| Alpine catchments are generally the headwater areas of large rivers all over the world. The hydrological factors in alpine catchment are sensitive extremely to the climate change, and their responses could be transferred to middle and lower reaches of the catchment through the runoff variation, which would further impact both the water quantity and quality in the whole catchment. Thus, it’s extremely important to investigate the runoff generation mechanism of the rivers originated from alpine catchments. Glaciers and snowmelt make great contribution to stream runoff in the Arctic and subarctic mountain areas, however stream runoff is mainly supplied by precipitation in mid and low latitudes alpine region. Therefore, the processes and mechanism of runoff generation transferred by rainwater is the important content of hydrology research in alpine region.In northwestern China, the large inland rivers cross over alpine area and arid region, the special mountain-basin structure determines the stream runoff distribution pattern that mountain is the runoff generation area and basin is the runoff dispassion area. It’s of great significance to study seasonal variation of runoff source and precipitation-runoff generation mechanism in alpine area, to better understand the formation and transformation of water resource in inland rivers, which provide the scientific fundamentals for reasonable utilization of water resources and relieving water shortage in northwestern China.In this dissertation, Hongnigou Watershed in upper stream of Heihe River has been chosen as the typical case study area. The water source and seasonal variation of stream runoff was analyzed by using environmental isotope and reactive solute tracers, dynamic change of runoff water source and flow route was quantitative identified according to the typical rainfall-runoff event in summer. Then physical processes based distributed rainfall-runoff hydrological model was developed to provide the scientific evidence for the runoff generation analysis, impact prediction of runoff process affected by climate change and anthropogenic activities, as well as water resource management in the alpine catchment. Four related studies are complicated in this dissertation:1. Seasonal variation of the runoff water sources determined by using isotopic tracersBased on the annual variation of δ18O values in river water, groundwater, soil water (including subsurface ice), precipitation and snowmelt in Hongnigou Watershed, the water source and seasonal variation of stream runoff could be deduced as follows: From March to May in spring, permafrost melt was the primary recharge source of river water. From June to September in summer, the main recharge sources of stream runoff were precipitation, overland flow and groundwater flow originated from precipitation. From the end of September to the middle of October, the primary source of river water was the groundwater recharged from precipitation in the end of summer and stored in the watershed. From the middle of October to the end of November, the river water generally originated from the residual groundwater during permafrost freezing, which had not transformed into subsurface ice. Owing to the isotopic fractionation effect during the transformation between solid phase and liquid phase, the δ18O value of this water was very depleted.2. The generation of rainfall-runoff event:evidence from environmental isotope and reactive tracersGenerally, the contributions from various rainfall-runoff events to stream runoff are relatively low (approximately 20%～40%), and only around 10%～18% finally flows into rivers, while the major contribution (60%～80%) is derived from the storage of previous rainfall events, which flows into river in the form of groundwater. At the different stages of rainfall-runoff event, the differences can be observed in the water sources and runoff generation:(1) at the initial stage, the rainfall mainly makes up the water loss of soil, which is difficult to form any runoff, and therefore nearly all rainfall in this period is contributed into river; (2) at the final stage, the rainfall is infiltrated into shallow groundwater in saturated zone, which leads to the ascent of water table and increase in saturated areaAs the increase of hydraulic gradient between groundwater and river water, the groundwater might recharge the river. In addition, when the soil water and low-lying land are fully recharged, the over-infiltration/storage of water would occur. Therefore, the contributions from pre-event and event water are both increasing, but the contribution percentage from pre-event water relatively decreases, while that from rainfall event increases; and (3) after the rainfall event, the runoff would keep in the same state for a while. Then, since the absence of rainfall source, the contributions from pre-event and event water would both decrease, but the contribution percentage of pre-event water is gradually increasing.3. Conceptual model of runoff generation in the small watershed of the mountainous steppe zone at the upstream of Heihe RiverIdentifying the runoff generation processes at the Hongnigou watershed by using environmental isotopes and reactive tracers, especially rainfall-runoff processes in summer, a conceptual model of runoff generation was developed. (1) A water-resisting floor was formed by the Permian and Cretaceous sandstone, mudstone, and argillaceous sandstone along the slope, which limited underground runoff (groundwater runoff and interflow) within the overlying thin residual-slope wash (1-4 m in thickness). From the up to the down of the hillside, the thickness of the residual-slope wash was increased, while the permeability was decreased, leading to the formation of unsaturated zone in the upper part and saturated zone in the lower part, and flooded wetlands on both side of the river during rainy season. (2) Rainfall was infiltrated into subsurface in the upper part of the hillside, flowed down to the lower part, and overflowed the surface under the intercept of aquitard and seasonally frozen soil. Finally, the overland flow fed the river via the flooded wetland. (3) The storage capacity of the two main water storage units (thin residual-slope wash at the upper part of the hillside and riverine wetland) were limited. Water in the soils was frozen in winter, resulting in drying up of the river. In spring, the frozen soil was melt and the gravitational water flowed into the river by underground runoff and overland flow, leading to limited water flow in the river. In autumn, river water was recharged by baseflow when the water in the wetland was frozen firstly, then the flow in the river was declined gradually to drying up again with the freeze of underground runoff in the residual-slope wash. (4) Characteristics of the rainfall-runoff processes in summer showed short water residence time and flow path and rapid runoff response to rainfall. Concentrations of environmental isotopes and reaction solute tracers corresponded well with the dynamic variations of runoff volume.4. Distributed rainfall-runoff model and runoff water source modelingDEM of the study area was built based on high resolution radio laser point cloud data using ArcGIS10.The boundary conditions, hydrographic net and vegetation coverage of the Hongni Ditch watershed were obtained with the hydraulic module of ArcGIS based on the DEM of Hulu Ditch. Further, based on the conceptual model of runoff yield and concentration, precipitation and runoff of Hongni Ditch, and watershed spatial attributes, a rainfall-runoff model was built using MIKE SHE. Results indicated that the NES coefficient of individual rainfall runoff and monthly runoff were0.75 and 0.74, the correlation coefficient (R2) were 0.7563 and 0.8158, indicating that this model predicted rainfall runoff of the catchment effectively. Additionally, this study simulated water source of the runoff. Results showed that underground runoff contributed 80% of the river runoff in monthly rainfall-runoff model and contributed 65% of individual rainfall runoff, which was consistent with the results of isotopic separation method (error<10%), further proved the availability of this model.The innovation of this dissertation is as follows:(1) Environmental isotopes and reactive solute tracing technique, combined with physical processes based distributed rainfall-runoff hydrological model were applied in this study. By comparing and verifying the results, we improved the precision of investigation on processes and mechanisms of rainfall runoff, and offered a new method for studying the hydrological processes in alpine catchments. (2) The study area is a typical headwater catchment, with major water storage units of "thin eluvial-colluvial soils and riverine wetland" that is widely distributed in the upper reaches of the Heihe River and Tibetan Plateau. By studying the rainfall runoff paths and the interaction of different water storage units, and developing the conceptual model of rainfall runoff generation, this research can provide a reference for hydrological processes studies in similar alpine catchments. |
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