A method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds

Abstract

The present invention discloses a method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds. The method includes collecting basic data on the watershed and setting up several monitoring points; conducting hydrological sampling during rainfall events; testing and analyzing the hydrogen and oxygen isotope composition and hydrochemical composition of water samples; classifying rainfall events; analyzing the hydrogen and oxygen isotope and hydrochemical characteristics of the hydrological process, and analyzing the rainfall-stratified runoff response pattern by combining the rainfall process, the change process of soil volumetric moisture content, and the change process of groundwater level; drawing an end-member mixing diagram to identify the main sources of stratified runoff, and constructing an end-member mixing model to divide the stratified runoff water source components. Through the analysis of hydrogen and oxygen isotope and hydrochemical characteristics of the hydrological process and the monitoring of multiple hydrological elements, the present invention can not only reveal the water migration process in the watershed and the runoff convergence path of each layer, but also provide a more in-depth analysis of the rainfall-stratified runoff response pattern under different rainfall patterns.

Description

Technical Field

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

[0002] Small and medium-sized rivers are widely distributed, characterized by strong spatial heterogeneity in rainfall and underlying surface areas, and short runoff generation and convergence periods. This makes flood forecasting for these rivers challenging, and forecast accuracy and forecast horizon are key issues that need to be addressed. The mechanism of mixed runoff generation in hydrological processes is a core issue in flood research, crucial for understanding the basin-scale hydrological cycle, revealing runoff generation and convergence mechanisms, and improving flood forecasting capabilities. However, due to the complex temporal and spatial interactions of different water sources within a basin (such as rainfall infiltration, groundwater recharge, and soil water redistribution), runoff formation exhibits multi-scale, multi-process coupling characteristics.

[0003] The rapid development of isotope hydrology and hydrochemical tracing 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 technology 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 the 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 runoff in the Qinling forest small watershed based on hydrogen and oxygen stable isotope tracing [J]. Journal of Earth Sciences and Environment, 2022, 44(03): 545-557; However, both studies lack a specific analysis of the water source components of surface runoff and subsurface 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, which lacks a specific analysis of the water source components of surface runoff and subsurface flow 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:

[0006] A method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds comprises the following steps:

[0007] S1. Collect and organize basic data of the study basin, set up several monitoring points in the study basin, and monitor hydrological and meteorological data before and during rainfall events;

[0008] S2. Carry out hydrological sampling during rainfall events according to the monitoring points in step S1;

[0009] S3, testing and analyzing the hydrogen and oxygen isotope composition and water chemical composition of the water sample in step S2;

[0010] S4. Based on the soil volumetric 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 the rainfall characteristics;

[0011] S5. Analyze hydrogen and oxygen isotope and water chemical characteristics during hydrological processes, and analyze the rainfall-stratified runoff response pattern based on the rainfall process, soil volumetric water content change process, and groundwater level change process to clarify the formation of stratified runoff and the water migration process;

[0012] 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 runoff generation mechanism of stratified runoff mixing in small and medium-sized watersheds.

[0013] Preferably, step S1 specifically includes:

[0014] S1.1. Collect and organize basic data on the watershed to be studied, including watershed climate characteristics, vegetation characteristics, topographic characteristics, soil type, soil layer thickness, surface elevation, and bedrock elevation data;

[0015] S1.2. Establish several rainfall monitoring points within the study basin;

[0016] Rainfall monitoring points are used to arrange rainfall collection devices and conduct high-resolution monitoring of rainfall.

[0017] S1.3. Based on the soil layer thickness and topographic characteristics of the study basin, deploy multiple moisture rapid measuring instruments in multiple layers of soil profiles to monitor changes in the volumetric moisture content of each layer, including monitoring changes in the volumetric moisture content of each layer before rainfall.

[0018] S1.4. Based on the spatial distribution characteristics of the surface elevation and bedrock elevation in the study basin, select multiple groundwater wells and deploy sensors to monitor groundwater level changes;

[0019] The selected groundwater wells should be evenly distributed or representative.

[0020] S1.5. Based on the characteristics of the soil layers and runoff in the basin, a stratified water collection tank is laid out at the outlet of the basin, extending toward the main stream. A stratified runoff monitoring device is connected to the tank. The water level is monitored through rectangular and triangular weirs, and the stratified runoff flow is calculated using the head-flow conversion relationship.

[0021] Soil layer characteristics refer to soil layer thickness and soil type; watershed runoff characteristics refer to the runoff characteristics within the watershed, which converges to low-lying areas within the watershed to form small rivers and ditches. Layers are selected based on soil layer characteristics, such as surface runoff, underground runoff from 0 to 50 cm, and underground runoff from 50 to 100 cm.

[0022] A stratified runoff monitoring device is installed at the basin outlet to guide the runoff of different layers through the collection tank, and then monitor the water level through rectangular weirs and triangular weirs.

[0023] Preferably, step S2 specifically includes:

[0024] S2.1. Deploy rainfall collection devices at the monitoring points in step S1.2 and perform sampling according to the frequency. When the rainfall intensity is high, the sampling frequency is shortened.

[0025] The sampling frequency can be selected from 30 to 60 minutes to capture the hydrogen and oxygen isotope and water chemical characteristics of rainfall at the peak rainfall intensity stage, thereby improving sample representativeness.

[0026] S2.2. Based on soil thickness and topographical characteristics, several locations are selected and clay heads are buried at different depths. These heads are connected to sampling bottles via hoses. During rainfall, the sampling bottles are vacuumed to extract soil water and sampled at a frequency.

[0027] The sampling frequency can be selected as 1~2 times / day;

[0028] S2.3. Conduct groundwater sampling at a frequency in the groundwater wells selected in step S1.4;

[0029] The sampling frequency can be selected as 1~2 times / day;

[0030] S2.4. Connect a runoff sampling device to each stratified water collection tank installed in step S1.5. Use a vacuum pump to extract stratified runoff water samples at a specified sampling frequency. Monitor the water level in real time to observe the runoff process. Reduce the sampling frequency when the flow rate is high.

[0031] The sampling frequency can be selected as once every 30 to 60 minutes to capture the hydrogen and oxygen isotopes and hydrochemical characteristics of runoff during the flood peak period, thereby improving sample representativeness.

[0032] S2.5. All experimental water samples were collected using 500 ml screw-capped bottles. The bottles were rinsed 2 to 3 times with water before sampling. Immediately after sampling, a portion 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 refrigerator at 4 ℃ for backup.

[0033] The filtered sample should be tested as soon as possible. If it cannot be tested in time, it should be stored in a 4°C refrigerator and restored to room temperature before being tested on the machine.

[0034] Preferably, step S3 specifically includes:

[0035] S3.1. Take a portion 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 the water sample before testing.

[0036] After long-term sampling, the test pen needs to be calibrated with pH standard buffer solution and conductivity standard solution to ensure data accuracy.

[0037] S3.2. Take a portion of the original sample and titrate it with methyl orange, phenolphthalein double indicator and hydrochloric acid standard solution to test the bicarbonate ion content;

[0038] 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 different concentrations of the four ions before testing, inject the different standard solutions into the ion chromatograph for analysis, and record the retention time and peak area corresponding to each concentration; draw a standard curve through quantitative analysis of the 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;

[0039] S3.4, test the other filtered sample 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 as relative differences, that is, compared with the stable isotopes of a certain standard substance.

[0040] The optional model of liquid water isotope analyzer is Picarro L2130-i.

[0041] Furthermore, the calculation formula for the bicarbonate ion content in step S3.2 is as follows:

[0042]

[0043] in, For sample HCO3 - Content, unit is mg / L; C HCL is the concentration of hydrochloric acid standard solution, in mol / L; V HCLThe volume of hydrochloric acid standard solution used for titration, in ml; V sample is the volume of water sample used for titration, in ml.

[0044] Furthermore, the five concentrations of standard solutions of the four ions in step S3.3 are: F - 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.

[0045] Furthermore, in step S3.4, D, 18 The formula for calculating the content of O stable isotopes in nature is as follows:

[0046]

[0047] 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.

[0048] Preferably, step S4 specifically includes:

[0049] S4.1. Repeat steps S2 and S3 to perform intensive sampling and water sample testing on the hydrological process of multiple rainfall events;

[0050] To ensure the rationality of the results, at least three rainfall events with complete rainfall runoff processes should be collected and tested;

[0051] S4.2. Summarize and study typical rainfall events with complete rainfall-runoff processes monitored in the basin;

[0052] S4.3. Determine the underlying surface state based on the results of pre-rainfall soil volumetric moisture monitoring in the study basin; determine rainfall characteristics based on the rainfall amount, rainfall intensity, and rainfall duration obtained from rainfall monitoring; and classify various rainfall patterns based on the combination of underlying surface state and rainfall characteristics to discuss the runoff generation mechanisms under different rainfall patterns.

[0053] The underlying surface conditions include drought and wetness, and the rainfall characteristics include short-term heavy rainfall, continuous rainfall, and multi-stage rainfall.

[0054] Preferably, step S5 specifically includes:

[0055] S5.1. Analyze the spatiotemporal distribution characteristics of hydrogen and oxygen isotopes in rainfall, soil water, groundwater, and stratified runoff. Analyze the differences and variations in hydrogen and oxygen isotope compositions among various water bodies. Preliminarily summarize runoff characteristics based on the temporal variations in hydrogen and oxygen isotopes in rainfall-stratified runoff.

[0056] 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;

[0057] S5.3. Analyze the infiltration and storage characteristics of soil water based on the dynamic changes in soil moisture during rainfall, combined with rainfall intensity and soil layers;

[0058] S5.4. Determine the relationship between groundwater recharge and response processes based on the dynamic changes in groundwater levels during rainfall, combined with rainfall amount and intensity;

[0059] S5.5. Based on the dynamic changes in 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.

[0060] Preferably, step S6 specifically includes:

[0061] S6.1. Select several stable isotopes and hydrochemical indicators as tracers and construct end-member mixing analysis diagrams for each water source and stratified runoff. The X-axis and Y-axis represent the concentrations of the two tracers, respectively. The tracers should be selected to be significantly different between water bodies and have no linear correlation between them. Use the end-member mixing analysis diagrams to identify the primary sources of stratified runoff under different rainfall patterns.

[0062] S6.2. Based on the water balance equation and the tracer mass concentration balance equation, an end-member mixing model is constructed to classify the source components of the stratified runoff and reveal the dynamic changes in the contribution of each water source to the runoff of each layer. The formula for the end-member mixing model is as follows:

[0063]

[0064] in, Q t represents the runoff flow at the basin outlet; Q i Representative iThe flow rate from each source to the basin outlet; Tracer in runoff from the outlet of the basin j concentration; Indicates the i Tracers in water sources j concentration;

[0065] 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 generation mechanism of the basin under various meteorological and underlying surface conditions.

[0066] Compared with the prior art, the present invention has the following beneficial effects:

[0067] 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 convergence paths of each layer, but also provide a more in-depth analysis of the rainfall-stratified runoff response under different rainfall patterns.

[0068] 2) Based on the end-member mixing 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 time 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 processes and improving flood forecasting capabilities.

[0069] 3) This invention can be applied to different types of closed basins, including those with varying climate, soil characteristics, topography, and other characteristics. Furthermore, its application in different basins can systematically analyze the mechanisms of runoff generation, providing a scientific reference for flood control in small and medium-sized rivers. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Figure 1 is a flow chart of the method of the present invention;

[0071] 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;

[0072] 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;

[0073] Figure 4 This is the rainfall-stratified runoff process diagram under the R1 rainfall event;

[0074] Figure 5 This is the rainfall-stratified runoff process diagram under the R2 rainfall event;

[0075] Figure 6 This is the rainfall-stratified runoff process diagram under the R3 rainfall event;

[0076] Figure 7 This is the time course change diagram of hydrogen and oxygen isotopes in water sources and runoff under the R1 rainfall event;

[0077] Figure 8 This is the time course change diagram of hydrogen and oxygen isotopes in water sources and runoff under the R2 rainfall event;

[0078] Figure 9 This is the time course change diagram of hydrogen and oxygen isotopes in water sources and runoff under the R3 rainfall event;

[0079] Figure 10 for 18 O-EC, 18 O-TDS, 18 O-CL - Endmember mixing analysis diagram;

[0080] Figure 11 The stratified runoff components of the R1 field are divided based on the two-end-member mixing model;

[0081] Figure 12 The runoff components of the R2 field are divided based on the two-end-member mixing model;

[0082] Figure 13 The runoff components of the R3 field are divided based on the two-end-member mixing model. DETAILED DESCRIPTION

[0083] The technical solution of the present invention is further described in detail below through embodiments and in conjunction with the accompanying drawings.

[0084] This example takes the Nandawan River Basin as an example and analyzes the mixed runoff generation mechanism according to this method.

[0085] 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:

[0086] S1. Collect and organize basic data of the Nandawan Basin, including the basin's climate characteristics, vegetation characteristics, topography 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 events.

[0087] The Nandawan Basin has an area of ​​7897 m 2Located in the Xiaosha River basin, a secondary tributary of the Chu River, a primary tributary of the Yangtze River, the area has a humid climate and abundant rainfall, with average annual precipitation ranging from 1000 mm to 1100 mm. The basin is densely wooded with a high density of vegetation between woodlands. The basin has a maximum ground elevation difference of 12.91 m, and slope gradients range from 6.7% to 17.1%. The soil type is mainly brown soil, mainly heavy loam to medium loam and clay loam, with a predominantly massive and prismatic structure. The surface elevation ranges from 38 m to 50 m, and the bedrock elevation ranges from 36 m to 40 m. The average soil thickness is 2.46 m, and is mostly 1 m near the basin outlet.

[0088] Based on the soil thickness and topographic characteristics of the Nandawan watershed, multi-layer soil profile rapid moisture meters were deployed in the watershed to monitor the soil volumetric moisture content at depths of 5, 10, 20, 30, 40, 50, 60, 75, and 100 cm, respectively.

[0089] 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 groundwater level changes.

[0090] Based on the soil layer characteristics and watershed confluence characteristics of the Nandawa watershed, three layers of water collection tanks were laid in the main drainage ditches within the watershed and connected to stratified runoff monitoring devices. Surface runoff (SR), subsurface runoff from 0 to 50 cm (SSR50), and subsurface runoff from 50 to 100 cm (SSR100) were monitored. The water level was monitored using rectangular and triangular weirs, and the stratified runoff flow was calculated using the head-discharge conversion relationship.

[0091] S2. Carry out hydrological sampling during rainfall events according to the monitoring points in step S1;

[0092] S3. The hydrogen and oxygen isotope compositions and water chemical composition of the water sample in step S2 were tested and analyzed. Some of the data are shown in Table 1.

[0093] Table 1 Hydrogen and oxygen isotope characteristics of stratified runoff during three rainfall events in the Nandawa Basin

[0094]

[0095] S4, judging the state of the underlying surface of the watershed based on the soil volumetric moisture content before rainfall monitored in step S1, and classifying the rainfall event in combination with rainfall characteristics;

[0096] In this example, three typical rainfall events in July and September 2024 in the basin are selected for analysis. The three rainfall events are named R1 (July 9, 16:08~July 12, 13:08), R2 (July 14, 07:33~July 14, 15:00), and R3 (September 16, 12:12~September 17, 13:22).

[0097] Figure 2 The temporal response characteristics of the 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 the shallow soil volumetric moisture content was at a low level only in the early stage of 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.

[0098] Figure 3 The temporal 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.

[0099] Figure 4 、 Figure 5 、 Figure 6 The rainfall-stratified runoff process of Nandawan under three typical rainfall events is shown. It can be found that the three rainfall basins show completely different runoff characteristics. In the wet-multi-stage 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 drought-continuous rainfall R3 event, the runoff SSR100>SR>SSR50.

[0100] S5. Analyze hydrogen and oxygen isotopes and water chemistry during hydrological processes. Combined with rainfall processes, soil volumetric water content changes, and groundwater level changes, analyze the rainfall-stratified runoff response patterns to clarify the formation of stratified runoff and the water migration process.

[0101] Figure 7 、 Figure 8 、 Figure 9 The rainfall and stratified runoff in Nandawa during three typical rainfall events are shown. 18 The temporal and spatial variation characteristics of O isotopes and soil water and groundwater 18 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 more enriched and closer to soil water and groundwater δ 18 The characteristics of O indicate 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 in this rainfall event respectively. 18The 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.

[0102] 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 runoff generation mechanism of stratified runoff mixing in small and medium-sized watersheds.

[0103] Figure 10 Demonstrated based on 18 O, EC, TDS, CL - The end-member mixing analysis diagram for the Nandawa hydrological process shows that the end-member mixing analysis diagram effectively identifies rainfall and the three types of runoff under the three rainfall patterns. However, the sampling data for soil water (SS) and groundwater (G) are quite repetitive, showing no significant differences and a high degree of overlap in the diagram. This is due to the lack of fixed groundwater in the study area. The groundwater is all shallow, rising and falling sharply after rainfall, and highly mixed with deeper soil water. Therefore, this example selects rainfall and soil water (including mixed soil water) as the primary end-members for runoff stratification in the study area, accurately identifying the dynamic changes in their contribution to stratified runoff under different meteorological and underlying surface conditions. In the diagram, SS and G represent the soil water and groundwater samples, respectively. The remaining data are the same as above. Figure 10 SS and G are the data of each water sample, Figure 7 、 Figure 8 、 Figure 9 Among them, RangeSS and RangeG are ranges.

[0104] Figure 11 、 Figure 12 、 Figure 13 The dynamic evolution of the contributions of rainfall and soil water to stratified runoff in the study basin under three typical rainfall events is presented. The results show that runoff in the study area exhibits a decreasing rainfall contribution and an increasing soil water contribution with increasing depth. Furthermore, runoff composition varies under different meteorological and underlying surface conditions. In the figure, PRContribution refers to the rainfall contribution, SSContribution refers to the soil water contribution, and Total Runoff refers to the total runoff.

[0105] Figure 11 The division of stratified runoff water source components in the R1 event based on the two-end-member mixing model was demonstrated. The results showed that in the wet-multi-stage rainfall R1 event, the runoff composition of SR and SSR50 was similar, the rainfall proportion in SSR100 was relatively small, and there was a longer water withdrawal process. The basin had a mixed runoff pattern of super-infiltration runoff and full storage runoff.

[0106] Figure 12The division of stratified runoff water source components in the R2 event based on the two-end-member 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 the over-infiltration runoff mode.

[0107] Figure 13 The division of runoff water source components in the R3 phase based on the two-end member mixed model was demonstrated. The results showed that in the drought-continuous rainfall R3 phase, under high-intensity rainfall, the proportion of rainfall in the runoff was high, and the proportion of soil water in the midstream after rainfall was high. The runoff in the early stage of rainfall in the basin was mainly due to excess infiltration, and the runoff in the later stage of rainfall was mainly due to storage.

[0108] 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-subsurface 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 hydrological and meteorological data before and during rainfall events; S2. Carry out hydrological sampling during 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 volumetric 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 the rainfall characteristics; S5. Analyze the hydrogen and oxygen isotope and water chemical characteristics of the hydrological process. Combined with the rainfall process, the changes in soil volumetric water content, and the changes in groundwater levels, analyze the rainfall-stratified runoff response pattern and clarify the formation of stratified runoff and the water migration process. Specifically, this includes: S5.

1. Analyze the spatiotemporal distribution characteristics of hydrogen and oxygen isotopes in rainfall, soil water, groundwater, and stratified runoff. Analyze the differences and variations in hydrogen and oxygen isotope compositions among various water bodies. Preliminarily summarize runoff characteristics based on the temporal variations in 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 in soil moisture during rainfall, combined with rainfall intensity and soil layers; S5.

4. Determine the relationship between groundwater recharge and response processes based on the dynamic changes in groundwater levels during rainfall, combined with rainfall amount and intensity; S5.

5. Based on the dynamic changes in runoff at different depths, combined with the rainfall process, soil moisture content changes, and groundwater level changes, analyze the rainfall-stratified runoff response under different rainfall patterns. Combined with the time course of hydrogen and oxygen isotopes and water chemistry changes during rainfall, discuss the runoff formation and water transport processes in the study area. S6. Draw an end-member mixing analysis diagram to identify the main sources of stratified runoff, construct an end-member mixing model, divide the stratified runoff water source components, clarify the dynamic changes in the water sources of each layer of runoff under different rainfall patterns, and analyze the runoff generation mechanism of stratified runoff mixing in small and medium-sized watersheds. The specific steps include: S6.

1. Select several stable isotopes and hydrochemical indicators as tracers and construct end-member mixing analysis diagrams for each water source and stratified runoff. The X-axis and Y-axis represent two tracers, respectively. The tracers should be selected to be distinct between water bodies and have no linear correlation between them. Use the end-member mixing analysis diagrams to identify the primary sources of stratified runoff under different rainfall patterns. S6.

2. Based on the water balance equation and the tracer mass concentration balance equation, an end-member mixing model is constructed to classify the source components of the stratified runoff and reveal the dynamic changes in the contribution of each water source to the runoff of each layer. The formula for 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 rate from each source to the basin outlet; Tracer in runoff from the outlet of the basin j concentration; Indicates the 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 generation mechanism of the basin under various meteorological and underlying surface conditions.

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 on the watershed to be studied, including watershed climate characteristics, vegetation characteristics, topographic characteristics, soil type, soil layer thickness, surface elevation, and bedrock elevation data; S1.

2. Establish several rainfall monitoring points within the study basin; 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 water content of each layer; S1.

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

5. Based on the characteristics of the soil layers and runoff in the basin, a stratified water collection tank is laid out at the outlet of the basin, extending toward the main stream. A stratified runoff monitoring device is connected to the tank. The water level is monitored through rectangular and triangular weirs, and the stratified runoff flow is calculated using 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. When the rainfall intensity is high, the sampling frequency is shortened. S2.

2. Based on soil thickness and topographical characteristics, select several locations and bury clay heads at different depths. Connect the heads to sampling bottles via hoses. During rainfall, vacuum the sampling bottles to extract soil water and sample at a specific frequency. S2.

3. Conduct groundwater sampling at a frequency in the groundwater wells selected in step S1.4; S2.

4. Connect a runoff sampling device to each stratified water collection tank installed in step S1.

5. Use a vacuum pump to extract stratified runoff water samples at a specified sampling frequency. Monitor the water level in real time and observe the runoff process. Reduce the sampling frequency when the flow rate is high. S2.

5. All experimental water samples were collected using 500 ml screw-capped bottles. Immediately after sampling, a portion of the original sample was taken for water chemical index and bicarbonate ion titration tests. Then, a portion of the water sample was filtered using a 0.22 μm PES glass fiber filter membrane into a centrifuge tube. 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 storage as a backup sample.

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 includes the following steps: S3.

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

2. Take a portion 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. Analyze the concentrations of fluoride, chloride, sulfate, and nitrate ions in a filtered sample using an ion chromatograph. Prepare standard solutions of each of the four ions at five different concentrations before testing. Inject the standard solutions into the ion chromatograph for analysis, recording the retention time and peak area for each concentration. Quantitatively analyze the standard solutions and plot a standard curve. Calculate the concentration of each ion in the sample by comparing the sample response with the standard curve. S3.4, test the other filtered sample 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 expressed as relative differences, that is, compared with the stable isotopes of a certain 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 for bicarbonate ions 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; 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, characterized in that: The five concentrations of standard solutions of the four ions in step S3.3 are: fluoride ion at 1 mg / L, 2 mg / L, 3 mg / L, 4 mg / L, and 5 mg / L, chloride ion at 2 mg / L, 4 mg / L, 6 mg / L, 8 mg / L, and 10 mg / L, sulfate ion at 10 mg / L, 20 mg / L, 30 mg / L, 40 mg / L, and 50 mg / L, and nitrate ion at 1 mg / L, 2 mg / L, 3 mg / L, 4 mg / L, and 5 mg / L.

7. The method for analyzing the mixed runoff generation mechanism of stratified runoff in small and medium-sized watersheds according to claim 4, characterized in that: In 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 adopts V-SMOW, namely 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 includes the following steps: S4.

1. Repeat steps S2 and S3 to perform 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 results of soil volumetric moisture monitoring before rainfall in the study basin; determine rainfall characteristics based on the rainfall amount, rainfall intensity, and rainfall duration obtained from rainfall monitoring; classify various rainfall patterns based on the combination of underlying surface state and rainfall characteristics, and discuss the runoff generation mechanisms under different rainfall patterns by category.

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