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CN-122022839-A - Method and system for identifying nitrogen and phosphorus loss pollution hot spot of river basin farmland and tracing water quality degradation

CN122022839ACN 122022839 ACN122022839 ACN 122022839ACN-122022839-A

Abstract

The invention discloses a method and a system for identifying nitrogen and phosphorus loss pollution hot spots of a river basin farmland and tracing water quality degradation, wherein a research river basin is divided into a plurality of landscape units based on a river basin process simulation model, and nitrogen and phosphorus loads of each landscape unit in a preset period are calculated; the method comprises the steps of constructing a directed communication network between landscape units based on landscape unit boundaries and terrain data, constructing a pollutant transmission directed graph, calculating pollutant transmission weights of directed edges according to terrain elevation differences, space distances and shared boundary lengths between the landscape units, constructing a pollutant transmission weight matrix, coupling nitrogen and phosphorus loads of each landscape unit with the pollutant transmission weight matrix, carrying out pollutant transmission calculation on the directed communication network to obtain contribution amounts of each landscape unit on target section nitrogen and phosphorus loads, sequencing the landscape units according to the contribution amounts, and identifying farmland nitrogen and phosphorus loss pollution hotspot units so as to realize space tracing of water quality degradation. The invention is suitable for hilly type watershed mainly polluted by agricultural non-point sources, and has simple operation method and convenient popularization and application.

Inventors

  • WANG YANAN
  • YANG GUISHAN
  • CHEN HONG
  • ZHANG CHUNLEI

Assignees

  • 河海大学

Dates

Publication Date
20260512
Application Date
20260128

Claims (10)

  1. 1. A method for identifying nitrogen and phosphorus loss pollution hot spots and tracing water quality degradation of a river basin farmland is characterized by comprising the following steps: step 1, dividing a research river basin into a plurality of landscape units based on a river basin process simulation model, and calculating nitrogen and phosphorus loads of each landscape unit in a preset period; Step 2, constructing a directed communication network between landscape units based on the boundaries of the landscape units and the topographic data, and constructing a pollutant transmission directed graph; Step 3, calculating pollutant transmission weights of the directed edges according to the terrain elevation difference, the space distance and the shared boundary length between the landscape units, and constructing a pollutant transmission weight matrix; Step 4, coupling the nitrogen and phosphorus loads of each landscape unit with a pollutant transmission weight matrix, and carrying out pollutant transmission calculation on a directional communication network to obtain contribution of each upstream landscape unit to the nitrogen and phosphorus loads of the target section; and 5, sequencing the view units according to the contribution quantity result, and identifying the farmland nitrogen and phosphorus loss pollution hotspot units, so that space tracing of water quality degradation is realized.
  2. 2. The method for identifying nitrogen and phosphorus loss pollution hot spots and tracing water quality degradation of a river basin farmland according to claim 1, wherein in the step 1, the hydrologic analysis and sub-river basin division of a research river basin are completed in a SWAT model by utilizing a digital elevation model DEM, river network data and river basin boundary data; automatically extracting a river network by setting a converging area threshold value to generate a plurality of sub-river basin units, wherein each sub-river basin unit is provided with a unique number, a corresponding water collecting area and a corresponding water outlet position; in the SWAT model, carrying out terrain preprocessing based on DEM data, wherein the terrain preprocessing comprises depression filling, slope direction and gradient calculation, converging path identification and flow direction and converging accumulation amount calculation; Setting a converging area threshold value in a SWAT model, extracting a river channel network in a research flow field, identifying a corresponding grid unit as a river channel starting point when the upstream converging area is larger than or equal to the threshold value, thereby generating a complete river network structure, dividing the river basin into a plurality of sub-river basin units based on the river network structure by the SWAT model, wherein each sub-river basin unit has a unique number and corresponds to a clear water collecting area, a topography boundary and a unique water outlet position.
  3. 3. The method for identifying nitrogen and phosphorus loss pollution hot spots and tracing water quality degradation of a river basin farmland according to claim 2, wherein after the division of the sub-river basin is completed, land utilization type data, soil type data and gradient data are imported into a SWAT model, wherein the gradient data are obtained by calculation of a DEM (digital elevation model), and are subjected to grading treatment according to a gradient grading rule; In each sub-basin, the SWAT model adopts a superposition analysis method of land utilization type, soil type and gradient level to further subdivide the space of the sub-basin, wherein space units with the same land utilization type, the same soil type and within the same gradient level range are divided into the same landscape units; And combining the space units in each sub-drainage area to generate a plurality of landscape units with homogeneous underlying surface characteristics, wherein each landscape unit inherits the hydrologic communication relation of the corresponding sub-drainage area and has unique identification numbers, spatial positions, area information and corresponding land utilization, soil and gradient attributes, and 98 landscape units are constructed in total.
  4. 4. The method for identifying nitrogen and phosphorus loss pollution hot spots and tracing water quality degradation of the river basin farmland according to claim 3, wherein after the construction of the landscape units is completed, nitrogen and phosphorus loads of the landscape units in an annual scale are calculated, a SWAT model firstly drives hydrologic process simulation of the landscape units in the annual scale according to meteorological data to calculate rainfall, runoff, transpiration and slope runoff of the landscape units, migration and conversion processes of nitrogen and phosphorus in the landscape units are further simulated, the nitrogen loads comprise nitrate nitrogen, ammonium nitrogen and organic nitrogen forms, sources comprise soil mineralization, fertilization input and runoff flushing, the phosphorus loads comprise soluble phosphorus and granular phosphorus, sources comprise soil erosion carrying and surface runoff dissolving, and the SWAT model obtains nitrogen and phosphorus output of the landscape units in the annual scale by coupling water output and nitrogen and phosphorus concentration calculation results.
  5. 5. The method for identifying nitrogen and phosphorus loss pollution hot spots and tracing water quality degradation of a river basin farmland according to claim 1, wherein in step 2, based on landscape unit boundaries and terrain data, constructing a directed communication network between landscape units specifically comprises the following steps: Step 21, based on the landscape unit vector data obtained by SWAT model division, reading the polygon boundary of each landscape unit, calculating the geometric center point of each polygon, regarding each landscape unit as a node in the graph theory, using the unique number as the node identification, using the geometric center coordinates as the position of the node in the plane, thereby obtaining the landscape unit node set ; Step 22, determining the adjacent relation between nodes by utilizing the space interrelationship between the polygons of the landscape units, judging whether a shared boundary exists between the polygons of the two landscape units or not through space index and geometric operation, and constructing an adjacent pair set I.e. only when the landscape unit With view unit When the two parts are directly adjacent in space, the edge connection is allowed to be established in the graph, so that the communication relationship is ensured to have definite geographic constraint; Step 23, performing elevation sampling on the geometric center point of each landscape unit to obtain a representative elevation value of the landscape unit For any pair of adjacent units When (when) When the water flow and the pollutants are considered to exist in the slave units under the driving of the topography Direction unit In the figures, i.e. a self-priming is established Pointing to When (1) is directed edge of When it is, then build up Pointing to Converting the geometric adjacency between all adjacent landscape elements into a directed edge set with explicit upstream-downstream direction Forming a directed graph 。
  6. 6. The method for identifying nitrogen and phosphorus loss pollution hot spots and tracing water quality degradation of a river basin farmland according to claim 1, wherein in step 3, for a landscape unit vector file obtained by pretreatment of a SWAT model, firstly calculating geometric center points of polygons of each landscape unit to obtain plane coordinates of nodes For each directed edge identified Calculating Euclidean distance between geometric centers of landscape units by using node coordinates Geometric center elevation difference between two landscape units Shared boundary length between landscape element polygons The landscape units are arranged according to the serial number sequence To Filling the first Line 1 A column, obtaining a pollutant transmission weight matrix; In which is derived from a unit Is according to the unit nitrogen and phosphorus loads of (2) Is distributed to each downstream landscape element.
  7. 7. The method for identifying and tracing the nitrogen and phosphorus loss pollution hot spots of the river basin farmland and degrading water quality according to claim 1, wherein in step 4, each node i is traversed from upstream to downstream in sequence according to topological order by step transmission and accumulation along the network, and the nitrogen and phosphorus load is updated for all downstream adjacent nodes j: Wherein Q N, j Q P, j respectively represents the nitrogen and phosphorus load quantity after upstream distribution is obtained by the j landscape units; Respectively representing nitrogen and phosphorus load quantities obtained by SWAT simulation of the j landscape units; Respectively represent the nitrogen and phosphorus loads of k upstream landscape units of j landscape units, and W N, ij and W P, ij respectively represent the nitrogen and phosphorus transmission coefficients between i and j landscape units.
  8. 8. The method for identifying nitrogen and phosphorus loss pollution hot spots and tracing water quality degradation of a river basin farmland according to claim 1, wherein in the step 5, in the step 4, total nitrogen Q N,i and total phosphorus Q P,i load of each node and pollution load contribution of landscape units to downstream nodes are recorded, the landscape units are ordered in descending order according to the load contribution of the upstream units, the first 80% is taken as a key management and control unit, and the upstream contribution of each landscape unit to the nitrogen and phosphorus load of an outlet section is calculated.
  9. 9. The method for identifying nitrogen and phosphorus loss pollution hot spots and tracing water quality degradation of the river basin farmland according to claim 8, wherein the upstream contribution of each landscape unit to the nitrogen and phosphorus load of the outlet section is specifically: CR N, r = C N,r→out / Q N,out CR P, r = C P,r→out / Q P,out Wherein CR N, r 、CR P, r represents the contribution of landscape unit r to nitrogen and phosphorus, C N,r→out 、C P,r→out represents the output of nitrogen and phosphorus of the landscape, and Q N,out 、Q P,out represents the total output of nitrogen and phosphorus.
  10. 10. A system for realizing the method for identifying nitrogen and phosphorus loss pollution hot spots and tracing water quality degradation of the river basin farmland according to claim 1, which is characterized by comprising a landscape unit dividing module, a water basin analysis module and a water basin analysis module, wherein the landscape unit dividing module is used for dividing a research river basin into a plurality of landscape units based on a river basin process simulation model and calculating nitrogen and phosphorus loads of each landscape unit in a preset period; The directional communication network construction module is used for constructing a directional communication network among the landscape units based on the landscape unit boundaries and the terrain data and constructing a pollutant transmission directional diagram; the pollutant transmission directed graph construction module calculates pollutant transmission weights of directed edges according to the terrain elevation difference, the space distance and the shared boundary length between the landscape units, and constructs a pollutant transmission weight matrix; The upstream contribution amount calculation module is used for coupling the nitrogen and phosphorus loads of each landscape unit with the pollutant transmission weight matrix, and carrying out pollutant output calculation on a directional communication network to obtain the contribution amount of each upstream landscape unit to the nitrogen and phosphorus loads of the target section; and the water quality degradation traceability module is used for sequencing the scenery units according to the contribution quantity result and identifying the farmland nitrogen and phosphorus loss pollution hotspot units, so that the space traceability of the water quality degradation is realized.

Description

Method and system for identifying nitrogen and phosphorus loss pollution hot spot of river basin farmland and tracing water quality degradation Technical Field The invention relates to the technical field of water environment protection and agricultural non-point source pollution control, in particular to a method and a system for identifying nitrogen and phosphorus loss pollution hot spots and tracing water quality degradation of a river basin farmland. Background River water quality deterioration is mainly due to excessive nitrogen input caused by human activity. The artificial nutrient salt load threatens the health of the aquatic ecosystem through eutrophication, and can cause the explosion of ecological algal bloom and the loss of biodiversity, thereby restricting the regional economic productivity. As a key space unit in the man-water coupling system, the landscape unit defined by different land utilization types, terrain positions and hydrologic processes in the river basin not only carries the nitrogen and phosphorus generation and conversion processes, but also directly determines the transfer path and efficiency of pollutants in the slope and river network by the spatial adjacent relation and elevation gradient of the nitrogen and phosphorus generation and conversion processes. The spatiotemporal evolution characteristics of the landscape elements constitute an important spatial analysis framework for understanding human-environment-water interactions. The dynamic change of the landscape unit comprises the increase and decrease of the areas of different landscape types and also relates to the recombination and adjustment of the combination mode and the interconnection relation of the landscape units. Existing researches show that the agricultural land and the area of the built-up area are always in a significant positive correlation with the nitrogen concentration of the river water body, the landscape configuration influences non-point source pollution by changing the flow converging path and the converging scale, in the process, the direct contact relationship between the source area and the water body is important, and the probability and the strength of pollutants reaching the water body can be changed in a space series connection mode between landscape units of different source areas and between the source area and the transition buffer unit. Therefore, on the drainage basin scale, the landscape units are regarded as nodes, the system quantifies geometric and topographic features such as shared boundaries, elevation differences, relative distances and the like between the landscape units, a landscape communication network with topological significance is constructed, and the multi-stage transmission process of non-point source pollution on the slope is facilitated to be disclosed. The length, elevation difference, slope flow direction distance and other indexes of the shared boundary among the landscape units provide a set of quantifiable space measurement factors for pollutant transport efficiency evaluation. A longer shared boundary means a wider mass exchange interface, a larger downhill elevation difference and a shorter sloping path correspond to stronger hydrodynamic conditions and higher transmission possibilities. However, the existing hydrologic-water quality model mainly regards the landscape units as mutually independent computing units in structure, and usually implicitly represents the space transportation process of pollutants only through sub-basin division, hydrologic response unit classification and main water channel path derivation based on DEM, but lacks explicit description of topology information among the landscape units. The communication degree between the pollution source unit and the peripheral buffer units and the confluence unit is difficult to be directly quantized in the model structure due to the concept and the gap on the model, and finally the accuracy and traceability of the non-point source pollution load and river water quality space coupling simulation are limited. In view of the above, it is necessary to provide a method for identifying the nitrogen and phosphorus loss pollution hot spot of the river basin farmland and tracing the degradation of water quality to intuitively quantify the pollutant transmission relationship between different landscape units and service the water resource management decision. Disclosure of Invention The invention aims to provide a river basin farmland nitrogen and phosphorus loss pollution hotspot identification and water quality degradation tracing method and system, which are suitable for hilly river basins mainly polluted by agricultural non-point sources, and are simple and convenient in operation method and convenient to popularize and apply. The technical scheme is that the method for identifying the nitrogen and phosphorus loss pollution hot spot and tracing the water quality degradation of the river basin farmland c