Water Sources Of Stream Runoff In Alpine Region And Their Seasonal Variations

Many inland rivers derive from alpine regions with middle and low latitudes.These alpine regions have essential water-conservation and regulation functions,which are often referred to as"water tower".For the inland rivers in the arid and semi-arid areas in northwest China,due to scarce precipitation and excessive evaporation in the middle and lower reaches,most stream runoff is generated in the alpine regions.The hydrological functions above are particularly important;they determine the amount of water to provide for the middle and lower reaches,then limit the social and economic development in turn.Therefore,it is of great practical significance to understand the formation mechanism of runoff in the alpine regions for the scientific management of water resources and sustainable development of the social economy in inland river basins.Unlike temperate regions,glaciers,snow,and permafrost are widely distributed in alpine regions.Solid water and its interaction with liquid and gaseous water play an essential role in the hydrological cycle,making the runoff formation process more complicated.The Qilian Mountains in the northern Qinghai-Tibet Plateau are dominated by alpine-canyon geomorphology.Their local height differences are significant,and the landscape is vertically zoned,resulting in a strong driving force of the water flow,complex underlying surface,and hydrogeological conditions.These characteristics make the runoff formation process more complicated.Many scientific questions remain unanswered.The most prominent questions are as follows:How much does groundwater contribute to stream runoff in such alpine regions dominated by bedrock and thin-layer weathering?In which aquifers is groundwater stored?By what kind of mechanism?Based on the above background,the Hulugou catchment in the headwaters of the Heihe river was chosen as our typical study site,where we conducted the study on water sources and their seasonal variation in the stream runoff.The Heihe River is a typical inland river basin in the northwestern region,and it is also a typical representative of the alpine regions in China.The alpine-gorge catchments widely exist in the headwaters of the Heihe River.The Quaternary porous aquifers in the piedmont alluvial plain in these catchments are the only way for runoff from the mountains to flow into the Heihe River.Therefore,the results in this study can be extended and applied to other alpine-gorge catchments.Firstly,based on hydrogeological data,the aquifers in the Hulugou catchment were classified into three types of porous aquifers over the frozen rock,in the permafrost zone,and in the seasonal frost zone according to sediment characteristics.Secondly,based on hydrological and meteorological observation data,the water input modes in different seasons in the catchment were identified as:seasonal snowmelt water(from November in winter to February next year),temporary snowmelt water(from late March to late April),glacier-snow meltwater(from May to October),and rainfall(from May to October).Then we determined the periods of hydrograph separation as(1)the periods from May 9 to June 1,2013(low flow season)and from July 9 to September 21,2013(high flow season),when the stream runoff was divided into three sources such as glacier-snow meltwater,rainwater,and groundwater;(2)the periods from March 1 to March 31,2014(spring snowmelt season),when the stream runoff was divided into two sources such as seasonal snowmelt water and groundwater.Then,we analyzed the hydrochemical and isotopic characteristics of potential water sources(endmembers)in hydrograph separations and found that there were significant differences in these characteristics of different water sources,which meets the basic assumptions of hydrograph separations.Then,the traditional isotope-based hydrograph separations(IHS)was used to estimate water sources in stream runoff in the low-flow season,high-flow season and spring melting season.The results show that in the low-flow season,groundwater contributed the most to the river runoff,and its total contribution percentage was90.85±0.59%,which was higher than those of glacier-snow meltwater(7.26±0.62%)and rainwater(1.89±0.25%).In the high flow season,groundwater was still the primary source of contribution to runoff,and its total contribution percentage was62.55±0.74%,whileglacier-snowmeltwater(24.34±3.01%)andrainwater(13.11±2.36%)contributed a considerate amount of water to runoff.In the early spring,groundwater was the highest contributor to stream runoff,and its total contribution percentage was 91.10±3.09%,while seasonal snowmelt water's contribution was minimal(8.90±3.09%).At the same time,the Bayesian Monte Carlo mixing models(BMC)was used to estimate water sources in stream runoff in the low-flow season and the high-flow season.The results showed that the groundwater had the highest contribution to the runoff in the low-flow season(73.50±3.67%),glacier-snow meltwater was the second contributor to runoff(16.68±2.44%),and rainwater was the smallest contributor(9.82±1.50%);in the high flow season,groundwater was still the highest contributor to runoff(85.72±0.60%),while the contributions of glacier-snow meltwater(7.51±0.83%)and rainwater(6.76±0.98%)were relatively small.Comparing the results of the above two methods,we found that groundwater dominated the Hulugou stream during not only the low-flow period but also the high-flow period,even though there were significant differences in the contribution ratio of each water source between the two methods.Based on the estimation contribution of glacier-snow meltwater,it is found that the seasonal variations of glacier-snow meltwater contribution to stream runoff are controlled by the temperature in the periglacial area,the size of glacier storage,and the porous aquifers over the frozen rock.There was a significant positive correlation between the contribution percentage of glacier-snow meltwater and air temperature near the front of glaciers.However,it did not change obviously in response to daily precipitation events.The glacial coverage of the Hulugou catchment is proximately7.75%,which holds the short-and intermediate-term glacier storages,but glacier-snow melt provides a considerable amount of water(IHS:~1.13×10~6 m~3,BMC:~3.53×10~5m~3)to the streams in the Hulugou catchment.The shallow conduit drainage networks within glaciers,together with porous aquifers over the frozen rock with large water-conduction capacity,can not only result in the short residence time of meltwater but also highly contribute to the high glacier-snow melt fraction in the warm season.Based on the estimation contribution of rainwater,it is found that the rainwater's contribution showed a highly positive correlation with the daily precipitation and rapid responses to the rainfall events,which is mainly attributed to the bedrock outcrops and permafrost.The bedrock outcrops,which cover~80%of the mountains in this catchment and have the lowest permeability and storage capacity,can highly contribute to the rapid runoff responses and high rainwater fraction to streams.Permafrost behaves like an aquitard with low temperature.During the late spring,the water table of suprapermafrost groundwater is very close to or even exceeds the surface.During the summer,the water table of suprapermafrost groundwater is closer to the surface.So the majority of the rainwater precipitating in a large area was transferred as saturation excess overland flow(in late spring)or infiltration excess overland flow(in summer)to streams.Based on the estimation contribution of groundwater,combined with analysis of changes in multiple tracers along the flow distance,the roles of porous aquifers in hydrological regulation are summarized as follows:(1)The aquifers over the frozen rock have a large water-conduction capacity during the warm season.The glacier-snow melt,precipitation,and much of surface runoff flow through the aquifer quickly into nearby streams and other lower aquifers.In the cold season,there was no water storage and water release because the aquifers have been drained.(2)In the permafrost zone,the suprapermafrost aquifers have a relatively large water-conduction capacity during the warm season and a minimum water-conduction capacity during the cold season.Also,the subpermafrost aquifers show a limited water-storage capacity because of the thick perennially frozen layers,which behave like an aquiclude or aquitard.(3)In the seasonal frost zone,the aquifers show a substantial water conduction capacity and water storage capacity during the warm season.However,during the cold season,the hydraulic gradient between the recharge and the discharge areas of the piedmont plain gradually decreases with groundwater release,this decreasing hydraulic gradient slows down the release of groundwater and exhibits a function of baseflow maintenance.Finally,we summarized the results above and built a conceptual model of the mechanism of runoff formation in alpine-gorge catchments,composed of“mountains”and“piedmont plain”,which indicates that bedrock mountain area and permafrost area are the critical regions of runoff-yield while the piedmont plain in seasonal frost zone is the confluent region.Glacier-snow meltwater and precipitation infiltrate into the moraine materials in the periglacial area quickly.Much of the water flows through moraine materials and discharges as springs,which becomes the source of rivers.The rest of the water recharges lower aquifers in the permafrost zone as supra-permafrost groundwater.In the permafrost zone,precipitation and permafrost meltwater infiltrate and become supra-permafrost groundwater.When flowing downwards,they will enter the stream channel as overland flow because of poor permeability at the foot of the slope and permafrost.Afterward,mountain runoff flows out and mostly infiltrates or laterally recharge into the aquifers at the top of the piedmont alluvial plain,then discharged as baseflow to the stream at the base of the piedmont plain.The innovations of this study are as follows:(1)the Qilian Mountains are characterized by the representative alpine-gorge landforms in the northern Qinghai-Tibet Plateau,where permafrost and seasonally frozen soil are widely distributed,and flow paths are complicated.In this study,three typical porous aquifers distributed along the frozen rock area,permafrost zone,and seasonal frost zone were identified,and their hydrological regulations in runoff formation were summarized.The switches and mechanism of the hydrological function with the annual freeze-thaw cycle were analyzed.The conceptual model of runoff formation in the“mountains-piedmont plain”type of Qilian Mountains was built.(2)In the application of the BMC method,previous studies usually lacked dynamic monitoring data of glacier-snow meltwater and described this endmember as a static variable.Therefore,the results of hydrograph separation were unable to reflect the dynamic contribution of glacier-snow meltwater to runoff.Based on the method,this study considered the dynamic input of glacier-snow meltwater,which was more scientific and reasonable in terms of mechanism and more accurate in terms of calculation results.