CN111723522B - Calculation method of exchange flux of dissolved organic carbon in lakes and rivers - Google Patents

Calculation method of exchange flux of dissolved organic carbon in lakes and rivers Download PDF

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CN111723522B
CN111723522B CN202010535291.XA CN202010535291A CN111723522B CN 111723522 B CN111723522 B CN 111723522B CN 202010535291 A CN202010535291 A CN 202010535291A CN 111723522 B CN111723522 B CN 111723522B
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刘�东
段洪涛
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Nanjing Institute of Geography and Limnology of CAS
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Abstract

A method for calculating dissolved organic carbon exchange flux of lakes and rivers, comprising: dividing lakes according to river zone positions to obtain different lake zones; collecting remote sensing data and synchronous actually measured DOC concentration data; performing atmospheric coarse correction on the remote sensing data; with atmospheric rough corrected remote sensing reflectivityR rc For input, actually measuring DOC concentration, and establishing a multi-layer backward feedback neural network for output, wherein the multi-layer backward feedback neural network comprises an input layer, three hidden layers and an output layer; the trained multilayer backward feedback neural network is utilized to remotely sense and invert the DOC concentration of the lake at different moments, the month average DOC concentration of each lake area is calculated, and the product of the river outlet lake flow and the month average DOC concentration of the river mouth adjacent to the lake area is the outlet lake DOC flux; the product of the river inflow and the measured average DOC concentration of the river is the DOC flux of the inflow. The method can realize real-time remote sensing estimation of the DOC concentration of the lake and remote sensing estimation of the DOC exchange flux of the lake and the river.

Description

Calculation method of exchange flux of dissolved organic carbon in lakes and rivers
Technical Field
The invention relates to a satellite remote sensing technology and the application field thereof, which mainly utilizes satellite remote sensing data and river actual measurement data which can be acquired in real time to realize remote sensing estimation of lake and river Dissolved Organic Carbon (DOC) exchange flux, namely, in a certain time: how many DOCs flow out of the lake into the river, and how many DOCs are transported by the river into the lake.
Background
The water body dissolved organic matter contains various organic molecules such as saccharides, fat, alkanes and the like, and is widely existed in water bodies of lakes and rivers, and the content flux of the water body dissolved organic matter is quantified by using Dissolved Organic Carbon (DOC). Dissolved organics are a source of energy for the survival of heterotrophic bacteria, and their decomposition consumes oxygen from the water and produces harmful substances, thereby destroying the water environment. The lake and the adjacent river have DOC exchange, the lake DOC is output by the river coming out of the lake, and the land source DOC is input into the lake by the river coming in the lake. Thus, there is a need to know the dynamics of lake DOC to better manage lake water quality. In order to estimate the DOC exchange flux of lakes and rivers, the traditional method is to calculate the DOC exchange flux according to the actual measured flow rate and DOC concentration of the rivers. However, because of the highly dynamic change of dissolved organic carbon in the river DOC, measured DOC data at a certain moment in time often cannot represent the average DOC concentration in that time, especially in the river under the influence of strong artificial activity in the eastern part of the river. Therefore, the remote sensing technology high-frequency observation advantage is necessary to be utilized to realize remote sensing estimation of the DOC exchange flux of the lake and the river. At present, the remote sensing of the lake DOC has the problem that the traditional remote sensing data band ratio cannot be well used for remote sensing monitoring of the lake DOC, and the lake DOC product is not reported.
Disclosure of Invention
In order to realize remote sensing estimation of lake and river DOC exchange flux, the invention constructs a calculation method of lake and river dissolved organic carbon exchange flux by means of high-time-resolution remote sensing data and field measured data in different seasons.
The technical scheme adopted by the invention is as follows:
a method for calculating dissolved organic carbon exchange flux of lakes and rivers, comprising:
dividing lakes according to river zone positions to obtain different lake zones;
collecting remote sensing data and synchronous actually measured DOC concentration data;
performing atmospheric coarse correction on the remote sensing data;
with atmospheric rough corrected remote sensing reflectivity R rc For input, actually measuring DOC concentration, and establishing a multi-layer backward feedback neural network for output, wherein the multi-layer backward feedback neural network comprises an input layer, three hidden layers and an output layer;
the trained multilayer backward feedback neural network is utilized to remotely sense and invert the DOC concentration of the lake at different moments, the month average DOC concentration of each lake area is calculated, and the product of the river outlet lake flow and the month average DOC concentration of the river mouth adjacent to the lake area is the outlet lake DOC flux; the product of the river inflow and the measured average DOC concentration of the river is the DOC flux of the inflow.
Further, the input layer comprises 5 nodes; selecting remote sensing reflectivity R of 5 wave bands of 469nm,555nm,645nm,859nm and 2130nm rc The data is taken as input.
Further, each hidden layer contains 10 nodes.
Further, the calculation modes of the hidden layer and the output layer node values are as follows:
y=f(-bias+∑w i ×x i )
fhidden=2/(1+e -2×x )-1
f output =a×x+b (1)
wherein x is the value of each node of the previous layer; omega is the weight; f (f) hidden And f output The activation functions used for the hidden layer and the output layer, respectively.
Further, the measured DOC concentration includes field measured DOC concentration data for different seasons.
Further, the matched satellite-ground synchronous data are screened for neural network training, and the screening mode is as follows: for all DOC concentration measured data, selecting clear sky and R without cloud influence according to time window + -3 hours and space window 3X 3 pixels rc Data.
Further, 70% of the satellite-to-ground synchronous data are randomly selected for training the neural network, and the rest 30% of the data are used for model verification.
Further, the lake area is divided in such a way that the boundary of the lake is internally buffered with a plurality of pixels; for each river region boundary, drawing a vertical drainage basin connecting line to determine the corresponding lake.
Further, the error transfer is carried out by adopting a Levenberg-Marquardt optimization method and a Bayesian normalization algorithm. Overfitting can be effectively avoided; and adjusting the weight according to the fed-back error.
Further, the remote sensing data is MODIS remote sensing data.
According to field measured data and remote sensing data in different seasons, a remote sensing algorithm of the lake DOC is constructed; dividing lakes according to river region positions to obtain different lake regions, and then calculating the month average DOC concentration of the different lake regions; and (3) sampling the actual measurement value of the DOC concentration of the river entering the lake and the river, adopting the remote sensing average value of the river mouth close to the lake area for the DOC of the river exiting the lake and the river, and multiplying the remote sensing average value by the flow to obtain the DOC flux of the river entering and exiting the lake of different rivers, namely the DOC exchange flux of the lake and the river. The applicant researches find that for an internal lake, the atmospheric correction is positive and negative, and larger errors can be caused, so that specific wave band remote sensing product data of atmospheric coarse correction is directly used, a lake DOC remote sensing algorithm is built by combining a multi-layer backward feedback neural network, finally lake river DOC concentration is calculated by utilizing the lake region division and the river mouth close to the lake region division, for flux estimation, the existing method utilizes actual DOC concentration and flux calculation at a certain position of the river, and the sampling data is usually only once in one season or month. However, the DOC concentration of the river is continuously changed, so that the calculated DOC flux is greatly uncertain. The invention estimates the DOC flux of the lake month by using a remote sensing method, and the DOC concentration used is the average value of DOC concentrations on different days of each month, thereby having obvious advantages in timeliness and representativeness. The method can realize real-time remote sensing estimation of the DOC concentration of the lake and remote sensing estimation of the DOC exchange flux of the lake and the river.
Drawings
FIG. 1 is a lake DOC remote sensing multilayer backward feedback neural network.
FIG. 2 is an R of measured DOC concentration and synchronous monitoring rc (469) Is a relationship of (3).
FIG. 3 is a Tai lake and its drainage basin partition.
Detailed Description
For a typical lake-Taihu lake in China, the DOC exchange flux of the lake and the river is estimated based on MODIS remote sensing data, and the specific implementation mode is further described with reference to the accompanying figures 1-3:
a method for calculating dissolved organic carbon exchange flux of lakes and rivers, comprising:
(1) The lakes are divided according to the river locations to obtain different lake areas, as shown in fig. 3. By buffering the lake boundary inwards by a plurality of pixels, for each river region boundary, a connecting line of the vertical river basin is drawn to determine the corresponding lake. For the Tai lake, the Tai lake boundary was buffered 5km inward (corresponding to 10 pels of MODIS data used) for lake region division.
(2) Collecting remote sensing data and synchronous actually measured DOC concentration data;
the remote sensing data is MODIS L1B level data downloaded by a network; the actually measured DOC concentration data is that a surface water sample is collected when no cloud exists on a sunny day, and the laboratory returns to carry out filtering measurement on the day so as to ensure synchronous acquisition of the synchronously actually measured DOC and remote sensing.
(3) Atmospheric rough correction of remote sensing data. The network downloaded MODIS data is a product which is not subjected to atmospheric correction, and more than 90% of the data signals are noise signals generated by the atmosphere and the like. Therefore, the SeaWiFS data analysis system (SeaDAS) is utilized to perform the rough atmospheric correction on the original remote sensing data so as to remove Rayleigh scattering influence of atmospheric molecules and the like, and the product R after the rough atmospheric correction is obtained rc 。R rc Values R comprising bands of central wavelengths 469nm,555nm, 640nm, 859nm, 460 nm,1640nm,2130nm rc (469),R rc (555),R rc (645),R rc (859),R rc (1240),R rc (1640) And R is rc (2130)。
(4) Matching of satellite-ground synchronous data. For all DOC concentration measured data of the Taihu lake, determining whether effective R with clear sky and no cloud influence exists according to time window + -3 hours and space window of 3X 3 pixels rc As a result.
(5) And training and checking the multi-layer backward feedback neural network.
(1) And randomly selecting 70% of the found star-ground synchronous matching data for training the multi-layer backward feedback neural network.
(2) Establishing a multi-layer backward feedback neural network model;
it is generally considered that too many hidden layers/nodes increase the computational load and increase the interference, and too few hidden layers/nodes are difficult to achieve the goal, but no related theoretical method has been determined at present regarding the number of hidden layers/nodes. The method adopts a mode of gradually increasing the number to carry out multiple attempts, and finally establishes a multi-layer backward feedback neural network model based on three hidden layers and 10 nodes in each layer.
The calculation modes of the node values of the hidden layer and the output layer are as follows:
y=f(-bias+∑w i ×x i )
f hidden =2/(1+e -2×x )-1
f output =a×x+b (1)
wherein x is the value of each node of the previous layer; omega is the weight; f (f) hidden And f output The activation functions used for the hidden layer and the output layer, respectively.
The output range of f (x) of the activation function used in the invention is-1, and a wider DOC range can be obtained through simulation.
(3) Screening an input wave band;
performing element analysis on the inverted lake, wherein the DOC has strong light absorption performance in a short wave band, and after preliminary test, R is calculated rc (469) Determining as input an approximate range of DOC concentrations (as shown in fig. 2); secondly, the phytoplankton production can release a large amount of DOC, and the sensitive wave band of the phytoplankton is R rc (555) And R is rc (645) The contribution of phytoplankton to the DOC will therefore be considered as input; thirdly, the atmospheric fine correction of the inland turbid water body has a difficult problem, and R is increased after test rc (859) And R is rc (1230) The wave band is used as input, DOC remote sensing can be realized based on the rough corrected remote sensing data, and the atmospheric fine correction difficulty is avoided or solved.
(4) Training a multi-layer backward feedback neural network model;
by R rc (469),R rc (555),R rc (645),R rc (859) And R is rc (2130) As an input value of the input layer node, the corresponding measured DOC concentration is taken as the value of the output layer node (fig. 1). Model training iteration is carried out, error transmission is carried out on the result of each cycle to the hidden layer and the output layer node, and the weight omega is adjusted according to the transmitted error delta until the output nodeThe fruit is stable to obtain a trained model. Error transfer uses a Levenberg-Marquardt optimization method and a Bayesian normalization algorithm.
(5) And (5) checking a model.
The model results were checked using the remaining 30% of the matched data, mean absolute percent error (MAPD), root Mean Square Error (RMSE), bias (bias) of 18.78%,1.38mg/L and-3.15%, respectively. MDPD, RMSE and bias are defined as in formula (2).
(6) DOC remote sensing of the Taihu day scale. Applying the trained multilayer backward feedback neural network to MODIS remote sensing R of each day rc And (5) data to obtain a DOC product with a daily scale.
(7) The average DOC concentration in different lake regions is calculated. Using the DOC results of each day, an arithmetic average calculation was performed to obtain an average DOC concentration per month. The average DOC concentration for different months in each lake region was further calculated from the partitions in fig. 3.
(8) Calculation of DOC exchange flux of lakes and rivers. For lake basin partitioning (fig. 3), each lake basin partitioning contains several rivers whose monthly in and out lake flow is available to the Tai lake authority. For the river of each river basin partition, the product of the lake outlet flow rate and the average DOC of the month of the river mouth close to the lake region is the lake outlet DOC flux, and the product of the lake inlet flow rate and the measured average DOC concentration of the river is the lake inlet DOC flux.

Claims (9)

1. A method for calculating the flux of dissolved organic carbon exchange between lakes and rivers, comprising:
dividing lakes according to river zone positions to obtain different lake zones;
collecting remote sensing data and synchronous actually measured DOC concentration data;
performing atmospheric coarse correction on the remote sensing data;
with atmospheric rough corrected remote sensing reflectivityR rc For input, actually measuring DOC concentration, and establishing a multi-layer backward feedback neural network for output, wherein the multi-layer backward feedback neural network comprises an input layer, three hidden layers and an output layer; the input layer comprises 5 nodes; selecting the remote sensing reflectivity of 5 wave bands of 469nm,555nm,645nm,859nm and 2130nmR rc Data is taken as input;
the trained multilayer backward feedback neural network is utilized to remotely sense and invert the DOC concentration of the lake at different moments, the month average DOC concentration of each lake area is calculated, and the product of the river outlet lake flow and the month average DOC concentration of the river mouth adjacent to the lake area is the outlet lake DOC flux; the product of the river inflow and the measured average DOC concentration of the river is the DOC flux of the inflow.
2. The method of claim 1, wherein each hidden layer comprises 10 nodes.
3. The method according to claim 1, wherein the implicit layer and output layer node values are calculated as follows:
(1)
wherein the method comprises the steps ofxValues for nodes of the previous layer; w is a weight;f hidden andf output the activation functions used for the hidden layer and the output layer, respectively.
4. The method of claim 1, wherein the measured DOC concentration comprises field measured DOC concentration data for different seasons.
5. The method of claim 1, wherein the matched satellite-to-ground synchronization data is screened for neural network training by: for all DOC concentration measured data, selecting clear sky and no cloud influence according to time window + -3 hours and space window 3 multiplied by 3 pixelsR rc Data.
6. The method of claim 1, wherein 70% of the satellite-to-ground synchronization data is randomly selected for training the neural network and the remaining 30% of the data is used for model verification.
7. The method of claim 1, wherein the lake region is partitioned in such a way that a lake boundary is buffered inward by a number of pixels; for each river region boundary, drawing a vertical drainage basin connecting line to determine the corresponding lake.
8. The method of claim 1 wherein the error is transferred using a Levenberg-Marquardt optimization method and a Bayesian normalization algorithm, and the weights are adjusted based on the fed back error.
9. The method of claim 1, wherein the telemetry data is MODIS telemetry data.
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