| Runoff mechanism that focuses on recharge sources of runoff and confluence pathways is a key problem of studies on surface runoff. A good understanding of formation mechanism of surface runoff adds to prediction of the influence by climate change and human activities, and scientific management of rainwater. A well knowledge of runoff formation is necessary to understand the impact of vegetation on its confluence pathways, as well as the control of redistribution of runoff recharge sources within a watershed on vegetation pattern.The formation of surface runoff during rainy season includes a variety of runoff generation and confluence processes within a watershed, which are different in temporal and special patterns. Traditional hydrological studies on formation mechanism of runoff were based on observation of rainfall, runoff and their statistical analysis. The black-box studies usually obtain inaccurate results that can not answer many important questions such as pathway after rainwater falling down the ground surface. Concentrated physical observation within a small watershed can provide more intuitive and detailed data, but it can only be carried out in the small-scale test fields due to high cost.Moreover, the same one physical parameter, such as groundwater table and soil moisture content, may produce similar responses in different runoff generation and confluence processes. They have no uniqueness and lead to multiple results. However, a combination of stable isotopic and chemical tracers can provide an important tool to trace surface runoff processes.Most previous studies focused on small-scale forested watersheds under mild climate in the developed countries, and on rainstorm runoff that serve flood prediction. As for runoff researches in the high-altitude and cold mountain areas, most studies were snowmelt runoff process in spring or runoff process under climate change, but studies on runoff process in rainy season that is partly recharged by glacier, hydrological cycle in the high-altitude mountain, and hydrological underlying surface located in different altitude areas are scarce. How much contribution from each water source in a hydrological unit at different height and how much difference of runoff process at different hydrological underlying surface in rainy season are controversial. Since the runoff in rainy season in high-altitude and cold mountain areas has important significance, the runoff process at hydrological underlying surface is beginning to be concerned.The Heihe River watershed is a typical inland river watershed in northwestern China. The Heihe river is originated from the high-altitude and cold Qilian Mountains that play an important role in water cycle in the regional and whole watershed as a vital recharge source of the river. The high-altitude mountain area has a cold climate and vertical vegetation zonation as well as complex underlying surface conditions, where widely distribute permafrost of low water permeability, loose covering of high water permeability, and chestnut soil that is rich in humus. A lot of work about hydrology and ecological hydrology were carried out in the upper reaches of the Heihe River and accumulated valuable data. Therefore, the mountain areas of the upper reaches of the Heihe River is an ideal site to carry out surface runoff processes in rainy season in high-altitude and cold mountain areas based on stable isotopic and chemical tracers.This study was conducted in a small watershed of Hulugou in the upper stream of the Heihe River using stable isotopic and chemical data, and aimed to identify water recharge sources of surface runoff in the rainy season at different landscape, to trace confluence pathway of rainwater and its dynamic changes, and to reveal the impact mechanism of the difference among hydrological underlying surface at the different landscape on runoff processes. It can be concluded from the following three aspects.1. Characteristic of water Environmental tracersCombining the landscape zoning with altitude, the study area can be divided into four zones. The four zones are alpine cold desert, alpine meadow, alpine shrubland, and alpine grasslands. Water samples for hydrochemical analysis were collected in four landscape belts in the study area, including70sets of rain samples,10sets of meltwater samples,55sets of river water samples, and25sets of spring samples. The deuterium and oxygen isotope, hydrochemical major elements, trace elements, and the dissolved inorganic carbon of different samples were tested and analyzed. Through the composite analysis of isotopic and water chemical test data, this article ascertained the following key conclusions.1) Characteristics of the deuterium and oxygen isotopeThe Hulugou watershed is located in the Northwest Inland. Stable isotopes δD and δ18O in rainfall show more negative than the standard atmospheric precipitation line on the basis of observation during the observation period. Based on the value of stable isotopes δ D and δ18O in rainfall, we obtained the deuterium oxygen relationship equation(δ D=9.395δ8O+22.36(R2=0.93)) of Hulugou watershed in rainy season. By regression analysis, the δ D and δ18O have good correlation in the equation. Compared with precipitation line collected from the nearest station, the slope intercept is larger. This indicates that gas-liquid two-phase isotopic disequilibrium fractionation degree is large during the formation of precipitation clouds in rainy season. High temperature in rainy season makes evaporation increases, this lead to ground water vapor enriched in heavy isotopes. Ground water vapor which is rich in heavy isotopes and rainfall that contains less heavy isotopes generating a strong exchange, showing apparent local moisture circulation effect.The stable isotopes8D and δ18O in rainfall in Hulugou watershed reduced as the elevation rise, namely precipitation isotope altitude effect. It also has negative correlation with rainfall, and rainfall effect is obvious. Rainfall isotope temporal and spatial heterogeneity is large,which is one of the factors of uncertain results when used for runoff divided.2) Characteristics of hydrochemical tracersWe analysis the hydrochemical tracers about snowmelt, rainfall, groundwater, Cl-in the water, total dissolved solids (TDS), dissolved Si and DIC in the study area during the rainy season, and found some regularity,a. Characteristics of Cl-in the waterConcentration of Cl-in the water is the minimum, through a short period of water-rock interaction, the snowmelt’s concentration is higher than rainfall, groundwater’s concentration is the maximum, and the river is lower than groundwater. Concentration of Cl-in the water is increased gradually along the process, which is called the process effect. In addition to the evaporation, concentration, scour and solution with the rock on the surface(the impact of the above factors is small),the more important reason may be the contribution rate of groundwater is increased along the process, showing the river’s characteristics of hydrochemica, that the groundwater is regarded as the supply of base flow. b.Characteristics of other hydrochemical tracersThe characteristics of total dissolved solids (TDS), dissolved Si and DIC is similar with Cl, and they have significant differences in the snowmelt, rainfall, groundwater and river, showing the characteristics that increase along the process.2. Identify water recharge sources of surface runoff in the rainy season at different Landscape beltIn the rainy season, the temperature is very high, the speed of ice and snow’s melting reached the peak of the year, rainfall runoff is actually the outcome of combined action of rainfall, snow’s melting water, groundwater, as a result, the time of ice resupply river in rainy season runoff is the biggest annual runoff. Based on the δD/Cl-segmenting the rainy season runoff with our height, the results show:during the study period, the contribution rate of ice and snow’s melting water that on alpine cold desert is between0-79%, the contribution of rainfall is between11-76%, the contribution of groundwater is between7-23%. The contribution of ice and snow’s melting water on alpine meadow is between0-77%, the rate of rainfall’s contribution between9-76%, the contribution rate of groundwater is between4-24%; on alpine shrubland, ice and snow melting water’s contribution rate is between0-56%, rainfall’s contribution rate is between18-67%, groundwater’s contribution rate is between11-44%; on alpine grasslands, ice and snow melting water’s contribution rate is between0-48%, rainfall’s contribution rate is between0-58%, groundwater’s contribution rate is between33-58%. Ice and snow melting water’s contribution on all landscape belts are the first falling and after rising, early and late rainy season is highest, in the late temporary snow melting water’s contribution is larger; Rainfall’s contribution rate is highest in the middle of the rainy season; Groundwater’s contribution rate volatiles smaller over time, the change along the process is obvious. In alpine cold desert and alpine meadow shrubland,the change is lower, and in alpine grasslands is the highest.3. The impact mechanism of the difference among hydrological underlying surface at the different landscape on runoff processes.Combined the study on convergence pathways tracer of rainfall dissolving Si and DIC, we found that the temporal and spatial variation of the convergence path tracer of rainfall is different at different heights area. It’s difficult to produce overland flow in the traditional sense because of the thick organic layer in alpine grasslands, which is conducive to rainfall infiltration. Rainfall occurred in the junction surface of an organic layer and an inorganic layer often produce inorganic layer upflow. There are certain restrictions on rainfall convergence and runoff caused by the frozen soil’s weak permeable in alpine shrub area:one is to limit the rainfall infiltrate and recharge deep groundwater,the other is to limit deep groundwater to transform into surface water. Springs appear because the rainfall infiltration rate increase, overland flow decrease and groundwater rise in the narrow bedrock after the melt of frozen soil. The permeability coefficient increased significantly and river runoff began to leak after the melt of frozen soil in alpine meadow shrub area. Snow meltwater in the upstream may recharge the area from non-normal river. The rainfall, part of which into fractured pore water and another import underground river, infiltrate into moraine, recharge surface runoff at last. |
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