Possible maximum flood calculation method suitable for areas without data

Abstract

The invention discloses a possible maximum flood calculation method suitable for a data-free area, and belongs to the field of drainage basin water circulation simulation and hydrological design. The method comprises the following steps: calculating a daily scale possible maximum rainfall result of a drainage basin of the data-free area by adopting an improved Hirschfield method; classifying and processing possible maximum rainfall calculation problems of areas without data; for small and medium-sized drainage basins with the drainage basin area smaller than or equal to 1000 km < 2 >, the 24-hour hour-by-hour possible maximum rainfall process is deduced in the mode that frequency calculation is coupled with a Chicago rainfall pattern on the basis of satellite rainfall inversion data; for a large watershed with the watershed area larger than 1000 km < 2 >, the possible maximum rainfall process of the next day scale of several days is deduced based on satellite rainfall inversion data in a same-frequency amplification mode. According to the method disclosed by the invention, a technical path with complete calculation is constructed, a more reasonable and reliable possible maximum flood calculation result of the watershed in the non-data region is obtained, and key and powerful theoretical guidance and technical support are provided for scale argumentation of a flood control system of water conservancy and hydropower engineering in the non-data region.

Description

Technical Field

[0001] The present invention belongs to the field of water cycle simulation and hydrological design of river basins, and more particularly, relates to a method for calculating the possible maximum flood in areas without data. Background Art

[0002] Calculating a basin's potential maximum flood primarily involves calculating the basin's maximum possible precipitation and the basin's runoff and confluence. Data-free areas are often defined as regions lacking measured precipitation and flow data. Calculating the potential maximum flood in data-free areas faces two major technical challenges: calculating the basin's potential maximum precipitation and calculating the basin's runoff and confluence.

[0003] Calculating the probable maximum precipitation in data-free areas is now common, using satellite precipitation inversion data and the Hirschfield method (also known as the "statistical estimation method"). This approach can yield results for the probable maximum precipitation on a 24-hour or daily scale, but lacks the technical means to derive the probable maximum precipitation process on a sub-daily scale (e.g., hourly or three-hourly).

[0004] For calculating runoff generation and confluence in data-free areas, the SCS curve number method has become an effective approach, often achieving good simulation results. However, the calculation of the CN parameter is cumbersome, making it difficult to apply in practice. Currently, inference formulas, geomorphic unit lines, or the SCS dimensionless unit line method are often used to calculate runoff generation and confluence in data-free areas. While inference formulas can yield peak results, they struggle to deduce flood processes. Furthermore, the required confluence parameter m and infiltration rate parameter μ are often determined empirically based on basin topography and geomorphic features, resulting in unsatisfactory peak calculation accuracy. For the geomorphic unit line method, the required characteristic velocity parameter at the basin outlet is crucial for its application. This parameter is influenced by basin topography and geomorphic influences, and a scientific and reasonable method for determining this parameter in data-free areas is currently lacking. Furthermore, the required basin average slope parameter for the SCS dimensionless unit line method is computationally difficult to calculate. There are currently two main calculation methods: Method 1 calculates the average value of the basin slope matrix as the basin average slope. This method ignores the problem that the ground elevation fluctuation belongs to the fractal structure, and the calculated value is greatly affected by the grid resolution, and the calculation results are often large and unstable. At present, some studies have proposed the basin kurtosis K value to reflect the average slope of the basin, but this indicator is not the average slope of the basin after all, and it is difficult to apply to the SCS dimensionless unit line. Method 2 uses the average gradient of the longest flow path in the basin as the average slope of the basin. This method assumes that the average inclination of the basin is consistent with the longest flow path. This assumption is also accidental to be established, which often results in the calculation of the average slope of the basin being too small.

[0005] Currently, there is limited research on coupling the calculation of possible maximum precipitation and runoff generation in data-deficient areas to calculate possible maximum floods in data-deficient areas. Only some research exists on methods for calculating possible maximum floods in data-deficient areas (where measured discharge data are available over a certain length). These methods use the Hirschfield method to calculate the possible maximum precipitation process on a daily scale in the basin. Based on a certain length of measured discharge data (e.g., 2–3 years), the parameters of the basin hydrological model are calibrated and the precipitation process is substituted into the calculation to derive the possible maximum daily discharge process. Based on this, the maximum daily discharge is converted to a peak through empirical judgment or a constructed relationship between daily discharge and peak flow, and the possible maximum flood is then derived. This approach fails to address the difficulty of calculating possible maximum floods in data-deficient areas without measured discharge data. Furthermore, the key technical approach for converting daily discharge to peak flow is highly empirical, which hinders the application and promotion of this method in data-deficient areas.

[0006] All of the above factors mean that calculating the possible maximum flood in areas with no data remains a key technical challenge in the industry, and it is currently difficult to implement. Summary of the Invention

[0007] The method of the present invention constructs a complete technical path for calculating the possible maximum flood in river basins in data-free areas, obtains more reasonable and reliable calculation results of the possible maximum flood in river basins in data-free areas, and provides key and powerful theoretical guidance and technical support for the scale demonstration of flood control systems of water conservancy and hydropower projects in data-free areas.

[0008] The present invention provides a method for calculating the maximum possible flood in areas without data, characterized in that the method is implemented in the following steps:

[0009] Step 1: Extract the watershed boundaries and longest flow paths based on globally available DEM data and the coordinate information of the outlets of watersheds in data-unavailable areas (hereinafter referred to as "watersheds"). Divide the watershed grids based on the spatial resolution and watershed boundaries of globally available satellite precipitation inversion data.

[0010] Step 2: Based on the satellite precipitation inversion data and the watershed grid, assuming that the time resolution of the satellite precipitation inversion data is a hour, extract the daily rainfall process of each grid point in the watershed and calculate the daily surface rainfall process of the watershed; according to the watershed area and the method of the present invention, determine the longest precipitation duration a max , further determine a i , the improved Hirschfield method is used to calculate the possible maximum precipitation results of the corresponding daily scale in the basin. The algorithm is as follows:

[0011] According to the basin area, the longest precipitation duration a can be determined by Table 1 below. max , when a max >1, between 1 and a maxSet several values between and divide the calculation period into m, then there is a calculation period sequence a i , where i is an integer from 0 to m-1, a0 is 1, a m-1 for a max ;

[0012] Table 1 The longest precipitation duration a max Determine the comparison table

[0013] <![CDATA[Watershed area (km 2 )]]> <![CDATA[a max Value (Daily)]]> ＜1000 1 1000～2000 3 2000～5000 5 5000～10000 7 More than 10,000 15 ;

[0014] Extract the daily precipitation data of each grid in the basin and at the boundary, and use the following formula to calculate a i Daily-scale gridded precipitation statistics

[0015]

[0016] In the above formula, is the maximum annual a of each grid i The maximum value in the daily rainfall series; n is the maximum annual rainfall in each grid i the number of years in the daily rainfall series; The maximum annual a of each grid after removing the maximum value i The average values of the daily rainfall series are:

[0017]

[0018] In the above formula, The maximum annual a of each grid after removing the maximum value i The value of the daily rainfall series in year j;

[0019] The maximum annual a of each grid after removing the maximum value i The standard deviation of the daily rainfall series is calculated as follows:

[0020]

[0021] Each grid The outer value, i.e. the maximum value, is used as the basin precipitation statistic Then deduce a i Maximum possible precipitation on a daily scale The calculation formula is as follows:

[0022]

[0023] In the above formula: The maximum annual a in the basin i Correction of the mean value calculated from the daily rainfall series; The maximum annual a in the basin iStandard deviation of the daily rainfall series.

[0024] Use the following formula to calculate:

[0025]

[0026] In the above formula: The maximum annual a in the basin i Mean values calculated from daily rainfall series; The maximum annual a in the basin i Coefficient of dispersion of daily rainfall series calculations; is the time period correction coefficient;

[0027] According to a i The number of data used in daily precipitation calculations determines the time period correction coefficient The value of ai and The value comparison relationship is shown in Table 2 below:

[0028] Table 2 Time period correction coefficient Determine the comparison table

[0029]

[0030] Step 3: According to the basin area, determine the calculation mode of the maximum precipitation process in the basin. For basins with an area greater than 1000km 2 For a basin with an area of less than or equal to 1000km, the maximum possible precipitation process for each hour in the basin is calculated by using the segmented same-frequency amplification method. 2 The maximum possible hourly precipitation process in the basin is calculated by coupling the frequency calculation with the Chicago rainfall pattern:

[0031] (1) When the basin area is less than or equal to 1000 km 2 When , the frequency calculation coupled with the Chicago rain pattern is used:

[0032] ①First, based on the basin boundary and satellite precipitation inversion data, the annual maximum daily precipitation series values of the basin are calculated;

[0033] ②Then, using the P-III curve and probability weighting method, derive the design results of the annual maximum daily surface precipitation for 10 frequencies, namely 0.01%, 0.05%, 0.1%, 1%, 2%, 5%, 10%, 20%, 50% and 99%.

[0034] The P-III curve method is a traditional method;

[0035] The definition of the probability weight moment in the probability weight method is:

[0036]

[0037] In the above formula: j is the order; M j is the j-th order overall probability weight moment; F(x) j is the probability weight; dF is the differential operator;

[0038] In the case of simple random samples, the formula for calculating the probability weight moment of the j-th order sample is:

[0039]

[0040] In the above formula: n is the number of samples; i is the serial number that arranges the samples in order from small to large; is the sample value corresponding to sequence number i; for The probability weight is generally in the following form:

[0041]

[0042] According to the above two equations, we can solve

[0043] Furthermore, the relationship between the distribution parameter and the probability weight moment can be obtained:

[0044] E(X)=M0

[0045]

[0046] C S =16.41u-13.5u 2 +10.72u 3 +94.54u 4

[0047] in:

[0048] H=3.545+29.85v-29.15v 2 +363.8v 3 +6093v 4

[0049]

[0050] In the above formula: E(X) is the sample mean; C V is the sample dispersion coefficient; C S is the sample skewness coefficient; H, v, u, and R are all intermediate variables in the calculation process;

[0051] ③ Based on the design results of the annual maximum daily surface precipitation of the above 10 frequencies and the Chicago rainfall pattern, the planning solution parameter optimization method was used to set other parameters. The rain peak coefficient parameter r in areas without data was directly used as 0.375, and the parameter A1 used the design result value of the 99% annual maximum daily surface precipitation. The total rainfall duration was 24 hours, that is, the parameter td was 1440, in minutes; tp was the product of td and r, that is, 540 minutes. The parameters C, b1, and n1 of the Chicago rainfall pattern rainstorm intensity formula were optimized;

[0052] Calculate the combined parameter aa1, the calculation formula is as follows:

[0053] aa1=A1(1+Cln(P))

[0054] Where: P is the recurrence period, in years, which is the reciprocal of the design frequency;

[0055] Furthermore, the rainfall time t is calculated d Total rainfall in P total :

[0056]

[0057] The time value t is calculated using the following formula i :

[0058] t i =i×60(i=0,1,2,…24)

[0059] When t i ≤t p When t i Accumulated rainfall at the time

[0060]

[0061] When t i >t p When t i Accumulated rainfall at the time

[0062]

[0063] On this basis, the adjacent t i With t i-1 Subtract the accumulated rainfall at each moment to get the possible maximum precipitation process result hour by hour;

[0064] (2) When the basin area is greater than 1000 km 2 When the same frequency amplification method is used:

[0065] According to the calculation period a set above i , the improved Hirschfield method is used to derive the a of the basin in the data-free area. i The maximum possible precipitation result on a daily scale is that the flood peak is mainly controlled by the maximum one-day precipitation. Based on the satellite precipitation inversion data, the top five values of the maximum one-day surface precipitation in the history of the basin are counted, and the maximum a in the corresponding year is further extracted. i The sub-daily precipitation process of the day; i The maximum possible precipitation result on daily scale is controlled, and the maximum a of the corresponding year is amplified in segments with the same frequency. i The next day scale precipitation process of the day, to ensure its maximum a i The daily precipitation is equal to a i The maximum possible precipitation result on a daily scale;

[0066] On day a0, i.e. day 1, the amplification coefficient FD0 of the precipitation process is:

[0067]

[0068] In the above formula: is the maximum possible precipitation in the basin on day a0; is the cumulative value of the sub-daily precipitation process on the maximum a0 day in the basin in the corresponding year, extracted based on satellite precipitation inversion data;

[0069] The rest i with a i-1 The amplification factor FD of the precipitation process between (i=1,2,…,m-1) days i for:

[0070]

[0071] In the above formula: For basin a i The maximum possible daily precipitation For basin a i-1 The maximum possible daily precipitation is the maximum a in the basin in the corresponding year extracted based on satellite precipitation inversion data i The cumulative value of the precipitation process on the next day, is the maximum a in the basin in the corresponding year extracted based on satellite precipitation inversion data i-1 The cumulative value of the precipitation process at the sub-daily scale on the day;

[0072] Through the above-mentioned amplification calculation, we obtained the results of five typical sub-daily maximum precipitation processes;

[0073] Step 4: Based on the globally available land use number grid map and the globally available soil category number grid map, resample to ensure that the spatial resolution of the land use number grid map and the soil category number grid map are consistent. Then, the two maps are clipped using the watershed boundary. The following watershed CN value determination method for grid feature matching is used to calculate the watershed curve number CN. s Value, the basin curve number CN s The SCS curve number runoff model is used to calculate the maximum possible precipitation process in the basin with an area greater than 1000km2. 2 Under these conditions, the maximum possible net rainfall process per hour in the basin or the basin area is less than or equal to 1000km 2 The maximum possible net rainfall process in the basin hourly under the following conditions:

[0074] (1) Method for determining watershed CN values based on grid element matching:

[0075] ① Unify the global soil number and land use number raster files to the same spatial resolution and crop them using watershed boundaries;

[0076] ② Based on the land use category and soil property classification of each grid within the watershed and boundary grids, and according to the corresponding relationship between Table 3 and Table 4 below, the CN value of each grid is calculated grid by grid using GIS tools to obtain the CN value under the watershed average and moderate soil moisture conditions;

[0077] Table 3 Comparison table of CN value determination under moderate soil moisture conditions

[0078]

[0079] Table 4 Comparison table of hydrological and soil grouping determination

[0080] Hydrological and soil grouping Soil texture group number A 1～3 B 4～6 C 7 D 8～12 ;

[0081] ③ Referring to the fact that the soil moisture content in the basin was at a high level before the maximum flood, CN was further corrected to obtain the CN value under the condition of moist soil in the basin, which was named CN s , which is calculated as follows:

[0082]

[0083] Step 5: Based on DEM data and watershed boundaries, extract the watershed elevation curve and watershed area and river length curve, and calculate the average slope of the watershed; calculate the average slope of the watershed, the longest flow path length of the watershed, the watershed area and the watershed CN. s Substituting the value into the dimensionless unit line of SCS, we get a basin area greater than 1000km 2The hourly and daily scale unit line or basin area is less than or equal to 1000km 2 The hourly unit line of the basin; the maximum possible net rainfall process of the basin obtained in step 4 is used as the input of the basin unit line, and the five typical basin areas of the basin are calculated to be greater than 1000km 2 The maximum possible flow process per hour or the basin area is less than or equal to 1000km 2 The maximum possible flow process of the watershed hour by hour is calculated as follows:

[0084] Extract the basin elevation curve, where the ordinate is the height difference between the lowest elevation of the basin block area and the lowest elevation of the basin, named ΔZ; the abscissa is the ratio of the basin block area above the elevation corresponding to the corresponding elevation difference to the basin area BA, named fz; and the basin area river length curve, where the ordinate is the river length from the basin block area to the basin outlet, named L, and the abscissa is the ratio of the sum of the basin block area and the upstream basin block area to the basin area BA, named fl; by analyzing the connotation of the basin elevation curve and the basin area river length curve, it is obtained that the area enclosed by the basin elevation curve is the average basin elevation difference ΔZ with area as the weight. b The basin area and river length curve is the average basin length L with area as weight. b , the average slope of the watershed Slp b Calculate according to the following formula:

[0085] Slp b =ΔZ b / L b

[0086]

[0087] In the above formula: nn is the number of watershed blocks, usually set to 20 to ensure the accuracy of integration; ΔZ i and ΔZ i-1 are the height differences between the lowest elevation of the i-th and i-1-th watershed area blocks and the lowest elevation of the watershed, ΔZ i >ΔZ i-1 , it is easy to know that ΔZ0=0; fz i-1 and fz i and ΔZ respectively i and ΔZ i-1 The ratio of the basin block area above the corresponding elevation to the basin area, fz i-1 >fz i , it is easy to know fz0=1; L i and L i-1 are the length of the river from the basin block area to the basin outlet, L i <L i-1 , YizhiL iis the longest flow path length LFP in the basin; fl i and fl i-1 are the ratios of the sum of the areas of the ith and i-1th basin blocks and the upstream basin blocks to the basin area, respectively; fl i >fl i-1 , it is easy to know that fl0=0;

[0088] Step 6: For drainage basins larger than 1000km 2 For a basin with an area of less than or equal to 1000km, the maximum possible flow in each hour is converted into a flood peak using the method of converting the next day's scale flow into the flood peak, and the conversion coefficient of the maximum hourly flow into the flood peak is calculated, and the maximum possible flood result of the basin is further calculated. For a basin with an area of less than or equal to 1000km 2 For a basin, the maximum possible flood result of the basin is calculated based on the maximum possible flow process of the basin hourly. The calculation method is as follows:

[0089] The obtained corrected curve number CN s , average slope of the watershed Slp b , the longest flow path length LFP and the basin area BA of the basin, and substituting them into the dimensionless unit line of SCS, we can get the 1h unit line of the basin;

[0090] When the drainage area of the study basin exceeds 1000km 2 , and when the temporal resolution of the satellite precipitation inversion data used is not 1 hour, the time interval of the possible maximum precipitation process on the next day scale in the data-free area obtained through the above steps will not be 1 hour, but the same as the temporal resolution of the satellite precipitation inversion data; therefore, assuming that the temporal resolution of the satellite precipitation inversion data is nh, 1<n<24, the unit line time period conversion method is used to combine the obtained 1-h unit line of the basin to further obtain the nh unit line of the basin; the possible maximum precipitation process of the basin in the data-free area is substituted into the SCS curve numerical runoff model to calculate the net rainfall process of the basin; the net rainfall process of the basin is used as input, and based on the basin nh unit line, the possible maximum flow process of the basin in each nh and the possible maximum nh flow of the basin are further obtained;

[0091] The maximum possible NH flow result of the basin is converted into the maximum possible flood result of the basin, that is, the flood peak; the conversion coefficient is obtained based on the statistical analysis of the basin 1h unit line and the basin NH unit line. The calculation formula is:

[0092] Q max =Q nh,max ×ZS

[0093]

[0094] Where: Q max The maximum possible flood result in the basin; Qnh,max is the maximum possible nh flow result of the basin; ZS is the conversion coefficient; q 1h,max is the maximum flow rate in the unit line of the basin in 1h; q nh,max is the maximum flow in the unit line of the basin nh.

[0095] Through the above calculations, we can obtain the possible maximum flood calculation results for the basin in the data-free area.

[0096] In view of the technical difficulties faced in the calculation of the possible maximum flood in the area without data, such as the difficulty in obtaining the possible maximum precipitation process at the next day scale, the inconvenience in determining the CN value of the SCS curve method for calculating the runoff of the basin in the area without data, the lack of a scientific and reasonable method to determine the average slope parameter of the basin using the SCS dimensionless unit line method for calculating the confluence of the basin in the area without data, and the difficulty in converting the next day scale flow results into flood peaks, a method for calculating the possible maximum flood in the area without data is proposed. This method uses the improved Hirschfield method to calculate the possible maximum precipitation results at the daily scale in the basin without data; the method classifies and processes the problem of calculating the possible maximum precipitation in the area without data. For basins with an area of less than or equal to 1000km 2 For small and medium-sized river basins, the maximum possible precipitation process for 24 hours is calculated by frequency calculation coupled with Chicago rain pattern based on satellite precipitation inversion data; for river basins with an area greater than 1000km 2 For a large river basin, the possible maximum precipitation process on the sub-daily scale is deduced based on the same frequency amplification method based on satellite precipitation inversion data; the basin CN value determination method oriented to grid element matching is used to improve the convenience and standardization of CN value calculation; based on the area-weighted integral idea, a new idea for basin average slope calculation is proposed, which solves the application problems in data-free areas such as the imprecise theory of SCS dimensionless unit line basin average slope calculation and the influence of grid resolution; based on the unit line time period conversion idea, this method proposes a calculation method for obtaining the possible maximum flood result of the basin by amplifying the possible maximum sub-daily average flow of the basin.

[0097] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0098] 1. Compared with the existing technology, the present invention can obtain the possible maximum precipitation process on the sub-daily scale, laying the foundation for the subsequent coupling with the SCS curve runoff model and the SCS dimensionless unit line model;

[0099] 2. This paper proposes a method for determining the CN value of a watershed based on grid element matching, which clarifies and standardizes the calculation path of the CN value, facilitating the promotion and application of the SCS curve runoff generation and the SCS dimensionless unit line model in areas without data.

[0100] 3. This invention innovatively proposes a new method for calculating the average slope of a watershed using the SCS dimensionless unit line model. The average slope of a watershed calculated by this new method is not affected by the grid resolution, and the physical concepts are clear and the calculation results are stable.

[0101] 4. Existing technologies can only solve the problem of calculating the maximum possible flood in areas with insufficient data. This invention, by constructing a complete technical system, sequentially solves key technical issues such as the difficulty in obtaining the maximum possible precipitation process at the next day scale when calculating the maximum possible precipitation, the inconvenience in determining the CN value of the SCS curve method for calculating runoff generation in watersheds in areas without data, the lack of a scientific and reasonable method for determining the average slope parameter of the watershed using the SCS dimensionless unit line method for calculating runoff in watersheds in areas without data, and the difficulty in converting next-day flow results into flood peaks. This solves the problem of calculating the maximum possible flood in areas without data, and can obtain reasonable and reliable results for calculating the maximum possible flood in watersheds in areas without data. BRIEF DESCRIPTION OF THE DRAWINGS

[0102] Figure 1 This is a flow chart of the implementation of the method for calculating the possible maximum flood of the present invention.

[0103] Figure 2 This is a rationality analysis diagram of the possible maximum precipitation results on a daily scale according to an embodiment of the present invention.

[0104] Figure 3 These are five typical 3-hourly maximum precipitation processes in the basin controlled by the Gulaohe Hydrological Station.

[0105] Figure 4 This is the elevation curve of the control basin.

[0106] Figure 5 This is the result diagram of area-river length curve.

[0107] Figure 6 This is the 1h unit line result diagram of the control basin.

[0108] Figure 7 This is the 3h unit line result map of the control basin of the Gulaohe Hydrological Station.

[0109] Figure 8 This is the result diagram of the possible maximum flow process of Gulao River in 3 hours.

[0110] Figure 9 This is the result diagram of the possible maximum flow process in Huangmaoling in 1 hour. DETAILED DESCRIPTION

[0111] The proposed method was used to calculate the maximum flood potential in data-free areas, using the Gulaohe and Huangmaoling hydrological stations on the Nanting River and Tengtiao River in Yunnan as the research targets. Since no measured rainfall and water flow data were available for the two basins or their adjacent areas, the two basins were treated as data-free areas lacking any measured precipitation or flow data.

[0112] The main implementation steps of the present invention are as follows, and the main process is shown in Figure 1 :

[0113] Step 1: Based on the globally available DEM data and the coordinates of the Gulaohe and Huangmaoling hydrological stations, the boundaries of the watersheds controlled by the two stations and the longest flow paths were extracted using GDEM V3. Based on the spatial resolution and watershed boundaries of globally available satellite precipitation inversion data, the watershed grids were divided using MSWEP V2.8 with a temporal resolution of 3 hours.

[0114] Step 2: Based on the satellite precipitation inversion data and the watershed grid, extract the daily rainfall process of each grid point in the watershed, and then calculate the daily surface rainfall process of the watershed; according to the watershed area and the method of the present invention, determine the longest precipitation duration a max , further determine a i The improved Hirschfield method is used to calculate the possible maximum daily precipitation results in the control basins of the Gulaohe and Huangmaoling hydrological stations. The calculation method is as follows:

[0115] According to the basin area, the longest precipitation duration a can be determined by Table 1 below. max , when a max >1, between 1 and a max Set several values between and divide the calculation period into m, then there is a calculation period sequence a i , where i is an integer from 0 to m-1, a0 is 1, a m-1 for a max ; The longest precipitation duration a max The comparison is shown in Table 1;

[0116] Extract the daily precipitation data of each grid in the basin and at the boundary, and use the following formula to calculate a i Daily-scale gridded precipitation statistics

[0117]

[0118] In the above formula, is the maximum annual a of each grid i The maximum value in the daily rainfall series; n is the maximum annual rainfall in each grid i the number of years in the daily rainfall series; The maximum annual a of each grid after removing the maximum value iThe average values of the daily rainfall series are:

[0119]

[0120] In the above formula, The maximum annual a of each grid after removing the maximum value i The value of the daily rainfall series in year j;

[0121] The maximum annual a of each grid after removing the maximum value i The standard deviation of the daily rainfall series is calculated as follows:

[0122]

[0123] Each grid The outer value, i.e. the maximum value, is used as the basin precipitation statistic Then deduce a i Maximum possible precipitation on a daily scale The calculation formula is as follows:

[0124]

[0125] In the above formula: The maximum annual a in the basin i Correction of the mean value calculated from the daily rainfall series; The maximum annual a in the basin i Standard deviation of the daily rainfall series.

[0126] Use the following formula to calculate:

[0127]

[0128] In the above formula: The maximum annual a in the basin i Mean values calculated from daily rainfall series; The maximum annual a in the basin i Coefficient of dispersion of daily rainfall series calculations; is the time period correction coefficient;

[0129] According to a i The number of data used in daily precipitation calculations determines the time period correction coefficient The value of a i Value and The value comparison relationship is shown in Table 2:

[0130] The calculation results are shown in Table 5, and the rationality analysis is shown in Figure 2 As shown:

[0131] Step 3: Based on the control basin area of Gulaohe Hydrological Station and Huangmaoling Hydrological Station, determine the basin of Gulaohe Hydrological Station and use the segmented same-frequency amplification method to calculate the five typical 3-hour maximum precipitation processes in the basin, such as Figure 3 As shown in the figure, the Huangmaoling hydrological station basin uses the frequency calculation coupled with the Chicago rainfall pattern to calculate the hourly maximum precipitation process in the basin. The calculation steps are as follows:

[0132] (1) The Huangmaoling hydrological station basin uses the frequency calculation coupled with the Chicago rainfall pattern:

[0133] ①First, based on the basin boundary and satellite precipitation inversion data, the annual maximum daily precipitation series values of the basin are calculated;

[0134] ②Then, using the P-III curve and probability weighting method, derive the design results of the annual maximum daily surface precipitation for 10 frequencies, namely 0.01%, 0.05%, 0.1%, 1%, 2%, 5%, 10%, 20%, 50% and 99%.

[0135] The P-III curve method is a traditional method;

[0136] The definition of the probability weight moment in the probability weight method is:

[0137]

[0138] In the above formula: j is the order; M j is the j-th order overall probability weight moment; F(x) j is the probability weight; dF is the differential operator;

[0139] In the case of simple random samples, the formula for calculating the probability weight moment of the j-th order sample is:

[0140]

[0141] In the above formula: n is the number of samples; i is the serial number that arranges the samples in order from small to large; is the sample value corresponding to sequence number i; for The probability weight is generally in the following form:

[0142]

[0143] According to the above two equations, we can solve

[0144] Furthermore, the relationship between the distribution parameter and the probability weight moment can be obtained:

[0145] E(X)=M0

[0146]

[0147] C S =16.41u-13.5u 2 +10.72u 3 +94.54u 4

[0148] in:

[0149] H=3.545+29.85v-29.15v 2 +363.8v 3 +6093v 4

[0150]

[0151] In the above formula: E(X) is the sample mean; C V is the sample dispersion coefficient; C S is the sample skewness coefficient; H, v, u, and R are all intermediate variables in the calculation process;

[0152] ③ Based on the design results of the annual maximum daily surface precipitation of the above 10 frequencies and the Chicago rainfall pattern, the planning solution parameter optimization method was used to set other parameters. The rain peak coefficient parameter r in areas without data was directly used as 0.375, and the parameter A1 used the design result value of the 99% annual maximum daily surface precipitation. The total rainfall duration was 24 hours, that is, the parameter td was 1440, in minutes; tp was the product of td and r, that is, 540 minutes. The parameters C, b1, and n1 of the Chicago rainfall pattern rainstorm intensity formula were optimized;

[0153] Calculate the combined parameter aa1, the calculation formula is as follows:

[0154] aa1=A1(1+Cln(P))

[0155] Where: P is the recurrence period, in years, which is the reciprocal of the design frequency;

[0156] Furthermore, the rainfall time t is calculated d Total rainfall in P total :

[0157]

[0158] The time value t is calculated using the following formula i :

[0159] t i =i×60(i=0,1,2,…24)

[0160] When t i ≤t p When ti The cumulative rainfall at the time P ti :

[0161]

[0162] When t i >t p When t i The cumulative rainfall at the time P ti :

[0163]

[0164] On this basis, the adjacent t i With t i-1 Subtract the accumulated rainfall at each moment to get the possible maximum precipitation process result hour by hour;

[0165] (2) The Gulao River Hydrological Station basin adopts the same frequency amplification method:

[0166] According to the calculation period a set above i , the improved Hirschfield method is used to derive the a of the basin in the data-free area. i The maximum possible precipitation result on a daily scale is that the flood peak is mainly controlled by the maximum one-day precipitation. Based on the satellite precipitation inversion data, the top five values of the maximum one-day surface precipitation in the history of the basin are counted, and the maximum a in the corresponding year is further extracted. i The sub-daily precipitation process of the day; i The maximum possible precipitation result on daily scale is controlled, and the maximum a of the corresponding year is amplified in segments with the same frequency. i The next day scale precipitation process of the day, to ensure its maximum a i The daily precipitation is equal to a i The maximum possible precipitation result on a daily scale;

[0167] On day a0, i.e. day 1, the amplification coefficient FD0 of the precipitation process is:

[0168]

[0169] In the above formula: is the maximum possible precipitation in the basin on day a0; is the cumulative value of the sub-daily precipitation process on the maximum a0 day in the basin in the corresponding year, extracted based on satellite precipitation inversion data;

[0170] The rest i with a i-1 The amplification factor FD of the precipitation process between (i=1,2,…,m-1) days i for:

[0171]

[0172] In the above formula: For basin a i The maximum possible daily precipitation For basin a i-1 The maximum possible daily precipitation is the maximum a in the basin in the corresponding year extracted based on satellite precipitation inversion data i The cumulative value of the precipitation process on the next day, is the maximum a in the basin in the corresponding year extracted based on satellite precipitation inversion data i-1 The cumulative value of the precipitation process at the sub-daily scale on the day;

[0173] Through the above-mentioned amplification calculation, we obtained the results of five typical sub-daily maximum precipitation processes;

[0174] Step 4: Based on the globally available land use number grid map, using the Modis MCD12Q1 data, and the globally available soil type number grid map, using the HWSD V1.2 version data, resample to ensure that the spatial resolution of the land use number grid map and the soil type number grid map are consistent. Then, the two maps are cropped using the watershed boundary. The calculation process is as follows:

[0175] (1) Method for determining watershed CN values based on grid element matching:

[0176] ① Unify the global soil number and land use number raster files to the same spatial resolution and crop them using watershed boundaries;

[0177] ② Based on the land use category and soil property classification of each grid within the watershed and boundary grids, and according to the correspondence between Table 3 and Table 4, the CN value of each grid was calculated grid by grid using GIS tools to obtain the CN value under the conditions of basin average and moderate soil moisture;

[0178] ③ Referring to the fact that the soil moisture content in the basin was at a high level before the maximum flood, CN was further corrected to obtain the CN value under the condition of moist soil in the basin, which was named CN s , which is calculated as follows:

[0179]

[0180] The calculated control basin of Gulaohe Hydrological Station and the curve number CN of Huangmaoling are obtained. s The values are 83.82 and 89.12 respectively; the control basin of Gulaohe hydrological station and the curve number of Huangmaoling are CN sThe maximum possible precipitation process of the two basins obtained in step 3 is obtained by using the SCS curve number runoff model to obtain the maximum possible net rainfall process every 3 hours in the basin controlled by the Gulaohe hydrological station and the maximum possible net rainfall process every hour in the basin controlled by the Huangmaoling hydrological station.

[0181] Step 5: Based on the DEM data and the boundaries of the two basins, extract the elevation curves of the two basins and the basin area river length curves. The ordinate is the height difference between the lowest elevation of the basin block area and the lowest elevation of the basin, named ΔZ; the abscissa is the ratio of the basin block area above the elevation corresponding to the corresponding elevation difference to the basin area BA, named fz; and the basin area river length curve, the ordinate is the river length from the basin block area to the basin outlet, named L, and the abscissa is the ratio of the sum of the basin block area and the upstream basin block area to the basin area BA, named fl; by analyzing the connotation of the basin elevation curve and the basin area river length curve, it is obtained that the area enclosed by the basin elevation curve is the average basin elevation difference ΔZ with area as the weight. b The basin area and river length curve is the average basin length L with area as weight. b , the average slope of the watershed Slp b Calculate according to the following formula:

[0182] Slp b =ΔZ b / L b

[0183]

[0184] In the above formula: nn is the number of watershed blocks, usually set to 20 to ensure the accuracy of integration; ΔZ i and ΔZ i-1 are the height differences between the lowest elevation of the i-th and i-1-th watershed area blocks and the lowest elevation of the watershed, ΔZ i >ΔZ i-1 , it is easy to know that ΔZ0=0; fz i-1 and fz i and ΔZ respectively i and ΔZ i-1 The ratio of the basin block area above the corresponding elevation to the basin area, fz i-1 >fz i , it is easy to know fz0=1; L i and L i-1 are the length of the river from the basin block area to the basin outlet, L i <L i-1 , YizhiL i is the longest flow path length LFP in the basin; fl i and fl i-1 are the ratios of the sum of the areas of the ith and i-1th basin blocks and the upstream basin blocks to the basin area, respectively; fli >fl i-1 , it is easy to know that fl0=0;

[0185] like Figure 4 and Figure 5 As shown in the figure, the average slopes of the basins controlled by the Gulaohe hydrological station and the Huangmaoling hydrological station are 1.05% and 1.89% respectively. The comparison between the average slopes of the basins calculated by the present invention and those calculated by the conventional method is shown in Table 6. The average slopes of the two basins, the longest flow path length of the basin, the basin area and the basin CN are calculated. s Substituting the value into the dimensionless unit line of SCS, we can obtain the hourly and 3-hour unit lines of the basin controlled by Gulaohe Hydrological Station and the hourly unit line of the basin controlled by Huangmaoling Hydrological Station, as shown in the following example: Figure 6 and Figure 7 As shown in the figure, the possible maximum net rainfall process of the two basins obtained in step 4 is used as the input of the unit lines of the two basins to calculate the five typical possible maximum flow processes every 3 hours in the basin controlled by the Gulaohe Hydrological Station and the possible maximum flow processes every hour in the basin controlled by the Huangmaoling Hydrological Station, as shown in the figure. Figure 8 and Figure 9 As shown;

[0186] Step 6: Based on the possible maximum flow process every 3 hours in the basin controlled by the Gulaohe Hydrological Station and the possible maximum flow process every hour in the basin controlled by the Huangmaoling Hydrological Station, calculate through the following steps:

[0187] The obtained corrected curve number CN s , average slope of the watershed Slp b , the longest flow path length LFP and the basin area BA of the basin, and substituting them into the dimensionless unit line of SCS, we can get the 1h unit line of the basin;

[0188] The drainage area of the Gulao River hydrological station exceeds 1000km 2 , and when the time resolution of the satellite precipitation inversion data used is not 1 hour, the time interval of the possible maximum precipitation process on the next day scale in the data-free area obtained through the above steps will not be 1 hour, but the same as the time resolution of the satellite precipitation inversion data; assuming that the time resolution of the satellite precipitation inversion data is 3 hours, the unit line time period conversion method is used to combine the obtained 1-hour unit line of the basin to further obtain the 3-hour unit line of the basin; the possible maximum precipitation process of the basin in the data-free area every 3 hours is substituted into the SCS curve runoff model to calculate the net rain process of the basin; the basin net rain process is used as input, and based on the basin 3-hour unit line, the possible maximum flow process of the basin every 3 hours and the possible maximum 3-hour flow of the basin are further obtained;

[0189] The maximum possible 3-hour flow result of the basin is converted into the maximum possible flood result of the basin, that is, the flood peak; the conversion coefficient is obtained based on the statistical analysis of the 1-hour unit line and the NH unit line of the basin. The calculation formula is:

[0190] Q max =Q 3h,max ×ZS

[0191]

[0192] Where: Q max The maximum possible flood result in the basin; Q 3h,max is the maximum possible nh flow result of the basin; ZS is the conversion coefficient; q 1h,max is the maximum flow rate in the unit line of the basin in 1h; q 3h,max It is the maximum flow in the 3h unit line of the basin.

[0193] Through the above calculations, we can obtain the possible maximum flood calculation results for the basin in the data-free area.

[0194] The conversion coefficient of the maximum 3-hour flow to the flood peak is 1.019, and the maximum possible flood result of the Gulao River hydrological station in the control basin is 3998m 3 According to the hourly maximum flow process of the Huangmaoling hydrological station control basin, the maximum flood result of the Huangmaoling hydrological station control basin is 1372m 3 / s. The comparison with the design flood of 10,000-year return period at the two stations is shown in Table 3. Judging from the ratio of the possible maximum flood to the 10,000-year return period flood, the results are reliable and reasonable.

[0195] The results after the implementation of the technical solution are shown in Figures 2 to 9 and Tables 5 to 7; from which we can see:

[0196] ① In terms of the calculation of the daily maximum possible precipitation, the results of the method of the present invention for the daily maximum possible precipitation in the control basins of the Gulaohe and Huangmaoling hydrological stations are generally close to those calculated in other areas with available data. The basin of the Gulaohe station is located in the middle of the point cluster, and the basin of the Huangmaoling station is a certain distance away from the center of the point cluster, but not far away.

[0197] ② In the calculation of the maximum possible precipitation process on the next day, for a basin with an area of more than 1000km, such as the Gulaohe Station basin, 2 The spatial distribution of precipitation in the basin has a certain degree of variability. To address this, the method of the present invention calculates five typical possible maximum precipitation processes, avoiding the problem of insufficient representativeness of a single process and difficulty in reflecting the "possible maximum", thereby improving the reliability of the results.

[0198] ③ The present invention calculates the average slope of the watershed by extracting the elevation curves and area-river length curves of the two watersheds, and compares the calculation results with those of the existing technical methods. The comparison shows that the calculation method of the present invention takes into account the area-weighted integral, and the calculation process fully considers the overall change trend of the watershed topography. The calculation results are insensitive to the spatial resolution of the DEM and the number of discrete integral segments. The results are stable and do not have the problems of the watershed slope matrix mean and the longest flow path gradient method being too large or too small. The calculation results are more reasonable and reliable.

[0199] ④ The method of the present invention can scientifically calculate the results of the unit line of the watershed, and the proposed method of converting the next-day scale flow into the flood peak is reasonable and feasible;

[0200] ⑤ The maximum possible flood result calculated by the method of the present invention is between 1 and 2 compared with the result of the 10,000-year flood. This ratio is also summarized from multiple regions with data, further proving the scientific rationality of the method of the present invention;

[0201] Comprehensive analysis shows that the present invention can obtain reasonable and reliable results of the possible maximum flood in the basin in the area without data. The present invention has explored a way to calculate the possible maximum flood in the area without data.

[0202] Table 5 The possible maximum precipitation results of the two basins on a daily scale calculated by this method

[0203]

[0204] Table 6 Comparison of the average slope of the watershed calculated by the present invention and the calculation results of the prior art method

[0205]

[0206]

[0207] Table 7 The possible maximum flood results of the two basins calculated by the present invention (discharge unit: m 3 / s)

[0208] River Basin mean Cv Cs / Cv P＝0.01％ Qmax Possible maximum flood / 10,000-year flood Gulaohe Hydrological Station Control Basin 586 0.52 4 3140 3998 1.27 Huangmaoling Hydrological Station Control Basin 204 0.42 4 846 1372 1.62

Claims

1. A method for calculating the maximum possible flood in areas without data, characterized by: The calculation method is implemented as follows: Step 1: Extract the watershed boundaries and longest flow paths based on globally available DEM data and the coordinates of watershed outlets in data-unavailable areas; and divide the watershed grids based on the spatial resolution and watershed boundaries of globally available satellite precipitation inversion data. Step 2: Based on the satellite precipitation inversion data and the watershed grid, assuming that the time resolution of the satellite precipitation inversion data is a hour, extract the daily rainfall process of each grid point in the watershed and calculate the daily surface rainfall process of the watershed; according to the watershed area and the method of the present invention, determine the longest precipitation duration a max , further determine a i , the improved Hirschfield method is used to calculate the possible maximum precipitation results of the corresponding daily scale in the basin. The algorithm is as follows: According to the basin area, the longest precipitation duration a can be determined by Table 1 below. max , when a max >1, between 1 and a max Set several values between and divide the calculation period into m, then there is a calculation period sequence a i , where i is an integer from 0 to m-1, a0 is 1, a m-1 for a max ; Table 1 The longest precipitation duration a max Determine the comparison table ； Extract the daily precipitation data of each grid in the basin and at the boundary, and use the following formula to calculate a i Daily-scale gridded precipitation statistics In the above formula, is the maximum annual a of each grid i The maximum value in the daily rainfall series; n is the maximum annual rainfall in each grid i the number of years in the daily rainfall series; The maximum annual a of each grid after removing the maximum value i The average values of the daily rainfall series are: In the above formula, The maximum annual a of each grid after removing the maximum value i The value of the daily rainfall series in year j; The maximum annual a of each grid after removing the maximum value i The standard deviation of the daily rainfall series is calculated as follows: Each grid The outer value, i.e. the maximum value, is used as the basin precipitation statistic Then deduce a i Maximum possible precipitation on a daily scale The calculation formula is as follows: In the above formula: The maximum annual a in the basin i Correction of the mean value calculated from the daily rainfall series; The maximum annual a in the basin i Standard deviation of the daily rainfall series. Use the following formula to calculate: In the above formula: The maximum annual a in the basin i Mean values calculated from daily rainfall series; The maximum annual a in the basin i Coefficient of dispersion of daily rainfall series calculations; is the time period correction coefficient; According to a i The number of data used in daily precipitation calculations determines the time period correction coefficient The value of ai and The value comparison relationship is shown in Table 2 below: Table 2 Time period correction coefficient Determine the comparison table Step 3: According to the basin area, determine the calculation mode of the maximum precipitation process in the basin. For basins with an area greater than 1000km 2 For a basin with an area of less than or equal to 1000km, the maximum possible precipitation process for each hour in the basin is calculated by using the segmented same-frequency amplification method. 2 For the basin, the frequency calculation coupled with the Chicago rainfall pattern is used to calculate the maximum possible hourly precipitation process in the basin: (1) When the basin area is less than or equal to 1000 km 2 When , the frequency calculation coupled with the Chicago rain pattern is used: ①First, based on the basin boundary and satellite precipitation inversion data, the annual maximum daily precipitation series values of the basin are calculated; ②Then, using the P-III curve and probability weighting method, derive the design results of the annual maximum daily surface precipitation for 10 frequencies, namely 0.01%, 0.05%, 0.1%, 1%, 2%, 5%, 10%, 20%, 50% and 99%. The P-III curve method is a traditional method; The definition of the probability weight moment in the probability weight method is: In the above formula: j is the order; M j is the j-th order overall probability weight moment; F(x) j is the probability weight; dF is the differential operator; In the case of simple random samples, the formula for calculating the probability weight moment of the j-th order sample is: In the above formula: n is the number of samples; i is the serial number that arranges the samples in order from small to large; is the sample value corresponding to sequence number i; for The probability weight is generally in the following form: According to the above two equations, we can solve Furthermore, the relationship between the distribution parameter and the probability weight moment can be obtained: E(X)=M0 C S =16.41u-13.5u 2 +10.72u 3 +94.54u 4 in: H=3.545+29.85v-29.15v 2 +363.8v 3 +6093v 4 In the above formula: E(X) is the sample mean; C V is the sample dispersion coefficient; C S is the sample skewness coefficient; H, v, u, and R are all intermediate variables in the calculation process; ③ Based on the design results of the annual maximum daily surface precipitation of the above 10 frequencies and the Chicago rainfall pattern, the planning solution parameter optimization method was used to set other parameters. The rain peak coefficient parameter r in areas without data was directly used as 0.375, and the parameter A1 used the design result value of the 99% annual maximum daily surface precipitation. The total rainfall duration was 24 hours, that is, the parameter td was 1440, in minutes; tp was the product of td and r, that is, 540 minutes. The parameters C, b1, and n1 of the Chicago rainfall pattern rainstorm intensity formula were optimized; Calculate the combined parameter aa1, the calculation formula is as follows: aa1=A1(1+Cln(P)) Where: P is the recurrence period, in years, which is the reciprocal of the design frequency; Furthermore, the rainfall time t is calculated d Total rainfall in P total : The time value t is calculated using the following formula i : t i =i×60(i=0,1,2,…24) When t i ≤t p When t i The cumulative rainfall at the time P ti : When t i >t p When t i The cumulative rainfall at the time P ti : On this basis, the adjacent t i With t i-1 Subtract the accumulated rainfall at each moment to get the possible maximum precipitation process result hour by hour; (2) When the basin area is greater than 1000 km 2 When the same frequency amplification method is used: According to the calculation period a set above i , the improved Hirschfield method is used to derive the a of the basin in the data-free area. i The maximum possible precipitation result on a daily scale is that the flood peak is mainly controlled by the maximum one-day precipitation. Based on the satellite precipitation inversion data, the top five values of the maximum one-day surface precipitation in the history of the basin are counted, and the maximum a in the corresponding year is further extracted. i The sub-daily precipitation process of the day; i The maximum possible precipitation result on daily scale is controlled, and the maximum a of the corresponding year is amplified in segments with the same frequency. i The next day scale precipitation process of the day, to ensure its maximum a i The daily precipitation is equal to a i The maximum possible precipitation result on a daily scale; On day a0, i.e. day 1, the amplification coefficient FD0 of the precipitation process is: In the above formula: is the maximum possible precipitation in the basin on day a0; is the cumulative value of the sub-daily precipitation process on the maximum a0 day in the basin in the corresponding year, extracted based on satellite precipitation inversion data; The rest i with a i-1 The amplification factor FD of the precipitation process between (i=1,2,…,m-1) days i for: In the above formula: For basin a i The maximum possible daily precipitation For basin a i-1 The maximum possible daily precipitation is the maximum a in the basin in the corresponding year extracted based on satellite precipitation inversion data i The cumulative value of the precipitation process on the next day, is the maximum a in the basin in the corresponding year extracted based on satellite precipitation inversion data i-1 The cumulative value of the precipitation process at the sub-daily scale on the day; Through the above-mentioned amplification calculation, we obtained the results of five typical sub-daily maximum precipitation processes; Step 4: Based on the globally available land use number grid map and the globally available soil category number grid map, resample to ensure that the spatial resolution of the land use number grid map and the soil category number grid map are consistent. Then, the two maps are clipped using the watershed boundary. The following watershed CN value determination method for grid feature matching is used to calculate the watershed curve number CN. s Value, the basin curve number CN s The SCS curve number runoff model is used to calculate the maximum possible precipitation process in the basin with an area greater than 1000km2. 2 Under these conditions, the maximum possible net rainfall process per hour in the basin or the basin area is less than or equal to 1000km 2 The maximum possible net rainfall process in the basin hourly under the following conditions: (1) Method for determining watershed CN values based on grid element matching: ① Unify the global soil number and land use number raster files to the same spatial resolution and use watershed boundaries to crop the soil number and land use number raster files; ② Based on the land use category and soil property classification of each grid within the watershed and boundary grids, and according to the corresponding relationship between Table 3 and Table 4 below, the CN value of each grid is calculated grid by grid using GIS tools to obtain the CN value under the watershed average and moderate soil moisture conditions; Table 3 Comparison table of CN value determination under moderate soil moisture conditions Table 4 Comparison table of hydrological and soil grouping determination ； ③ Referring to the fact that the soil moisture content in the basin was at a high level before the maximum flood, CN was further corrected to obtain the CN value under the condition of moist soil in the basin, which was named CN s , which is calculated as follows: Step 5: Based on DEM data and watershed boundaries, extract the watershed elevation curve and watershed area and river length curve, and calculate the average slope of the watershed; calculate the average slope of the watershed, the longest flow path length of the watershed, the watershed area and the watershed CN. s Substituting the value into the dimensionless unit line of SCS, we get a basin area greater than 1000km 2 The hourly and daily scale unit line or basin area is less than or equal to 1000km 2 The hourly unit line of the basin; the maximum possible net rainfall process of the basin obtained in step 4 is used as the input of the basin unit line, and the five typical basin areas of the basin are calculated to be greater than 1000km 2 The maximum possible flow process per hour or the basin area is less than or equal to 1000km 2 The maximum possible flow process of the watershed hour by hour is calculated as follows: Extract the basin elevation curve, where the ordinate is the height difference between the lowest elevation of the basin block area and the lowest elevation of the basin, named ΔZ; the abscissa is the ratio of the basin block area above the elevation corresponding to the corresponding elevation difference to the basin area BA, named fz; and the basin area river length curve, where the ordinate is the river length from the basin block area to the basin outlet, named L, and the abscissa is the ratio of the sum of the basin block area and the upstream basin block area to the basin area BA, named fl; by analyzing the connotation of the basin elevation curve and the basin area river length curve, it is obtained that the area enclosed by the basin elevation curve is the average basin elevation difference ΔZ with area as the weight. b The basin area and river length curve is the average basin length L with area as weight. b , the average slope of the watershed Slp b Calculated according to the following formula: Slp b =ΔZ b / L b In the above formula: nn is the number of watershed blocks, usually set to 20 to ensure the accuracy of integration; ΔZ i and ΔZ i-1 are the height differences between the lowest elevation of the i-th and i-1-th watershed area blocks and the lowest elevation of the watershed, ΔZ i >ΔZ i-1 , it is easy to know that ΔZ0=0; fz i-1 and fz i and ΔZ respectively i and ΔZ i-1 The ratio of the basin block area above the corresponding elevation to the basin area, fz i-1 >fz i , it is easy to know fz0=1; L i and L i-1 are the length of the river from the basin block area to the basin outlet, L i <L i-1 , YizhiL i is the longest flow path length LFP in the basin; fl i and fl i-1 are the ratios of the sum of the areas of the ith and i-1th basin blocks and the upstream basin blocks to the basin area, respectively; fl i >fl i-1 , it is easy to know that fl0=0; Step 6: For drainage basins larger than 1000km 2 For a basin with an area of less than or equal to 1000km, the maximum possible flow in each hour is converted into a flood peak using the method of converting the next day's scale flow into the flood peak, and the conversion coefficient of the maximum hourly flow into the flood peak is calculated, and the maximum possible flood result of the basin is further calculated. For a basin with an area of less than or equal to 1000km 2 For a basin, the maximum possible flood result of the basin is calculated based on the maximum possible flow process of the basin hourly. The calculation method is as follows: The obtained corrected curve number CN s , average slope of the watershed Slp b , the longest flow path length LFP and the basin area BA, are substituted into the dimensionless unit line of SCS to obtain the 1h unit line of the basin; When the drainage area of the study basin exceeds 1000km 2 , and when the temporal resolution of the satellite precipitation inversion data used is not 1 hour, the time interval of the possible maximum precipitation process on the next day scale in the data-free area obtained through the above steps will not be 1 hour, but the same as the temporal resolution of the satellite precipitation inversion data; therefore, assuming that the temporal resolution of the satellite precipitation inversion data is nh, 1<n<24, the unit line time period conversion method is used to combine the obtained 1-h unit line of the basin to further obtain the nh unit line of the basin; the possible maximum precipitation process of the basin in the data-free area is substituted into the SCS curve numerical runoff model to calculate the net rainfall process of the basin; the net rainfall process of the basin is used as input, and based on the basin nh unit line, the possible maximum flow process of the basin in each nh and the possible maximum nh flow of the basin are further obtained; The maximum possible NH flow result of the basin is converted into the maximum possible flood result of the basin, that is, the flood peak; the conversion coefficient is obtained based on the statistical analysis of the basin 1h unit line and the basin NH unit line. The calculation formula is: Q max =Q nh,max ×ZS Where: Q max The maximum possible flood result in the basin; Q nh,max is the possible maximum nh flow result of the basin; ZS is the conversion coefficient; q 1h,max is the maximum flow rate in the unit line of the basin in 1h; q nh,max is the maximum flow in the unit line of the basin nh. Through the above calculations, we can obtain the possible maximum flood calculation results for the basin in the data-free area.

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