CN117572533B - Method for calculating rainfall frequency of composite surface of upstream river basin of village - Google Patents
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Abstract
The invention discloses a village upstream basin composite surface rainfall frequency calculating method, which comprises the following steps: step 1, determining the upstream river basin range of villages; step 2, determining average confluence time of upstream watershed of village; step 3, calculating the hour-by-hour face rainfall of the upstream basin of the village; step 4, calculating the composite surface rainfall of the upstream basin of the village; step 5, calculating a statistical characteristic value of the p c sample; step 6, determining a frequency distribution curve of p c; and 7, calculating the composite surface rainfall in the reproduction period. The method improves the reliability, accuracy and adaptability of the hydrological weather forecast, reduces the river basin weather and hydrological risks, and simultaneously provides scientific basis for the high-precision forecast of the small river basin under various climate types.
Description
Technical Field
The invention belongs to the technical field of hydrological weather forecast, and particularly relates to a method for calculating the rainfall frequency of a composite surface of an upstream river basin of a village.
Background
With the aggravation of climate change, climate risks such as extreme precipitation are increasingly increased, and the safety and development of a hydrologic system are threatened. In order to reduce the associated risk, analog computation of the rainfall frequency is increasingly important. The rainfall frequency calculation is an important means for exploring and recognizing the regional rainfall process, can provide important help for weather forecast, hydrologic forecast and watershed hydrologic risk avoidance, and has obvious flood control significance for small watershed such as village mountain land and the like. However, analog computation is often accompanied by a number of uncertainties due to limitations of the data or existing methods, and then extreme rainfall forecasting under its guidance would present a certain risk. At present, regression fit of rainfall frequency, regional climate model and global climate model by using historical rainfall data is a common calculation method. However, these methods have certain disadvantages and disadvantages such as poor predictive power, low reproducibility, excessive spatial scale, etc. One compensates or downscales by adding post bias correction to compensate for some of the predicted shortfalls, but this is difficult to substantially overcome. Particularly, for the rainfall frequency analysis of the small-basin space scale, how to realize stable and reliable calculation is a technical problem which needs to be solved at present.
Disclosure of Invention
The invention aims to provide a method for calculating the rainfall frequency of a village upstream basin composite surface, so as to solve the technical problems.
In order to achieve the above purpose, the present invention provides the following technical solutions:
The invention discloses a village upstream basin composite surface rainfall frequency calculating method, which comprises the following steps:
step 1, determining the upstream river basin range of villages: determining the upstream river basin range of villages according to the DEM data, and calculating the river basin area A;
Step 2, determining average confluence time of upstream watershed of village: extracting the average drop J of the longest confluence path of the village upstream drainage basin according to the village upstream drainage basin range determined in the step 1, and further determining the average confluence time tau;
Step 3, calculating the hour-by-hour face rainfall of the upstream river basin of the village: collecting historical rainfall sequence data of rainfall stations in the upstream river basin range of villages, and calculating the hourly rainfall of the upstream river basin of the villages by adopting a Thiessen polygon method
Step 4, calculating the composite surface rainfall of the upstream river basin of the village: according to the average drainage basin confluence time tau determined in the step 2 and the village upstream drainage basin hour-by-hour face rainfall determined in the step3Calculating the daily face rainfall p D-1,pD-2,…,pD-15 of the previous 15 days and the accumulated face rainfall p s of the previous tau hours corresponding to the hour face rainfall of the upstream watershed of the village, calculating the early-stage influence rainfall p p of each hour according to an improved early-stage influence rainfall empirical formula, and further calculating the compound face rainfall p c of the upstream watershed of the village;
Step 5, calculating a statistical characteristic value of the p c sample: based on the hourly composite surface rainfall p c of the village upstream basin obtained in the step 4, selecting the annual maximum p c value to form a p c sample, and calculating the statistical characteristic value of the p c sample;
Step 6, determining a frequency distribution curve of p c: selecting a pearson III-type frequency curve, calculating curve parameters based on the statistical characteristic value of p c, and determining a frequency distribution curve of p c;
Step 7, calculating the composite surface rainfall in the reproduction period: and calculating the composite surface rainfall in the reproduction period according to the frequency distribution curve of p c.
Further, the specific process of determining the upstream river basin range of the village according to the DEM data and calculating the river basin area a in the step 1 is as follows: taking villages as centers, collecting DEM data in the range of 10km around the villages, and selecting DEM data with the maximum resolution as basic data; based on the basic data, sequentially performing operations of filling, flowing and setting river network density in ArcGIS software; using the village position as a drainage point, drawing an upstream drainage basin range of the village by using a watershed tool in hydrologic analysis, and calculating a drainage basin area A in ArcGIS software.
Further, the specific process of determining the average confluence time τ in the step 2 is: the regional empirical formula is adopted to calculate parameters N and K of instantaneous unit lines of the upstream river basin of the village, and then average confluence time tau=N.K is determined;
the Nash instantaneous unit line formula is as follows:
Wherein N is the number of linear reservoirs; k is the storage constant of the linear reservoir; Γ is a gamma function, Γ (N) = (N-1) +.! ;
the regional empirical formula is:
wherein A is the upstream river basin area of village; j is the average drop of the longest confluence path; and I is rainfall intensity.
Further, in the step 3, the step of collecting the historical rainfall sequence data of the rainfall stations in the upstream basin of the village is specifically to collect and identify all the rainfall station codes in the upstream basin of the village, extract the historical rainfall sequence data of the rainfall stations from a database according to the codes, screen the extracted rainfall station data, remove stations without data and stations with abnormal data, obtain screened station information, and set n stations;
The hour-by-hour face rainfall of the upstream river basin of village is calculated by adopting a Thiessen polygon method The specific process of (2) is as follows: in ArcGIS software, n Thiessen polygons are created according to the n site information obtained after screening, the areas of the polygons are automatically identified by the software, the hour-by-hour face rainfall of the upstream river basin of village is calculated based on the Thiessen polygon method, and the calculation formula is as follows:
in the method, in the process of the invention, Hour-by-hour rainfall for village upstream watershed; p i and a i are the measured rainfall at the rainfall station in the ith Thiessen polygon and the area of the ith Thiessen polygon, i=1, 2, …, n, respectively; a is the area of the upstream watershed in village.
Further, in the step 4, the specific process of calculating the early-stage influence rainfall p p per hour according to the improved early-stage influence rainfall empirical formula, and further calculating the composite surface rainfall p c per hour of the upstream basin of the village is as follows:
For the face rainfall at the time of D day H, the corresponding face rainfall p D-1,pD-2,…,pD-15 day by day in the first 15 days is the face rainfall of D-1, D-2, … and D-15 days; the accumulated surface rainfall p s of the former tau hours is the sum of the surface rainfall of H-1, H-2, … and H-tau for tau hours;
According to the front 15 days daily face rainfall p D-1,pD-2,…,pD-15 corresponding to the H-time face rainfall, the early-stage influence rainfall in the H-time is calculated by an empirical formula shown in the formula (4):
Wherein p p is the earlier-stage influence rainfall; k is an early rainfall influence coefficient; e m is the drainage basin evaporation capacity, and W m is the drainage basin soil water storage capacity;
An empirical formula for improving k based on basin daily potential vapor deposition: collecting data of net radiation, humidity, wind speed and temperature of the upstream basin of village day by day, calculating the daily potential evaporation amount ET o,D-i of D-1, D-2, … and D-15 days by using a PM formula, and determining the maximum potential evaporation amount ET omax＝max(ETo,D-1,ETo,D-2,…,ETo,D-15, wherein the evaporation capacity E m of the basin in D-1, D-2, … and D-15 days is represented;
The PM formula is:
Wherein ET o is the potential vapor emission amount; r n is ground net radiation; g is soil heat flux; gamma is the hygrometer constant; t is the daily average temperature; u 2 is the wind speed at the height of 2 m; e s is saturated water vapor pressure; e a is the actual water vapor pressure; delta is the slope of the steam pressure curve;
the empirical formula for improving the early rainfall impact coefficient k is:
substituting the formula (6) into the formula (4), and obtaining an improved early-stage influence rainfall empirical formula which is as follows:
The composite surface rainfall per hour p c of the upstream watershed of villages is calculated as follows:
pc＝pp×ps (8)
wherein p c is the composite surface rainfall of village upstream watershed per hour; p p is the earlier-stage influence rainfall; p s is the accumulated face rainfall for the first τ hours.
Further, in the step 5, the mean E x, the variance D x, the variance coefficient C v, and the bias coefficient C s of the p c sample are calculated according to the following formulas:
where p cj is the p c value of the j-th event in the sample, j=1, 2, …, m; m is the total number of samples.
Further, in the step 6, the curve parameters include a curve shape parameter, a curve scale parameter and a curve position parameter, and the calculation formulas are respectively as follows:
Wherein a 0 is a curve shape parameter; alpha is a curve scale parameter; beta is a curve position parameter;
the frequency distribution curve of p c is:
Wherein P (X) is the frequency of X.gtoreq.x; Γ is a gamma function, Γ (α) = (α -1) +.! .
Further, in the step 7, the composite surface rainfall of the calculated reproduction period is specifically: according to the corresponding frequency of the reproduction period, namely 1/reproduction period and the deviation coefficient C s, looking up a hydrological phi value table to obtain a standardized Pearson III type variable phi p, and further calculating the composite surface rainfall x p of the reproduction period, wherein the calculation formula is as follows:
xp＝Ex(1+CvΦp) (17)
wherein x p is the composite surface rainfall in the reproduction period.
The beneficial effects of the invention are as follows: according to the village upstream basin composite surface rainfall frequency calculating method, the simulation rate of participating in calculation is increased, the conceptualization process is enhanced, uncertainty caused by a single sequence is avoided, and the reliability of a calculating result is improved; meanwhile, the evaporation factors are considered, an improved empirical formula is provided, the accuracy of the calculation process is improved, and the calculated rainfall frequency has wider regional and climate adaptability by combining the evaporation with the rainfall frequency analysis. The method improves reliability, accuracy and adaptability of the hydrological weather forecast, reduces the hydrological weather risk of the upstream river basin of villages, and simultaneously provides scientific basis for high-precision forecast of small river basins under various climate types.
The invention will be described in further detail with reference to the drawings and the detailed description.
Drawings
FIG. 1 is a flow chart of the method of the present invention.
Detailed Description
The invention discloses a village upstream basin composite surface rainfall frequency calculating method, as shown in fig. 1, comprising the following steps:
Step 1, determining the upstream river basin range of villages: and determining the upstream river basin range of the village according to the DEM data, and calculating the river basin area A.
Taking villages as centers, collecting DEM data in the range of 10km around the villages, and selecting DEM data with the maximum resolution as basic data; based on the basic data, operations of filling, flowing and setting river network density are sequentially performed in ArcGIS software. Using the village position as a drainage point, drawing an upstream drainage basin range of the village by using a watershed tool in hydrologic analysis, and calculating a drainage basin area A in ArcGIS software.
Step 2, determining average confluence time of upstream watershed of village: and (3) extracting the average drop J of the longest confluence path of the village upstream drainage basin according to the village upstream drainage basin range determined in the step (1), and further determining the average confluence time tau.
Based on the village upstream basin range in the step 1, extracting the longest confluence path average ratio drop J of the village upstream basin in ArcGIS software, and according to the theory that when the product of parameters N and K in Nash instantaneous unit lines is unit line lag, the parameters N and K of the village upstream basin instantaneous unit lines are calculated by adopting a regional empirical formula, and further determining the average confluence time tau=N.K.
The Nash instantaneous unit line formula is as follows:
Wherein N is the number of linear reservoirs and reflects the comprehensive energy regulating capacity of the river basin; k is the storage constant of the linear reservoir and reflects the confluence time characteristic of the river basin; Γ is a gamma function, Γ (N) = (N-1) +.! .
The regional empirical formula is:
wherein A is the upstream river basin area of village; j is the average drop of the longest confluence path; and I is rainfall intensity.
Step 3, calculating the hour-by-hour face rainfall of the upstream river basin of the village: collecting historical rainfall sequence data of rainfall stations in the upstream river basin range of villages, and calculating the hourly rainfall of the upstream river basin of the villages by adopting a Thiessen polygon method
Collecting historical rainfall sequence data of rainfall stations in the upstream basin range of villages, specifically collecting and identifying all rainfall station codes in the upstream basin range of villages, and extracting the historical rainfall sequence data (hour-by-hour data) of the rainfall stations from a database according to the codes; and screening the extracted rainfall station data, removing stations without data and stations with abnormal data, obtaining screened station information, and setting the screened station information as n stations.
In ArcGIS software, n Thiessen polygons are created according to the n site information obtained after screening, the areas of the polygons are automatically identified by the software, the hour-by-hour face rainfall of the upstream river basin of village is calculated based on the Thiessen polygon method, and the calculation formula is as follows:
in the method, in the process of the invention, Hour-by-hour rainfall for village upstream watershed; p i and a i are the measured rainfall at the rainfall station in the ith Thiessen polygon and the area of the ith Thiessen polygon, i=1, 2, …, n, respectively; a is the area of the upstream watershed in village.
Step 4, calculating the composite surface rainfall of the upstream river basin of the village: according to the average drainage basin confluence time tau determined in the step 2 and the village upstream drainage basin hour-by-hour face rainfall determined in the step 3, the front 15 day face rainfall p D-1,pD-2,…,pD-15 and the front tau hour accumulated face rainfall p s corresponding to the village upstream drainage basin hour-by-hour face rainfall are calculated, the hourly front-stage influence rainfall p p is calculated according to the improved front-stage influence rainfall empirical formula, and then the village upstream drainage basin hourly composite face rainfall p c is calculated.
For the face rainfall at the time of D day H, the corresponding face rainfall p D-1,pD-2,…,pD-15 day by day in the first 15 days is the face rainfall of D-1, D-2, … and D-15 days; the accumulated surface rainfall p s of the previous tau hours is the sum of the surface rainfall of H-1, H-2, … and H-tau for tau hours.
According to the front 15 days daily face rainfall p D-1,pD-2,…,pD-15 corresponding to the H-time face rainfall, the early-stage influence rainfall in the H time is calculated by the following empirical formula:
Wherein p p is the earlier-stage influence rainfall; k is an early rainfall influence coefficient; e m is the drainage basin evaporation capacity, and W m is the drainage basin soil water storage capacity.
The actual calculation of the early-middle rainfall influence coefficient k generally takes a fixed empirical value. Taking into account the basin evaporation capacity variation, an empirical formula for improving k based on the basin daily potential evaporation capacity. The day-by-day data of net radiation, humidity, wind speed, temperature, etc. of the upstream basin of the village is collected, the daily potential evaporation amount ET o,D-i of D-1, D-2, …, D-15 days is calculated by using Penman-montetith formula (abbreviated as PM formula), and the maximum potential evaporation amount ET omax＝max(ETo,D-1,ETo,D-2,…,ETo,D-15) is determined, which approximately represents the evaporation capacity E m of the basin in D-1, D-2, …, D-15 days.
The PM formula is:
Wherein ET o is the potential vapor emission amount; r n is ground net radiation; g is soil heat flux; gamma is the hygrometer constant; t is the daily average temperature; u 2 is the wind speed at the height of 2 m; e s is saturated water vapor pressure; e a is the actual water vapor pressure; delta is the slope of the steam pressure curve.
The empirical formula for improving the early rainfall impact coefficient k is:
Substituting k calculated by the formula (6) into the formula (4), the improved early-stage influence rainfall empirical formula is:
According to the calculated p p and p s, the composite surface rainfall p c of the upstream drainage basin of the village per hour is calculated as follows:
pc＝pp×ps (8)
wherein p c is the composite surface rainfall of village upstream watershed per hour; p p is the earlier-stage influence rainfall; p s is the accumulated face rainfall for the first τ hours.
Step 5, calculating a statistical characteristic value of the p c sample: based on the hourly composite surface rainfall p c of the village upstream basin obtained in the step 4, selecting the annual maximum p c value to form a p c sample, calculating the statistical characteristic value of the p c sample, wherein the statistical characteristic value comprises the mean E x, the variance D x, the variation coefficient C v and the bias coefficient C s of the p c sample, and the calculation formulas are respectively as follows:
where p cj is the p c value of the j-th event in the sample, j=1, 2, …, m; m is the total number of samples.
Step 6, determining a frequency distribution curve of p c: selecting a pearson III-type frequency curve, calculating curve parameters based on the statistical characteristic value of p c, and determining a frequency distribution curve of p c;
And (3) selecting a pearson III-type frequency curve commonly used in China, and calculating curve parameters including curve shape parameters, curve scale parameters and curve position parameters based on the statistical characteristic value of the p c obtained in the step (5), so as to determine a frequency distribution curve of the p c.
The calculation formulas of the curve shape parameter, the curve scale parameter and the curve position parameter are respectively as follows:
Wherein a 0 is a curve shape parameter; alpha is a curve scale parameter; beta is a curve position parameter.
The frequency distribution curve of p c is determined according to the above description:
Wherein P (X) is the frequency of X.gtoreq.x; Γ is a gamma function, Γ (α) = (α -1) +.! .
Step 7, calculating the composite surface rainfall in the reproduction period: and calculating the composite surface rainfall in the reproduction period, such as the composite surface rainfall in the reproduction period of 1000 years, 500 years, 100 years and the like according to the frequency distribution curve of p c.
According to the corresponding frequency of the reproduction period (namely 1/reproduction period) and the deviation coefficient C s, looking up a hydrological phi value table to obtain a standardized Pearson III type variable phi p, and further calculating the composite surface rainfall x p of the reproduction period, wherein the calculation formula is as follows:
xp＝Ex(1+CvΦp) (17)
wherein x p is the composite surface rainfall in the reproduction period.
Finally, it should be noted that the above description is only for the purpose of illustrating the technical solution of the present invention and not for the purpose of limiting the same, and that although the present invention has been described in detail with reference to the preferred arrangement, it will be understood by those skilled in the art that modifications and equivalents may be made thereto without departing from the spirit and scope of the technical solution of the present invention.
Claims (2)
1. A method for calculating a composite surface rainfall frequency of an upstream watershed of a village, comprising the steps of:
step 1, determining the upstream river basin range of villages: determining the upstream river basin range of villages according to the DEM data, and calculating the river basin area A;
Step 2, determining average confluence time of upstream watershed of village: extracting the average drop J of the longest confluence path of the village upstream drainage basin according to the village upstream drainage basin range determined in the step 1, and further determining the average confluence time tau;
Step 3, calculating the hour-by-hour face rainfall of the upstream river basin of the village: collecting historical rainfall sequence data of rainfall stations in the upstream river basin range of villages, and calculating the hourly rainfall of the upstream river basin of the villages by adopting a Thiessen polygon method
Step 4, calculating the composite surface rainfall of the upstream river basin of the village: according to the average drainage basin confluence time tau determined in the step 2 and the village upstream drainage basin hour-by-hour face rainfall determined in the step3Calculating the daily face rainfall p D-1,pD-2,…,pD-15 of the previous 15 days and the accumulated face rainfall p s of the previous tau hours corresponding to the hour face rainfall of the upstream watershed of the village, calculating the early-stage influence rainfall p p of each hour according to an improved early-stage influence rainfall empirical formula, and further calculating the compound face rainfall p c of the upstream watershed of the village;
Step 5, calculating a statistical characteristic value of the p c sample: based on the hourly composite surface rainfall p c of the village upstream basin obtained in the step 4, selecting the annual maximum p c value to form a p c sample, and calculating the statistical characteristic value of the p c sample;
Step 6, determining a frequency distribution curve of p c: selecting a pearson III-type frequency curve, calculating curve parameters based on the statistical characteristic value of p c, and determining a frequency distribution curve of p c;
Step 7, calculating the composite surface rainfall in the reproduction period: according to the frequency distribution curve of p c, calculating the composite surface rainfall in the reproduction period;
The specific process of determining the upstream river basin range of village and calculating the river basin area A according to the DEM data in the step 1 is as follows: taking villages as centers, collecting DEM data in the range of 10km around the villages, and selecting DEM data with the maximum resolution as basic data; based on the basic data, sequentially performing operations of filling, flowing and setting river network density in ArcGIS software; using the position of the village as a drainage point, drawing the upstream drainage basin range of the village by using a watershed tool in hydrologic analysis, and calculating the drainage basin area A in ArcGIS software;
The specific process of determining the average confluence time tau in the step 2 is as follows: according to the theory that when the product of parameters N and K in Nash instantaneous unit lines is unit line lag, the product is equivalent to average basin conflux time tau, the regional empirical formula is adopted to calculate parameters N and K of the instantaneous unit lines of the upstream basin of villages, and further the average conflux time tau=N.K is determined;
the Nash instantaneous unit line formula is as follows:
Wherein N is the number of linear reservoirs; k is the storage constant of the linear reservoir; Γ is a gamma function, Γ (N) = (N-1) +.! ;
the regional empirical formula is:
wherein A is the upstream river basin area of village; j is the average drop of the longest confluence path; i is rainfall intensity;
Collecting historical rainfall sequence data of rainfall stations in the upstream drainage basin range of villages in the step 3, namely collecting and identifying all rainfall station codes in the upstream drainage basin range of villages, extracting the historical rainfall sequence data of the rainfall stations from a database according to the codes, screening the extracted rainfall station data, removing stations without data and stations with abnormal data, obtaining screened station information, and setting the station information as n stations;
The hour-by-hour face rainfall of the upstream river basin of village is calculated by adopting a Thiessen polygon method The specific process of (2) is as follows: in ArcGIS software, n Thiessen polygons are created according to the n site information obtained after screening, the areas of the polygons are automatically identified by the software, the hour-by-hour face rainfall of the upstream river basin of village is calculated based on the Thiessen polygon method, and the calculation formula is as follows:
in the method, in the process of the invention, Hour-by-hour rainfall for village upstream watershed; p i and a i are the measured rainfall at the rainfall station in the ith Thiessen polygon and the area of the ith Thiessen polygon, i=1, 2, …, n, respectively; a is the area of the upstream river basin of village;
In the step 4, the early-stage influence rainfall p p per hour is calculated according to the improved early-stage influence rainfall empirical formula, and the specific process for calculating the composite surface rainfall p c per hour of the upstream basin of the village is as follows:
For the face rainfall at the time of D day H, the corresponding face rainfall p D-1,pD-2,…,pD-15 day by day in the first 15 days is the face rainfall of D-1, D-2, … and D-15 days; the accumulated surface rainfall p s of the former tau hours is the sum of the surface rainfall of H-1, H-2, … and H-tau for tau hours;
According to the front 15 days daily face rainfall p D-1,pD-2,…,pD-15 corresponding to the H-time face rainfall, the early-stage influence rainfall in the H-time is calculated by an empirical formula shown in the formula (4):
Wherein p p is the earlier-stage influence rainfall; k is an early rainfall influence coefficient; e m is the drainage basin evaporation capacity, and W m is the drainage basin soil water storage capacity;
An empirical formula for improving k based on basin daily potential vapor deposition: collecting data of net radiation, humidity, wind speed and temperature of the upstream basin of village day by day, calculating the daily potential evaporation amount ET o,D-i of D-1, D-2, … and D-15 days by using a PM formula, and determining the maximum potential evaporation amount ET omax＝max(ETo,D-1,ETo,D-2,…,ETo,D-15, wherein the evaporation capacity E m of the basin in D-1, D-2, … and D-15 days is represented;
The PM formula is:
Wherein ET o is the potential vapor emission amount; r n is ground net radiation; g is soil heat flux; gamma is the hygrometer constant; t is the daily average temperature; u 2 is the wind speed at the height of 2 m; e s is saturated water vapor pressure; e a is the actual water vapor pressure; delta is the slope of the steam pressure curve;
the empirical formula for improving the early rainfall impact coefficient k is:
substituting the formula (6) into the formula (4), and obtaining an improved early-stage influence rainfall empirical formula which is as follows:
The composite surface rainfall per hour p c of the upstream watershed of villages is calculated as follows:
pc＝pp×ps (8)
Wherein p c is the composite surface rainfall of village upstream watershed per hour; p p is the earlier-stage influence rainfall; p s is the accumulated face rainfall for the first τ hours;
in the step 6, the curve parameters include curve shape parameters, curve scale parameters and curve position parameters, and the calculation formulas are respectively as follows:
Wherein a 0 is a curve shape parameter; alpha is a curve scale parameter; beta is a curve position parameter;
the frequency distribution curve of p c is:
Wherein P (X) is the frequency of X.gtoreq.x; Γ is a gamma function, Γ (α) = (α -1) +.! ;
The composite surface rainfall of the calculated reproduction period in the step 7 is specifically: according to the corresponding frequency of the reproduction period, namely 1/reproduction period and the deviation coefficient C s, looking up a hydrological phi value table to obtain a standardized Pearson III type variable phi p, and further calculating the composite surface rainfall x p of the reproduction period, wherein the calculation formula is as follows:
xp＝Ex(1+CvΦp) (17)
wherein x p is the composite surface rainfall in the reproduction period.
2. The method for calculating the composite surface rainfall frequency of the village upstream basin according to claim 1, wherein in the step 5, the mean E x, the variance D x, the variance coefficient C v and the bias coefficient C s of the p c sample are calculated according to the following formulas:
where p cj is the p c value of the j-th event in the sample, j=1, 2, …, m; m is the total number of samples.
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