Persistence of fertilization effects on soil organic carbon in degraded alpine wetlands in the Yellow River source region

In the restoration of degraded wetlands,fertilization can improve the vegetation-soil-microorganisms complex,thereby affecting the organic carbon content.However,it is currently unclear whether these effects are sustainable.This study employed Biolog-Eco surveys to investigate the changes in vegetation characteristics,soil physicochemical properties,and soil microbial functional diversity in degraded alpine wetlands of the source region of the Yellow River at 3 and 15 months after the application of nitrogen,phosphorus,and organic mixed fertilizer.The following results were obtained:The addition of nitrogen fertilizer and organic compost significantly affects the soil organic carbon content in degraded wetlands.Three months after fertilization,nitrogen addition increases soil organic carbon in both lightly and severely degraded wetlands,whereas after 15 months,organic compost enhanced the soil organic carbon level in severely degraded wetlands.Structural equation modeling indicates that fertilization decreases the soil pH and directly or indirectly influences the soil organic carbon levels through variations in the soil water content and the aboveground biomass of vegetation.Three months after fertilization,nitrogen fertilizer showed a direct positive effect on soil organic carbon.However,organic mixed fertilizer indirectly reduced soil organic carbon by increasing biomass and decreasing soil moisture.After 15 months,none of the fertilizers significantly affected the soil organic carbon level.In summary,it can be inferred that the addition of nitrogen fertilizer lacks sustainability in positively influencing the organic carbon content.

Influences of pika and simulated grazing disturbances on bare patches of alpine meadow in the Yellow River Source Zone

Bare patches in alpine meadow are the main manifestation of its degradation.The change of bare patches in an alpine meadow in the Yellow River Source Zone during 2018-2019 was studied in relation to the disturbances caused by plateau pika(Ochotona curzoniae)population and simulated grazing via artificial mowing both independently and interactively.The disturbance was set at three levels of high,medium and no disturbance(control group).Bare patches were mapped by from unmanned aerial vehicle(UAV)images with fine resolution of 1 cm obtained in August 2018 and August 2019 in ArcGIS.The results showed that the total area of bare patches decreased by 112.05 m2 in sub-plots devoid of pika disturbance but increased by 126.37 m2 in other subplots.The highest rate of increase is 89.02%.The individual effect of pika exceeds the joint effect of pika and mowing disturbances.The sole effect of mowing is lower than the joint effect of pika disturbance and intensive mowing,but higher than the joint influence of pika disturbance and moderate mowing.Strong pika disturbance(14 per sub-plot)caused the influence of mowing from moderate to intensive to increase by five-fold.The area of bare patches treated with moderate mowing and no pika disturbance decreased at the highest pace(-37.22%).Intensive mowing and medium density pikas(100 pikas/ha)are considered the thresholds at which the bare patches start to expand.Even if the meadow is mowed at the medium and high intensity,the area of bare patches can be significantly reduced if plateau pika population is controlled to a low level.ANOVA analysis and longterm macro-scale satellite-derived results reveal that pika disturbance is more important in causing the bare patches to change than simulated grazing.Therefore,it is more important to control the number of pikas than to reduce grazing intensity to prevent the expansion of bare patches in the degraded alpine meadow in the study area.

Grass-livestock balance based grassland ecological carrying capability and sustainable strategy in the Yellow River Source National Park, Tibet Plateau, China

The Yellow River Source National Park(YRSP)is one of the most sensitive and fragile ecological regions in the world.The historical intensive grazing and climate change have resulted in ecological degradation that threatens the wildlife and livestock.Exploring the sustainable strategy is urgent for policy makers to meet the demands for wild ungulates and livestock.In our study,the grassland ecological carrying capability(GECC)was assessed based on the updated grass-livestock balance that considered the grass competition from wild ungulates.The balances between grass and livestock,and GECC and grassland pressure index(GPI)in the YRSP were measured through overlay analysis and geostatistic analysis.The results showed that:(1)the ratio of livestock to wild ungulates in the research area was approximately 4.56:1,in which the proportion of livestock was 81.75%and the actual number of livestock was 33.84×104 standard sheep units;(2)Under the scenario of minimum grazing utilisation rate,the theoretical grazing capacity and GECC were 37.83×104 standard sheep units and−0.13,respectively.Under the maximum grazing utilisation rate,the theoretical grazing capacity and GECC were 41.93×104 standard sheep units and−0.21,respectively.Since GECC in both scenarios were both less than 0,the grassland was considered to be in surplus and the livestock was not overloaded.However,GPI in the two scenarios were 0.87 and 0.79,respectively,both of which exceeded the warning line of 0.70.Based on GECC,we recommend that the sustainable strategy in YRSP is either to increase the supplementary feeding about 6.40×104 standard sheep units or reduce the grazing livestock by about 3.50×10^(4) standard sheep units.

Large-scale characteristics of thermokarst lakes across the source area of the Yellow River on the Qinghai-Tibetan Plateau

As significant evidence of permafrost degradation,thermokarst lakes play an important role in the permafrost regions by regulating hydrology,ecology,and biogeochemistry.In the Sources Area of the Yellow River(SAYR),permafrost degradation has accelerated since the 1980s,and numerous thermokarst lakes have been discovered.In this paper,we use Sentinel-2 images to extract thermokarst lake boundaries and perform a regional-scale study on their geometry across the permafrost region in the SAYR.We also explored the spatiotemporal variations and potential drivers from the perspectives of the permafrost,climate,terrain and vegetation conditions.The results showed that there were 47,518 thermokarst lakes in 2021 with a total area of 190.22×106 m^(2),with an average size of 4,003.3 m^(2).The 44,928 ponds(≤10,000 m^(2))predominated the whole lake number(94.1%)but contributed to a small portion of the total lake area(28.8%).With 2,590 features(5.9%),small-sized(10,000 to 100,000 m^(2))and large-sized lakes(>100,000 m^(2))constituted up to 71.2%of the total lake area.Thermokarst lakes developed more significantly in warm permafrost regions than in cold permafrost areas;74.1%of lakes with a total area of 119.6×106 m^(2)(62.9%),were distributed in warm permafrost regions.Most thermokarst lakes were likely to develop within the elevation range of 4,500~4,800 m,on flat terrain(slope<10°),on SE and S aspects and in alpine meadow areas.The thermokarst lakes in the study region experienced significant shrinkage between 1990 and 2021,characterized by obvious lake drainage;the lake numbers decreased by 5418(56.1%),with a decreasing area of 58.63×106 m^(2)(49.0%).This shrinkage of the thermokarst lake area was attributable mainly to the intensified degradation of rich-ice permafrost thawing arising from continued climate warming,despite the wetting climatic trend.

Dataset of Comparative Observations for Land Surface Processes over the Semi-Arid Alpine Grassland against Alpine Lakes in the Source Region of the Yellow River

Thousands of lakes on the Tibetan Plateau(TP) play a critical role in the regional water cycle, weather, and climate. In recent years, the areas of TP lakes underwent drastic changes and have become a research hotspot. However, the characteristics of the lake-atmosphere interaction over the high-altitude lakes are still unclear, which inhibits model development and the accurate simulation of lake climate effects. The source region of the Yellow River(SRYR) has the largest outflow lake and freshwater lake on the TP and is one of the most densely distributed lakes on the TP. Since 2011,three observation sites have been set up in the Ngoring Lake basin in the SRYR to monitor the lake-atmosphere interaction and the differences among water-heat exchanges over the land and lake surfaces. This study presents an eight-year(2012–19), half-hourly, observation-based dataset related to lake–atmosphere interactions composed of three sites. The three sites represent the lake surface, the lakeside, and the land. The observations contain the basic meteorological elements,surface radiation, eddy covariance system, soil temperature, and moisture(for land). Information related to the sites and instruments, the continuity and completeness of data, and the differences among the observational results at different sites are described in this study. These data have been used in the previous study to reveal a few energy and water exchange characteristics of TP lakes and to validate and improve the lake and land surface model. The dataset is available at National Cryosphere Desert Data Center and Science Data Bank.

Geodiversity in the Yellow River source zone

It is a key premise of ＇ecosystem approaches＇ to natural resources management that we must have an appropriate understanding of biodiversity values, and controls upon them, if we wish to manage them effectively. These biodiversity values, and associated ecosystem functionality, vary with space and time and are tied directly to landscape-scale relationships and evolutionary traits. In riverine systems, nested hierarchical principles provide a useful platform to assess relationships between landscape components across a range of scales. These understandings are most instructively synthesized through catchment-scale analyses. This paper outlines a rationale for systematic catchment-wide appraisals of river geodiversity. An initial application of these principles is presented for the Yellow River source zone in Qinghai Province, western China. Geo-ecological relationships are outlined for five broad sections of the trunk stream, highlighting implications for the management of these individual landscape compartments and for the system as a whole.

Characteristics of grassland degradation and driving forces in the source region of the Yellow River from 1985 to 2000

The source region of the Yellow River is located in the middle east of the Tibetan Plateau in northwest China. The total area is about 51,700 km^2, mainly covered by grassland （79%）, unused land （16%） and water （4%）. The increasing land utilization in this area has increased the risk of environmental degradation. The land use/cover data （1985 and 2000） provided by the Data Center of Resources and Environment in the Chinese Academy of Sciences were used to analyze the land cover change in the source region of the Yellow River. DEM （1：250,000） data, roads and settlement data were used to analyze the spatial characteristics of grasslands degradation. The ArcGIS 9 software was used to convert data types and do the overlay, reclassification and zonal statistic analysis. Results show that grassland degradation is the most important land cover change in the study area, which occupied 8.24% of the region＇s total area. Human activities are the main causes of the grassland degradation in the source region of the Yellow River： 1） the degradation rate is higher on the sunny slope than on the shady slope; 2） the grassland degradation rate decreases with an increase in the elevation, and it has a correlation coefficient of -0.93; 3） the nearer to the settlements the grassland is, the higher the degradation rate. Especially within a distance range of 12 km to the settlements, the grassland degradation rate is highly related with the distance, with a coefficient of -0.99; and 4） in the range of 4 km, the degradation rate decreases with the increase of distance to the roads, with a correlation coefficient of -0.98. Besides some physical factors, human activities have been the most important driving forces of the grassland degradation in the source region of the Yellow River since 1985. To resolve the degradation problems, population control is essential, and therefore, it can reduce the social demand of livestock products from the grassland. To achieve sustainable development, it needs to improve the management of grassland ecosystem.

Impacts of degrading permafrost on streamflow in the source area of Yellow River on the Qinghai-Tibet Plateau,China

Many observations in and model simulations for northern basins have confirmed an increased streamflow from degrading permafrost,while the streamflow has declined in the source area of the Yellow River(SAYR,above the Tanag hydrological station)on the northeastern Qinghai-Tibet Plateau,West China.How and to what extent does the degrading permafrost change the flow in the SAYR?According to seasonal regimes of hydrological processes,the SAYR is divided intofour sub-basins with varied permafrost extents to detect impacts of permafrost degradation on the Yellow River streamflow.Results show that permafrost degradation may have released appreciable meltwater for recharging groundwater.The potential release rate of ground-ice melt-water in the Sub-basin 1(the headwater area of the Yellow River(HAYR),above the Huangheyan hydrological station)is the highest(5.6 mm per year),contributing to 14.4%of the annual Yellow River streamflow at Huangheyan.Seasonal/intra-and annual shifts of streamflow,a possible signal for the marked alteration of hydrological processes by permafrost degradation,is observed in the HAYR,but the shifts are minor in other sub-basins in the SAYR.Improved hydraulic connectivity is expected to occur during and after certain degrees of permafrost degradation.Direct impacts of permafrost degradation on the annual Yellow River streamflow in the SAYR at Tanag,i.e.,from the meltwater of ground-ice,is estimated at 4.9%that of the annual Yellow River discharge at Tanag,yet with a high uncertainty,due to neglecting of the improved hydraulic connections from permafrost degradation and the flow generation conditions for the ground-ice meltwater.Enhanced evapotranspiration,substantial weakening of the Southwest China Autumn Rain,and anthropogenic disturbances may largely account for the declined streamflow in the SAYR.

Influencing factors of water resources in the source region of the Yellow River

Taking the source region of the Yellow River as a study area and based on the data from Madoi Meteorological Station and Huangheyan Hydrological Station covering the period 1955-2005, this paper analyses the changing trends of surface water resources, climate and frozen ground and reveals their causes. Results show that there exist frequent fluctuations from high to low water flow in the 51-year period. In general, the discharge has shown a de- clining trend in the 51 years especially since the 1990s. The annual distribution shows one peak which, year on year is getting smaller. （1） Precipitation has a significant and sustained influence on discharge. （2） A sharp rise of temperature resulted in the increase of evaporation and the decrease of discharge, which has a greater effect than on ice-snow melting. （3） Frozen ground tends to be degraded markedly. There is a significant positive correlation be- tween the permafrost thickness and the discharge. （4） Evaporation rates are significantly increasing, leading to the decrease of discharge. 70% of the discharge reduction resulted from climate change, and the remaining 30% may have been caused by human activities.

Estimation of water balance in the source region of the Yellow River based on GRACE satellite data

Water storage has important significance for understanding water cycles of global and local domains and for monitoring climate and environmental changes. As a key variable in hydrology, water storage change represents the sum of precipitation, evaporation, surface runoff, soil water and groundwater exchanges. Water storage change data during the period of 2003-2008 for the source region of the Yellow River were collected from Gravity Recovery and Climate Experiment （GRACE） satellite data. The monthly actual evaporation was estimated according to the water balance equation. The simulated actual evaporation was significantly consistent and correlative with not only the observed pan （20 cm） data, but also the simulated results of the version 2 of Simple Biosphere model. The average annual evaporation of the Tangnaihai Basin was 506.4 mm, where evaporation in spring, summer, autumn and winter was 130.9 mm, 275.2 mm, 74.3 mm and 26.1 mm, and accounted for 25.8%, 54.3%, 14.7% and 5.2% of the average annual evaporation, respectively, The precipitation increased slightly and the actual evaporation showed an obvious decrease. The water storage change of the source region of the Yellow River displayed an increase of 0.51 mm per month from 2003 to 2008, which indicated that the storage capacity has significantly increased, probably caused by the degradation of permafrost and the increase of the thickness of active layers. The decline of actual evaporation and the increase of water storage capacity resulted in the increase of river runoff.

Different Responses of Vegetation to Frozen Ground Degradation in the Source Region of the Yellow River from 1980 to 2018

Frozen ground degradation under a warming climate profoundly influences the growth of alpine vegetation in the source region of the Qinghai-Tibet Plateau.This study investigated spatiotemporal variations in the frozen ground distribution,the active layer thickness(ALT)of permafrost(PF)soil and the soil freeze depth(SFD)in seasonally frozen soil from 1980 to 2018 using the temperature at the top of permafrost(TTOP)model and Stefan equation.We compared the effects of these variations on vegetation growth among different frozen ground types and vegetation types in the source region of the Yellow River(SRYR).The results showed that approximately half of the PF area(20.37%of the SRYR)was projected to degrade into seasonally frozen ground(SFG)during the past four decades;furthermore,the areal average ALT increased by 3.47 cm/yr,and the areal average SFD decreased by 0.93 cm/yr from 1980 to 2018.Accordingly,the growing season Normalized Difference Vegetation Index(NDVI)presented an increasing trend of 0.002/10 yr,and the increase rate and proportion of areas with NDVI increase were largest in the transition zone where PF degraded to SFG(the PF to SFG zone).A correlation analysis indicated that variations in ALT and SFD in the SRYR were significantly correlated with increases of NDVI in the growing season.However,a rapid decrease in SFD(<-1.4 cm/10 yr)could have reduced the soil moisture and,thus,decreased the NDVI.The NDVI for most vegetation types exhibited a significant positive correlation with ALT and a negative correlation with SFD.However,the steppe NDVI exhibited a significant negative correlation with the SFD in the PF to SFG zone but a positive correlation in the SFG zone,which was mainly limited by water condition because of different change rates of the SFD.

Eco-environment range in the source regions of the Yangtze and Yellow rivers

Based on geographical and hydrological extents delimited, four principles are identified, as the bases for delineating the ranges of the source regions of the Yangtze and Yellow rivers in the paper. According to the comprehensive analysis of topographical characteristics, climate conditions, vegetation distribution and hydrological features, the source region ranges for eco-environmental study are defined. The eastern boundary point is Dari hydrological station in the upper reach of the Yellow River. The watershed above Dari hydrological station is the source region of the Yellow River which drains an area of 4.49×10 4 km 2 . Natural environment is characterized by the major topographical types of plateau lakes and marshland, gentle landforms, alpine cold semi-arid climate, and steppe and meadow vegetation in the source region of the Yellow River. The eastern boundary point is the convergent site of the Nieqiaqu and the Tongtian River in the upstream of the Yangtze River. The watershed above the convergent site is the source region of the Yangtze River, with a watershed area of 12.24×10 4 km 2 . Hills and alpine plain topography, gentle terrain, alpine cold arid and semi-arid climate, and alpine cold grassland and meadow are natural conditions in the source region of the Yangtze River.

Evolution of the freeze-thaw cycles in the source region of the Yellow River under the influence of climate change and its hydrological effects

As an important water source and ecological barrier in the Yellow River Basin,the source region of the Yellow River(above the Huangheyan Hydrologic Station)presents a remarkable permafrost degradation trend due to climate change.Therefore,scientific understanding the effects of permafrost degradation on runoff variations is of great significance for the water resource and ecological protection in the Yellow River Basin.In this paper,we studied the mechanism and extent of the effect of degrading permafrost on surface flow in the source region of the Yellow River based on the monitoring data of temperature and moisture content of permafrost in 2013–2019 and the runoff data in 1960–2019.The following results have been found.From 2013 to 2019,the geotemperature of the monitoring sections at depths of 0–2.4 m increased by 0.16°C/a on average.With an increase in the thawing depth of the permafrost,the underground water storage space also increased,and the depth of water level above the frozen layer at the monitoring points decreased from above 1.2 m to 1.2–2 m.64.7%of the average multiyear groundwater was recharged by runoff,in which meltwater from the permafrost accounted for 10.3%.Compared to 1960-1965,the runoff depth in the surface thawing period(from May to October)and the freezing period(from November to April)decreased by 1.5 mm and 1.2 mm,respectively during 1992–1997,accounting for 4.2%and 3.4%of the average annual runoff depth,respectively.Most specifically,the decrease in the runoff depth was primarily reflected in the decreased runoff from August to December.The permafrost degradation affects the runoff within a year by changing the runoff generation,concentration characteristics and the melt water quantity from permafrost,decreasing the runoff at the later stage of the permafrost thawing.However,the permafrost degradation has limited impacts on annual runoff and does not dominate the runoff changes in the source region of the Yellow River in the longterm.

Climate transformation to warm-humid and its effect on river runoff in the source region of the Yellow River

The change characteristics and trends of the regional climate in the source region of the Yellow River, and the response of runoff to climate change, are analyzed based on observational data of air temperature, precipitation, and runoff at 10 main hydrological and weather stations in the region. Our results show that a strong signal of climate shift from warm-dry to warm-humid in the western parts of northwestern China （Xinjiang） and the western Hexi Corridor of Gansu Province occurred in the late 1980s, and a same signal of climate change occurred in the mid-2000s in the source region of the Yellow River located in the eastern part of northwestern China. This climate changeover has led to a rapid increase in rainfall and stream runoff in the latter region. In most of the years since 2004 the average annual precipitation in the source region of the Yellow River has been greater than the long-term average annual value, and after 2007 the runoff measured at all of the hydrologic sections on the main channel of the Yellow River in the source region has also consistently exceeded the long-term average annual because of rainfall increase. It is difficult to determine the prospects of future climate change until additional observations and research are conducted on the rate and temporal and spatial extents of climate change in the region. Nevertheless, we predict that the climate shift from warm-dry to warm-humid in the source region of the Yellow River is very likely to be in the decadal time scale, which means a warming and rainy climate in the source region of the Yellow River will continue in the coming decades.

Changes in stress within grassland ecosystems in the three counties of the source regions of the Yangtze and Yellow Rivers

Based on a database of more than 40 years of second production process and energy flow records for Maduo,Qumalai and Yushu counties,a dynamic model of the stress within grassland ecosys-tems was established using a nonlinear regression method for this source regions of the Yangtze and Yel-low Rivers.The results show that dynamic curves of stress within grassland ecosystems in the three coun-ties were in the shape of an inverted 'U' during the period 1965-2007.It also revealed that the variation in actual amount of livestock inventories reflected the general trends of the stress within the grassland eco-systems in the source regions,although there were many other factors for the increase or reduction in grassland ecosystem stress.

An evaluation of soil moisture from AMSR-E over source area of the Yellow River, China

In this study,in-situ soil moisture measurements are used to evaluate the accuracy of three AMSR-E soil moisture prod ucts from NASA(National Aeronautics and Space Administration),JAXA(Japanese Aerospace Exploration Agency)and VUA(Vrije University Amsterdam and NASA)over Maqu County,Source Area of the Yellow River(SAYR),China.Re sults show that the VUA soil moisture product performs the best among the three AMSR-E soil moisture products in the study area,with a minimum RMSE(root mean square error)of 0.08(0.10)m3/m3 and smallest absolute error of 0.07(0.08)m3/m3 at the grassland area with ascending(descending)data.Therefore,the VUA soil moisture product is used to describe the spatial variation of soil moisture during the 2010 growing season over SAYR.The VUA soil moisture product shows that soil moisture presents a declining trend from east south(0.42 m3/m3)to west north(0.23 m3/m3),with good agreement with a general precipitation distribution.The center of SAYR presents extreme wetness(0.60 m3/m3)dur ing the whole study period,especially in July,while the head of SAYR presents a high level soil moisture(0.23 m3/m3)in July,August and September.

Spatial-temporal variations in near-surface soil freeze-thaw cycles in the source region of the Yellow River during the period 2002–2011 based on the Advanced Microwave Scanning Radiometer for the Earth Observing System(AMSR-E) data

Detecting near-surface soil freeze-thaw cycles in high-altitude cold regions is important for understanding the Earth＇s surface system, but such studies are rare. In this study, we detected the spatial-temporal variations in near-surface soil freeze-thaw cycles in the source region of the Yellow River（SRYR） during the period 2002–2011 based on data from the Advanced Microwave Scanning Radiometer for the Earth Observing System（AMSR-E）. Moreover, the trends of onset dates and durations of the soil freeze-thaw cycles under different stages were also analyzed. Results showed that the thresholds of daytime and nighttime brightness temperatures of the freeze-thaw algorithm for the SRYR were 257.59 and 261.28 K, respectively. At the spatial scale, the daily frozen surface（DFS） area and the daily surface freeze-thaw cycle surface（DFTS） area decreased by 0.08% and 0.25%, respectively, and the daily thawed surface（DTS） area increased by 0.36%. At the temporal scale, the dates of the onset of thawing and complete thawing advanced by 3.10（±1.4） and 2.46（±1.4） days, respectively; and the dates of the onset of freezing and complete freezing were delayed by 0.9（±1.4） and 1.6（±1.1） days, respectively. The duration of thawing increased by 0.72（±0.21） day/a and the duration of freezing decreased by 0.52（±0.26） day/a. In conclusion, increases in the annual minimum temperature and winter air temperature are the main factors for the advanced thawing and delayed freezing and for the increase in the duration of thawing and the decrease in the duration of freezing in the SRYR.

Regional climate response to global warming in the source region of the Yellow River and its impact on runoff

The source region of the Yellow River has experienced obvious climate and discharge changes in recent decades due to global warming, which largely affects the water resources and ecological and environmental security in the Yellow River basin. This study analyzed the changes in runoff and several climate factors in the source region of the Yellow River based on the observed discharges at the Tangnag hydrological station, routine meteorological data from China Meteorological Administration(CMA) stations within and near this source region, and several evaporation datasets. The results indicate that the runoff in the source region was relatively abundant from 1960 to 1989 and then declined sharply afterward. It recovered slightly after 2005 but was still below normal—10% less than that during 1960–1989. Similarly, the precipitation amounts in the source region were relatively low in the 1990s, but they increased significantly after 2003, with an average increase of 31.4 mm or 6% more when compared to that in 1960–1989. In addition, the temperatures in the source region continued to rise from 1960 to 2017, and the evaporation levels also showed an upward trend after 1990. The influences of the spatial and temporal variations in climatic factors on runoff in the source region were then further analyzed. The results indicate that the decreases in precipitation and the number of days of heavy rainfall in the source region from 1990 to 2002 were important reasons for the lower runoff during this period. After 2003, the precipitation in the southeastern part of the source region, which is a key area for runoff generation,increased only to a limited extent, but the evaporation in the entire source region generally increased with increasing temperature,which might have led to the low capacity for actual runoff production in each subbasin and persistent low runoff in the source region. Therefore, such a climate response to global warming in the source region might be unfavorable for increased runoff in the future. The above analysis provides a valuable reference for the future planning and management of water resources in the source region of the Yellow River and the entire Yellow River Basin in the context of warming.

Changes in lake area and water level in response to hydroclimate variations in the source area of the Yellow River:a case study from Lake Ngoring

In the north-eastern Qinghai-Tibet Plateau(QTP),the source area of the Yellow River(SAYR)has been experiencing significant changes in climatic and environmental conditions in recent decades.To date,few studies have combined modern hydrological conditions with paleoclimate records to explore the mechanism(s)of these changes.This study seeks to improve understanding of hydrological variability on decadal and centennial timescales in the SAYR and to identify its general cause.We first determined annual fluctuations in the surface area of Lake Ngoring from 1985 to 2020 using multi-temporal Landsat images.The results show that lake surface area changes were generally consistent with variations in precipitation,streamflow and the regional dry-wet index in the SAYR,suggesting that the water balance of the Lake Ngoring area is closely associated with regional hydroclimate changes.These records are also comparable to the stalagmite δ^(18)O monsoon record,as well fluctuations in the Southern Oscillation Index(SOI).Moreover,an association of high TSI(total solar insolation)anomalies and sunspot numbers with the expansion of Lake Ngoring surface area is observed,implying that solar activity is the key driving factor for hydrologic variability in the SAYR on a decadal timescale.Following this line of reasoning,we compared the δ^(13)C org-based lake level fluctuations of Lake Ngoring for the last millennium,as previously reported,with the hydroclimatic history and the reconstructed TSI record.We conclude that the hydrological regime of Lake Ngoring has been mainly controlled by centennial fluctuations in precipitation for the last millennium,which is also dominated by solar activity.In general,it appears that solar activity has exerted a dominant influence on the hydrological regime of the SAYR on both decadal and centennial timescales,which is clearly manifested in the variations of lake area and water level of Lake Ngoring.

Response of runoff to climate change and its future tendency in the source region of Yellow River

This study examines the hydrological and meteorological data of the source region of the Yellow River from 1956 to 2010 and future climate scenarios from regional climate model （PRECIS） during 2010-2020. Through analyzing the flow variations and revealing the climate causes, it predicts the variation trend for future flows. It is found that the annual mean flow showed a decreasing trend in recent 50 years in the source region of the Yellow River with quasi-periods of 5a, 8a, 15a, 22a and 42a; the weakened South China Sea summer monsoon induced precipitation decrease, as well as evaporation increase and frozen soil degeneration in the scenario of global warming are the climate factors, which have caused flow decrease. Based on the regional climate model PRECIS prediction, the flows in the source region of the Yellow River are likely to decrease generally in the next 20 years.