Small watershed flood risk early warning method

Abstract

The invention discloses a small watershed flood risk early warning method, which comprises the steps of constructing a digital watershed bottom plate based on watershed DEM data, and obtaining a flow direction grid, a flow grid, a river reach watershed range and a river reach data set; trying and determining a rainstorm formula of the area where the drainage basin is located; performing 1-5km space rainfall grid interpolation based on the integrated rainfall station to obtain a small watershed unit, a river reach watershed range and peripheral rainfall grids; calculating the surface average rainfall in different time periods; calculating an early-stage rainfall capacity and an early-stage rainfall index based on the grid mode; extracting an early rainfall index of the river reach basin according to the small basin unit and the river reach basin range; calculating rainfall recurrence periods of the small watershed units and the river reach; flood risk early warning is carried out on the small watershed unit; carrying out risk early warning on the river flood; flood risk early warning is carried out on the flood control object; the problem that large-scale application and popularization are not facilitated due to lack of hydrological data of most small watersheds is solved.

Description

Technical Field

[0001] The invention belongs to the technical field of flood forecasting, and in particular relates to a small watershed flood risk early warning method. Background Art

[0002] Generally speaking, flood risk warning for small watersheds requires quantitative determination of flood peak flow or water level at the watershed section through hydrological forecasting models before risk warning. However, due to the lack of hydrological data in most small watersheds, higher requirements are put forward for the quantification of flood risks and calibration of hydrological forecasting models, which is not conducive to large-scale application and promotion. It is necessary to propose some new ideas and methods to simplify the flood risk analysis of small watersheds, so as to facilitate large-scale application and promotion. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a small watershed flood risk warning method to solve the technical problems that the prior art has a relatively lack of hydrological data in most small watersheds, which puts forward higher requirements on the quantification of flood risks and the calibration of hydrological forecast models, and is not conducive to large-scale application and promotion.

[0004] The technical solution of the present invention is:

[0005] A small watershed flood risk early warning method, the method comprising:

[0006] Step 1: construct a digital watershed base plate based on the watershed DEM data to obtain the flow direction grid, flow rate grid, river section watershed range and river section dataset;

[0007] Step 2: Based on historical rainstorm data, calculate and determine the rainstorm formula for the area where the basin is located;

[0008] Step 3: Based on the integrated rainfall stations, 1-5 km spatial rainfall grid interpolation is performed to obtain the small watershed unit and river section watershed range and surrounding rainfall grids;

[0009] Step 4: Calculate the average rainfall in different periods;

[0010] Step 5, calculating the previous rainfall amount and previous rainfall index based on the grid mode;

[0011] Step 6, extracting the previous rainfall index of the river section basin according to the small watershed unit and the river section basin range;

[0012] Step 7, calculate the rainfall return period of small watershed units and river sections;

[0013] Step 8: Conduct flood risk warning for small watershed units;

[0014] Step 9: Conduct risk warning for river floods;

[0015] Step 10: Issue flood risk warnings to flood control targets.

[0016] The methods for constructing a digital basin floor based on basin DEM data include:

[0017] Step 1.1, obtain 30-meter DEM data of the watershed, perform depression filling, flow direction analysis and flow analysis to form terrain depression filling, flow direction and flow raster results, and perform water system correction on DEM according to the actual water system line before DEM depression filling;

[0018] Step 1.2: Based on the watershed analysis, construct the river network topology of the small watershed. When extracting the water system, identify the water system grid based on the catchment area of ​​5 square kilometers. Then, divide the river sections based on the water system topology and take the confluence as the node, and calculate the scope and area of ​​the river section basin. When there is a reservoir project on the river section, split the river section where the reservoir project is located to form two new river sections. Extract small watershed units based on 5-15 square kilometers or take the small watershed range as the small watershed unit, and establish the association between the small watershed unit and the river section.

[0019] Step 1.3, extract the scope of each river section basin, establish the relationship between each river section and its corresponding basin, and establish the relationship of "riverside flood control object-river section-river section basin".

[0020] Methods for calculating and determining the rainstorm formula for the area where the basin is located include:

[0021] The basin rainstorm formula is established using historical rainstorm data to meet the calculation needs of real-time rainstorm recurrence periods of different durations. The formula is:

[0022]

[0023] Y pt is the rainstorm value with a period of t and a return period of p. a1, a2, b1 and b2 are parameter values, which are obtained by calculating the design rainstorm value with a typical period of length and a typical return period.

[0024] The method for generating rainfall grids for small watershed units, river section watershed ranges and surrounding areas includes: interpolating 1-5km rainfall grids based on integrated rain gauges, fusing them with meteorological rainfall grid products, and performing regional and surrounding rainfall grid interpolation calculations based on small watershed units and river section watershed ranges to obtain rainfall grids for small watershed units, river section watershed ranges and surrounding areas.

[0025] The method for calculating the average surface rainfall in different time periods includes: extracting the average surface rainfall of small watershed units and river section basins according to the time period length of 1h, 2h, 3h, 4h, 6h, 9h, 12h, 18h and 24h, and obtaining a set of cumulative rainfall values ​​of 1 hour, 2 hours, 3 hours, 4 hours, 6 hours, 9 hours, 12 hours, 18 hours and 24 hours for each small watershed unit and river section basin.

[0026] 11. A small watershed flood risk early warning method according to claim 1, characterized in that: the method for calculating the previous rainfall and the previous rainfall index based on the grid mode comprises:

[0027] Step 5.1, construct the Pa grid set of the region and calculate the hourly antecedent rainfall of the region;

[0028] P a，t =kP t-1 +k 2 P t-2 +……+k n P t-n

[0029] P a,t represents the previous rainfall index during period t;

[0030] P t-1 represents the rainfall in period t-1;

[0031] n is the number of previous rainfall periods that affect this runoff;

[0032] k is the hour reduction factor;

[0033] Calculate pa using the formula:

[0034] P a,t+1 = k(P t +P a，t )

[0035] Using the above formula, we only need to create a Pa field in the spatial grid database table and calculate P every hour based on the precipitation in the previous period. a Value storage;

[0036] Step 5.2: Based on the previous rainfall P a Basin relative rainfall index P xa calculate;

[0037] Hourly rolling calculation P a After the value is obtained, retrieve the series P of this period a The maximum value P in a,t,max , when P a,t,max Greater than the maximum previous rainfall in the basin P a,max When P a,t,max Replace P a,max ; Then calculate the relative rainfall index P of each grid in the basin every hour xa The formula is as follows:

[0038] P xa =P a / P a,max

[0039] When P xa When ≥0.8, the basin is considered to be wet and basically saturated in the early stage;

[0040] When 0.5≤P xa When <0.8, the basin is considered to be relatively moist in the early stage;

[0041] When 0.2≤P xa When <0.5, the basin is considered to be relatively dry in the early stage;

[0042] When P xa When <0.2, the basin is considered to be dry in the early stage;

[0043] Step 5.3, P a and P a,max Reset; Reset P a By default, the calculation is based on the last 15 days. a,max In P a Manual adjustment is performed after reset, but the value cannot be less than the reset P a The maximum value calculated during the process.

[0044] The method for calculating the rainfall return period of small watershed units and river sections includes: comprehensively converting the rainfall of small watershed units and river sections based on the previous rainfall, then converting the comprehensive rainfall of small watershed units and river sections into point rainfall using the point-to-surface conversion relationship, and finally calculating the rainfall return period using the rainstorm formula; specifically including:

[0045] Step 7.1, according to the previous rainfall index of the small watershed unit and the river section basin, convert the average surface rainfall of the small watershed unit and the river section basin in different periods into the comprehensive surface rainfall;

[0046] When P xa When ≥0.8, the basin surface precipitation before determining the rainstorm frequency is *1.2;

[0047] When 0.5≤P xa <0.8, basin surface precipitation*1.1;

[0048] When 0.2≤P xa <0.5, basin surface precipitation*1.0;

[0049] When P xa <0.2, basin surface precipitation*0.9;

[0050] Step 7.2, convert the comprehensive surface rainfall of the small watershed unit and the river section basin into point rainfall, and perform rainfall point-to-surface conversion calculation according to the location of the watershed and the rainfall accumulation period through the point-to-surface conversion interpolation table in the rain-flood method;

[0051] Step 7.3: Use the rainstorm formula corresponding to the watershed to calculate the rainstorm recurrence period based on the point rainfall in the watershed. When using the rainstorm formula, determine the rainstorm formula corresponding to different rainfall period lengths based on the actual situation of the rainstorm formula set.

[0052] Methods for early warning of flood risks for small watershed units include:

[0053] According to the calculated rainstorm recurrence period in different time periods, flood risk warning is carried out for small watershed units according to the principle of the same frequency of rainstorm and flood; the flood classification warning indicators for small watershed units include:

[0054] When P≧50-year return period, a major flood may occur, and a red alert corresponds to level 1;

[0055] When 20≤P<50 years, a major flood may occur, and an orange warning corresponds to level 2;

[0056] When 5≤P<20-year return period, moderate flood may occur, yellow warning, corresponding to level 3;

[0057] When 2≤P<5-year return period, a minor flood may occur, and a blue warning corresponds to level 4;

[0058] When P < 2 years, there is no warning, corresponding to level 0;

[0059] P represents the recurrence period of heavy rain in different time periods. When the warning value is reached in more than one time period, the highest warning level will be used for warning.

[0060] Methods for early warning of river flood risks include:

[0061] According to the calculated rainstorm recurrence period in different periods of the river section basin, and in accordance with the principle of the same frequency of rainstorm and flood, flood risk warning is carried out for the corresponding river section. The flood classification warning indicators of the river section include:

[0062] When P≧50-year return period, a major flood may occur, and a red alert corresponds to level 1;

[0063] When 20≤P<50 years, a major flood may occur, and an orange warning corresponds to level 2;

[0064] When 5≤P<20-year return period, moderate flood may occur, yellow warning, corresponding to level 3;

[0065] When 2≤P<5-year return period, a minor flood may occur, and a blue warning corresponds to level 4;

[0066] When P < 2 years, there is no warning, corresponding to level 0;

[0067] P represents the recurrence period of heavy rain in different time periods. When the warning value is reached in more than one time period, the highest warning level will be used for warning.

[0068] The flood risk warning methods for flood control objects include: issuing flood warnings for flood control objects based on the rainstorm recurrence period set of river sections and the flood control capabilities of villages along the river and important protection objects; issuing a red warning when the rainstorm recurrence period P exceeds the flood control capabilities of villages along the river and important protection objects; and issuing corresponding flood risk warnings when a flood warning is issued in the river channel.

[0069] The beneficial effects of the present invention are:

[0070] The present invention is mainly based on the idea of ​​generalizing the same frequency of rainfall and flood in small watersheds. It uses the basin rainstorm formula combined with the previous rainfall index to calculate the cumulative rainfall return period in different periods, and divides the warning indicators based on the flood level return period to achieve small watershed flood risk warning. The basin rainstorm formula proposed by the present invention helps to unify the reasoning process of "rainstorm-return period" of the basin, and it is easier to describe the impact of the previous rainfall in the basin through the normalized previous rainfall index.

[0071] The present invention extracts watersheds and rivers based on DEM terrain data. Establish a small watershed (catchment area 300km 2 The following) and river segment extraction methods (such as taking 5 square kilometers of catchment area as the starting point, for every increase of 5 square kilometers or 5 kilometers of river length or 10% increase in area, or a tributary confluence, a reservoir project, extract a basin boundary and a river segment), establish the relationship between each river segment and the corresponding basin, and extract the scope of each corresponding basin. The extraction of basins and rivers can be based on the small basins divided by the mountain torrent disaster investigation and evaluation project. Collect the flood control capacity of each riverside protection object, and establish the corresponding relationship between the riverside protection object and the river section and basin.

[0072] The present invention is based on the real-time precipitation monitoring data of rainfall stations in and around the basin, and uses spatial interpolation and other methods to associate it with the basin surface in a grid manner. The grid corresponding to each extracted basin is determined, and the average precipitation of the corresponding basin is generalized by using the arithmetic average method of the precipitation of each grid in the basin.

[0073] The time scale for calculation of average precipitation in a watershed is divided into cumulative calculations of 1 hour, 2 hours, 3 hours, 4 hours, 6 hours, 9 hours, 12 hours, 18 hours, and 24 hours according to the size of the watershed area.

[0074] The present invention calculates the previous rainfall of each small watershed every hour, and analyzes the change level of the previous rainfall of the watershed in the historical series. By comparing the previous rainfall with the historical maximum previous rainfall, the dynamic previous rainfall index of the small watershed is analyzed and calculated.

[0075] The present invention adopts historical rainstorm data to establish a basin rainstorm formula to meet the needs of rapid calculation of real-time rainstorm recurrence periods of different durations.

[0076] The present invention determines the long-term comprehensive real-time surface precipitation in different periods of the basin based on the previous precipitation index and real-time precipitation, and converts it into point rainfall using the point-to-surface conversion method. Based on the point rainfall converted from the surface rainfall and the rainstorm formula, the recurrence period p corresponding to the surface rainfall water volume is calculated. Finally, the rainfall-flood frequency method is used to determine the flood risk warning level of the small basin.

[0077] The present invention provides flood warning for the river section corresponding to the outlet of the basin according to the calculated rainstorm recurrence period in different time periods and the principle of the same frequency of rainstorm and flood.

[0078] The present invention superimposes a base map to dynamically and real-time display small watersheds, river sections, and flood control objects along the river with flood risks, and outputs warning content through reports and charts.

[0079] The present invention provides flood warnings for villages along the river and important flood prevention objects according to the flood prevention capabilities of the villages along the river and important flood prevention objects (how many years of floods they can prevent) and the flood frequency of the river section.

[0080] The method of quantitatively determining the relationship between rainfall characteristics and flood risk levels in small watersheds of the present invention simplifies and unifies the "rainfall-return period" reasoning process through the rainstorm formula; and unifies the factors affecting the degree of previous rainfall through the normalized previous rainfall index.

[0081] It solves the technical problems that the lack of hydrological data in most small watersheds puts higher requirements on the quantification of flood risks and the calibration of hydrological forecast models, which is not conducive to large-scale application and promotion. DETAILED DESCRIPTION

[0082] Generally, in small and medium-sized watersheds, rain and flood have the same frequency, but the previous precipitation will affect the incoming water. Therefore, the present invention considers the rain and flood frequency affected by the previous precipitation and proposes a small watershed flood warning method.

[0083] The invention is designed for 300km 2 The following small watersheds are generally divided into two parts: "pre-processing" and "flood risk analysis". Risk analysis calculation is based on pre-processing, and the basin flood risk is determined by real-time analysis of the basin rainfall recurrence period, so as to achieve flood risk warning for the basin, river section and flood control objects along the river.

[0084] Specifically include:

[0085] Step 1: Digital watershed analysis: Based on the watershed DEM, analyze the watershed range and river section composition, as well as the rainstorm zone and administrative divisions where the watershed is located. If necessary, modify the DEM according to the actual water system line.

[0086] This step mainly builds the digital basin base plate based on DEM data. Contents include:

[0087] (1) Basin analysis. Obtain 30-meter DEM data of the basin, perform depression filling, flow direction analysis, and flow analysis, and generate terrain depression filling, flow direction, and flow raster results. If necessary, the DEM can be corrected for water system based on the actual water system before DEM depression filling.

[0088] (2) Generate river network. Construct river network topology of small watersheds based on watershed analysis. When extracting water systems, consider using 5 square kilometers of catchment area as the benchmark for water system grid identification. Then, based on the water system topology, divide the river sections with the confluence as the node, and calculate the range and area of ​​the river section basin. In addition, when there is a reservoir project on the river section, consider splitting the river section where the reservoir project is located to form two new river sections; extract small watershed units based on 5-15 square kilometers or use the small watershed range in the results of mountain torrent investigation and evaluation as small watershed units, and establish the relationship between small watershed units and river sections.

[0089] (3) River segment extraction and configuration. Extract the scope of each river segment’s watershed and establish the relationship between each river segment and its corresponding watershed; collect data on the flood control capabilities of each riverside protection object, such as mountain torrent disaster investigation and evaluation, and establish the relationship between “riverside flood control object – river segment – ​​river segment watershed”.

[0090] The main inputs of this step include: 30-meter DEM, small watershed unit layer, flood control object layer, county-level administrative area layer and regional actual water system line layer.

[0091] Among them, the small watershed unit layer is used to establish topological association with river sections and flood control objects; the flood control object layer indicates the geographical location and flood control capacity (flood control standards) of the flood control objects, and can be associated and bound with river sections; the administrative area layer is used to establish associations with small watershed units, flood control objects and river sections; the actual regional water system line is an actual water system line layer manually marked based on surveying and mapping or satellite images, which is mainly used for water system correction of DEM.

[0092] The main outputs of this step include: flow direction grid, flow rate grid, river section watershed range and river section dataset. The river section watershed range is the watershed range above the river section outlet control section; the river section dataset includes the river section basic information, river section range identification and river section flood control object identification information set.

[0093] Step 2: Calibration of rainstorm formula: Based on historical rainstorm data, calculate and determine the rainstorm formula for the basin area. Existing results can be used, or the rain-flood method can be used for trial calculation and calibration. Early warning small watershed units and river sections support setting of one or more statistical periods of long-gradient rainstorm formulas.

[0094] This step uses historical rainstorm data to establish a basin rainstorm formula to meet the needs of calculating the return period of real-time rainstorms of different durations. The formula is:

[0095]

[0096] Among them, Y pt is the rainstorm value with a period of t and a return period of p. a1, a2, b1 and b2 are parameter values, which can be obtained by calculating the design rainstorm value with a typical period of length and a typical return period.

[0097] For a given watershed, the rainstorm formula can directly adopt the existing calibration results. Similarly, the rainstorm formula of the watershed can be calibrated by predetermining the corresponding relationship between the typical period, the typical recurrence period and the design rainstorm value through the rain-flood method, and then calculating and deriving the four parameters a1, a2, b1, and b2.

[0098] In order to ensure the adaptability of the rainstorm formula, the river section rainstorm formula can be set according to the characteristics of the basin rainstorm, with four levels of 10min-1 hour, 1 hour-6 hours, 6 hours-12 hours, and 12 hours-24 hours, or it can be set regardless of the time period.

[0099] The input of this step mainly includes: rainfall and flood calculation data and river section watershed range set. The rainfall and flood calculation data refers to the mean and CV value grids of heavy rain in each period; the river section watershed range set refers to the set of watershed ranges above the river section outlet control section, through which the mean and CV value of heavy rain in the corresponding period of the river section can be extracted.

[0100] The main outputs of this step are: a set of rainstorm formulas for small watershed units and a set of rainstorm formulas for river sections. The parameter information of the rainstorm formulas for all small watershed units and river sections is stored.

[0101] Step 3: Rainfall grid generation: 1-5 km spatial rainfall grid interpolation is performed based on the integrated rainfall stations. Fusion with meteorological rainfall grid products can be considered to improve the accuracy of spatial rainfall data.

[0102] This step interpolates the 1-5km rainfall grid based on the integrated rainfall stations. Fusion with meteorological rainfall grid products. Regional and surrounding rainfall grid interpolation calculations are performed based on small watershed units and river section watershed ranges. In order to optimize the use of computing power, consider designing a shared rainfall generation scheme for adjacent areas.

[0103] The input of this step mainly includes: the location of rain gauges associated with small watershed units and river section basins and real-time rainfall data.

[0104] The main outputs of this step are: small watershed units and river section watershed ranges and surrounding rainfall rasters.

[0105] Step 4. Calculate the average rainfall in different time periods: extract the average rainfall in small watershed units and river section basins according to the time period length of 1h, 2h, 3h, 4h, 6h, 9h, 12h, 18h, and 24h.

[0106] The source of rainfall data in this step is essentially based on the real-time precipitation monitoring data of rainfall stations in and around the small watershed units and river sections, and is interpolated to the watershed surface in a grid manner using methods such as spatial interpolation. One or more grid indexes corresponding to the small watershed units and river section watershed ranges should be determined in advance, and the average precipitation within the corresponding watershed range should be generalized using the arithmetic average method of precipitation of each grid within the watershed range.

[0107] Rainfall data can also use the mature rainfall raster data products of the national meteorological department, or can use local station data with meteorological rainfall raster data products and radar echoes to generate rainfall data grids according to its own data conditions.

[0108] The time scale of average precipitation in the basin is divided into 1 hour, 2 hours, 3 hours, 4 hours, 6 hours, 9 hours, 12 hours, 18 hours and 24 hours according to the size of the basin. When calculating the return period of rainfall in the subsequent river section basin, the return period of the cumulative rainfall corresponding to all the above time periods should be calculated.

[0109] The input of this step mainly includes: 24-hour rainfall grid sequence, small watershed unit watershed range set and river section watershed range set. Among them, the 24-hour rainfall grid sequence must be the rainfall grid data of 24 consecutive hours; the small watershed unit watershed range set and the river section watershed range set can correspond to the rainfall grid number and area weight corresponding to each river section watershed in advance, but the premise is that the rainfall grid data framework must be fixed.

[0110] The output of this step is mainly: the average rainfall set of different time periods for small watershed units and river sections. Each small watershed unit and river section contains a set of cumulative rainfall values ​​for 1 hour, 2 hours, 3 hours, 4 hours, 6 hours, 9 hours, 12 hours, 18 hours and 24 hours.

[0111] Step 5: Calculation of previous rainfall in the watershed: Calculate previous rainfall and previous rainfall index based on the grid mode. Use hourly iterative calculation method to calculate previous rainfall.

[0112] This step calculates the previous rainfall index on an hourly basis for small watershed units, river section basins and surrounding areas, and analyzes the regional previous rainfall and previous rainfall index.

[0113] (1) Construct the Pa grid set of the region and calculate the hourly antecedent rainfall of the region.

[0114] P a，t =kPt-1 +k 2 P t-2 +…+k n P t-n

[0115] P a,t — represents the previous rainfall index for period t

[0116] P t-1 — represents the rainfall in period t-1

[0117] n—the number of previous rainfall periods that affect this runoff, usually 15 days and 15*24 hours.

[0118] k—hourly reduction coefficient. For example, the daily reduction coefficient in a certain area is about 0.85, which is converted into an hourly reduction coefficient k=0.85^(1 / 24), that is, k=0.993251254.

[0119] For ease of calculation, the formula is used to calculate pa:

[0120] P a，t+1 = k(P t +P a，t )

[0121] Using the above formula, we only need to create a Pa field in the spatial grid database table and calculate P every hour based on the precipitation in the previous period. a Value storage.

[0122] (2) Based on the previous rainfall P a Basin relative rainfall index P xa calculate

[0123] Hourly rolling calculation P a After the value is obtained, retrieve the series P of this period a The maximum value P in a,t,max , when P a,t,max Greater than the maximum previous rainfall in the basin P a,max When P a,t,max Replace P a,max Then calculate the relative rainfall index P of each grid in the basin every hour xa The formula is as follows:

[0124] P xa =P a / P a,max

[0125] When P xa When ≥0.8, the basin is considered to be wet in the early stage and basically saturated.

[0126] When 0.5≤P xa When <0.8, the basin is considered to be relatively moist in the early stage.

[0127] When 0.2≤P xa When <0.5, the basin is considered to be relatively dry in the early stage.

[0128] When P xa When <0.2, the basin is considered to be dry in the early stage.

[0129] (3)P a and P a,max Reset.

[0130] After a period of time, the model may cause calculation deviations due to parameter and data quality reasons. It is necessary to provide corresponding operations to reset P a and P a,max Reset. Reset P a By default, the calculation is based on the last 15 days, but manual adjustment of the forward calculation days is also supported. a,max Can be in P a Manual adjustment is performed after reset, but the value cannot be less than the reset P a The maximum value calculated during the process.

[0131] The inputs of this step mainly include: hourly rainfall grid, previous hour antecedent rainfall grid and historical maximum antecedent rainfall.

[0132] The output of this step mainly includes: basin previous rainfall raster, maximum previous rainfall, and basin previous rainfall index raster.

[0133] Step 6: Extract the previous rainfall index of the river section: Extract the previous rainfall index of the river section basin based on the small watershed unit and the river section basin range.

[0134] This step is based on the calculation results of previous rainfall, and extracts the corresponding previous rainfall index according to the small watershed units and river section basin range, which is used for the subsequent conversion of the comprehensive average rainfall in each period of the basin.

[0135] The input of this step mainly includes: basin previous rainfall index grid, small basin unit basin range set and river section basin range set. In the small basin unit basin range set and river section basin range set, the associated index of the previous rainfall index grid unit needs to be generated to facilitate the rapid extraction and calculation of the average previous rainfall index.

[0136] The output of this step is: a collection of previous rainfall indices for small watershed units and river sections.

[0137] Step 7. Calculate the rainfall return period of small watershed units and river sections: Based on the previous rainfall, perform a comprehensive conversion of the surface rainfall of small watershed units and river sections, and then use the point-to-surface conversion relationship to convert the comprehensive surface rainfall of small watershed units and river sections into point rainfall. Finally, use the heavy rain formula to calculate the rainfall return period.

[0138] This step includes:

[0139] (1) According to the previous rainfall index of small watershed units and river sections, the average surface rainfall of small watershed units and river sections in different periods is converted into comprehensive surface rainfall.

[0140] The calculation of comprehensive surface rainfall considering the impact of previous rainfall is mainly divided into the following situations:

[0141] Considering the previous precipitation, when P xa When ≥0.8, the basin is considered to be moist and basically saturated in the early stage. Before determining the frequency of heavy rain, the basin surface precipitation is *1.2.

[0142] When 0.5≤P xa When <0.8, it is considered that the basin is relatively moist in the early stage, and the basin surface precipitation is *1.1 before determining the frequency of heavy rain.

[0143] When 0.2≤P xa When <0.5, it is considered that the basin was relatively dry in the early stage, and the basin surface precipitation was *1.0 before determining the frequency of heavy rain.

[0144] When P xa When <0.2, the basin is considered to be dry in the early stage, and the basin surface precipitation is * 0.9 before determining the frequency of heavy rain.

[0145] (2) According to the point-to-surface conversion coefficient, the comprehensive surface rainfall of the small watershed unit and river section basin is converted into point rainfall. Usually, the point-to-surface conversion calculation table in the rain-flood method can be used to perform rainfall point-to-surface conversion calculations based on the location of the watershed and the rainfall accumulation period.

[0146] (3) Finally, use the rainstorm formula corresponding to the basin to calculate the rainstorm recurrence period based on the point rainfall in the basin. When using the rainstorm formula, the rainstorm formula corresponding to different rainfall periods should be determined based on the actual situation of the rainstorm formula set.

[0147] The inputs of this step mainly include: a set of heavy rain formulas for small watershed units, a set of heavy rain formulas for river section basins, a set of previous rainfall indexes for small watershed units, a set of previous rainfall indexes for river section basins, rainfall point-to-surface conversion coefficients, a set of average surface rainfall for small watershed units in different time periods, and a set of average surface rainfall for river section basins in different time periods.

[0148] The output of this step is: the set of rainstorm return periods of small watershed units and the set of rainstorm return periods of river section basins.

[0149] Step 8. Flood risk warning for small watershed units: Analyze the flood risk of small watershed units based on the frequency of rainfall and flood.

[0150] This step is to conduct flood risk warning for small watershed units based on the calculated rainstorm recurrence period in different time periods and the principle of the same frequency of rainstorms and floods. The flood classification warning indicators for small watershed units are as follows:

[0151] When P≧50-year return period, a severe flood may occur, corresponding to a red alert, level 1.

[0152] When 20≤P<50 years, a major flood may occur, with an orange warning corresponding to level 2.

[0153] When 5≤P<20 years, moderate flood may occur, yellow warning, corresponding to level 3.

[0154] When 2≤P<5 years, minor floods may occur, and a blue warning, corresponding to level 4, is issued.

[0155] When P<2 years, there is no warning, corresponding to level 0.

[0156] P represents the recurrence period of heavy rain in different time periods.

[0157] The warning values ​​were reached in multiple time periods, and warnings were issued at the highest warning level.

[0158] The input of this step mainly includes: small watershed rainstorm return period set and small watershed flood classification warning indicators.

[0159] The output of this step is mainly: a set of warning levels for small watershed units.

[0160] Step 9. River flood risk warning: Analyze the flood risk of each river section based on the frequency of rainfall and flood.

[0161] This step is to issue flood risk warnings for the corresponding river sections based on the calculated rainstorm recurrence period at different time periods in the river section basin and the principle of the same frequency of rainstorms and floods. The flood classification warning indicators for river sections are as follows:

[0162] When P≧50-year return period, a severe flood may occur, corresponding to a red alert, level 1.

[0163] When 20≤P<50 years, a major flood may occur, with an orange warning corresponding to level 2.

[0164] When 5≤P<20 years, moderate flood may occur, yellow warning, corresponding to level 3.

[0165] When 2≤P<5 years, minor floods may occur, and a blue warning, corresponding to level 4, is issued.

[0166] When P<2 years, there is no warning, corresponding to level 0.

[0167] P represents the recurrence period of heavy rain in different time periods.

[0168] The warning values ​​were reached in multiple time periods, and warnings were issued at the highest warning level.

[0169] The input of this step mainly includes: the rainstorm recurrence period set of river section basin and the flood classification warning index of small basin.

[0170] The output of this step is mainly: river section warning level set.

[0171] Step 10. Flood risk warning for flood control objects: Analyze the flood risk of flood control objects based on the frequency of rainfall and flood.

[0172] This step is to issue a flood warning for flood control objects based on the rainstorm recurrence period set of the river section and the flood control capacity of the villages along the river and important protection objects (how many years of flood protection they can defend). When the rainstorm recurrence period P exceeds the flood control capacity of the villages along the river and important protection objects, a red warning is issued. When other flood warnings are issued in the river, corresponding flood risk warnings are issued.

[0173] The input of this step mainly includes: a river section warning level set and a river section flood control object information set (including flood control capacity information).

[0174] The output of this step is mainly: the flood risk level of the flood control object.

[0175] Step 11: Warning display, including:

[0176] (1) Based on GIS, the distribution of small watershed units is generally displayed. The basic layers include administrative regions, water systems, rainfall stations, hydrological stations (water level stations), etc.

[0177] (2) Use the GIS base map as the main application framework to load and display relevant functional modules.

[0178] (3) Based on user favorites, the flood risk information of the small watershed of concern is displayed.

[0179] (4) Real-time display of early warning statistics for small watersheds, flood control objects and river sections.

[0180] (5) The rainfall grid and radar echo map can be displayed in time sequence.

[0181] (6) Display rainfall data integration and model operation status tracking information.

Claims

1. A small watershed flood risk early warning method, characterized by: The method comprises: Step 1: construct a digital watershed base plate based on the watershed DEM data to obtain the flow direction grid, flow rate grid, river section watershed range and river section dataset; Step 2: Based on historical rainstorm data, calculate and determine the rainstorm formula for the area where the basin is located; Step 3: Based on the integrated rainfall stations, 1-5 km spatial rainfall grid interpolation is performed to obtain the small watershed unit and river section watershed range and surrounding rainfall grids; Step 4: Calculate the average rainfall in different time periods; Step 5, calculating the previous rainfall amount and previous rainfall index based on the grid mode; Step 6, extracting the previous rainfall index of the river section basin according to the small watershed unit and the river section basin range; Step 7, calculate the rainfall return period of small watershed units and river sections; Step 8: Conduct flood risk warning for small watershed units; Step 9: Conduct risk warning for river floods; Step 10: Issue flood risk warnings to flood control targets.

2. A small watershed flood risk early warning method according to claim 1, characterized in that: The methods for constructing a digital basin floor based on basin DEM data include: Step 1.1, obtain 30-meter DEM data of the watershed, perform depression filling, flow direction analysis and flow analysis to form terrain depression filling, flow direction and flow raster results, and perform water system correction on DEM according to the actual water system line before DEM depression filling; Step 1.2: Based on the watershed analysis, construct the river network topology of the small watershed. When extracting the water system, identify the water system grid based on the catchment area of ​​5 square kilometers. Then, divide the river sections based on the water system topology and take the confluence as the node, and calculate the scope and area of ​​the river section basin. When there is a reservoir project on the river section, split the river section where the reservoir project is located to form two new river sections. Extract small watershed units based on 5-15 square kilometers or take the small watershed range as the small watershed unit, and establish the association between the small watershed unit and the river section. Step 1.3, extract the scope of each river section basin, establish the relationship between each river section and its corresponding basin, and establish the "riverside flood control object-river section-river section basin" relationship.

3. A small watershed flood risk early warning method according to claim 1, characterized in that: Methods for calculating and determining the rainstorm formula for the area where the basin is located include: The basin rainstorm formula is established using historical rainstorm data to meet the calculation needs of real-time rainstorm recurrence periods of different durations. The formula is: Y pt is the rainstorm value with a period of t and a return period of p. a1, a2, b1 and b2 are parameter values, which are obtained by calculating the design rainstorm value with a typical period of length and a typical return period.

4. A small watershed flood risk early warning method according to claim 1, characterized in that: The method for generating rainfall grids for small watershed units, river section watershed ranges and surrounding areas includes: interpolating 1-5km rainfall grids based on integrated rain gauges, fusing them with meteorological rainfall grid products, and performing regional and surrounding rainfall grid interpolation calculations based on small watershed units and river section watershed ranges to obtain rainfall grids for small watershed units, river section watershed ranges and surrounding areas.

5. A small watershed flood risk early warning method according to claim 1, characterized in that: The method for calculating the average surface rainfall in different time periods includes: extracting the average surface rainfall of small watershed units and river section basins according to the time period length of 1h, 2h, 3h, 4h, 6h, 9h, 12h, 18h and 24h, and obtaining a set of cumulative rainfall values ​​of 1 hour, 2 hours, 3 hours, 4 hours, 6 hours, 9 hours, 12 hours, 18 hours and 24 hours for each small watershed unit and river section basin.

6. A small watershed flood risk early warning method according to claim 1, characterized in that: The methods for calculating the previous rainfall amount and previous rainfall index based on the grid model include: Step 5.1, construct the Pa grid set of the region and calculate the hourly antecedent rainfall of the region; P a，t =kP t-1 +k 2 P t-2 +…+k n P t-n P a，t represents the previous rainfall index during period t; P t-1 represents the rainfall in period t-1; n is the number of previous rainfall periods that affect this runoff; k is the hour reduction factor; Calculate pa using the formula: P a，t+1 =k(P t +P a，t ) Using the above formula, we only need to create a Pa field in the spatial grid database table and calculate P every hour based on the precipitation in the previous period. a Value storage; Step 5.2: Based on the previous rainfall P a Basin relative rainfall index P xa calculate; Hourly rolling calculation P a After the value is obtained, retrieve the series P of this period a The maximum value P in a，t，max , when P a，t，max Greater than the maximum previous rainfall in the basin P a，max When P a，t，max Replace P a，max ; Then calculate the relative rainfall index P of each grid in the basin every hour xa The formula is as follows: P xa =P a / P a，max When P xa When ≥0.8, the basin is considered to be wet and basically saturated in the early stage; When 0.5≤P xa When <0.8, the basin is considered to be relatively wet in the early stage; When 0.2≤P xa When <0.5, the basin is considered to be relatively dry in the early stage; When P xa When <0.2, the basin is considered to be dry in the early stage; Step 5.3, P a and P a，max Reset; Reset P a By default, the calculation is based on the last 15 days. a，max In P a Manual adjustment is performed after reset, but the value cannot be less than the reset P a The maximum value calculated during the process.

7. A small watershed flood risk early warning method according to claim 6, characterized in that: The method for calculating the rainfall return period of small watershed units and river sections includes: comprehensively converting the rainfall of small watershed units and river sections based on the previous rainfall, then converting the comprehensive rainfall of small watershed units and river sections into point rainfall using the point-to-surface conversion relationship, and finally calculating the rainfall return period using the rainstorm formula; specifically including: Step 7.1, according to the previous rainfall index of the small watershed unit and the river section basin, convert the average surface rainfall of the small watershed unit and the river section basin in different periods into the comprehensive surface rainfall; When P xa When ≥0.8, the basin surface precipitation before determining the rainstorm frequency is *1.2; When 0.5≤P xa When <0.8, the basin surface precipitation is *1.1; When 0.2≤P xa When <0.5, the basin surface precipitation is *1.0; When P xa When <0.2, the basin surface precipitation is *0.9; Step 7.2, convert the comprehensive surface rainfall of the small watershed unit and the river section basin into point rainfall, and perform rainfall point-to-surface conversion calculation according to the location of the watershed and the rainfall accumulation period through the point-to-surface conversion interpolation table in the rain-flood method; Step 7.3: Use the rainstorm formula corresponding to the watershed to calculate the rainstorm recurrence period based on the point rainfall in the watershed. When using the rainstorm formula, determine the rainstorm formula corresponding to different rainfall period lengths based on the actual situation of the rainstorm formula set.

8. A small watershed flood risk early warning method according to claim 1, characterized in that: Methods for early warning of flood risks for small watershed units include: According to the calculated rainstorm recurrence period in different time periods, flood risk warning is carried out for small watershed units according to the principle of the same frequency of rainstorm and flood; the flood classification warning indicators for small watershed units include: When P≧50-year return period, a major flood may occur, and a red alert corresponds to level 1; When 20≤P<50 years, a major flood may occur, and an orange warning corresponds to level 2; When 5≤P<20-year return period, moderate flood may occur, yellow warning, corresponding to level 3; When 2≤P<5-year return period, a minor flood may occur, and a blue warning corresponds to level 4; When P < 2 years, there is no warning, corresponding to level 0; P represents the recurrence period of heavy rain in different time periods. When the warning value is reached in more than one time period, the highest warning level will be used for warning.

9. A small watershed flood risk early warning method according to claim 1, characterized in that: Methods for early warning of river flood risks include: According to the calculated rainstorm recurrence period in different periods of the river section basin, and in accordance with the principle of the same frequency of rainstorm and flood, flood risk warning is carried out for the corresponding river section. The flood classification warning indicators of the river section include: When P≧50-year return period, a major flood may occur, and a red alert corresponds to level 1; When 20≤P<50 years, a major flood may occur, and an orange warning corresponds to level 2; When 5≤P<20-year return period, moderate flood may occur, yellow warning, corresponding to level 3; When 2≤P<5-year return period, a minor flood may occur, and a blue warning corresponds to level 4; When P < 2 years, there is no warning, corresponding to level 0; P represents the recurrence period of heavy rain in different time periods. When the warning value is reached in more than one time period, the highest warning level will be used for warning.

10. A small watershed flood risk early warning method according to claim 1, characterized in that: The flood risk warning methods for flood control objects include: issuing flood warnings for flood control objects based on the rainstorm recurrence period set of river sections and the flood control capabilities of villages along the river and important protection objects; issuing a red warning when the rainstorm recurrence period P exceeds the flood control capabilities of villages along the river and important protection objects; and issuing corresponding flood risk warnings when a flood warning is issued in the river channel.

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