extensive and profound eco-hydrological impacts.

There is a recognition that permafrost degradation (melt of ground ice or presence of taliks) may have important implications for changes in wetlands and lakes (Jin et al., 2009; Wang Y. et al., 2018; Shi et al., 2020; Cao W. et al., 2021). These changes, in turn, are accelerating the permafrost thaw by transferring more thermal energy via heat convection and advection (Sjöberg et al., 2016; Luo et al., 2018). An electrical resistivity tomography study shows that sbu- permafrost groundwater moves towards to talik zones and this brings the heat to the surrounding permafrost as well, and hence, a further thaw of permafrost (Gao et al., 2019).

The plant ecosystem in the SAYR is fragile due to its simple structure. Main vegetation types here include alpine paludal meadows, alpine meadows, and alpine steppes. Borehole temperatures and other data have revealed a good correlation between vegetation distribution and permafrost development (Figure 2). Positions of these boreholes are shown in Figure 2A and their corresponding ground temperature curves are shown in Figure 2B. Borehole A is located at the edge of a degrading alpine meadow with positive temperature at depths of 0–12 m. The zero curtain phenomenon has been observed at depths of 12–24 m. Stable positive temperatures are recorded at depths greater than 24 m. Thus, this borehole site is underlain by degrading permafrost on the margin of thawing. Borehole B is located in a paludal meadow underlain by 18-m-thick, very warm (−0.5°C) permafrost. Borehole C is set in an alpine desert steppe and ground temperature here is always positive, indicating a site of talik.

In the SAYR, as illustrated in Figure 2, paludal and alpine meadows are generally areas where permafrost develops and desert steppes are generally underlain by seasonal frost or talik. With permafrost degradation, the zone of alpine meadows retreated while alpine desert steppes expanded into the former ground of alpine meadows. Additionally, alpine desert steppes are dry and covered by a thin layer of steppe soil (crust), under which, are glaciofluvial sand, pebbles, and gravel. In the severely cold, dry, and windy environment, the sparse and cushion-like vegetation is often disrupted, so land desertification processes are prone to occur, resulting in land desertification as evidenced by mobilizing sand dunes or drifting sand (Hu et al., 2013; Li et al., 2016d).

Climate warming and permafrost degradation have changed the distributive features of surface waters and deepened and elongated the flow paths of groundwater, resulting in a series of vegetation successions from alpine marsh meadows to alpine meadows, and further to alpine steppes, resulting in the blackened soil spots, or even alpine deserts (Figure 3) (Mu et al., 2018; Jin et al., 2020; Jin X.-Y. et al., 2021). Permafrost, as an aquitard, blocks the downward movement of water from infiltration and stores water at the bottom of the active layer, generally at depths of about 1–2 m, which could be utilized by plants (Gao Z. et al., 2020). With a thickening of the active layer and the deepening of the permafrost table, water moves further downward, and the surface layer of soil becomes drier (Wang et al., 2006; Jin et al., 2020). This may accelerate the grassland degradation, especially in low-lying valley zones underlain by the isolated patches of alpine permafrost, where the soil water infiltration is dominated by lateral flow and downward movements (Sun et al., 2019; Jin et al., 2020; Cao H. et al., 2021). These land surface changes, furthermore, have resulted in the reduction of vegetation coverage.

As the active layer deepens progressively, the plant species richness, vegetation coverage, and underground biomass in alpine paludal meadows and the coverage of Cyperaceae decreases, but the coverage of forbs species increases (Jin et al., 2020). With further degradation of alpine steppes, the similarity index among communities decreases, and aboveground biomass significantly decreases in grasses, while it first increases and then decreases in forbs (Wang et al., 2012). In addition to ALT, an acceleration of ground freeze–thaw cycles (at 5 cm in depth) under the changing climate, could reduce the activity of root cells of plants and even leads to the death of plant root and rhizospheric microorganisms, thereby lowering the nitrogen fixation ability (Man et al., 2019). Moreover, ground thaw at a depth of 5 cm has advanced at rates of 0.1–1.5 days/year (2003–2015), and the maximum depth of frost penetration has decreased at rates of 0.7–3.1 cm/year, resulting in an earlier start date of the vegetation growing season (Gao B. et al., 2020).

All these aspects have led to the stresses of alpine vegetation from physiological droughts, accelerating grassland degradation. If effective measures are not taken in time to mitigate further grassland degradation, the imbalance between the permafrost environment and alpine vegetation in this region will result in positive feedback between permafrost degradation and grassland degradation (Figure 4). The consequence will be the extensive, persistent, and intensifying deterioration of the ecological environment, such as the degradation of grasslands and land desertification. These changes will lead to an increase in surface albedo, a decrease in the convection effect of the near-surface air flows, and an increase in near-surface ground temperatures. In addition, the patterns of atmospheric circulations over the Qinghai-Tibet Plateau will change, resulting in extensive environmental impacts.

Recent remote sensing studies have found increasing normalized difference vegetation index (NDVI) and plant productivity in the SAYR on the northeastern Qinghai-Tibet Plateau, Southwest China (Ge et al., 2018; Bai et al., 2020; Wang M. et al., 2020). Using the TTOP model simulations and remote sensing data, a recent study has showed that from 1980 to 2018, half of the areal extent of alpine permafrost in the SAYR (where permafrost accounts for 20.37% in areal extent) has been converted to seasonal frost, but the NDVI in the growing season has been increasing at an average rate of 0.002/10a, and; the increase in NDVI mainly occurred in the transitional zone between permafrost and seasonal frost (Wang R. et al., 2020). However, this does not necessarily signal the recovery of vegetation upon permafrost thaw in the HAYR. First, changes in vegetation species are difficult to determine based on the increase in NDVI, and the increased NDVI is likely due to the increase in weeds. In addition, the increase in NDVI is likely to be a short-term effect of permafrost degradation. In a long run, under a weak increase in precipitation, permafrost thaw will eventually lead to aridification of the environment, resulting in the degradation of alpine grasslands. However, the timely and proper implementation of ecological protection programs and establishment and prudent operation of national parks may have mitigated the vegetation degradation to some extents (e.g., Zhao et al., 2018).