| With the development of high technology and the improvement of the strategic demand for ensuring water safety in a changing environment,the distributed hydrological model will inevitably expand from the traditional basin scale to the national or regional scale.It is of great theoretical and practical significance to construct a large-scale distributed hydrological model to quantitatively reveal the hydrological response and water resources effects under the influence of different climates,topography,vegetation and aerated zone structures in a large spatial range.However,compared with small and medium-scale watersheds,large-scale areas generally involve multiple climatic-hydrological zones and geological and geomorphic units,resulting in significant spatial differences in water and thermal conditions and underlying ground structures,which cannot be fully reflected in traditional watershed models.Therefore,the study hopes to make full use of existing observing systems and model technologies to carry out detailed simulation of large-scale distributed hydrological processes.On the basis of revealing the complex mechanisms of runoff generation of large-scale areas affected by different climatic and underlying conditions,the watershed model WEP-L was improved from four aspects:calculation unit determination,model input,spatial parameterization and simulation method.During the research process,China was selected as the study area,and the national model WEP-CN was constructed.Then,a total of 127 hydrological stations were selected nationwide,considering different climatic-hydrological zones,upstream and downstream watersheds,and data accessi-bility.Using the monthly runoff data from 1960-2015 of these hydrological stations,the improved performance and universality of the model WEP-CN was verified.Finally,the model was used to quantitatively compare and analyze the evolution of water resources from 1960 to 2015 in 21 typical watersheds of different climatic and geological regions across the country.The main results are as follows:(1)The spatial difference of the mechanisms of runoff generation in large-scale region was revealed,which shows the coexistence of saturation-excess and infiltration-excess mechanisms,the coexistence of cold and non-cold modes,the coexistence of mountain and plain models,and the coexistence of soil and rock models.On the basis,the existing(semi-)distributed hydrological models were investigated from the unit division,the mechanisms of runoff generation,confluence calculation,structural characteristics,and applicable scale,and then the model WEP-L was selected.Finally,based on WEP-L,the structure design and implementation scheme of large-scale distributed hydrological model were completed.(2)China is a typical large-scale area,with water and thermal conditions and vegetation varying strongly in time and space.The average annual temperature of the national basic meteorological sites from 1960 to 2015 was between-5.4 and 27.8 ?,and gradually rose from northern China to southern China.Over the years,the annual temperature of these sites generally showed a significant growth trend.China's average annual precipitation was 654.2 mm,which gradually increased along the northwest-southeast direction.The annual potential evapotranspiration was mainly between 400-1400mm,which was high in the northwest inland,and low in the northeast area,the upper reaches of southwestern rivers,and the upper and middle reaches of the Yangtze River.The multi-year trend of potential evapotranspiration varied significantly among different regions.The annual average NDVI was more than 0.6 in the north and 0.6-0.8 in the south of China,and showed vertical zoning characteristics with increasing altitude.In addition,the regression relationships between temperature,precipitation and elevation were established in each secondary area of water resources.It is found that there is a strong negative correlation between temperature and elevation in mountainous areas with high altitude difference.The correlation between precipitation and elevation was poor,and the correlation coefficients had both positive and negative values.(3)In the process of watershed subdivision in large-scale and complicated terrain area,the traditional method has some shortcomings in accurately describing the spatial extent of the study area,determining the outlet of the basin,and extracting the river network of the inflow area,which commonly uses a single catchment area threshold to extract the river network and divide the watersheds based on surface runoff and flow model.Consequently,a watershed subdivision method was proposed based on automatic recognition of the outlets of basin and the fusion of river network with variable catchment area thresholds.The method mainly includes the determination of basin outlets,the maximum and minimum catchment area thresholds,the multi-threshold virtual river network integration and watershed subdivision and coding.Compared with the traditional methods,the method has the following advantages:?it can automatically identify the watershed outlet,which is accurate and fast;?it initially solves the distortion problem of river network extraction in the inflow area,and can accurately describe the topological relationship between the sub-watersheds;?it accurately control the number of sub-watersheds while accurately fitting the range of the study area,taking into account the accuracy and efficiency requirements of the model simulation.Taking the whole country as an example,compared with the traditional method,the method accurately extracts the river network of the inflow area.Moreover,the method controls 170 000 km2 of land area more than the traditional method,which the the number of sub-watersheds is only increased by 3 406.(4)In order to effectively reduce the computing redundancy,many sub-watersheds located in the non-runoff areas were eliminated,and then a total of 19,406 sub-watersheds and 81 687 contour bands were subdivided nationwide.According to the regression relationship of temperature,precipitation and elevation established based on the secondary area of water resources,the spatial three-dimensional interpolation and plane interpolation algorithm are used to realize the distribution of meteorological input in these sub-watersheds.In order to reflect the spatial heterogeneity of water and thermal conditions and vegetation,river hydrodynamic conditions,and underlying surface characteristics,a multi-scale spatial parameterization scheme of "nested zones of climate zones and the tertiary area of water resources——sub-watersheds——contour bands——mosaic block" was proposed.Aiming at the special structure of the aerated zone in the expansive loess area,karst development zone and cold zone,the two-layer aeration structure of the original model "soil-bedrock" was improved,and the soil water characteristic parameters were corrected.Finally,a large-scale distributed hydrological model WEP-CN was constructed.The verification results showed that most hydrological stations have a runoff NSE value greater than 0.7 and a RE value between-10%and 10%.Consequently,the model has a good improvement effect and can be applied to hydrological process simulation of multiple climatic-hydrological zones and geological and geomorphic units.(5)The spatial difference in water and thermal factors such as annual precipitation,temperature,and actual evapotranspiration in the watersheds is in good agreement with climate zones.The precipitation in the northern watersheds was less and more concentrated than in the humid basins in south China.Futhermore,there was a significant positive correlation between the precipitation concentration degree and the latitude of the watershed.The annual average evapotranspiration was consistent with the change of annual average temperature,increasing from the cold zone to the tropics.The annual quantity and composition of runoff are not only related to climatic characteristics,but also affected and restricted by different geological conditions.In terms of the multi-year changes in annual runoff,the watersheds located in the northern mountainous area and affected by frozen soil was increasing,the watersheds located in the Huanghuaihai warm temperate mountainous area and the southwestern subtropical mountainous area was obviously reduced,and the watersheds around the Qinghai-Tibet Plateau and the southeastern mountainous area had little change.In the northern watersheds,the proportion of river base flow was relatively large,and the existence of frozen soil played a role in increasing the base flow.While the runoff in the southern watersheds was dominated by surface runoff,and the regulation and storage of the surface karst belt in the southwestern karst mountain contributed to the increase of river base flow.On the whole,climate change mostly contributed to the reduce of runoff in the southern basin,while it played a role in increasing in the cold regions.Land use changes played a significant role in reducing runoff in the Weihe River Basin and the Haihe River Basin,while its effect in the southern basin was not obvious. |
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