Medium and small watershed layered runoff mixed runoff production mechanism analysis method

Abstract

The invention discloses a medium and small watershed layered runoff mixed runoff production mechanism analysis method, which comprises the following steps: collecting basic data of a research watershed, and arranging a plurality of monitoring points; hydrological sampling in rainfall of the session is carried out; testing and analyzing hydrogen and oxygen isotope composition and water chemical composition of the water sample; classifying the rainfall events; analyzing hydrogen and oxygen isotopes and hydrochemical characteristics in a hydrological process, and analyzing a rainfall-layered runoff response rule by combining a rainfall process, a soil volumetric water content change process and an underground water level change process; and drawing an end member mixed graph to identify the main source of the layered runoff, and constructing an end member mixed model to divide layered runoff water source components. Through analysis of hydrogen and oxygen isotopes and water chemical characteristics in the hydrological process and monitoring of various hydrological elements, the water migration process of the watershed and runoff confluence paths of all layers can be disclosed, and rainfall-layered runoff response rules in different rainfall modes can be analyzed more deeply.

Description

Technical Field

[0001] The invention belongs to the technical field of watershed runoff generation mechanism analysis, and specifically relates to a method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds. Background Art

[0002] Small and medium-sized rivers are widely distributed, with strong spatial heterogeneity of rainfall and underlying surface, and short runoff generation and convergence time. This makes flood forecasting of small and medium-sized rivers difficult, and forecast accuracy and forecast period become key issues that need to be solved urgently. The mixed runoff generation mechanism in the hydrological process is the core issue of flood research, which is of great significance for understanding the hydrological cycle process at the basin scale, revealing the runoff generation and convergence mechanism, and improving flood forecasting capabilities. However, due to the complex interaction of different water sources in the basin (such as rainfall infiltration, groundwater recharge and soil water redistribution, etc.) in time and space, runoff formation shows the characteristics of multi-scale and multi-process coupling.

[0003] The rapid development of isotope hydrology and hydrochemical tracer technology has provided new scientific means for runoff segmentation and analysis of runoff generation mechanisms. Stable isotopes (such as hydrogen and oxygen isotopes) and hydrochemical solute tracing techniques can quantitatively separate the relative contributions of various water sources in runoff by accurately measuring the isotopic fingerprints and hydrochemical composition of different water sources. Existing studies have used hydrogen and oxygen isotopes and hydrochemistry to analyze the runoff composition of small and medium-sized watersheds, such as existing literature 1: Qu Simin et al. Analysis of typhoon rain runoff composition in Hemuqiao Basin based on hydrogen and oxygen isotope tracing technology [J]. Hydropower Energy Science, 2022, 40(08): 32-36; existing literature 2: Ma Tianwen et al. Analysis of water source of Qinling forest small watershed runoff based on hydrogen and oxygen stable isotope tracing [J]. Journal of Earth Sciences and Environment, 2022, 44(03): 545-557; However, both lack a specific analysis of the water source components of surface runoff and subsoil flow in the basin, and do not consider the analysis of the mixed runoff generation mechanism of the basin. Summary of the invention

[0004] Aiming at the shortcomings of the above-mentioned prior art that lacks specific analysis of the water source components of surface runoff and subsoil flow runoff in the basin and does not consider the analysis of the mixed runoff generation mechanism of the basin, the present invention provides a method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized basins.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions: A method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds comprises the following steps: S1. Collect and organize basic data of the study basin, set up several monitoring points in the study basin, and monitor the hydrological and meteorological data before and during rainfall; S2. Carry out hydrological sampling in rainfall events according to the monitoring points in step S1; S3, testing and analyzing the hydrogen and oxygen isotope composition and water chemical composition of the water sample in step S2; S4, based on the soil volume moisture content before rainfall monitored in step S1, determine the state of the underlying surface of the watershed, and classify the rainfall events in combination with rainfall characteristics; S5. Analyze the hydrogen and oxygen isotopes and water chemical characteristics in the hydrological process, analyze the rainfall-stratified runoff response law in combination with the rainfall process, the change process of soil volume water content and the change process of groundwater level, and clarify the formation of stratified runoff and the water migration process; S6. Draw end-member mixing diagrams to identify the main sources of stratified runoff, construct end-member mixing models to divide the water source components of stratified runoff, clarify the dynamic changes in the water sources of each layer of runoff components under different rainfall patterns, and analyze the flow generation mechanism of stratified runoff mixing in small and medium-sized watersheds.

[0006] Preferably, step S1 specifically includes: S1.1. Collect and organize basic data of the basin to be studied, including basin climate characteristics, vegetation characteristics, topographic characteristics, soil type, soil layer thickness, surface elevation, and bedrock elevation data; S1.2. Set up several rainfall monitoring points in the study basin; Rainfall monitoring points are used to arrange rainfall collection devices and conduct high-resolution monitoring of rainfall.

[0007] S1.3. Based on the soil layer thickness and topographic characteristics of the study basin, multiple moisture rapid measuring instruments are deployed in the multi-layer soil profile to monitor the changes in the volumetric moisture content of each layer of soil, including monitoring the changes in the volumetric moisture content of each layer of soil before rain; S1.4. Based on the spatial distribution characteristics of the surface elevation and bedrock elevation of the study basin, select multiple groundwater wells and deploy sensors to monitor the changes in groundwater levels; The selected groundwater wells should be evenly distributed or representative.

[0008] S1.5. Based on the research on the characteristics of the soil layer and the confluence characteristics of the basin, a stratified water collection tank is arranged at the outlet of the basin extending toward the main stream, and a stratified runoff monitoring device is connected. The water level is monitored through rectangular weirs and triangular weirs, and the stratified runoff flow is calculated through the head-flow conversion relationship.

[0009] Soil layer characteristics refer to the thickness of the soil layer and soil type; basin runoff characteristics refer to the runoff characteristics within the basin, which converges to low-lying areas in the basin to form small rivers and ditches. The stratification is selected based on the soil layer characteristics, such as surface runoff, underground runoff of 0-50 cm, underground runoff of 50-100 cm, etc.

[0010] A stratified runoff monitoring device is installed at the basin outlet to guide the runoff of different layers through the sump, and then monitor the water level through rectangular weirs and triangular weirs.

[0011] Preferably, step S2 specifically includes: S2.1, deploy rainfall collection devices at the monitoring points in step S1.2, and perform sampling according to the frequency, shortening the sampling frequency when the rainfall intensity is high; The sampling frequency can be selected to be once every 30 to 60 minutes to capture the hydrogen and oxygen isotopes and water chemical characteristics of rainfall at the peak rainfall intensity stage, thus improving the representativeness of the samples; S2.2. Based on the soil layer thickness and terrain characteristics, several locations are selected and clay heads are buried at different depths. The sampling bottles are connected through hoses. During rainfall, the sampling bottles are evacuated to extract soil water and samples are taken according to the frequency. The sampling frequency can be selected as 1~2 times / day; S2.3, sampling groundwater at a frequency in the several groundwater wells selected in step S1.4; The sampling frequency can be selected as 1~2 times / day; S2.4. Connect a runoff sampling device to each stratified water collection tank arranged in step S1.5, extract stratified runoff water samples according to the sampling frequency through a vacuum pump, monitor the water level in real time to observe the runoff process, and shorten the sampling frequency when the flow rate is large; The sampling frequency can be selected to be once every 30 to 60 minutes to capture the hydrogen and oxygen isotopes and water chemical characteristics of the runoff during the flood peak stage and improve the representativeness of the samples; S2.5. All experimental water samples were collected using 500 ml screw-mouth bottles. The sampling bottles were rinsed 2-3 times with water samples before sampling. Immediately after sampling, part of the original sample was taken for water chemical indicators and HCO3 - Titration test, then take part of the original sample and filter it into a centrifuge tube using a 0.22 μm PES glass fiber filter membrane. Divide the filtered liquid of each sample into two parts, seal the filtered sample and store the remaining original sample in a plastic bottle and place it in a 4 ℃ refrigerator for backup.

[0012] The filtered samples should be tested as soon as possible. If they cannot be tested in time, they should be stored in a 4 ℃ refrigerator and restored to room temperature before being tested on the machine.

[0013] Preferably, step S3 specifically includes: S3.1. Take part of the original sample and use a water quality test pen to test parameters such as pH, conductivity (EC), total dissolved solids (TDS) and salinity. Rinse the sample cup 2 to 3 times with water sample before testing. After long-term sampling, the test pen needs to be calibrated with pH standard buffer solution and conductivity standard solution to ensure data accuracy.

[0014] S3.2. Take part of the original sample and titrate it with methyl orange, phenolphthalein double indicator and hydrochloric acid standard solution to test the bicarbonate ion content; S3.3. Test a portion of the filtered sample for fluoride ions (F - ), chloride ion (CL - ), sulfate ion (SO4 2- ) and nitrate ions (NO3 - ) concentration, prepare standard solutions of five concentrations of four ions before testing, inject different standard solutions into ion chromatograph for analysis, record the retention time and peak area corresponding to each concentration; draw a standard curve through quantitative analysis of standard solutions, and then calculate the concentration of each ion in the sample by comparing the response value of the sample with the standard curve; S3.4, another filtered sample was tested by liquid water isotope analyzer for δD, δ 18 O, D, 18 The content of O stable isotopes in nature is very small and is usually expressed by relative difference, that is, by comparison with a stable isotope of a standard substance.

[0015] The optional model of liquid water isotope analyzer is Picarro L2130-i.

[0016] Furthermore, the calculation formula for the bicarbonate ion content in step S3.2 is as follows:

[0017] in, For sample HCO3 - Content, in mg / L; C HCL is the concentration of hydrochloric acid standard solution, in mol / L; V HCL is the volume of hydrochloric acid standard solution used for titration, in ml; V sample is the volume of water sample used for titration, in ml.

[0018] Furthermore, the standard solutions of five concentrations of four ions in step S3.3 are: - 1 mg / L, 2 mg / L, 3 mg / L, 4 mg / L, 5 mg / L, CL - 2 mg / L, 4 mg / L, 6 mg / L, 8 mg / L, 10 mg / L, SO4 2- 10 mg / L, 20 mg / L, 30 mg / L, 40 mg / L, 50 mg / L, NO3 - They are 1 mg / L, 2 mg / L, 3 mg / L, 4 mg / L and 5 mg / L respectively.

[0019] Further, in step S3.4, D,18 The formula for calculating the content of O stable isotopes in nature is as follows:

[0020] in, i D or 18 O; R sample , R reference They represent the stable isotope ratio of the sample to be tested and the stable isotope ratio of the standard sample respectively. The standard sample generally uses V-SMOW, which is the Vienna Standard Mean Ocean Water.

[0021] Preferably, step S4 specifically includes: S4.1, repeating steps S2 and S3 to perform intensive sampling and water sample testing on the hydrological process of multiple rainfall events; To ensure the rationality of the results, at least three rainfall events with complete rainfall-runoff processes should be collected and tested; S4.2. Summarize and study typical rainfall events with complete rainfall-runoff processes monitored in the basin; S4.3. Determine the underlying surface state based on the monitoring results of soil volumetric moisture content before rainfall in the study basin; determine rainfall characteristics based on rainfall amount, rainfall intensity, and rainfall duration from rainfall monitoring; classify rainfall patterns into multiple patterns based on the combination of underlying surface state and rainfall characteristics, and discuss the runoff generation mechanism under different rainfall patterns by category.

[0022] The underlying surface conditions include drought and wetness, and the rainfall characteristics include short-term heavy rainfall, continuous rainfall and multi-stage rainfall.

[0023] Preferably, step S5 specifically includes: S5.1. Analyze the temporal and spatial distribution characteristics of hydrogen and oxygen isotopes in rainfall, soil water, groundwater and stratified runoff, analyze the differences and changing patterns of hydrogen and oxygen isotope compositions among water bodies, and preliminarily summarize the runoff characteristics through the temporal changes of hydrogen and oxygen isotopes in rainfall-stratified runoff; S5.2. Analyze the hydrochemical characteristics of rainfall, soil water, groundwater and stratified runoff, and determine the hydrochemical characteristics and response characteristics of different water bodies through conductivity, total dissolved solids and chloride ion indicators; S5.3. Analyze the infiltration and storage characteristics of soil water based on the dynamic changes of soil water content during rainfall, combined with rainfall intensity and soil layers; S5.4. Determine the relationship between groundwater recharge and response process based on the dynamic changes of groundwater level during rainfall, combined with rainfall amount and rainfall intensity; S5.5. Based on the dynamic changes of runoff at different depths, combined with the rainfall process, soil moisture content change process and groundwater level change process, the rainfall-stratified runoff response law under different rainfall patterns is analyzed. Combined with the time-course changes of hydrogen and oxygen isotopes and water chemistry during rainfall, the basin runoff formation and water migration process are discussed and studied.

[0024] Preferably, step S6 specifically includes: S6.1. Select several stable isotopes and water chemical indicators as tracers, and construct end-member mixing analysis diagrams for each water source and stratified runoff. The X-axis and Y-axis are the contents of two tracers, respectively. The selection of tracers requires that there are obvious differences between water bodies and no linear correlation between tracers. The main sources of stratified runoff under different rainfall patterns are identified through end-member mixing analysis diagrams. S6.2. An end-member mixing model is constructed based on the water balance equation and the tracer mass concentration balance equation to divide the stratified runoff water source components and reveal the dynamic change process of the contribution rate of each water source to each layer of runoff. The formula of the end-member mixing model is as follows:

[0025] in, Q t represents the runoff flow at the basin outlet; Q i Representative i The flow from a water source to the basin outlet; Tracer in runoff representing the outlet of the basin j concentration; Indicates i Tracers in water sources j concentration; S6.3. Based on the dynamic changes in the contribution rate of each water source to stratified runoff, combined with the rainfall process, soil moisture content change process and groundwater level change process, identify the runoff patterns in different time periods and analyze the mixed runoff mechanism of the basin under various meteorological-substrate conditions.

[0026] Compared with the prior art, the present invention has the following beneficial effects: 1) Through the analysis of hydrogen and oxygen isotopes and water chemical characteristics of hydrological processes and the monitoring of multiple hydrological elements, the present invention can not only reveal the water migration process in the basin and the runoff confluence paths of each layer, but also can more deeply analyze the rainfall-stratified runoff response law under different rainfall modes.

[0027] 2) Based on the end-member mixed model, the present invention can accurately identify the dynamic changes in the contribution rate of each water source in the basin to the runoff of each layer, dynamically identify the runoff generation pattern of the basin at different periods, and analyze the mixed runoff generation mechanism of the basin under various meteorological-substrate conditions, providing scientific support for a comprehensive understanding of the basin's hydrological process and improving flood forecasting capabilities.

[0028] 3) The present invention can be applied to different types of closed basins, including basins with different basin characteristics such as climate, soil characteristics, and topography. Moreover, the application in different basins can systematically analyze the basin flow generation mechanism and provide scientific reference for flood prevention and control of small and medium-sized rivers. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a flow chart of the method of the present invention; Figure 2 The time-history response diagram of soil volumetric water content during typical rainfall events, where (a) shows the time-history response diagram of soil volumetric water content during rainfall events R1 and R2, and (b) shows the time-history response diagram of soil volumetric water content during rainfall event R3; Figure 3 The time-history response diagram of the groundwater level during typical rainfall events, where (a) shows the time-history response diagram of the groundwater level during the R1 and R2 rainfall events, and (b) shows the time-history response diagram of the groundwater level during the R3 rainfall event; Figure 4 This is the rainfall-stratified runoff process diagram under the R1 rainfall event; Figure 5 This is the rainfall-stratified runoff process diagram under the R2 rainfall event; Figure 6 This is the rainfall-stratified runoff process diagram under the R3 rainfall event; Figure 7 This is the time course change diagram of hydrogen and oxygen isotopes in each water source and runoff under the R1 rainfall event; Figure 8 This is the time course change diagram of hydrogen and oxygen isotopes in each water source and runoff under the R2 rainfall event; Fig. 9 This is the time course change diagram of hydrogen and oxygen isotopes in each water source and runoff under the R3 rainfall event; Fig.10 for 18 O-EC, 18 O-TDS, 18 O-CL - Endmember mixing analysis diagram; Fig.11 The stratified runoff components of R1 field are divided based on the two-end member mixing model; Fig.12 It is the division of stratified runoff components of R2 field based on two-end member mixing model; Fig.13 It is the division of stratified runoff components in R3 field based on the two-endmember mixing model. DETAILED DESCRIPTION

[0030] The technical solution of the present invention is further described in detail below through embodiments and in conjunction with the accompanying drawings.

[0031] This embodiment takes the Nandawa Basin as an example and analyzes the mixed runoff generation mechanism according to this method.

[0032] like Figure 1 As shown, the steps of a method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds of the present invention include: S1. Collect and organize the basic data of the Nandawa Basin, including the basin's climate characteristics, vegetation characteristics, terrain characteristics, soil type, soil layer thickness, surface elevation, and bedrock elevation data; set up monitoring points in the study basin to monitor the hydrological and meteorological data of rainfall processes.

[0033] The Nandawan Basin has an area of ​​7897 m 2 It is located in the Xiaosha River Basin, a secondary tributary of the Chu River, a first-level tributary of the Yangtze River. It has a humid climate and abundant rainfall, with an average annual precipitation of 1000 mm to 1100 mm. There are dense forests in the basin, and the vegetation density between the woodlands is relatively high. The maximum ground height difference in the basin is 12.91 m, and the slope gradient is between 6.7% and 17.1%. The soil type is mainly brown soil, mainly heavy loam to medium loam and clay loam, and the soil structure is mainly blocky and prismatic. The surface elevation is between 38 m and 50 m, the bedrock surface elevation is between 36 m and 40 m, the average soil layer thickness is 2.46 m, and it is mostly 1 m near the basin outlet.

[0034] Based on the soil thickness and topographic characteristics of the Nandawan Basin, multi-layer soil profile moisture rapid testers were deployed in the basin to monitor the soil volumetric moisture content at depths of 5, 10, 20, 30, 40, 50, 60, 75, and 100 cm respectively.

[0035] Based on the spatial distribution characteristics of the surface elevation and bedrock elevation in the Nandawan Basin, seven groundwater wells were selected to deploy sensors to monitor changes in groundwater levels.

[0036] Based on the soil layer characteristics and watershed confluence characteristics of the Nandawan Basin, three-layer water collection troughs were arranged in the main water collection ditches in the basin and connected to stratified runoff monitoring devices to monitor surface runoff (SR), underground runoff of 0-50 cm (SSR50) and underground runoff of 50-100 cm (SSR100). The water level was monitored by rectangular weirs and triangular weirs, and the stratified runoff flow was calculated through the head-flow conversion relationship.

[0037] S2. Carry out hydrological sampling in rainfall events according to the monitoring points in step S1; S3. Testing and analyzing the hydrogen and oxygen isotope composition and water chemical composition of the water sample in step S2. Some data are shown in Table 1.

[0038] Table 1 Hydrogen and oxygen isotope characteristics of stratified runoff during three rainfall events in the Nandawa Basin

[0039] S4, judging the state of the underlying surface of the watershed based on the soil volume moisture content before rainfall monitored in step S1, and classifying the rainfall events in combination with rainfall characteristics; In this embodiment, three typical rainfall events in July and September 2024 in the basin are selected for analysis. The three rainfall events are named R1 (7 / 9 16:08~7 / 12 13:08), R2 (7 / 14 07:33~7 / 14 15:00), and R3 (9 / 1612:12~9 / 17 13:22).

[0040] Figure 2 The time-course response characteristics of soil volumetric moisture content in the Nandawa Basin during three typical rainfall events are shown. It can be found that the soil in the basin was in a relatively moist state during the R1 and R2 events, and only the shallow soil volumetric moisture content was at a low level before the rainfall. During the R3 event, the soil in the basin was in a relatively dry state. The soil volumetric moisture content gradually increased with the progress of the rainfall process, and the shallow soil volumetric moisture content responded rapidly.

[0041] Figure 3 The time-course response characteristics of the groundwater level in the Nandawa Basin during three typical rainfall events are shown. It can be found that the groundwater in the basin is mostly in a dry state. After the rainfall, the water levels of some wells rise and fall sharply, indicating that the water storage function of the basin is relatively poor.

[0042] Figure 4 , Figure 5 , Figure 6 The rainfall-stratified runoff process of Nandawa under three typical rainfall events is shown. It can be found that the three rainfall basins show completely different runoff production and convergence characteristics. In the wet-multi-segment rainfall R1 event, the runoff SR>SSR100>SSR50, in the wet-short-term heavy rainfall R2 event, the runoff SR>SSR50>SSR100, and in the dry-continuous rainfall R3 event, the runoff SSR100>SR>SSR50.

[0043] S5. Analyze the hydrogen and oxygen isotopes and water chemical characteristics in the hydrological process, analyze the rainfall-stratified runoff response law in combination with the rainfall process, the change process of soil volume water content and the change process of groundwater level, and clarify the formation of stratified runoff and the water migration process; Figure 7 , Figure 8 , Fig. 9 The rainfall and stratified runoff in Nandawa during three typical rainfall events are shown. 18 Temporal and spatial variation characteristics of O isotopes and soil water and groundwater18 The distribution characteristics of O isotopes show that in different rainfall events, 18 O isotopes show different fluctuation characteristics with the occurrence of rainfall, and stratified runoff also shows different hysteresis; stratified runoff shows an increase with depth. 18 O isotope is richer and closer to soil water and groundwater δ 18 O, indicating that soil water and groundwater provide more replenishment for deep runoff; when the rainfall intensity is high, the runoff δ 18 O is closer, indicating that the flood peak water source composition is more similar. In the figure, Range SS and Range G represent the soil water and groundwater δ 18 The fluctuation range of O, PR, SR, SSR50 and SSR100 represent the water samples collected from rainfall, surface, 0-50 cm underground and 50-100 cm underground runoff, respectively.

[0044] S6. Draw end-member mixing analysis diagrams to identify the main sources of stratified runoff, construct end-member mixing models, divide the water source components of stratified runoff, clarify the dynamic changes in the water sources of each layer of runoff under different rainfall patterns, and analyze the flow generation mechanism of stratified runoff mixing in small and medium-sized watersheds.

[0045] Fig.10 Demonstrated based on 18 O, EC, TDS, CL - The end-member mixed analysis diagram of the hydrological process in Nandawa shows that in the three rainfall modes, the end-member mixed analysis diagram effectively identifies rainfall and three types of runoff, but the sampling data of soil water SS and groundwater G are relatively repetitive, that is, there is no obvious difference, and they are highly overlapped in the diagram. This is related to the fact that there is no fixed groundwater in the study area. The groundwater is all shallow groundwater, which rises and falls sharply after rainfall and is highly mixed with deep soil water. Therefore, this embodiment selects rainfall and soil water (including mixed soil water) as the main end-members for runoff division in the study area, and accurately identifies the dynamic changes in the contribution rate of rainfall and soil water to stratified runoff under different meteorological-substrate conditions. In the figure, SS and G represent the water samples collected for soil water and groundwater, respectively, and the rest are the same as above. Fig.10 SS and G are the data of each water sample. Figure 7 , Figure 8 , Fig. 9 Among them, RangeSS and RangeG are ranges.

[0046] Fig.11 , Fig.12 , Fig.13The dynamic changes of the contribution rates of rainfall and soil water to stratified runoff under three typical rainfall events in the study basin are shown. The results show that the runoff in the study area as a whole shows the characteristics of decreasing rainfall contribution rate and increasing soil water contribution rate as the depth increases. At the same time, the runoff composition under different meteorological-substrate conditions is also different. In the figure, PRContribution refers to the rainfall contribution part, SS Contribution refers to the soil water contribution part, and Total Runoff refers to the total runoff.

[0047] Fig.11 The division of stratified runoff water source components in the R1 event based on the two-endmember mixing model was demonstrated. The results showed that in the wet-multi-segment rainfall R1 event, the runoff composition of SR and SSR50 was similar, the proportion of rainfall in SSR100 was relatively small, and there was a longer water withdrawal process. The basin had a mixed runoff generation pattern of over-infiltration and full-storage runoff.

[0048] Fig.12 The division of stratified runoff water source components in the R2 event based on the two-endmember mixing model was demonstrated. The results showed that in the wet-short-term heavy rainfall R2 event, the proportion of rainfall in SR and SSR50 was quite high, and the proportion in SSR100 was relatively small. The basin was mainly in an over-infiltration runoff production mode.

[0049] Fig.13 The division of stratified runoff water source components in the R3 event based on the two-endmember mixed model was demonstrated. The results showed that in the drought-continuous rainfall R3 event, under high-intensity rainfall, the proportion of rainfall in the runoff was relatively high, and the proportion of soil water in the midflow after rainfall was relatively high. The runoff was mainly caused by excess infiltration in the early stage of rainfall in the basin, and by full storage in the later stage of rainfall.

[0050] In summary, the method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds provided by the embodiment of the present invention not only solves the arbitrariness of traditional runoff segmentation methods and the ambiguity of the composition of stratified runoff water sources, but can also accurately identify the dynamic changes in the contribution rate of each water source to stratified runoff under different meteorological-substrate conditions, and further analyzes the mixed runoff generation mechanism of the watershed.

Claims

1. A method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds, characterized in that: The following steps are involved: S1. Collect and organize basic data of the study basin, set up several monitoring points in the study basin, and monitor the hydrological and meteorological data before and during rainfall; S2. Carry out hydrological sampling in rainfall events according to the monitoring points in step S1; S3, testing and analyzing the hydrogen and oxygen isotope composition and water chemical composition of the water sample in step S2; S4, based on the soil volume moisture content before rainfall monitored in step S1, determine the state of the underlying surface of the watershed, and classify the rainfall event in combination with rainfall characteristics; S5. Analyze the hydrogen and oxygen isotopes and water chemical characteristics in the hydrological process, combine the rainfall process, the change process of soil volume water content and the change process of groundwater level, analyze the rainfall-stratified runoff response law, and clarify the formation of stratified runoff and the water migration process; S6. Draw end-member mixing analysis diagrams, identify the main sources of stratified runoff, construct end-member mixing models, divide the water source components of stratified runoff, clarify the dynamic changes in the water sources of each layer of runoff under different rainfall patterns, and analyze the flow generation mechanism of stratified runoff mixing in small and medium-sized watersheds.

2. The method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds according to claim 1 is characterized in that: The step S1 specifically includes the following steps: S1.

1. Collect and organize basic data of the basin to be studied, including basin climate characteristics, vegetation characteristics, topographic characteristics, soil type, soil layer thickness, surface elevation, and bedrock elevation data; S1.

2. Set up several rainfall monitoring points in the study basin; S1.

3. Based on the soil layer thickness and topographic characteristics of the study basin, multiple moisture rapid testers are deployed in the multi-layer soil profile to monitor the changes in the volumetric moisture content of each layer of soil; S1.

4. Based on the spatial distribution characteristics of the surface elevation and bedrock elevation of the study basin, select multiple groundwater wells and deploy sensors to monitor the changes in groundwater levels; S1.

5. Based on the research on the characteristics of the soil layer and the confluence characteristics of the basin, a stratified water collection tank is arranged at the outlet of the basin extending toward the main stream, and a stratified runoff monitoring device is connected. The water level is monitored through rectangular weirs and triangular weirs, and the stratified runoff flow is calculated through the head-flow conversion relationship.

3. The method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds according to claim 2 is characterized in that: The step S2 specifically includes the following steps: S2.1, deploy rainfall collection devices at the monitoring points in step S1.2, and perform sampling according to the frequency, shortening the sampling frequency when the rainfall intensity is high; S2.

2. Based on the soil layer thickness and terrain characteristics, several locations are selected and clay heads are buried at different depths. The sampling bottles are connected through hoses. During rainfall, the sampling bottles are evacuated to extract soil water and samples are taken according to the frequency. S2.3, sampling groundwater at a frequency in the several groundwater wells selected in step S1.4; S2.

4. Connect a runoff sampling device to each stratified water collection tank arranged in step S1.5, extract stratified runoff water samples according to the sampling frequency through a vacuum pump, monitor the water level in real time, observe the runoff process, and shorten the sampling frequency when the flow rate is large; S2.

5. The experimental water samples were collected using 500 ml screw-capped bottles. After sampling, part of the original sample was immediately taken for water chemical index and bicarbonate ion titration test. Then part of the water sample was filtered into a centrifuge tube using a 0.22 μm PES glass fiber filter membrane. The filtered liquid of each sample was divided into two parts. The filtered sample was sealed and stored, and the remaining original sample was stored in a plastic bottle and placed in a 4 ℃ refrigerator for backup.

4. The method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds according to claim 3 is characterized in that: The step S3 specifically comprises the following steps: S3.

1. Take part of the original sample and use a water quality test pen to test the pH, conductivity, total dissolved solids concentration and salinity parameters. Rinse the sample cup 2 to 3 times with water sample before testing. S3.

2. Take part of the original sample and titrate it with methyl orange, phenolphthalein double indicator and hydrochloric acid standard solution to test the bicarbonate ion content; S3.

3. Test the concentrations of fluoride ions, chloride ions, sulfate ions and nitrate ions in a filtered sample by ion chromatography. Prepare standard solutions of five concentrations of four ions before testing. Inject different standard solutions into the ion chromatograph for analysis. Record the retention time and peak area corresponding to each concentration. Draw a standard curve through quantitative analysis of the standard solution, and then calculate the concentration of each ion in the sample by comparing the response value of the sample with the standard curve. S3.4, another filtered sample was tested by liquid water isotope analyzer for δD, δ 18 O, D, 18 The content of O stable isotopes in nature is very small and is usually expressed by relative difference, that is, by comparison with a stable isotope of a standard substance.

5. The method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds according to claim 4 is characterized in that: The calculation formula of bicarbonate ion in step S3.2 is as follows: ； in, is the bicarbonate ion content of the sample, in mg / L; is the concentration of hydrochloric acid standard solution, in mol / L; is the volume of hydrochloric acid standard solution used for titration, in ml; is the volume of water sample used for titration, in ml.

6. The method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds according to claim 4 is characterized in that: The five concentrations of standard solutions of the four ions in step S3.3 are: 1 mg / L, 2 mg / L, 3 mg / L, 4 mg / L, and 5 mg / L for fluoride ions; 2 mg / L, 4 mg / L, 6 mg / L, 8 mg / L, and 10 mg / L for chloride ions; 10 mg / L, 20 mg / L, 30 mg / L, 40 mg / L, and 50 mg / L for sulfate ions; and 1 mg / L, 2 mg / L, 3 mg / L, 4 mg / L, and 5 mg / L for nitrate ions.

7. The method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds according to claim 4 is characterized in that: Step S3.4 D, 18 The formula for calculating the content of O stable isotopes in nature is as follows: ； in, i D or 18 O; They represent the stable isotope ratio of the sample to be tested and the stable isotope ratio of the standard sample respectively. The standard sample generally uses V-SMOW, which is the Vienna Standard Mean Ocean Water.

8. The method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds according to claim 4 is characterized in that: The step S4 specifically comprises the following steps: S4.1, repeating steps S2 and S3, and performing intensive sampling and water sample testing on the hydrological process of multiple rainfall events; S4.

2. Summarize and study typical rainfall events with complete rainfall-runoff processes monitored in the basin; S4.

3. Determine the underlying surface state based on the monitoring results of soil volumetric moisture content before rainfall in the study basin; determine rainfall characteristics based on rainfall amount, rainfall intensity, and rainfall duration from rainfall monitoring; classify various rainfall patterns according to the combination of underlying surface state and rainfall characteristics, and discuss the runoff generation mechanism under different rainfall patterns by category.

9. The method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds according to claim 8 is characterized in that: The step S5 specifically comprises the following steps: S5.

1. Analyze the temporal and spatial distribution characteristics of hydrogen and oxygen isotopes in rainfall, soil water, groundwater and stratified runoff, analyze the differences and changing patterns of hydrogen and oxygen isotope compositions among water bodies, and preliminarily summarize the runoff characteristics through the temporal changes of hydrogen and oxygen isotopes in rainfall-stratified runoff; S5.

2. Analyze the hydrochemical characteristics of rainfall, soil water, groundwater and stratified runoff, and determine the hydrochemical characteristics and response characteristics of different water bodies through conductivity, total dissolved solids and chloride ion indicators; S5.

3. Analyze the infiltration and storage characteristics of soil water based on the dynamic changes of soil water content during rainfall, combined with rainfall intensity and soil layers; S5.

4. Determine the relationship between groundwater recharge and response process based on the dynamic changes of groundwater level during rainfall, combined with rainfall amount and rainfall intensity; S5.

5. Based on the dynamic changes of runoff at different depths, combined with the rainfall process, soil moisture content change process and groundwater level change process, the rainfall-stratified runoff response law under different rainfall patterns is analyzed. Combined with the changes in hydrogen and oxygen isotopes and water chemistry time course during rainfall, the runoff formation and water migration process in the study area are discussed.

10. The method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds according to claim 9, characterized in that: The step S6 specifically comprises the following steps: S6.

1. Select several stable isotopes and water chemical indicators as tracers, and construct end-member mixing analysis diagrams for each water source and stratified runoff. The X-axis and Y-axis are two tracers respectively. The selection of tracers requires that there are obvious differences between water bodies and no linear correlation between tracers. The main sources of stratified runoff under different rainfall patterns are identified through end-member mixing analysis diagrams. S6.

2. An end-member mixing model is constructed based on the water balance equation and the tracer mass concentration balance equation to divide the stratified runoff water source components and reveal the dynamic change process of the contribution rate of each water source to each layer of runoff. The formula of the end-member mixing model is as follows: ； in, Q t represents the runoff flow at the basin outlet; Q i Representative i The flow from a water source to the basin outlet; Tracer in runoff representing the outlet of the basin j concentration; Indicates i Tracers in water sources j concentration; S6.

3. Based on the dynamic changes in the contribution rate of each water source to stratified runoff, combined with the rainfall process, soil moisture content change process and groundwater level change process, identify the runoff patterns in different time periods and analyze the mixed runoff mechanism of the basin under various meteorological-substrate conditions.

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