CN113779041B - A method for dividing and coding small watersheds - Google Patents

Abstract

The invention discloses a small drainage basin dividing and encoding method which comprises the steps of basic data collection and arrangement, small drainage basin dividing, small drainage basin basic attribute extraction, small drainage basin unified encoding, space topological relation establishment, river cross section extraction, space association relation establishment, step-by-step merging of large drainage basins, small drainage basin standardized unit line extraction and smooth decryption of data results. The invention realizes small drainage basin division and coding, and can better describe the topological relation of the small drainage basins in the small drainage basin coding aiming at the national range.

Description

A method for dividing and coding small watersheds

Technical field

The invention belongs to the technical field of hydrological data processing, and specifically relates to a small watershed division and encoding method.

Background technique

Distributed hydrological model modeling requires a large amount of geographical information data. Commonly used methods are used to divide and code small watersheds for a small and medium-sized watershed or a specific region. Currently, there is no national-scale small watershed dividing and coding method. The main reason is that there is currently no unified coding rule for divided small watersheds. Therefore, when there is a large amount of data for the classification of small watersheds nationwide, it is difficult for data users to conduct rapid searches and queries. At the same time, the divided small watersheds cannot support parallel computing of large-scale distributed hydrological models.

Contents of the invention

In order to solve the problem of dividing and coding small watersheds nationwide, the present invention provides a method of dividing and coding small watersheds.

The present invention is realized through the following technical solutions. A method for dividing and encoding small watersheds includes the following steps:

S1. Basic data collection and sorting: including data inspection, data sorting, coordinate conversion, cropping, splicing, and spatial matching steps;

S2. Small watershed division: including river burning processing, river embankment processing, filling processing, flow direction calculation, slope calculation, catchment area calculation, river definition, river segment definition, watershed grid definition, watershed boundary extraction, and river lines. Extraction, watershed outlet point extraction, river intersection extraction, attention point processing, watershed area adjustment, watershed splitting, watershed merging, reservoir and lake watershed processing steps;

S3. Extraction of basic attributes of small watersheds: including extraction of related attributes of watersheds, rivers, and nodes.

S4, unified coding of small watersheds;

S5. Establishment of spatial topological relationships: The water surface boundaries of reservoirs with an area exceeding 0.5 km ² and lakes with an area exceeding 1 km ² are treated as separate watershed surfaces, and the watershed boundaries where the reservoirs and lakes are located should completely include the water surface boundaries of the reservoirs and lakes. There is no overlapping phenomenon with the reservoir boundary, and the spatial topological relationship between the reservoir, lake basin and the original river channel, inflow channel and outflow channel is established;

S6. River cross-section extraction: Use original DEM data to extract river cross-sections 50-200 meters upstream and downstream of the basin outlet node;

S7. Establishment of spatial correlation: Establish correlation between small watersheds, administrative divisions, monitoring sites, and water conservancy projects;

S8. Merge large watersheds step by step:

S9. Small watershed standardized unit line extraction: including small watershed underlying surface slope roughness extraction, small watershed underlying surface infiltration characteristics extraction, small watershed standardized unit line extraction;

S10. Smoothing and dedensification processing of data results: The vector data of small watershed division results are smoothed.

Further preferably, in step S2, watershed merging refers to merging one by one from upstream to downstream, first tributaries and then main streams, and each merged watershed is saved in the same layer file.

Further preferably, in step S4, the unified coding method for small watersheds is as follows:

(1) Based on the coded rivers in the "Chinese River Code" (SL249-2012), find the river with the largest drainage area from the outlet of the river upstream as the main stream, segment it according to the outlet node of the small drainage basin, and code it from top to bottom. ; The tributaries that merge into this main stream are also coded from top to bottom; then this tributary is used as the main stream of the lower tributary, and is coded step by step according to this principle;

(2) For independent water systems that are not coded in the "Chinese River Code" (SL249-2012), the first 7 digits of the code will be compiled according to the coding rules of the "Chinese River Code" (SL249-2012), and then the last 9 digits will be compiled according to the small watershed coding rules. code;

(3) If the name, location, and flow direction of the rivers coded in the "China River Code" (SL249-2012) are inconsistent with those in the 1:50,000 DLG, make corresponding modifications or recode them according to the superior rivers;

(4) The watershed code after level-by-level merging is the same as the code of the most downstream small watershed.

To further optimize, in step S4, the coding rules are: 1) All external river segments (river segments not joined by other river segments) are the first level; 2) Two river segments of the same level (let their level be k) converge, The level of the new river channel formed is k+1; 3) If a river segment with level k is added to a higher-level river segment, the higher-level river segment will increase by 1 level.

Further preferably, in step S4, the encoding is defined:

a Coding objects: rivers with a drainage area greater than ^500km2 , as well as rivers where large, important and medium-sized reservoirs and sluices are located. For areas where the drainage area is difficult to determine, a river length of 30km is used as the standard;

The b code adopts a mixed code of Latin letters and numbers, with a total of 8 digits, which respectively indicate the basin, water system, number and category of the river;

The format and meaning of c code:

Code format: ABTFFSSY.

A - 1 letter indicating the project category, the value is A;

BT - 2-digit letters represent water system zoning code, implemented in SL 213-2012;

FFSS - 4 digits or letters representing the number of any river, the value range of F and S is 0~9, A~Y;

Y - 1 digit indicates the river category.

When the code digits are not enough or for river network areas where it is difficult to distinguish the upstream and downstream relationships, the restrictions on FFSS are cancelled. The order of canceling the restriction conditions is: cancel the restriction that the second S of SS is 0. If it is still not satisfied, cancel the restrictions on FFSS. 00~09 are used as restrictions on the main stream or different river section codes of the main stream.

Small watershed coding definition:

The number of coding digits for small watersheds is 16, and the value of each bit is uppercase letters A-Z, lowercase letters a-z, or numbers 0-9. The same code is used for watersheds and river sections;

The encoding structure is: FBTFSSHHHXXXXXX

F——1 bit, which is the classification code, W: watershed, A: river course, Q: node;

BT - 2-digit letters represent water system zoning code, implemented in SL 213-2012;

FFSS - 4 digits or letters representing the number of any river, the value range of F and S is 0~9, A~Y;

HHH——This segment represents the mainstream encoding, the default is 1 bit. When there are too many main river sections, code points will be automatically added backwards, up to 3 digits, numbers 1-9, uppercase letters A-Z, and lowercase letters a-z;

XXXXXX——This paragraph represents the coding of the tributaries below the main stream. If there are no tributaries, it is filled with 0; the lower main branches and tributaries are coded step by step according to this principle, with uppercase letters A-Z and lowercase letters a-z.

Further preferably, in step S5, the spatial topological relationship is established as follows:

(1) Establish the upstream and downstream topological relationships of the water system through the FRVCD and TRVCD fields in the river segment layer attribute table. FRVCD indicates the code of the upper reaches of the river that flows into the river, and TRVCD indicates the code of the lower reaches of the outflow;

(2) Based on the water catchment relationship between the upstream and downstream river sections, establish the upstream and downstream topological relationship of the small watershed through the IWSCD and OWSCD fields in the small watershed layer attribute table; IWSCD represents the watershed code that flows into the watershed, and OWSCD represents the downstream watershed code that flows out. ;

(3) The topological relationship between small watersheds and river sections is established through the BWSCD field in the river section layer attribute table. This field BWSCD represents the watershed code where the river section is located;

(4) The water catchment relationship between the outlet nodes of the small watershed is established through the FNDCD and TNDCD fields in the node layer attribute table to establish the upstream and downstream topological relationships. FNDCD represents the node code that flows into the node, and TNDCD represents the downstream node code that flows out;

(5) The spatial topological relationship between the node and the small watershed and river channel is established through the AWSCD and ARVCD fields in the node layer attribute table. AWSCD represents the watershed code set that merges into the node, and ARVCD represents the river channel code set that merges into the node;

(6) A database is used to establish the relationship between small watersheds, county and rural administrative divisions, automatic monitoring sites shared with the national flash flood disaster monitoring and early warning platform, and water conservancy projects such as large reservoirs, hydropower stations, and sluices.

The technical effect of the present invention: realizes the division and coding of small watersheds, and can better describe the topological relationships of small watersheds in nationwide small watershed coding.

Description of the drawings

Figure 1 is a flow chart of the present invention.

Figure 2 is a schematic diagram of merging small watersheds step by step.

Figure 3 is a small watershed coding structure diagram.

Detailed ways

The present invention will be further explained in detail below in conjunction with the accompanying drawings.

A method for dividing and coding small watersheds, including the following steps:

S1. Basic data collection and sorting: including data checking, data sorting, coordinate conversion, cropping, splicing, and spatial matching steps. The data includes 16 layers: small watershed surface, small watershed boundary, small watershed river section, and the longest confluence of small watershed Path, small watershed outlet node, small watershed river section outlet section, step-by-step merge of small watersheds I watershed surface, step-by-step merge of small watersheds I watershed lines, step-by-step merge of small watersheds I river sections, step-by-step merge of small watersheds I longest confluence Paths, residential areas, embankments, reservoirs, point-shaped water system ancillary facilities, linear water system ancillary facilities, and monitoring sites;

S2. Small watershed division: including river burning processing, river embankment processing, filling processing, flow direction calculation, slope calculation, catchment area calculation, river definition, river segment definition, watershed grid definition, watershed boundary extraction, and river lines. Extraction, watershed outlet point extraction, river intersection extraction, attention point processing, watershed area adjustment, watershed splitting, watershed merging, reservoir and lake watershed processing steps;

Using 1:50,000 DEM and DLG data provided by the National Basic Geographic Information, as well as hydrological monitoring site and water conservancy project data, combined with high-resolution image data, it mainly focuses on the water system of the "Chinese River Code" (SL 249-2012), combined with Administrative divisions at provincial and county levels should be implemented to rationally divide small watersheds. The plane coordinate system adopts the national geodetic coordinate system: WGS84; the projection method adopts the 6-degree zoning Gauss-Krüger projection; the elevation datum adopts the 1985 national elevation datum.

The small watershed is divided into hilly areas with a ground slope of ≥2 degrees across the country, and areas with local slopes of less than 2 degrees also need to be treated accordingly. The final divided small watershed should cover the flash flood disaster prevention and control area. Two adjacent small watersheds should be spliced without gaps and overlaps. The sum of the areas of the small watersheds should be equal to the total area of the region.

In principle, the area of small watersheds should be controlled between 10-50km2. In special cases, it should not be less than 3km2 or greater than 100km2 (to maintain the correct topological relationship of the water system, there are exceptions for a small number of fragments). Focus on considering factors such as reservoirs, hydropower stations, sluices, hydrological stations, villages and towns, residential areas, and topographic and geomorphological characteristics, and set nodes for dividing small watersheds.

On the basis of the division of small watersheds, the watersheds are gradually merged into the watersheds of the lowest-level rivers in the "Chinese River Code" (SL 249-2012) (Note: The lowest-level rivers stipulated in the Chinese River Code Standard are: the watershed area is greater than 500km2 or the length is greater than 30km of rivers, as well as rivers where large and important medium-sized reservoirs and sluices are located).

Watershed merging refers to merging one by one from upstream to downstream, first tributaries and then main streams. Each merged watershed is saved in the same layer file, as shown in Figure 2.

The boundaries of small watersheds should be consistent with the natural watershed of the terrain, and the error should not exceed 1 grid.

The error between the extracted river channel (river segment) lines and the actual river channel lines (river lines in the 1:50,000 DLG data) does not exceed 1 raster.

S3. Extraction of basic attributes of small watersheds: including extraction of related attributes of watersheds, rivers, and nodes.

S4, unified code for small watersheds:

Follow the following small watershed coding principles:

(1) Unification principle: The unified coding of small watersheds is expanded on the basis of the national third-level river codes;

(2) Unique principle: ensure the uniqueness of the coding of each small watershed in hilly areas nationwide;

(3) Stability principle: The coding system ensures that no major changes will occur within a long period of time;

(4) Compatibility principle: The coding must be compatible with the current system to ensure minimal system changes;

(5) Expansion principle: When the small watershed is adjusted or increased in the future, the coding scheme can be expanded;

(6) Principle of topological correctness: reflect the logical connections of watersheds at all levels and accurately reflect surface water confluence relationships;

(7) Principle of hierarchical recursion: hierarchical compilation and step-by-step recursion based on the topological relationships contained or juxtaposed by watersheds at each level;

(8) Top-down principle: Watersheds at the same level are coded in sequence from top to bottom, from left bank to right bank, according to the direction of water flow.

encoding rules:

(1) River basin classification:

The coding scheme takes into account the actual needs of flash flood disaster prevention and control, as well as the complexity of providing basic data for establishing distributed hydrological models. With reference to the Strahler grading scheme, an optimized river grading scheme is proposed, as shown in the figure below. The main principles are: 1) All external river segments (river segments not joined by other river segments) are the first level; 2) Two river segments of the same level (let their level be k) converge, and the level of the new river channel formed is k+ 1; 3) If a river segment with level k is added to a higher-level river segment, the higher-level river segment will increase by 1 level.

(2) Coding definition of "China River Code" (SL 249-2012)

a coding object. Rivers with a drainage area greater than 500km2, as well as rivers where large and important medium-sized reservoirs and sluices are located. For areas where the drainage area is difficult to determine, a river length of 30km is used as the standard.

The b code adopts a mixed code of Latin letters (I, O, Z discarded) and numbers, with a total of 8 digits, which respectively represent the basin, water system, number and category of the river.

The format and meaning of c code:

Code format: ABTFFSSY.

A——One letter indicates the project category, and the value is A.

BT - 2-digit letters represent the water system zoning code, implementing SL 213-2012.

FFSS - 4-digit numbers or letters represent the number of any river. The value range of F and S is 0~9, A~Y.

Y - 1 digit indicates the river category.

Note: When there are not enough code digits or for river network areas where it is difficult to distinguish the upstream and downstream relationships, the restriction on FFSS is cancelled. The order of canceling the restriction conditions is: cancel the restriction that the second S of SS is 0. If it is still not satisfied, cancel the restriction on 00 to 09 in FF as the main stream or different river section codes of the main stream.

River Code FFSS Field Regulations

(3) Definition of small watershed coding

The number of small watershed coding digits is 16, and the value of each bit is an uppercase letter (A-Z), a lowercase letter (a-z) or a number (0-9). The same coding is used for river basins and river sections. The coding structure is shown in Figure 3. The coding specifications for small watersheds are shown in the table below.

Small watershed coding definition:

The number of coding digits for small watersheds is 16, and the value of each bit is uppercase letters A-Z, lowercase letters a-z, or numbers 0-9. The same code is used for watersheds and river sections;

The encoding structure is: FBTFSSHHHXXXXXX

F——1 bit, which is the classification code, W: watershed, A: river course, Q: node;

BT - 2-digit letters represent water system zoning code, implemented in SL 213-2012;

FFSS - 4 digits or letters representing the number of any river, the value range of F and S is 0~9, A~Y;

HHH——This segment represents the mainstream encoding, the default is 1 bit. When there are too many main river sections, code points will be automatically added backwards, up to 3 digits, numbers 1-9, uppercase letters A-Z, and lowercase letters a-z;

XXXXXX——This paragraph represents the coding of the tributaries below the main stream. If there are no tributaries, it is filled with 0; the lower main branches and tributaries are coded step by step according to this principle, with uppercase letters A-Z and lowercase letters a-z.

Code bit regulations for small watersheds

Encoding method:

(1) Based on the coded rivers in the "Chinese River Code" (SL249-2012), find the river with the largest drainage area from the outlet of the river upstream as the main stream, segment it according to the outlet node of the small drainage basin, and code it from top to bottom. ; The tributaries that merge into this main stream are also coded from top to bottom; then this tributary is used as the main stream of the lower-level tributaries, and is coded step by step according to this principle.

(2) For independent water systems (rivers that flow into the sea alone, inland rivers, etc.) that are not coded in the "Chinese River Code" (SL249-2012), the first 7 digit codes should be compiled according to the coding rules of the "Chinese River Code" (SL249-2012) , and then compile the last 9-digit code according to the small watershed coding rules.

(3) If the names, locations, and flow directions of the rivers coded in the "China River Code" (SL249-2012) are inconsistent with those in the 1:50,000 DLG, make corresponding modifications or recode them according to the superior rivers.

(4) The watershed code after level-by-level merging is the same as the code of the most downstream small watershed.

S5. Establishment of spatial topological relationships: The water surface boundaries of reservoirs with an area exceeding 0.5 km ² and lakes with an area exceeding 1 km ² are treated as separate watershed surfaces, and the watershed boundaries where the reservoirs and lakes are located should completely include the water surface boundaries of the reservoirs and lakes. There is no overlapping phenomenon with the reservoir boundary, and the spatial topological relationship between the reservoir, lake basin and the original river channel, inflow channel and outflow channel is established.

(1) Establish the upstream and downstream topological relationship of the water system through the (FRVCD, TRVCD) fields in the river segment layer attribute table. FRVCD indicates the code of the upper reaches of the river that flows into the river, and TRVCD indicates the code of the lower reaches of the outflow.

(2) Based on the water catchment relationship between the upstream and downstream river sections, establish the upstream and downstream topological relationship of the small watershed through the (IWSCD, OWSCD) fields in the small watershed layer attribute table. IWSCD represents the basin code that flows into the basin, and OWSCD represents the downstream basin code that flows out.

(3) The topological relationship between small watersheds and river sections is established through the (BWSCD) field in the river section layer attribute table. This field (BWSCD) indicates the watershed code where the river segment is located.

(4) The water catchment relationship between small watershed outlet nodes is to establish the upstream and downstream topological relationships through the (FNDCD, TNDCD) fields in the node layer attribute table. FNDCD represents the node code that is imported into the node, and TNDCD represents the downstream node code that flows out.

(5) The spatial topological relationship between nodes and small watersheds and rivers is established through the (AWSCD, ARVCD) fields in the node layer attribute table. AWSCD represents the watershed code set that flows into the node, and ARVCD represents the river channel code set that flows into the node.

(6) Established in the form of a database the relationships between small watersheds (river sections) and county and rural administrative divisions (residential areas), automatic monitoring sites shared with the national flash flood monitoring and early warning platform, and large reservoirs, hydropower stations, sluices and other water conservancy projects. relation.

S6. River cross-section extraction:

(1) Based on the original DEM data and combined with the small watershed layer, find the outlet node of the watershed.

(2) Extract river channel cross-sections 50-200 meters upstream and downstream of the watershed outlet node through three-dimensional spatial analysis tools.

(3) When setting the DEM data processing scope, the connection relationship with adjacent watersheds should be considered to ensure the integrity of the watershed.

S7. Establishment of spatial correlation relationship:

(1) Establish the upstream and downstream topological relationship of the water system through the (FRVCD, TRVCD) fields in the river segment layer attribute table. FRVCD indicates the code of the upper reaches of the river that flows into the river, and TRVCD indicates the code of the lower reaches of the outflow.

(2) Based on the water catchment relationship between the upstream and downstream river sections, establish the upstream and downstream topological relationship of the small watershed through the (IWSCD, OWSCD) fields in the small watershed layer attribute table. IWSCD represents the basin code that flows into the basin, and OWSCD represents the downstream basin code that flows out.

(3) The topological relationship between small watersheds and river sections is established through the (BWSCD) field in the river section layer attribute table. This field (BWSCD) indicates the watershed code where the river segment is located.

(4) The water catchment relationship between small watershed outlet nodes is to establish the upstream and downstream topological relationships through the (FNDCD, TNDCD) fields in the node layer attribute table. FNDCD represents the node code that is imported into the node, and TNDCD represents the downstream node code that flows out.

(5) The spatial topological relationship between nodes and small watersheds and rivers is established through the (AWSCD, ARVCD) fields in the node layer attribute table. AWSCD represents the watershed code set that flows into the node, and ARVCD represents the river channel code set that flows into the node.

(6) Established in the form of a database the relationships between small watersheds (river sections) and county and rural administrative divisions (residential areas), automatic monitoring sites shared with the national flash flood monitoring and early warning platform, and large reservoirs, hydropower stations, sluices and other water conservancy projects. relation.

S8. Merge large watersheds step by step:

From upstream to downstream, first the tributaries and then the main stream, they are merged one by one, and each merged watershed is saved in the same layer file.

S9. Small watershed standardized unit line extraction: including small watershed underlying surface slope roughness extraction, small watershed underlying surface infiltration characteristics extraction, small watershed standardized unit line extraction. Use land use and vegetation type data to analyze the corresponding relationship between land use types and the comprehensive flow velocity coefficient of the small watershed slope, perform rasterization processing, and generally determine the comprehensive flow velocity coefficient value of the small watershed slope; use soil texture data to analyze the soil texture The corresponding relationship between the type and the infiltration characteristics of the small watershed slope is rasterized, the infiltration characteristic parameters of the small watershed are generally determined, and the comprehensive velocity coefficient and infiltration characteristics data attribute table of the small watershed slope is established. Standardized unit line extraction requirements: Based on the National Heavy Rain Atlas, calculate the standardized unit lines corresponding to different rain intensity levels in small watersheds at different time periods (including 10 minutes, 30 minutes, and 60 minutes) and the corresponding flood peak modulus and watershed convergence time of each unit line. .

S10. Smoothing and dedensification processing of data results: The vector data of small watershed division results are smoothed.

The above are only implementation examples of the present invention, and are not intended to limit the present invention in any form. Although the present invention has been disclosed as above with preferred implementation examples, it is not intended to limit the present invention. Any skilled person familiar with the art , without departing from the scope of the technical solution of the present invention, the structure and technical content disclosed above can be used to make slight changes or modifications to equivalent implementation examples with equivalent changes. However, any simple modifications, equivalent changes, and modifications made to the above implementation examples based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the technical solution of the present invention.

Claims (6)

1. The small drainage basin dividing and encoding method is characterized by comprising the following steps:

s1, collecting and sorting basic data: the method comprises the steps of data inspection, data arrangement, coordinate conversion, cutting, splicing and space matching;

s2, small drainage basin division: the method sequentially comprises the steps of river firing treatment, river embankment treatment, depression filling treatment, flow direction calculation, gradient calculation, water collecting area calculation, river definition, river section definition, river basin grid definition, river basin boundary extraction, river line extraction, river basin outlet point extraction, river basin intersection extraction, attention point treatment, river basin area adjustment, river basin splitting, river basin merging and reservoir and lake river basin treatment;

s3, extracting basic attributes of the small drainage basin: extracting relevant attributes of a river basin, a river channel and a node;

s4, uniformly coding the small drainage basins;

s5, establishing a space topological relation: for areas exceeding 0.5km ² Is more than 1km in area ² The boundary of the lake water surface of the water tank is treated as an independent river basin surface, and the boundary of the river basin where the water tank and the lake are positioned completely comprises the boundary of the water tank and the lake water surface,the method has no superposition phenomenon with reservoir boundaries, and establishes the spatial topological relation between reservoirs and lake river courses and original river courses, inflow river courses and outflow river courses;

s6, extracting the cross section of the river channel: extracting river cross sections at 50-200 meters upstream and downstream of the outlet nodes of the river basin by adopting original DEM data;

s7, establishing a spatial association relation: establishing an association relationship between a small river basin and an administrative division, a monitoring station and a hydraulic engineering;

s8, merging large drainage basins step by step:

s9, extracting a small drainage basin standardized unit line: the method comprises the steps of extracting the slope roughness of the undersea surface of the small drainage basin, extracting the undersea surface seepage characteristic of the undersea surface of the small drainage basin and extracting the standardized unit line of the small drainage basin;

s10, smooth decryption processing of data results: and carrying out smoothing treatment on vector data of small-drainage-basin division results.

2. The method of claim 1, wherein in step S2, the watershed merging means that upstream to downstream, tributary-first and then main stream are merged one by one, and each merged watershed is stored in the same layer file.

3. The small-drainage-basin dividing and encoding method according to claim 1, wherein in step S4, the small-drainage-basin unified encoding method is as follows:

(1) Based on the coded river in Chinese river code (SL 249-2012), searching the river with the largest river basin area from the river outlet to the upstream as a main stream, and coding from top to bottom according to the sections of small river basin outlet nodes; the tributaries that merge into the main stream are also encoded from top to bottom; then taking the branch as a main stream of a lower branch, and coding step by step according to the principle;

(2) Independent water systems without codes in Chinese river codes (SL 249-2012) firstly compile a front 7-bit code according to the coding rule of the Chinese river codes (SL 249-2012), and then compile a rear 9-bit code according to the small river coding rule;

(3) When the coded river in the Chinese river code (SL 249-2012) is inconsistent with the name, the position and the flow direction of the river in the 1:5 DLG, the coded river is modified correspondingly or recoded according to the superior river;

(4) The stream domain codes after step-by-step combination are the same as the downstream-most small stream domain codes.

4. A small watershed segmentation and coding method according to claim 3, wherein in step S4, the coding rule: 1) All the external river sections are first-stage river sections, and the external river sections are river sections without other river sections; 2) The two river channel sections with the same level and k are converged, and the level of the formed new river channel is k+1; 3) If the river channel segment with the level k is added into the river channel segment with the higher level, the river channel segment with the higher level is increased by 1 level.

5. The small watershed segmentation and coding method according to claim 1, wherein in step S4, the coding definition:

a coding object, the area of the river basin is more than 500km ² River of large, important medium-sized reservoirs and floodgates, and for areas where the river basin area is difficult to determine, the river length is taken as a standard of 30 km;

the code b adopts Latin letters and numbers to be mixed and coded, 8 bits are used for respectively representing the river basin, water system, number and category of the river;

c format and meaning of code:

code format: ABTFFSSY;

a-1 bit letter represents engineering category, and takes value A;

BT-2 bit letters represent water system partition codes, and SL 213-2012 is executed;

FFSS-4 digits or letters represent the number of any river, and the value range of F, S is 0-9 and A-Y;

y-1 digits represent river class;

when the number of code bits is insufficient or for river network areas in which the upstream-downstream relationship is not easily distinguished, the limitation on FFSS is canceled, and the order of canceling the limitation conditions is as follows: canceling the limitation of SS that the second S is 0, canceling the limitation of 00-09 in FF as the codes of different river segments of the main stream or the main stream when the limitation is still not satisfied;

small-watershed coding definition:

the number of the coding digits of the small river basin is 16, the value of each digit is uppercase letters A-Z, lowercase letters a-Z or numbers 0-9, and the river basin and the river reach adopt the same coding;

the coding structure is as follows: FBTFFSSHHHHXXXXXXXX

F, 1 bit is a classification code, W is a river basin, A is a river channel, and Q is a node;

BT-2 bit letters represent water system partition codes, and SL 213-2012 is executed;

FFSS-4 digits or letters represent the number of any river, and the value range of F, S is 0-9 and A-Y;

hhh—represents dry stream coding, defaulting to 1 bit; when the main stream river reach is too many, automatically adding code bits, at most 3 bits, numbers 1-9, uppercase letters A-Z and lowercase letters a-Z backwards;

XXXXXX-indicates the coding of the substream below the main stream, filled with 0 when no substream is present; the lower stem and branch stream is encoded step by step according to the principle, with capital letters A-Z and lowercase letters a-Z.

6. The method for small basin division and coding according to claim 1, wherein in step S5, the spatial topological relation is established as follows:

(1) Establishing a water system upstream and downstream topological relation through FRVCD and TRVCD fields in a river reach layer attribute table; FRVCD represents the upper run code of the incoming run, TRVCD represents the lower run code of the outgoing run;

(2) Establishing a small river basin upstream and downstream topological relation through IWSCD and OWSCD fields in a small river basin layer attribute table according to the upstream and downstream catchment relation of the river basin; IWSCD represents the basin code that merges into the basin, OWSCD represents the downstream basin code that flows out;

(3) The topological relation between the small river basin and the river reach is that the relation is established through BWSCD fields in a river reach layer attribute table, and BWSCD represents the river basin code of the river reach;

(4) The water collecting relation between the small drainage basin outlet nodes is that an upstream and downstream topological relation is established through FNDCD and TNDCD fields in a node layer attribute table, wherein FNDCD represents node codes which are converged into the node, and TNDCD represents downstream node codes which are flown out;

(5) The spatial topological relation between the node and the small river basin and the spatial topological relation between the node and the river channel are established through AWSCD and ARVCD fields in a node layer attribute table, wherein AWSCD represents a river basin code set converged into the node, and ARVCD represents a river channel code set converged into the node;

(6) And the association relation between the small river basin and administrative division of county and country, the automatic monitoring site shared to the national mountain torrent disaster monitoring and early warning platform and the hydraulic engineering is established in a database mode.