Method for evaluating water holding capacity of bioretention facility

Abstract

The invention belongs to the technical field of rainwater treatment, discloses a method for evaluating the water holding capacity of a bioretention facility, and aims to effectively evaluate the water holding capacity of the bioretention facility and provide scientific guidance for subsequent construction of the bioretention facility. According to the scheme, the method comprises the following steps: collecting a soil water sample and a rainfall inflow water sample of the bioretention facility; the outflow flow of the bioretention facility in the rainfall event is continuously monitored, and outflow water samples in different time periods are collected; respectively carrying out oxyhydrogen stable isotope detection analysis on the collected soil water sample, the rainfall inflow water sample and the outflow water samples in different time periods; respectively calculating the soil moisture in the effluent water sample at different time periods and the composition proportion of the rainfall inflow water component according to the detection analysis result, and dividing the soil moisture outflow process from the outflow flow; and evaluating the water holding capacity of the bioretention facility according to the soil moisture outflow process.

Description

Technical Field

[0001] The invention belongs to the technical field of rainwater treatment, and in particular relates to a method for evaluating the water holding capacity of a biological retention facility. Background Art

[0002] As an important part of the low-impact development technology of sponge cities, bioretention facilities have good control effects on hydrological effects such as total water reduction, peak flow reduction, and delayed outflow. A large number of experiments have been carried out at home and abroad to study them, and a series of results have been achieved. However, existing studies often regard bioretention facilities as a black box, measuring the control effect of bioretention facilities by comparing the difference between inflow and outflow, while the impact on soil water and the migration process of water in bioretention are not discussed.

[0003] The water holding capacity of a biological retention facility has a significant impact on its water volume control and water quality purification capabilities. Too high a water holding capacity will result in high internal water content when rainfall flows in, affecting its infiltration capacity and water storage capacity. Too low a water holding capacity will shorten the contact time between pollutants and fillers, and contact time is crucial for the adsorption and biological processes responsible for removing soluble pollutants. The shortened contact time between pollutants and fillers will affect the water quality purification capabilities of the biological retention facility.

[0004] Existing research has focused less on the water holding capacity of bioretention facilities, and there is a lack of methods to evaluate the water holding capacity of bioretention facilities. Summary of the invention

[0005] The technical problem to be solved by the present invention is to propose a method for evaluating the water holding capacity of a biological retention facility, so as to effectively evaluate the water holding capacity of the biological retention facility and provide scientific guidance for the subsequent construction of the biological retention facility.

[0006] The technical solution adopted by the present invention to solve the above technical problems is:

[0007] A method for evaluating the water holding capacity of a bioretention facility, comprising:

[0008] Collect soil water samples from the bioretention facility and inflow water samples from this rainfall;

[0009] The outflow flow of the bioretention facility during this rainfall event was continuously monitored, and outflow water samples were collected at different times;

[0010] The collected soil water samples, inflow water samples of this rainfall, and outflow water samples at different times were tested and analyzed for hydrogen and oxygen stable isotopes;

[0011] According to the test and analysis results, the composition ratio of soil moisture in the outflow water samples at different periods and the inflow moisture of this rainfall are calculated respectively, and the outflow process of soil moisture is divided from the outflow flow rate;

[0012] The water holding capacity of bioretention facilities is evaluated based on the soil moisture outflow process.

[0013] Furthermore, the method for collecting soil water samples of the bioretention facility includes any one of the following:

[0014] ① Collect soil samples from each layer of the bioretention facility and extract soil water by vacuum condensation extraction;

[0015] ② Extract soil water through a soil water collector;

[0016] ③ Collect inflow water samples from the last rainfall event as soil water in this rainfall event;

[0017] ④ Collect water samples from the late outflow period of the last rainfall event as soil water in this rainfall event;

[0018] ⑤ Collect water samples at the beginning of outflow in this rainfall event as soil water in this rainfall event.

[0019] Furthermore, the late outflow period refers to the last 10 minutes of outflow; and the beginning of outflow refers to 0-5 minutes after outflow.

[0020] Furthermore, the method for collecting outflow water samples at different time periods includes:

[0021] After the bioretention facility has a stable outflow, collect the first outflow water sample;

[0022] Then, the outflow water samples were continuously collected according to the set time intervals, and sampling was stopped when the outflow flow rate was less than 1 mL / min.

[0023] Further, the continuously collecting the outflow water sample according to the set time interval includes:

[0024] During the continuous collection period, data were collected every 2 minutes for the first 15 minutes, and every 4 minutes from 15 to 51 minutes. The subsequent sampling time intervals were adjusted according to the outflow flow rate.

[0025] Furthermore, the composition ratio of soil moisture in outflow water samples at different time periods and inflow moisture of this rainfall is calculated according to the detection and analysis results, including

[0026] For the outflow water samples of each period, according to the test and analysis results, the two-end member mixture model is used to calculate the composition ratio of soil moisture in the outflow water samples of this period and the inflow moisture of this rainfall;

[0027] The two-end member mixture model formula is expressed as:

[0028]

[0029] Among them, Q is the outflow water flow corresponding to the outflow water sample; V is the volume of the outflow water sample, t is the collection time of the water sample; Q e is the inflow water flow corresponding to the inflow water sample of this rainfall, F is the catchment area, is the runoff coefficient, h is the rainfall depth; δ Q is the stable isotope deviation of the outflow water sample in parts per thousand; δ e is the stable isotope deviation of the inflow water sample of this rainfall; δ pe is the stable isotope deviation of the inflow water other than this rainfall, i.e. soil water sample; f e is the contribution rate of this rainfall, that is, the proportion of the inflow water of this rainfall in the outflow water;

[0030] Non-current precipitation, i.e. the proportion of soil water in outflow water = 1-f e .

[0031] Furthermore, the water holding capacity of the bioretention facility is evaluated based on the soil water outflow process, specifically including:

[0032] Calculation of the total soil water content V in the bioretention facility based on the soil water outflow process in ;

[0033] According to the soil moisture outflow process, calculate the time T when the soil moisture content in the outflow is higher than 80% 80 and the time T that accounts for less than 20% 20 , and then calculate the time ΔT required for the soil moisture content in the flow to drop from 80% to 20%:

[0034] ΔT=T 20 -T 80

[0035] Total soil water content V in The larger the value is, the stronger the water storage capacity of the bioretention facility is. The time when the proportion of soil moisture in the outflow is higher than 80% is T 80 and the time T that accounts for less than 20% 20 The longer the time ΔT required for the soil moisture content in the outflow to drop from 80% to 20%, the stronger the water holding capacity of the bioretention column.

[0036] The beneficial effects of the present invention are:

[0037] The present invention collects soil water samples of the biological retention facility, inflow water samples of this rainfall, and outflow water samples at different periods of this rainfall event, and performs hydrogen and oxygen stable isotope detection and analysis on each of the collected water samples, thereby calculating the composition ratio of soil moisture in outflow water samples at different periods and inflow water of this rainfall respectively according to the detection and analysis results, and dividing the soil moisture outflow process from the outflow flow, and finally evaluating the water holding capacity of the biological retention facility according to the soil moisture outflow process. This scheme can measure the water holding capacity of the biological retention facility to a certain extent, which helps to provide a basis for judging the water holding capacity of the biological retention facility and provide scientific guidance for the subsequent construction of biological retention facilities. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a flow chart of the method for evaluating the water holding capacity of the bioretention facility in the present invention;

[0039] Figure 2 Schematic diagram of bioretention columns 1# and 2# in the experimental example of the present invention;

[0040] Figure 3 is the δ of the bioretention column 1# at different intervals of days in the experimental example of the present invention. 2 H two-end member mixing calculation result curve diagram, where: Figure 3 (a), (b), and (c) correspond to intervals of 1 day, 4 days, and 7 days, respectively;

[0041] Figure 4 is the δ of the bioretention column 1# at different intervals of days in the experimental example of the present invention. 18 O two-end member mixing calculation result curve diagram, where, Figure 4 (a), (b), and (c) correspond to intervals of 1 day, 4 days, and 7 days, respectively;

[0042] Figure 5 is the δ of the bioretention column 2# at different intervals of days in the experimental example of the present invention. 2 H two-end member mixing calculation result curve diagram, where: Figure 5 (a), (b), and (c) correspond to intervals of 1 day, 4 days, and 7 days, respectively;

[0043] Figure 6 is the δ of the bioretention column 2# at different intervals of days in the experimental example of the present invention. 18 O two-end member mixing calculation result curve diagram, where, Figure 6 (a), (b), and (c) correspond to intervals of 1 day, 4 days, and 7 days, respectively;

[0044] Figure 7 It is a schematic diagram of the outflow process division of each experimental scenario in the experimental example of the present invention, wherein: Figure 7A, B, and C in the figure correspond to the outflow process of bioretention column 1# at intervals of 1 day, 4 days, and 7 days, respectively. Figure 7 D, E, and F in the figure correspond to the outflow process division of bioretention column 2# at intervals of 1 day, 4 days, and 7 days, respectively. DETAILED DESCRIPTION

[0045] The present invention aims to provide a method for evaluating the water holding capacity of a biological retention facility, so as to achieve an effective evaluation of the water holding capacity of a biological retention facility and provide scientific guidance for the subsequent construction of biological retention facilities. The core idea is to divide the outflow water according to the hydrogen and oxygen stable isotopes and judge the water holding capacity of the biological retention facility according to the change process of the proportion of soil moisture in the outflow. Specifically, the method collects soil water samples of the biological retention facility, the inflow water samples of this rainfall, and the outflow water samples at different periods of this rainfall event, and performs hydrogen and oxygen stable isotope detection and analysis on each collected water sample, thereby calculating the composition ratio of the soil moisture in the outflow water samples at different periods and the inflow moisture of this rainfall according to the detection and analysis results, and dividing the soil moisture outflow process from the outflow flow, and finally evaluating the water holding capacity of the biological retention facility according to the soil moisture outflow process.

[0046] For specific implementation, see Figure 1 The method for evaluating the water holding capacity of a bioretention facility provided by the present invention comprises the following steps:

[0047] S1. Collect soil water samples from the bioretention facility and inflow water samples from this rainfall;

[0048] In this step, soil water samples of the bioretention facility can be collected in any of the following ways:

[0049] ① Collect soil samples from each layer of the bioretention facility and extract soil water by vacuum condensation extraction;

[0050] ② Extract soil water through a soil water collector;

[0051] ③ Collect inflow water samples from the last rainfall event as soil water in this rainfall event;

[0052] ④ Collect water samples from the late outflow period (the last 10 minutes of outflow) of the last rainfall event as soil water in this rainfall event;

[0053] ⑤ Collect water samples at the beginning of outflow (0-5 minutes after outflow) in this rainfall event as soil water in this rainfall event.

[0054] The soil water samples collected in this step and the rainfall inflow water samples will be used for subsequent hydrogen and oxygen stable isotope testing to determine the proportion of soil water and rainfall inflow water in the outflow flow.

[0055] S2. Continuously monitor the outflow flow of the bioretention facility during this rainfall event and collect outflow water samples at different times;

[0056] In this step, the first water sample is collected after a stable outflow appears in the bioretention facility. Then, a fixed time interval is set to continuously collect outflow water samples. The collection method is dense at the beginning and sparse at the end. The water samples are collected every 2 minutes in the first 15 minutes, and every 4 minutes from 15 to 51 minutes. The sampling interval is adjusted according to the outflow flow rate. Sampling is stopped when the outflow flow rate is less than 1 mL / min.

[0057] Use a 0.22μm polyethersulfone (PES) filter to filter each water sample into a 2mL chromatographic injection bottle and seal it. During the filtration process, ensure that the water sample fills the sample bottle and there are no bubbles in the bottle. After sealing, store it in a refrigerator at low temperature. Monitor the average outflow flow rate in each interval by volume method, such as using a 200mL measuring cylinder to measure the outflow volume during the sampling interval, and calculate the average outflow flow rate in each interval according to the following formula:

[0058]

[0059] Where: Q is the average outflow flow rate in the time interval (mL / min); V is the outflow volume in the time interval (mL); t is the duration of the time interval (min).

[0060] S3. Conduct hydrogen and oxygen stable isotope detection and analysis on the collected soil water samples, inflow water samples of this rainfall, and outflow water samples at different times;

[0061] In this step, all water samples collected are filtered in the stable isotope laboratory and then subjected to high temperature cracking for hydrogen and oxygen isotope testing. The instrument can be an element analyzer connected in series with a stable isotope gas mass spectrometer (such as Flashed HT+MAT253). 2 H and δ 18 The O test accuracy was 2‰ and 0.2‰, respectively, and the Vienna Standard Mean Ocean Water (VSMOW) was selected as the reference.

[0062] S4. Calculate the composition ratio of soil moisture in outflow water samples at different time periods and inflow moisture of this rainfall according to the detection and analysis results, and divide the soil moisture outflow process from the outflow flow;

[0063] In this step, the stable isotope two-end member mixing model is used for water partitioning. Due to the short experimental period and low temperature, the effects of evaporation and exchange on isotope evolution are small and can be ignored. At this time, mixing becomes the main factor causing isotope evolution.

[0064] The endmember mixing model is a common tool for quantifying the impact of mixing. Two-endmember mixing is the simplest mixing system, that is, the mixing of two different water bodies, such as atmospheric precipitation and other types of water bodies. The stable isotope two-endmember mixing model is based on the principle of mass conservation. When the two types of water sources (this precipitation Q e and non-current precipitation Q pe ), there is the mass balance equation at that moment:

[0065] Q=Q e +Q pe

[0066] δ Q Q=δ e Q e +δ pe Q pe

[0067] Wherein, Q is the flow rate of the water body after mixing, which corresponds to the flow rate of the outflowing water in the present invention; Q e , Q pe Respectively represent the flow of this rainfall and non-this rainfall, and in the present invention, they correspond to the flow of inflow water and the flow of soil water; δ Q ,δ e With δ pe Represent the mixed water body Q and the current precipitation Q respectively. e Q pe The stable isotope deviation of 1,000 parts per million can be obtained:

[0068]

[0069] Among them, fe represents the current precipitation Q e Contribution rate; 1-f e This is not the current precipitation. pe contribution rate.

[0070] That is, in the two-end member mixing model, the contribution of each potential water source to the mixed water body can be obtained by testing the stable isotope deviation of the mixed water body and the stable isotope deviation of the two potential water sources. Then, the present invention can obtain the composition ratio of soil moisture in the outflow water samples of different time periods and the inflow moisture of this rainfall according to the two-end member mixing model.

[0071] When the ratio of soil moisture in outflow water to rainfall inflow moisture in different periods is determined, the soil moisture outflow process, that is, the process of soil moisture ratio change, can be known.

[0072] S5. Evaluate the water holding capacity of the bioretention facility based on the soil water outflow process;

[0073] In this step, the total soil water content v in the bioretention facility can be calculated based on the soil water outflow process. in ; The time T when the soil moisture content in the outflow is higher than 80% can be calculated based on the soil moisture outflow process 80 and the time T that accounts for less than 20% 20 , and then calculate the time ΔT required for the soil moisture content in the flow to drop from 80% to 20%:

[0074] ΔT=T 20 --T 80

[0075] Finally, the total soil water content V in The larger the value is, the stronger the water storage capacity of the bioretention facility is. The time when the proportion of soil moisture in the outflow is higher than 80% is T 80 and the time T that accounts for less than 20% 20 The longer the time ΔT required for the soil moisture content in the outflow to drop from 80% to 20%, the stronger the water holding capacity of the bioretention column.

[0076] Experimental example

[0077] 1. Introduction of experimental device:

[0078] The bioretention facility used in the experiment is a bioretention column, which is made of PVC. The whole column is composed of 5 sections of 30 cm long and 11 cm in diameter PVC pipes and 4 sections of 11 cm diameter extended double threaded expansion joints. The bioretention column is divided into four main structures from top to bottom: gravel layer (300 mm), filler layer (600 mm), planting layer (300 mm), and aquifer (300 mm). A permeable geotextile is laid between the gravel layer and the filler layer as a filter layer. A perforated blind pipe is installed 10 cm from the bottom of the gravel layer as a drainage pipe. The perforated blind pipe is made of PVC pipe with a diameter of 2 cm, and is perforated on one side. At the same time, the surface is wrapped with permeable geotextile to prevent soil particles from entering the perforated blind pipe and causing blockage. The composition of the bioretention column filler is shown in Table 1. Above the planting layer is the aquifer, which is 30 cm thick and is not filled with fillers. Soil water collectors were installed in the middle of the bioretention column planting layer (45 cm depth), the middle of the filler layer (90 cm depth), and the middle of the gravel layer (135 cm depth). Figure 2 The installation method is to drill a hole in the soil column wall, insert the soil water collector horizontally into the middle of the soil column, reserve a soil water sampling tube outside the soil column wall, and then use AB glue to seal the column wall.

[0079] Table 1 Bioretention column structure and filler composition

[0080]

[0081] 2. Introduction of experimental plan:

[0082] The experiment simulates a 60-minute rainfall with a return period of 2 years in Wuhan. The rainfall intensity and depth are determined according to the "Wuhan Rainstorm Intensity Formula and Design Rainstorm Type":

[0083]

[0084] Where: q is the design rainstorm intensity (mm / min); P(a) is the rainfall return period; t is the rainfall duration (min); h is the rainfall depth (mm).

[0085] The calculation method of inflow V is:

[0086]

[0087] Where: F is the catchment area (m 2 ); is the runoff coefficient, that is, the ratio of runoff to rainfall (roof, concrete pavement); other symbols have the same meanings as before.

[0088] The catchment area F is designed to have a bioretention surface area of ​​0.009847 m 2 10 times, which is 0.09847m 2 , runoff coefficient According to the Technical Guidelines for Sponge City Planning in Wuhan, 0.9 is taken. Combined with the previous formula, the total inflow is calculated to be 3944 ml. In order to facilitate the setting of inflow intensity and improve the accuracy of water volume control, the total inflow in the experiment is set to 4000 ml, as shown in Table 2. In the experiment, the inflow is in the form of uniform rainfall, and the calculated inflow flow is 66.67 ml / min. In this experiment, the inflow point is set 20 cm above the bottom of the bioretention column aquifer to ensure uniform inflow.

[0089] Table 2 Inflow flow design table

[0090]

[0091] The experiment designed three interval days of 1d, 4d and 7d, and designed 6 experimental scenarios, as shown in Table 3.

[0092] Table 3 Experimental scenario design table

[0093] Experimental scenario Bioretention Column Interval days A 1# 1d B 1# 4d C 1# 7d D 2# 1d E 2# 4d F 2# 7d

[0094] (1) Collection of soil water and inflow water samples:

[0095] Before the experiment, 10 mL of influent water sample was collected and sealed in a polyethylene centrifuge tube for temporary storage. A 20 mL syringe was used to extract about 3 mL of soil water sample from each layer of soil water collector and then placed in a polyethylene centrifuge tube for temporary storage. After the above samples were collected, the water samples were filtered into a 2 mL chromatographic injection bottle using a 0.22 μm polyethersulfone (PES) filter and sealed. During filtration, the water sample filled the sample bottle and there were no bubbles in the bottle to prevent evaporation of the water sample and cause hydrogen and oxygen stable isotope fractionation. The above water samples were sealed and placed in a refrigerator for low-temperature storage.

[0096] (2) Outflow monitoring and water sampling:

[0097] After a stable outflow appears at the bottom of the bioretention column, the first water sample is collected using a polyethylene cup. Then, a fixed time interval is set to continuously collect all outflow water samples. The collection method is dense at the beginning and sparse at the end. The samples are collected every 2 minutes in the first 15 minutes and every 4 minutes from 15 to 51 minutes. The sampling interval is adjusted according to the outflow flow rate. Sampling is stopped when the outflow flow rate is less than 1 mL / min. About 36 water samples are collected for each experiment. After the water samples are collected, a 200 mL measuring cylinder is used to measure the outflow volume during the sampling interval, and the average outflow flow rate in each interval is calculated according to the formula.

[0098]

[0099] Where: Q is the average outflow flow rate in the time interval (mL / min); V is the outflow volume in the time interval (mL); t is the duration of the time interval (min).

[0100] Then use a 0.22 μm polyethersulfone (PES) filter to filter each water sample into a 2 mL chromatographic injection bottle and seal it. During the filtration process, ensure that the water sample fills the sample bottle and there are no bubbles in the bottle. After sealing, store it in a refrigerator at low temperature.

[0101] (3) Isotope detection and water classification:

[0102] The collected water samples were tested for isotopes and the formula δ Q Q=δ e Q e +δ pe Q pe The water sample components were divided and the composition ratio of soil water and runoff water in the water sample was calculated. Among them, the δ 2 The calculation results of H two-end member mixing are respectively shown in Figure 3 (a), (b), (c); δ of bioretention column 1# at intervals of 1, 4, and 7 days respectively 18 The calculation results of O two-end member mixing are respectively shown in Figure 4 (a), (b), (c); δ of bioretention column 2# at intervals of 1, 4, and 7 days respectively 2 The calculation results of H two-end member mixing are respectively shown in Figure 5 (a), (b), (c); δ of bioretention column 2# at intervals of 1, 4, and 7 days respectively 18 The calculation results of O two-end member mixing are respectively shown in Figure 6 (a), (b), and (c) in the figure.

[0103] Finally, the outflow flow was divided according to the composition ratio of soil water and runoff water in the water samples at each time period, and the soil water outflow process and runoff outflow process were obtained. Figure 7 .

[0104] (4) Water holding capacity assessment:

[0105] T for six experimental scenarios 80 , T 20 , ΔT and V in The statistical results are shown in Table 4:

[0106] Table 4 Experimental results statistics

[0107]

[0108] ( a According to δ 2 The calculation results after H is divided into flows; b According to δ 18 O calculation results after dividing the flow)

[0109] From Table 4, we can see that the T of the 1# bioretention column in the experimental results 80 , T 20 and ΔT are longer than those of 2# bioretention column, indicating that the water holding capacity of 1# bioretention column is stronger than that of 2# bioretention column.

[0110] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for evaluating the water holding capacity of a bioretention facility, characterized in that: include: Collect soil water samples from the bioretention facility and inflow water samples from this rainfall; The outflow flow of the bioretention facility during this rainfall event was continuously monitored, and outflow water samples were collected at different times; The collected soil water samples, inflow water samples of this rainfall, and outflow water samples at different times were tested and analyzed for hydrogen and oxygen stable isotopes; According to the test and analysis results, the composition ratio of soil moisture in the outflow water samples at different periods and the inflow moisture of this rainfall are calculated respectively, and the outflow process of soil moisture is divided from the outflow flow rate; The water holding capacity of bioretention facilities is evaluated based on the soil moisture outflow process.

2. A method for evaluating the water holding capacity of a bioretention facility according to claim 1, characterized in that: The method for collecting soil water samples of the bioretention facility includes any of the following: ① Collect soil samples from each layer of the bioretention facility and extract soil water by vacuum condensation extraction; ② Extract soil water through a soil water collector; ③ Collect inflow water samples from the last rainfall event as soil water in this rainfall event; ④ Collect water samples from the late outflow period of the last rainfall event as soil water in this rainfall event; ⑤ Collect water samples at the beginning of outflow in this rainfall event as soil water in this rainfall event.

3. A method for evaluating the water holding capacity of a bioretention facility as claimed in claim 2, characterized in that: The late outflow period refers to the last 10 minutes of outflow; and the beginning of outflow refers to 0-5 minutes after outflow.

4. A method for evaluating the water holding capacity of a bioretention facility according to claim 1, characterized in that: The method for collecting outflow water samples at different time periods comprises: After the bioretention facility has a stable outflow, collect the first outflow water sample; Then, the outflow water samples were continuously collected according to the set time intervals, and sampling was stopped when the outflow flow rate was less than 1 mL / min.

5. A method for evaluating the water holding capacity of a bioretention facility as claimed in claim 4, characterized in that: The continuously collecting the outflow water sample according to the set time interval comprises: During the continuous collection period, data were collected every 2 minutes for the first 15 minutes, and every 4 minutes from 15 to 51 minutes. The subsequent sampling time intervals were adjusted according to the outflow flow rate.

6. A method for evaluating the water holding capacity of a bioretention facility according to any one of claims 1 to 5, characterized in that: The composition ratio of soil moisture in outflow water samples at different time periods and inflow moisture of this rainfall is calculated according to the detection and analysis results, including For the outflow water samples of each period, according to the test and analysis results, the two-end member mixture model is used to calculate the composition ratio of soil moisture in the outflow water samples of this period and the inflow moisture of this rainfall; The two-end member mixture model formula is expressed as: Among them, Q is the outflow water flow corresponding to the outflow water sample; V is the volume of the outflow water sample, t is the collection time of the water sample; Q e is the inflow water flow corresponding to the inflow water sample of this rainfall, F is the catchment area, is the runoff coefficient, h is the rainfall depth; δ Q is the stable isotope deviation of the outflow water sample in parts per thousand; δ e is the stable isotope deviation of the inflow water sample of this rainfall; δ pe is the stable isotope deviation of the inflow water other than this rainfall, i.e. soil water sample; f e is the contribution rate of this rainfall, that is, the proportion of the inflow water of this rainfall in the outflow water; Non-current precipitation, i.e. the proportion of soil water in outflow water = 1-f e .

7. A method for evaluating the water holding capacity of a bioretention facility according to claim 6, characterized in that: The evaluation of the water holding capacity of the bioretention facility based on the soil water outflow process specifically includes: Calculation of the total soil water content V in the bioretention facility based on the soil water outflow process in ; According to the soil moisture outflow process, calculate the time T when the soil moisture content in the outflow is higher than 80% 80 and the time T that accounts for less than 20% 20 , and then calculate the time ΔT required for the soil moisture content in the flow to drop from 80% to 20%: ΔT=T 20 -T 80 Total soil water content V in The larger the value is, the stronger the water storage capacity of the bioretention facility is. The time when the proportion of soil moisture in the outflow is higher than 80% is T 80 and the time T that accounts for less than 20% 20 The longer the time ΔT required for the soil moisture content in the outflow to drop from 80% to 20%, the stronger the water holding capacity of the bioretention column.

Images