Xiaoyan Li

Groundwater plays a crucial role in maintaining baseflow in rivers and ensuring water supply, particularly in alpine regions where the freeze–thaw (FT) cycle exerts a strong influence on hydrological processes. However, a systematic understanding is still lacking regarding how FT processes affect the composition of groundwater recharge sources and the transitions among recharge pathways. This study takes the Qinghai Lake basin (QLB) as a case study and combines water isotope and hydrometeorological data to quantify the dynamic characteristics of groundwater recharge sources and pathways during the FT periods. The study found that soil water (57.0 %–76.3 %) was the dominant source of groundwater recharge during the FT periods, followed by rainfall (13.8 %–26.1 %) and snowmelt (7.9 %–22.0 %). The thawing process enhances the vertical connectivity of the soil profile, facilitating the recharge of groundwater from snowmelt and the 60–90 cm soil layer. Furthermore, the lc-excess value of groundwater gradually shifts from values closer to soil water to those closer to precipitation, indicating that piston flow gradually weakens during the process of groundwater recharge by soil water, while preferential flow intensifies, resulting in a pattern where piston flow and preferential flow coexist. Spatially, in the middle and upper regions dominated by permafrost, groundwater is primarily recharged by water from the 0–60 cm soil layer traveling along longer hydrological pathways, whereas in the downstream regions of the basin dominated by seasonal frozen ground, groundwater is primarily recharged by rapid infiltration from the 30–90 cm soil layer. Our research demonstrates that in alpine permafrost regions, freeze–thaw processes regulate water storage and transport, thereby further influencing the recharge sources and pathways of shallow groundwater.

Groundwater plays a crucial role in maintaining baseflow in rivers and ensuring water supply, particularly in alpine regions where the freeze–thaw (FT) cycle exerts a strong influence. However, a systematic understanding is still lacking regarding how FT processes affect the composition of groundwater recharge sources and the transitions among recharge pathways. This study takes the Qinghai Lake basin (QLB) as a case example and combines water isotope and hydrometeorological data to quantify the dynamic characteristics of groundwater recharge sources and pathways during the FT periods. The study found that soil water (57.0 %–76.3 %) is not only the main recharge source for groundwater during the FT periods but also serves as a key transitional reservoir linking rainfall 13.8 %–26.1 %) and snowmelt (7.9 %–22.0 %) to groundwater recharge. The thawing process enhances the vertical connectivity of the soil profile, facilitating the recharge of groundwater from snowmelt and the 60–90 cm soil layer. Furthermore, the lc–excess value of groundwater gradually shifts from values closer to soil water to those closer to precipitation, indicating that piston flow gradually weakens during the process of groundwater recharge by soil water, while preferential flow intensifies, resulting in a pattern where piston flow and preferential flow coexist. Spatially, in the middle and upper regions dominated by permafrost, groundwater is primarily recharged by water from the 0–60 cm soil layer traveling along longer hydrological pathways, whereas in the downstream regions of the basin dominated by seasonal frozen ground, groundwater is primarily recharged by rapid infiltration from the 30–90 cm soil layer. Our research demonstrates that in alpine permafrost regions, freeze-thaw processes regulate water storage and transport, thereby further influencing the recharge sources and pathways of shallow groundwater.

Groundwater plays a crucial role in maintaining baseflow in rivers and ensuring water supply, particularly in alpine regions where the freeze–thaw (FT) cycle exerts a strong influence. However, a systematic understanding is still lacking regarding how FT processes affect the composition of groundwater recharge sources and the transitions among recharge pathways. This study takes the Qinghai Lake basin (QLB) as a case example and combines water isotope and hydrometeorological data to quantify the dynamic characteristics of groundwater recharge sources and pathways during the FT periods. The study found that soil water (57.0 %–76.3 %) is not only the main recharge source for groundwater during the FT periods but also serves as a key transitional reservoir linking rainfall 13.8 %–26.1 %) and snowmelt (7.9 %–22.0 %) to groundwater recharge. The thawing process enhances the vertical connectivity of the soil profile, facilitating the recharge of groundwater from snowmelt and the 60–90 cm soil layer. Furthermore, the lc–excess value of groundwater gradually shifts from values closer to soil water to those closer to precipitation, indicating that piston flow gradually weakens during the process of groundwater recharge by soil water, while preferential flow intensifies, resulting in a pattern where piston flow and preferential flow coexist. Spatially, in the middle and upper regions dominated by permafrost, groundwater is primarily recharged by water from the 0–60 cm soil layer traveling along longer hydrological pathways, whereas in the downstream regions of the basin dominated by seasonal frozen ground, groundwater is primarily recharged by rapid infiltration from the 30–90 cm soil layer. Our research demonstrates that in alpine permafrost regions, freeze-thaw processes regulate water storage and transport, thereby further influencing the recharge sources and pathways of shallow groundwater.

Groundwater plays a crucial role in maintaining baseflow in rivers and ensuring water supply, particularly in alpine regions where the freeze–thaw (FT) cycle exerts a strong influence. However, a systematic understanding is still lacking regarding how FT processes affect the composition of groundwater recharge sources and the transitions among recharge pathways. This study takes the Qinghai Lake basin (QLB) as a case example and combines water isotope and hydrometeorological data to quantify the dynamic characteristics of groundwater recharge sources and pathways during the FT periods. The study found that soil water (57.0 %–76.3 %) is not only the main recharge source for groundwater during the FT periods but also serves as a key transitional reservoir linking rainfall 13.8 %–26.1 %) and snowmelt (7.9 %–22.0 %) to groundwater recharge. The thawing process enhances the vertical connectivity of the soil profile, facilitating the recharge of groundwater from snowmelt and the 60–90 cm soil layer. Furthermore, the lc–excess value of groundwater gradually shifts from values closer to soil water to those closer to precipitation, indicating that piston flow gradually weakens during the process of groundwater recharge by soil water, while preferential flow intensifies, resulting in a pattern where piston flow and preferential flow coexist. Spatially, in the middle and upper regions dominated by permafrost, groundwater is primarily recharged by water from the 0–60 cm soil layer traveling along longer hydrological pathways, whereas in the downstream regions of the basin dominated by seasonal frozen ground, groundwater is primarily recharged by rapid infiltration from the 30–90 cm soil layer. Our research demonstrates that in alpine permafrost regions, freeze-thaw processes regulate water storage and transport, thereby further influencing the recharge sources and pathways of shallow groundwater.

The Qinghai Lake Basin (QLB) serves as a crucial ecological barrier on the Qinghai–Tibet Plateau, making high-precision mapping of land use/land cover (LULC) essential for eco-hydrological research within the basin. In this study, multitemporal Sentinel-1 radar and Sentinel-2 optical imagery from 2024, DEM-derived terrain information, and features derived from these sources were used to produce a 10-m resolution LULC map for the QLB using a support vector machine classifier. The Level-1 and Level-2 LULC datasets of QLB (QLBLC-10) achieved sample-based apparent overall accuracies (OAs) of 91.95% and 91.24%, respectively, and kappa coefficients of 0.90 for both. In contrast, the area-weighted apparent overall accuracy (OA_w) decreased to 81.50 ± 2.09% (95% confidence interval), indicating that class-area imbalance and small-area classes affect map-level performance. The ablation study confirms the contribution of multisource temporal information and terrain constraints to alpine LULC classification. The OA increased from 77.07% with single-temporal Sentinel-2 to 91.24% when multitemporal Sentinel-1/2 data and DEM-derived features were added, while the kappa coefficient increased from 0.75 to 0.90. The comparison with existing products shows that QLBLC-10 outperforms existing global and regional LULC datasets in representing alpine land cover patterns in the QLB. The LULC system proposed in this study is tailored to the QLB, and the presented LULC classification strategy enhances discrimination among major alpine vegetation types, including temperate and alpine steppes, alpine meadows, and alpine shrublands. It provides an up-to-date (2024) LULC dataset for ecosystem monitoring and land management across the QLB.

Groundwater plays a crucial role in maintaining baseflow in rivers and ensuring water supply, particularly in alpine regions where the freeze–thaw (FT) cycle exerts a strong influence. However, a systematic understanding is still lacking regarding how FT processes affect the composition of groundwater recharge sources and the transitions among recharge pathways. This study takes the Qinghai Lake basin (QLB) as a case example and combines water isotope and hydrometeorological data to quantify the dynamic characteristics of groundwater recharge sources and pathways during the FT periods. The study found that soil water (57.0 %–76.3 %) is not only the main recharge source for groundwater during the FT periods but also serves as a key transitional reservoir linking rainfall 13.8 %–26.1 %) and snowmelt (7.9 %–22.0 %) to groundwater recharge. The thawing process enhances the vertical connectivity of the soil profile, facilitating the recharge of groundwater from snowmelt and the 60–90 cm soil layer. Furthermore, the lc–excess value of groundwater gradually shifts from values closer to soil water to those closer to precipitation, indicating that piston flow gradually weakens during the process of groundwater recharge by soil water, while preferential flow intensifies, resulting in a pattern where piston flow and preferential flow coexist. Spatially, in the middle and upper regions dominated by permafrost, groundwater is primarily recharged by water from the 0–60 cm soil layer traveling along longer hydrological pathways, whereas in the downstream regions of the basin dominated by seasonal frozen ground, groundwater is primarily recharged by rapid infiltration from the 30–90 cm soil layer. Our research demonstrates that in alpine permafrost regions, freeze-thaw processes regulate water storage and transport, thereby further influencing the recharge sources and pathways of shallow groundwater.

Groundwater plays a crucial role in maintaining baseflow in rivers and ensuring water supply, particularly in alpine regions where the freeze–thaw (FT) cycle exerts a strong influence. However, a systematic understanding is still lacking regarding how FT processes affect the composition of groundwater recharge sources and the transitions among recharge pathways. This study takes the Qinghai Lake basin (QLB) as a case example and combines water isotope and hydrometeorological data to quantify the dynamic characteristics of groundwater recharge sources and pathways during the FT periods. The study found that soil water (57.0 %–76.3 %) is not only the main recharge source for groundwater during the FT periods but also serves as a key transitional reservoir linking rainfall 13.8 %–26.1 %) and snowmelt (7.9 %–22.0 %) to groundwater recharge. The thawing process enhances the vertical connectivity of the soil profile, facilitating the recharge of groundwater from snowmelt and the 60–90 cm soil layer. Furthermore, the lc–excess value of groundwater gradually shifts from values closer to soil water to those closer to precipitation, indicating that piston flow gradually weakens during the process of groundwater recharge by soil water, while preferential flow intensifies, resulting in a pattern where piston flow and preferential flow coexist. Spatially, in the middle and upper regions dominated by permafrost, groundwater is primarily recharged by water from the 0–60 cm soil layer traveling along longer hydrological pathways, whereas in the downstream regions of the basin dominated by seasonal frozen ground, groundwater is primarily recharged by rapid infiltration from the 30–90 cm soil layer. Our research demonstrates that in alpine permafrost regions, freeze-thaw processes regulate water storage and transport, thereby further influencing the recharge sources and pathways of shallow groundwater.

Recent decades have witnessed a concurrent increase in global vegetation greenness and soil dryness, with the latter manifested through intensified soil droughts and declining soil moisture trends. Despite previous insight into the damage of soil dryness on vegetation, whether and to what extent vegetation greening has in turn exacerbated soil dryness remains virtually unknown. This limits our ability to anticipate water resource crises and adapt to climate change. Here we evaluated the effects of vegetation greenness on soil drought (ESD) and soil moisture trend (EST) from 1982 to 2020. Results indicated that ESD and EST were negative in 71% and 59% of vegetated places, respectively, mainly manifested as exacerbated soil drought and accelerated soil drying trend induced by vegetation greening. Across the aridity gradient, both effects peaked in the semi‐arid or dry sub‐humid zone before diminishing toward the humid zone. And vegetation exerted stronger ESD and EST than both effects of precipitation and temperature in 19% and 44% of vegetated places, respectively. Notably, negative ESD and EST significantly intensified over time, driven primarily by increasing transpiration dominance in evapotranspiration partitioning. Yet, some Earth system models poorly simulated these effects of vegetation on soil dryness and even distorted their temporal evolution. These findings highlight the urgent need for sustainable ecological management to mitigate hydrological risk associated with global greening.