A nested watershed flood simulation method considering two-way coupling

Abstract

The application discloses a nested basin flood simulation method considering bidirectional coupling, relates to the technical field of hydrology, hydrodynamic model coupling and nested basin flood simulation, and comprises the following steps: based on the spatial structure characteristics and the water quantity secondary infiltration of the overflow flood area participating in the soil water mechanism, coupling hydrology and hydrodynamic modules, and constructing a small-scale flood model; based on space, time and coupled boundaries, nesting the small-scale flood model and a preset large-scale flood model to obtain a nested basin flood simulation model; and using the nested basin flood simulation model to simulate the hydrology and hydrodynamics of a to-be-measured basin to obtain a flood inundation area evaluation result. The small-scale flood model constructed by the application integrates multiple hydrology and hydrodynamic modules, supports the secondary infiltration of water overflowing to a flood area and participating in the soil water process, realizes the bidirectional coupling of hydrology and hydrodynamics between a river channel and a flood area, and provides mechanism support for improving the local simulation precision in the nested simulation.

Description

Technical Field

[0001] This invention relates to the field of hydrological and hydrodynamic model coupling and nested watershed flood simulation technology, specifically, to a nested watershed flood simulation method considering bidirectional coupling. Background Technology

[0002] Floods often occur and evolve over large-scale watersheds, but their prevention and control measures must be implemented precisely on a smaller scale. Therefore, nested watershed flood simulation methods have gradually become an important means of simulating the causes and evolution of floods, capable of simultaneously taking into account both the overall characteristics of the watershed and the requirements for local accuracy.

[0003] Among existing nested flood simulation methods, representative schemes such as the Global Applicable Computational Framework for Integrated Hydrological Hydrodynamic Modelling (GLOFRIM) model use a large-scale hydrological model to unidirectionally transfer surface runoff to the hydrodynamic model for runoff calculation. This fails to simulate the process of river floods overflowing into the floodplain and then infiltrating again, which can easily lead to an overestimation of the floodplain inundation range.

[0004] No effective solutions have yet been proposed to address the problems in the relevant technologies. Summary of the Invention

[0005] To address the problems in related technologies, this invention proposes a nested watershed flood simulation method that considers bidirectional coupling, in order to overcome the technical problem of unidirectional hydrological-hydrodynamic coupling in existing nested flood simulation methods.

[0006] Therefore, the specific technical solution adopted by the present invention is as follows:

[0007] A nested watershed flood simulation method considering bidirectional coupling, the method comprising the following steps:

[0008] S1. Based on spatial structural characteristics, the secondary infiltration of water in the floodplain into the soil water mechanism, and the coupling of hydrological and hydrodynamic modules, a small-scale flood model is constructed; the hydrological and hydrodynamic modules include a runoff generation module, a soil evapotranspiration module, a seepage module, a soil interflow module, a slope runoff module, a river runoff module, and a groundwater module.

[0009] S2. Based on space, time and coupling boundary, the small-scale flood model is nested with the pre-set large-scale flood model to obtain a nested watershed flood simulation model.

[0010] S3. Using a nested watershed flood simulation model, the hydrology and hydrodynamics of the watershed under test are simulated to obtain the assessment results of the flood inundation area.

[0011] Furthermore, based on spatial structural characteristics, the secondary infiltration mechanism of overflow floodplain water into soil water, and the coupling of hydrological and hydrodynamic modules, a small-scale flood model is constructed, including the following steps:

[0012] S11. Spatial discretization is performed based on grid cells with fine resolution to construct spatial structural features; the spatial structural features include the surface layer and the soil layer;

[0013] S12. When water reaches the surface, a runoff generation module is constructed based on the infiltration calculation mechanism of the Green-Ampt model.

[0014] S13. Based on the soil evapotranspiration model, and combined with the presence or absence of surface water, construct a soil evapotranspiration module.

[0015] S14. When soil seepage occurs, a seepage module is constructed based on the seepage calculation mechanism of the distributed hydrology-soil-vegetation model.

[0016] S15. When saturated interflow occurs in the soil layer, construct a soil lateral interflow module based on the interflow calculation mechanism of the distributed hydrology-soil-vegetation model.

[0017] S16. When runoff exchange occurs between adjacent surface units, a slope runoff module is constructed based on the slope runoff calculation mechanism of the two-dimensional flood evolution model.

[0018] S17. Based on the river channel calculation mechanism of the two-dimensional flood evolution model and the mechanism of secondary infiltration of overflow floodplain water into soil water, and combined with the comparison results of river channel water depth and floodplain elevation, a river channel runoff module is constructed.

[0019] S18. When the moisture content of the lowest soil layer exceeds its field capacity, a groundwater module is constructed based on the recharge and discharge mechanism of the nonlinear groundwater reservoir.

[0020] S19. Couple the runoff generation module, soil evapotranspiration module, seepage module, interflow module, slope runoff module, river runoff module, and groundwater module to construct a small-scale flood model.

[0021] Furthermore, the soil layers are divided into three layers: the upper soil layer, the lower soil layer, and the deep soil layer. When the actual number of soil layers is more than three, all soil layers at the third layer and below the effective depth are merged into the deep soil layer.

[0022] Furthermore, based on the soil evapotranspiration model and considering the presence or absence of surface water, a soil evapotranspiration module is constructed, including:

[0023] Determine the evapotranspiration path based on the surface water accumulation status;

[0024] When there is standing water on the local surface, the actual evapotranspiration is calculated based on the potential evapotranspiration rate.

[0025] When there is no standing water on the surface, the actual evapotranspiration is calculated layer by layer according to the soil evapotranspiration model, based on the three-layer structure of the upper soil layer, the lower soil layer, and the deep soil layer.

[0026] Furthermore, leakage phenomena include:

[0027] When the moisture content of the upper soil layer is greater than the field capacity and the moisture content of the lower soil layer is less than the soil porosity, water from the upper soil layer will seep vertically into the lower soil layer.

[0028] Furthermore, interflow phenomena in saturated soils include:

[0029] When the water content of any soil layer reaches or exceeds the field capacity and lateral drainage is available, water flows laterally along the direction of maximum hydraulic gradient under the drive of gravity and water potential gradient, enters the adjacent simulation unit, and forms interflow in the soil under saturation.

[0030] Furthermore, based on the river channel calculation mechanism of the two-dimensional flood evolution model and the mechanism of secondary infiltration of overflow floodplain water into soil water, combined with the comparison results of river channel water depth and floodplain elevation, a river runoff module is constructed, including:

[0031] When the river channel depth is lower than the floodplain elevation, the water flow in the river channel will not overflow into the floodplain. The river channel calculation mechanism based on the two-dimensional flood evolution model is used to calculate the water flow in the river channel.

[0032] When the river channel depth is higher than the floodplain elevation, the water in the river overflows into the floodplain. Based on the river channel calculation mechanism of the two-dimensional flood evolution model, the overflow volume and the lateral flow of the floodplain are calculated.

[0033] Based on the mechanism of secondary infiltration of overflow floodplain water into soil water, the overflow water is input into the corresponding floodplain as surface water and participates in the subsequent infiltration process.

[0034] Furthermore, the formula for calculating the water flow in the river channel is:

[0035] ;

[0036] The formula for calculating lateral discharge in floodplains is:

[0037] ;

[0038] The formula for calculating overflow volume is:

[0039] ;

[0040] In the formula, The flow rate on the river grid from time t to time t+Δt; Let t be the hydraulic radius of the river channel; The cross-sectional area of ​​the river channel. The flow rate of the floodplains on both sides of the river channel; w c,flow The width of the river channel; The slope of the water surface in the floodplain; The water depth in the floodplain; The water depth is above the riverbed elevation; z c,i This refers to the elevation of the riverbed bottom. z f,i Elevation of the floodplain.

[0041] Furthermore, based on spatial, temporal, and coupling boundaries, the small-scale flood model is nested with a pre-set large-scale flood model to obtain a nested watershed flood simulation model, including:

[0042] S21. Explicitly nest and couple the small-scale flood model with the pre-set large-scale flood model in space;

[0043] S22. Nest and couple the small-scale flood model with the pre-set large-scale flood model in time;

[0044] S23. The small-scale flood model and the pre-set large-scale flood model are coupled at the boundary to transfer river flow bidirectionally.

[0045] S24. Based on the spatial nesting, temporal nesting, and coupling boundary nesting of the small-scale flood model and the preset large-scale flood model, a nested watershed flood simulation model is obtained.

[0046] Furthermore, the explicit spatial nesting and coupling of the small-scale flood model with the pre-set large-scale flood model includes:

[0047] Spatially, spatial discretization is performed within key sub-basins using a model suitable for small-scale flooding, while generalization is performed in other regions using a model suitable for pre-set large-scale flooding, thus achieving spatial nesting between the small-scale flooding model and the pre-set large-scale flooding model.

[0048] The beneficial effects of this invention are as follows: The small-scale model constructed by this invention when performing nested coupling integrates multiple hydrological and hydrodynamic modules, including a runoff generation module, a soil evapotranspiration module, a seepage module, a soil interflow module, a slope runoff module, a river runoff module, and a groundwater module. This supports the re-infiltration of water overflowing into the floodplain and its participation in soil water processes, realizing bidirectional coupling of hydrology and hydrodynamics between the river and the floodplain, thus providing a mechanism to improve the accuracy of local simulations in nested simulations. Attached Figure Description

[0049] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0050] Figure 1 This is a flowchart of a nested watershed flood simulation method considering bidirectional coupling according to an embodiment of the present invention;

[0051] Figure 2 This is a schematic diagram of the spatial discretization of the large-scale model and the small-scale model in a specific embodiment of a nested watershed flood simulation method considering bidirectional coupling according to an embodiment of the present invention.

[0052] Figure 3 This is a schematic diagram of runoff input from a large-scale model to a small-scale model in a specific embodiment of a nested watershed flood simulation method considering bidirectional coupling according to an embodiment of the present invention.

[0053] Figure 4 This refers to the inundation extent of the Omaha sub-basin during the March 2019 flood event, based on a nested watershed flood simulation method considering bidirectional coupling according to an embodiment of the present invention. Detailed Implementation

[0054] To further illustrate the various embodiments, the present invention provides accompanying drawings, which are part of the disclosure of the present invention. These drawings are mainly used to illustrate the embodiments and can be used in conjunction with the relevant descriptions in the specification to explain the operating principles of the embodiments. With reference to these drawings, those skilled in the art should be able to understand other possible implementation methods and the advantages of the present invention. The components in the drawings are not drawn to scale, and similar component symbols are generally used to represent similar components.

[0055] According to an embodiment of the present invention, a nested watershed flood simulation method considering bidirectional coupling is provided.

[0056] The present invention will now be further described in conjunction with the accompanying drawings and specific embodiments, such as... Figures 1-4As shown, according to an embodiment of the present invention, a nested watershed flood simulation method considering bidirectional coupling includes the following steps:

[0057] S1. Based on spatial structural characteristics, the secondary infiltration of water in the floodplain into the soil water mechanism, and the coupling of hydrological and hydrodynamic modules, a small-scale flood model (i.e., small-scale model) is constructed; the hydrological and hydrodynamic modules include a runoff generation module, a soil evapotranspiration module, a seepage module, a soil interflow module, a slope runoff module, a river runoff module, and a groundwater module.

[0058] It should be explained that the coupled hydrology and hydrodynamic module, which includes a runoff generation module, a soil evapotranspiration module, a seepage module, a soil interflow module, a slope runoff module, a river runoff module, and a groundwater module, constructs a small-scale flood model that couples hydrology and hydrodynamics in two ways. The small-scale flood model performs a refined simulation of local conditions.

[0059] In this optional embodiment, based on spatial structural characteristics, the secondary infiltration of floodwaters into the soil water mechanism, and the coupling of hydrological and hydrodynamic modules, the construction of a small-scale flood model includes the following steps:

[0060] S11. Spatial discretization is performed based on grid cells with fine resolution to construct spatial structural features; the spatial structural features include the surface layer and the soil layer;

[0061] Specifically, spatial discretization is performed based on high-resolution grid cells, and each grid cell is divided vertically into a surface layer (above ground) and multiple soil layers, forming a spatial simulation structure with soil columns as the basic simulation unit. Using this method, key sub-basins can be discretized into... I OK J A grid consisting of columns of soil. Each soil column contains... L The soil layer is connected to its adjacent soil columns in four directions. In the following text, it will be referred to as... i The row number indicates the soil column. j Indicates the column number. k Indicates adjacent directions. For ease of understanding, meters (m) and seconds (s) will be used consistently as the basic units of spatial distance and time in the following text.

[0062] S12. When water reaches the surface, a runoff generation module is constructed based on the infiltration calculation mechanism of the Green-Ampt model.

[0063] Specifically, the integrated Green-Ampt runoff generation module works as follows: when rainfall reaches the point where water enters the surface layer of the simulated unit (including rainfall, adjacent grid runoff, or interflow), the amount of infiltration must be deducted before the remaining water will generate surface runoff. The runoff generation module simulates the surface infiltration process, allowing water to penetrate to the uppermost soil layer. The formula for calculating the infiltration rate is:

[0064] ;

[0065] In the formula: f max_inf,t for t Maximum infiltration rate at any given time (m / s); I p,t for t Cumulative infiltration (meters) at any given time. k s is the saturated hydraulic conductivity (m / s); θ 1,ini and θ 1,t These are the initial moisture content of the top layer of soil and t The water content at any given time (cubic meters / cubic meter). The actual infiltration rate is calculated based on the maximum infiltration rate and the water depth in the simulation unit, using the following formula:

[0066] ;

[0067] In the formula, f act_inf,t This represents the actual infiltration depth (meters). h t The depth of surface water accumulation (in meters) on the simulated unit.

[0068] S13. Based on the soil evapotranspiration model, and combined with the presence or absence of surface water, construct a soil evapotranspiration module.

[0069] S14. When soil seepage occurs, a seepage module is constructed based on the seepage calculation mechanism of the distributed hydrology-soil-vegetation model.

[0070] S15. When saturated interflow occurs in the soil layer, construct a soil lateral interflow module based on the interflow calculation mechanism of the distributed hydrology-soil-vegetation model.

[0071] S16. When runoff exchange occurs between adjacent surface units, a slope runoff module is constructed based on the slope runoff calculation mechanism of the two-dimensional flood evolution model.

[0072] Specifically, the slope runoff calculation module, which integrates the two-dimensional flood propagation model (LISFLOOD-FP), simulates the runoff exchange process between adjacent simulation units on a land surface slope, thus realizing slope runoff confluence. The formula for calculating surface runoff between adjacent simulation units is as follows:

[0073] ;

[0074] ;

[0075] In the formula, Δ t For time step; for t Time to t +Δ t At time 1, adjacent grid cells (the first) i and i Flow rate (cubic meters per second) between grid cells (+1); Δ x This refers to the width of the grid cell; g It is the acceleration due to gravity; q t The flow rate of the previous time step. Q t Divide by cell width Δ x ; S t for t Hydraulic gradient between adjacent grid cells at any given time; n The coefficient of friction on the grid cell; for t The water flow depth between grid cells at any given time; For grid cells i The water depth; z i For grid cells i The elevation.

[0076] S17. Based on the river channel calculation mechanism of the two-dimensional flood evolution model and the mechanism of secondary infiltration of overflow floodplain water into soil water, and combined with the comparison results of river channel water depth and floodplain elevation, a river channel runoff module is constructed.

[0077] S18. When the moisture content of the lowest soil layer exceeds its field capacity, a groundwater module is constructed based on the recharge and discharge mechanism of the nonlinear groundwater reservoir.

[0078] Specifically, an integrated nonlinear groundwater reservoir calculation module is used. When the moisture in the lowest soil layer exceeds the field capacity, the excess water can seep into the groundwater reservoir. While receiving recharge from seepage, the groundwater layer, acting as a slow-responding water source, continuously provides baseflow directly to the main channel of the sub-basin, achieving a time-delayed response and regulation of river runoff. The formula for calculating the flow rate of groundwater outflow into the river channel is:

[0079] ;

[0080] In the formula, Q g The rate (in meters) at which groundwater replenishes the river channel; k g This is the groundwater retreat coefficient (an empirical parameter, dimensionless). S gw Groundwater storage (meters); λ This is the groundwater recession index (an empirical parameter, dimensionless).

[0081] S19. Couple the runoff generation module, soil evapotranspiration module, seepage module, interflow module, slope runoff module, river runoff module, and groundwater module to construct a small-scale flood model.

[0082] In this optional embodiment, the soil layers are divided into three layers, including an upper soil layer, a lower soil layer, and a deep soil layer. When the actual number of soil layers is more than three, all soil layers at the third layer and below the effective depth are merged into the deep soil layer.

[0083] In this optional embodiment, based on the soil evapotranspiration model and considering the presence or absence of surface water, the soil evapotranspiration module is constructed as follows:

[0084] Determine the evapotranspiration path based on the surface water accumulation status;

[0085] When there is standing water on the local surface, the actual evapotranspiration is calculated based on the potential evapotranspiration rate.

[0086] When there is no standing water on the surface, the actual evapotranspiration is calculated layer by layer according to the soil evapotranspiration model, based on the three-layer structure of the upper soil layer, the lower soil layer, and the deep soil layer.

[0087] Specifically, the soil evapotranspiration module integrating the Xin'anjiang model calculates surface water evaporation based on the potential evapotranspiration rate for each simulation unit when surface water is present. When surface water is absent, the soil evapotranspiration process is calculated layer by layer from top to bottom using the Xin'anjiang three-layer soil evapotranspiration model, based on the soil's three-layer structure (from top to bottom: upper soil layer, lower soil layer, and deep soil layer; when there are more than three soil layers, all layers from the third layer down are considered the deep soil layer of the Xin'anjiang model) and their soil moisture content. In the Xin'anjiang model, the formula for calculating the upper soil evapotranspiration rate is:

[0088] ;

[0089] In the formula, e 1,t This refers to the evapotranspiration rate of the upper soil layer; e p,t Potential evapotranspiration rate; h t This refers to the depth of surface water. θ 1,t The value represents the water content of the upper soil layer. All variables are in meters.

[0090] When the moisture content of the surface and upper soil layers is sufficient to meet the potential evapotranspiration demand, the actual evapotranspiration rate is prioritized; otherwise, the evapotranspiration rate is equal to the remaining moisture content of the surface and upper soil layers.

[0091] The formula for calculating the evapotranspiration rate of the lower soil layer is:

[0092] ;

[0093] In the formula, e 2,t This refers to the evapotranspiration of the middle soil layer; θ 2,t This refers to the water content of the middle soil layer; θ 2,max This represents the maximum water storage capacity of the middle soil layer. c This represents the evapotranspiration coefficient of deep soil layers.

[0094] The calculation of evapotranspiration in the middle soil layer depends on the remaining potential evapotranspiration in the upper soil layer. When the upper soil layer can already meet all evapotranspiration requirements... e 2,t Take zero; conversely, evapotranspiration will transfer to the middle soil layer, depending on the water content of the middle soil layer and its maximum capacity. θ 2,max The water content is allocated based on the ratio of water content to the evapotranspiration coefficient of the deep soil layer. If the middle soil layer has a higher water content, it can meet more of the evapotranspiration demand; if it is insufficient, it can only provide limited water compensation.

[0095] The formula for calculating the evapotranspiration rate of deep soil is:

[0096] ;

[0097] In the formula, e 3,t This refers to the evapotranspiration of deep soil layers.

[0098] Evapotranspiration in deep soil layers e 3,t It is activated only when the moisture content of the middle soil layer is insufficient: when the moisture content of the middle soil layer is too low to meet the remaining potential evapotranspiration demand, it is replenished by the deeper layers; otherwise, it is set to zero.

[0099] In this optional embodiment, the leakage phenomenon includes:

[0100] When the moisture content of the upper soil layer is greater than the field capacity and the moisture content of the lower soil layer is less than the soil porosity, water from the upper soil layer will seep vertically into the lower soil layer.

[0101] It needs to be explained that the infiltration module of the integrated Distributed Hydrology-Soil Vegetation Model (DHSVM) allows water in the upper soil layer to seep vertically to the lower layer when the moisture content of the upper soil layer is greater than the field capacity and the moisture content of the lower soil layer is less than the soil porosity. The infiltration rate is calculated based on Darcy's law, using the following formula:

[0102] ;

[0103] In the formula, For the first l Vertical seepage rate of soil moisture (m / s); The vertical saturated hydraulic conductivity of the soil (m / s); m It is the void distribution index; Soil moisture content; This refers to the soil saturation water content. It refers to the soil field water holding capacity.

[0104] In this optional embodiment, interflow in saturated soil includes:

[0105] When the water content of any soil layer reaches or exceeds the field capacity and lateral drainage is available, water flows laterally along the direction of maximum hydraulic gradient under the drive of gravity and water potential gradient, enters the adjacent simulation unit, and forms interflow in the soil under saturation.

[0106] It needs to be explained that the lateral interflow module in the integrated distributed hydrological-soil-vegetation model works as follows: when the water content of a soil layer in a simulation unit exceeds the field capacity, the excess water will move laterally along the direction of maximum hydraulic gradient and enter adjacent simulation units, forming saturated interflow. The formula for calculating the saturated interflow rate is:

[0107] ;

[0108] In the formula, For the simulation unit in t Always towards k The velocity of soil flow in the direction (m³ / s); w k For the simulation unit in k Outflow width (meters) in the direction of flow; For the simulation unit in t Always towards k Hydraulic gradient in the direction; For the simulation unit in t Always towards k Soil permeability in the direction of soil movement. The calculation formula is:

[0109] ;

[0110] In the formula, k s The saturated hydraulic conductivity (m / s) of the top layer of soil. γ is the exponential decay constant of hydraulic conductivity with depth; z t For the simulation unit in t Groundwater level depth (meters) at any given time; D Total thickness (meters) of all soil layers.

[0111] In this optional embodiment, based on the river channel calculation mechanism of the two-dimensional flood evolution model and the mechanism of secondary infiltration of overflow floodplain water into soil water, and combined with the comparison results of river channel depth and floodplain elevation, a river runoff module is constructed, including:

[0112] When the river channel depth is lower than the floodplain elevation, the water flow in the river channel will not overflow into the floodplain. The river channel calculation mechanism based on the two-dimensional flood evolution model is used to calculate the water flow in the river channel.

[0113] When the river channel depth is higher than the floodplain elevation, the water in the river overflows into the floodplain. Based on the river channel calculation mechanism of the two-dimensional flood evolution model, the overflow volume and the lateral flow of the floodplain are calculated.

[0114] Based on the mechanism of secondary infiltration of overflow floodplain water into soil water, the overflow water is input into the corresponding floodplain as surface water and participates in the subsequent infiltration process.

[0115] Specifically, the river channel calculation module integrating the two-dimensional flood evolution model calculates the river runoff process. This method uses existing sub-gridization technology to simplify the river channel into one-dimensional river channel units embedded in the two-dimensional grid of the floodplain, and treats the river cross-section as an idealized rectangular cross-section. When the river channel depth is lower than the floodplain elevation, the water flow in the river channel will not overflow into the floodplain. At this time, the flow rate in the river channel is calculated using the following formula:

[0116] ;

[0117] In the formula, The flow rate (m³ / s) on the river grid from time t to time t+Δt. Let t be the hydraulic radius of the river channel (in meters); The cross-sectional area of ​​the river channel (square meters).

[0118] When the river channel depth is higher than the floodplain elevation, the water in the river overflows into the floodplain. At this time, the river discharge includes the sum of the water flow within the river channel and the water flow in the floodplains on both sides of the river. The formula for calculating the water flow in the floodplains on both sides of the river is:

[0119] ;

[0120] In the formula, The flow rate (cubic meters per second) in the floodplains on both sides of the river channel. w c,flow The width of the river channel (meters); This refers to the slope of the floodplain. The formula for calculating the overflow volume is:

[0121] ;

[0122] In the formula, The water depth in the floodplain; The water depth is above the riverbed elevation; z c,i This refers to the elevation of the riverbed bottom. z f,i Elevation of the floodplain.

[0123] Based on this, it supports the re-infiltration of water overflowing into the floodplain and its participation in soil water processes. The water depth overflowing into the floodplain is added to the water accumulation depth of the floodplain simulation unit in S12, which supports the re-infiltration of water overflowing into the floodplain and its participation in soil water processes, thus realizing the two-way coupling of hydrology and hydrodynamics between the river channel and the floodplain.

[0124] S2. Based on space, time and coupling boundary, the small-scale flood model is nested with the preset large-scale flood model (i.e., large-scale model) to obtain a nested watershed flood simulation model.

[0125] It should be explained that while existing large-scale models are used to simulate the overall hydrological and hydrodynamic processes of the watershed, small-scale models take over the detailed simulation of key sub-watersheds during specific periods. Simultaneously, a bidirectional transfer method for flow boundaries between the large-scale and small-scale models is established, enabling their coordinated operation. This method retains the ability of large-scale models to simulate overall flood evolution trends while utilizing small-scale models for detailed local simulations, making it widely applicable to scenarios such as regional flood risk assessment and refined watershed management.

[0126] In this optional embodiment, based on space, time, and coupling boundaries, a small-scale flood model is nested with a pre-set large-scale flood model to obtain a nested watershed flood simulation model, including:

[0127] S21. Explicitly nest and couple the small-scale flood model with the pre-set large-scale flood model in space;

[0128] S22. Nest and couple the small-scale flood model with the pre-set large-scale flood model in time;

[0129] S23. The small-scale flood model and the pre-set large-scale flood model are coupled at the boundary to transfer river flow bidirectionally.

[0130] S24. Based on the spatial nesting, temporal nesting, and coupling boundary nesting of the small-scale flood model and the preset large-scale flood model, a nested watershed flood simulation model is obtained.

[0131] In this optional embodiment, explicitly nesting and coupling the small-scale flood model with the preset large-scale flood model in space includes:

[0132] Spatially, spatial discretization is performed within key sub-basins using a model suitable for small-scale flooding, while generalization is performed in other regions using a model suitable for pre-set large-scale flooding, thus achieving spatial nesting between the small-scale flooding model and the pre-set large-scale flooding model.

[0133] Specifically, the constructed small-scale model is explicitly nested spatially with the existing large-scale model. The small-scale model is used to simulate one or more key sub-basins and is spatially discretized using a dense grid. The remaining sub-basins are simulated by the large-scale model, which uses spatial units suitable for large-scale simulation, such as elevation bands. The key sub-basins typically refer to key catchment areas prone to flooding, and therefore require special attention and detailed simulation in flood simulation.

[0134] Spatially, the boundary line of the key sub-basin is used as the dividing line of the spatial discretization scheme: the key sub-basin is spatially discretized at high resolution based on S11, and the remaining sub-basins adopt spatial units suitable for generalized simulation at a large scale, such as elevation bands and hydrological response units, to achieve spatial coverage of the entire watershed.

[0135] In terms of time, the use of small-scale models can cover the entire time period, or it can be limited to specific important periods such as the flood peak. Large-scale models are used to simulate the entire domain during other periods to achieve a balance between efficiency and accuracy.

[0136] At the coupling boundary, considering that large-scale models typically employ "m 3 / d”m 3 The flow rate unit is " / h", while the small-scale model uses "m". 3 " / s" is the unit of flow rate. When coupling the models, unit conversion for boundary flows is necessary. When the large-scale model is located upstream of the small-scale model, the outflow rate of the large-scale model (in meters) should be converted. 3 / d、m 3 The flow rate is converted from / h to per second (divided by multiples such as 86400, 3600, etc.) and used as the upper boundary inflow condition of the small-scale model. When the small-scale model is upstream of the large-scale model, the outflow rate is summarized according to the step size of the large-scale model and used as the inflow rate of the large-scale model.

[0137] S3. Using a nested watershed flood simulation model, the hydrology and hydrodynamics of the watershed under test are simulated to obtain the assessment results of the flood inundation area.

[0138] It should be noted that the present invention will be further described in detail below with reference to implementation examples.

[0139] like Figure 1 The above is an overall flowchart of the present invention, and the specific implementation steps are as follows:

[0140] Step 1: Construct a small-scale flood model with bidirectional coupling of hydrology and hydrodynamics: Integrate the core modules of mainstream models such as the Green-Ampt model, the Xin'anjiang model, the two-dimensional flood propagation model (LISFLOOD-FP), and the distributed hydrology-soil-vegetation model (DHSVM) to construct a small-scale flood model with bidirectional coupling of hydrology and hydrodynamics.

[0141] Step 1.1: Spatial discretization is performed based on fine-resolution grid cells, and each grid cell is divided into a surface layer and multiple soil layers in the vertical direction to form a simulation structure with soil columns as the basic unit.

[0142] Step 1.2: Integrate the Green-Ampt runoff model: When water enters the surface layer of a grid cell (including rainfall, runoff from neighboring grid cells, or interflow), the runoff module is invoked to simulate the surface infiltration process, allowing water to enter the uppermost soil layer through infiltration.

[0143] Step 1.3: Integrate the Xin'anjiang model evapotranspiration module: When there is surface water, the surface water is evaporated according to the potential evapotranspiration rate; when there is no surface water, the Xin'anjiang model is used to simulate the evapotranspiration process of three soil layers. If there are more than three soil layers, the third layer and all layers below it are processed according to the evapotranspiration calculation method of the third layer in the Xin'anjiang model.

[0144] Step 1.4: Integrating the seepage module of the distributed hydrology-soil-vegetation model: When the moisture content of the upper soil layer is greater than the field capacity and the moisture content of the lower soil layer is less than the soil porosity, the water in the upper soil layer can seep vertically to the lower layer.

[0145] Step 1.5: Interflow module of the integrated distributed hydrological-soil-vegetation model: Assuming that there are lateral water transport channels between soil layers, the distributed hydrological-soil-vegetation model is used to simulate the interflow process. When the water content of a certain soil layer exceeds the field capacity, the excess water will move laterally along the slope, forming an interflow.

[0146] Step 1.6: Integrate the hydrodynamic calculation module in the LISFLOOD-FP two-dimensional flood evolution model to simulate the runoff exchange process between adjacent grids on the surface slope, realizing slope confluence; simultaneously, use the subgrid river channel calculation module to calculate the river runoff process; and simulate the bidirectional exchange of surface water flow between the floodplain and the river channel, dynamically reflecting the flooded area between the river channel and the floodplain. Support the re-infiltration of water overflowing into the floodplain and its participation in soil water processes, realizing the bidirectional coupling of hydrology and hydrodynamics between the river channel and the floodplain.

[0147] Step 1.7: Integrate the groundwater calculation module. When the moisture in the lowest soil layer exceeds the field capacity, the excess can seep into the groundwater reservoir. While receiving recharge from seepage, the groundwater layer, as a slow-responding water source, continuously provides baseflow to the main channel of the sub-basin, achieving time-delayed response and regulation of river runoff.

[0148] Step 2, Spatial Nesting: The small-scale model constructed in Step 1 is explicitly nested in space with the existing large-scale model. The small-scale model is used to simulate one or more key sub-basins and is spatially discretized using a dense grid. The remaining sub-basins are simulated by the large-scale model, which uses spatial units suitable for large-scale simulation, such as elevation bands.

[0149] Step 2.1: Select key sub-basins that are crucial to people's lives and property safety within the large river basin. Spatially discretize these sub-basins based on Step 1.1 above. The remaining sub-basins are discretized using existing large-scale models. For example, a large river basin may contain 50 sub-basins, of which 5 contain densely populated towns or economic development zones and are designated as key sub-basins. The remaining 45 sub-basins are sparsely populated natural watersheds and are designated as general sub-basins.

[0150] like Figure 2 The diagram illustrates the spatial discretization of large-scale and small-scale models in a specific embodiment. In this example, the large-scale model uses the height above the nearest drainage (HAND) as the basic simulation unit to simulate flood discharge in the Missouri River basin from 2010 to 2019; the small-scale model uses a high-resolution raster as the simulation unit to simulate the flood inundation range of the Omaha sub-basin (located in the middle reaches of the Missouri River).

[0151] Step 3, temporal coupling: The small-scale model can be used to cover the entire time period or limited to specific important periods such as the flood peak. The large-scale model is used to simulate the entire other time periods to achieve a balance between efficiency and accuracy.

[0152] Step 3.1: Within the large-scale simulation cycle, set one or more small-scale simulation time periods. Only during these time periods will the small-scale model be activated to simulate the key sub-basins described in Step 2.1; during the remaining time periods, the entire basin will be simulated using the large-scale model. For example, the total simulation period for a large basin is from January 1, 2010, 00:00:00 to December 31, 2019, 23:59:59. The five key sub-basins experience frequent flooding from June to August each year, while the remaining seasons are relatively dry. The period from June 1, 2010, 00:00:00 to August 31, 2019, 23:59:59 for the five key sub-basins is designated as the small-scale simulation time period. When the large-scale model simulates these time periods, the small-scale model will automatically be activated to simulate the five key sub-basins instead of the large-scale model. During the remaining time periods, the entire basin will be simulated using the large-scale model.

[0153] Step 4, Flow Exchange: Bidirectional transfer of boundary flow between large and small scale models.

[0154] Step 4.1: Considering that large-scale models typically use "m"3 / d”m 3 The flow rate unit is " / h", while the small-scale model uses "m". 3 " / s" is the flow rate unit. Unit conversion is required during model coupling. When the large-scale model is located upstream of the small-scale model, the outflow rate of the large-scale model (in meters) should be converted. 3 / d、m 3 The flow rate is converted from / h to per second (divided by multiples such as 86400, 3600, etc.) and used as the upper boundary inflow condition of the small-scale model. When the small-scale model is upstream of the large-scale model, the outflow rate is summarized according to the step size of the large-scale model and used as the inflow rate of the large-scale model.

[0155] like Figure 3 As shown, the runoff input from the large-scale model to the small-scale model in the embodiment is illustrated. The period from 2010 to 2014 is the calibration period, with an NSE of 0.78; the period from 2015 to 2019 is the validation period, with an NSE of 0.74. The dots in the figure represent observed runoff points, and the dashed lines represent best-simulated runoff values.

[0156] Step 5: Use a nested watershed flood simulation model to simulate the hydrology and hydrodynamics of the watershed under test and obtain the assessment results of the flood inundation area.

[0157] like Figure 4 As shown, the inundation extent of the Omaha sub-basin during the March 2019 flood event (simulated by a small-scale model) is presented and compared with Sentinel-2 satellite imagery from the same period. Blue areas represent regions classified as inundated by both satellite imagery and simulation results; green areas represent regions classified as inundated only by satellite imagery; and red areas represent regions classified as inundated only by simulation results.

[0158] In summary, by utilizing the above-mentioned technical solutions of this invention, the small-scale model constructed during nested coupling integrates multiple hydrological and hydrodynamic modules, including a runoff generation module, a soil evapotranspiration module, an infiltration module, a soil interflow module, a slope runoff module, a river runoff module, and a groundwater module. This supports the re-infiltration of water overflowing into the floodplain and its participation in soil water processes, achieving bidirectional coupling of hydrology and hydrodynamics between the river and the floodplain. This provides a mechanism to improve the accuracy of local simulations in nested simulations.

[0159] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A nested watershed flood simulation method considering bidirectional coupling, characterized in that, The method includes the following steps: S1. Based on spatial structural characteristics, the secondary infiltration of water in floodplains into soil water mechanisms, and the coupling of hydrological and hydrodynamic modules, a small-scale flood model is constructed. The hydrological and hydrodynamic modules include runoff generation, soil evapotranspiration, seepage, interflow, slope runoff, river runoff, and groundwater modules. The specific steps are as follows: S11. Spatial discretization is performed based on grid cells with fine resolution to construct spatial structural features; the spatial structural features include the surface layer and the soil layer; S12. When water reaches the surface, a runoff generation module is constructed based on the infiltration calculation mechanism of the Green-Ampt model. S13. Based on the soil evapotranspiration model, and combined with the presence or absence of surface water, construct a soil evapotranspiration module. S14. When soil seepage occurs, a seepage module is constructed based on the seepage calculation mechanism of the distributed hydrology-soil-vegetation model. S15. When saturated interflow occurs in the soil layer, construct a soil lateral interflow module based on the interflow calculation mechanism of the distributed hydrology-soil-vegetation model. S16. When runoff exchange occurs between adjacent surface units, a slope runoff module is constructed based on the slope runoff calculation mechanism of the two-dimensional flood evolution model. S17. Based on the river channel calculation mechanism of the two-dimensional flood evolution model and the mechanism of secondary infiltration of overflow floodplain water into soil water, and combined with the comparison results of river channel water depth and floodplain elevation, a river channel runoff module is constructed. S18. When the moisture content of the lowest soil layer exceeds its field capacity, a groundwater module is constructed based on the recharge and discharge mechanism of the nonlinear groundwater reservoir. S19. Couple the runoff generation module, soil evapotranspiration module, seepage module, interflow module, slope runoff module, river runoff module and groundwater module to construct a small-scale flood model; S2. Based on space, time and coupling boundary, the small-scale flood model is nested with the pre-set large-scale flood model to obtain a nested watershed flood simulation model. S3. Using a nested watershed flood simulation model, the hydrology and hydrodynamics of the watershed under test are simulated to obtain the assessment results of the flood inundation area.

2. The nested watershed flood simulation method considering bidirectional coupling according to claim 1, characterized in that, The soil layers are divided into three layers: an upper soil layer, a lower soil layer, and a deep soil layer. When the actual number of soil layers is more than three, all soil layers at or below the third effective depth are merged into the deep soil layer.

3. The nested watershed flood simulation method considering bidirectional coupling according to claim 2, characterized in that, The soil evapotranspiration module, constructed based on the soil evapotranspiration model and considering the presence or absence of surface water, includes: Determine the evapotranspiration path based on the surface water accumulation status; When there is standing water on the local surface, the actual evapotranspiration is calculated based on the potential evapotranspiration rate. When there is no standing water on the surface, the actual evapotranspiration is calculated layer by layer according to the soil evapotranspiration model, based on the three-layer structure of the upper soil layer, the lower soil layer, and the deep soil layer.

4. The nested watershed flood simulation method considering bidirectional coupling according to claim 2, characterized in that, The leakage phenomenon includes: When the moisture content of the upper soil layer is greater than the field capacity and the moisture content of the lower soil layer is less than the soil porosity, water from the upper soil layer will seep vertically into the lower soil layer.

5. A nested watershed flood simulation method considering bidirectional coupling according to claim 2, characterized in that, The interflow phenomenon in saturated soil includes: When the water content of any soil layer reaches or exceeds the field capacity and lateral drainage is available, water flows laterally along the direction of maximum hydraulic gradient under the drive of gravity and water potential gradient, enters the adjacent simulation unit, and forms interflow in the soil under saturation.

6. The nested watershed flood simulation method considering bidirectional coupling according to claim 1, characterized in that, The river runoff module, constructed based on the river channel calculation mechanism of the two-dimensional flood evolution model and the mechanism of secondary infiltration of overflow floodplain water into soil water, combined with the comparison results of river channel depth and floodplain elevation, includes: When the river channel depth is lower than the floodplain elevation, the water flow in the river channel will not overflow into the floodplain. The river channel calculation mechanism based on the two-dimensional flood evolution model is used to calculate the water flow in the river channel. When the river channel depth is higher than the floodplain elevation, the water in the river overflows into the floodplain. Based on the river channel calculation mechanism of the two-dimensional flood evolution model, the overflow volume and the lateral flow of the floodplain are calculated. Based on the mechanism of secondary infiltration of overflow floodplain water into soil water, the overflow water is input into the corresponding floodplain as surface water and participates in the subsequent soil water infiltration process.

7. A nested watershed flood simulation method considering bidirectional coupling according to claim 6, characterized in that, The formula for calculating the water flow in the river channel is: ； The formula for calculating the lateral discharge in the floodplain is as follows: ； The formula for calculating the overflow volume is: ； In the formula, The flow rate on the river grid from time t to time t+Δt; Let t be the hydraulic radius of the river channel; The cross-sectional area of ​​the river channel. The flow rate of the floodplains on both sides of the river channel; w c,flow The width of the river channel; The slope of the water surface in the floodplain; The water depth in the floodplain; The water depth above the riverbed elevation; z c,i This refers to the elevation of the riverbed bottom; z f,i Elevation of the floodplain.

8. The nested watershed flood simulation method considering bidirectional coupling according to claim 1, characterized in that, The nested watershed flood simulation model, based on space, time, and coupling boundaries, combines a small-scale flood model with a pre-set large-scale flood model, resulting in the following: S21. Explicitly nest and couple the small-scale flood model with the pre-set large-scale flood model in space; S22. Nest and couple the small-scale flood model with the pre-set large-scale flood model in time; S23. The small-scale flood model and the pre-set large-scale flood model are coupled at the boundary to transfer river flow bidirectionally. S24. Based on the spatial nesting, temporal nesting, and coupling boundary nesting of the small-scale flood model and the preset large-scale flood model, a nested watershed flood simulation model is obtained.

9. A nested watershed flood simulation method considering bidirectional coupling according to claim 8, characterized in that, The explicit spatial nesting and coupling of the small-scale flood model with the pre-set large-scale flood model includes: Spatially, spatial discretization is performed within key sub-basins using a model suitable for small-scale flooding, while generalization is performed in other regions using a model suitable for pre-set large-scale flooding, thus achieving spatial nesting between the small-scale flooding model and the pre-set large-scale flooding model.

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