Set the analysis domain
Define the horizontal grid extent from polygons representing the target area.
Overview
Overview of the National Water-Cycle Model
The National Water-Cycle Model is a wide-area water-cycle model for evaluating surface-water and groundwater flow across Japan. It is built on GETFLOWS, the GEneral purpose Terrestrial fluid-FLOW Simulator. The model integrates terrain, drainage networks, geology, soils, land use, vegetation, meteorology, and observations on a grid basis, and represents surface-water flow, groundwater flow, recharge, spring discharge, and evapotranspiration within a single three-dimensional model domain.
Terrain
Defines ground-surface geometry and catchment conditions.
Water
Sets surface-water boundaries and water-body conditions.
Geology
Assigns hydraulic properties from topsoil, cover layers, bedrock, weathering, and loosened zones.
Climate
Provides precipitation, potential evapotranspiration, and daily meteorological forcing.
Specifications
Core Specifications
The representative configuration shown here couples surface water and groundwater in GETFLOWS and uses a two-phase water-air formulation. Under average forcing conditions, the model evaluates a natural equilibrium state without anthropogenic water use.
| Name | National Water-Cycle Model |
|---|---|
| Simulator | GETFLOWS, the GEneral purpose Terrestrial fluid-FLOW Simulator |
| Fluid system | Coupled surface-subsurface flow with two-phase water-air flow |
| Domain | Japan, subject to the coverage of the source datasets including islands |
| Horizontal resolution | 0.25 arc-minutes × 0.25 arc-minutes, approximately 500 m in the representative configuration |
| Vertical discretization | 25 layers |
| Lower boundary | Elevation -3000 m |
| Analysis condition | Natural equilibrium under average forcing conditions, without anthropogenic water use |
| Steady meteorological forcing | Year-round constant forcing from JMA Mesh Climatology 2020 precipitation and CHELSA V2.1 potential evapotranspiration corrected by an empirical coefficient of 0.65 |
| Fluid properties: water | Density 1,000 kg/m³, viscosity 1.0×10⁻³ Pa·s, compressibility 4.5×10⁻¹⁰ 1/Pa |
| Fluid properties: air | Density 1.22 kg/m³, viscosity 1.82×10⁻⁵ Pa·s, with compressibility inversely proportional to pressure |
| Boundary conditions | Standard atmospheric pressure at the atmosphere boundary, no-flow bottom and side boundaries, and tide fixed at MSL 0 m |
Input Data
Input Data
The model converts external datasets prepared for Japan, together with model-derived estimates and derivative datasets, into grid-level terrain, water-body, geology, soil, land-use, vegetation, meteorological, and observation information. This section focuses on the role of each dataset as model input.
The external data-source list is provided on a separate page.
| Category | Dataset | Role in the model |
|---|---|---|
| Meteorology and climate | Normal precipitation | Precipitation from JMA Mesh Climatology 2020 is converted to annual mean daily values and used as steady precipitation input. |
| Meteorology and climate | Normal potential evapotranspiration | CHELSA v2.1 potential evapotranspiration is converted to annual mean daily values and used as steady evapotranspiration input. |
| Meteorology and climate | Daily precipitation | JMA analyzed precipitation is assigned to model grids and used as water-input time series for transient analysis. |
| Meteorology and climate | Daily meteorology | NARO Agro-Meteorological Grid Square Data and ERA5-derived pressure variables are used for evapotranspiration, snowmelt, and water-balance evaluation. |
| Meteorology and climate | Future climate input | ISIMIP / CMIP6 GCM inputs are assigned to the national model for comparing future conditions. |
| Terrain and water system | Land elevation | Fundamental Geospatial Data DEM is used to build the ground surface, terrain relief, and the upper surface of subsurface structure. |
| Terrain and water system | Bathymetry and ocean area | GEBCO is used to define marine cells, bathymetry, and boundary conditions including coastal areas. |
| Terrain and water system | River network and bed elevation | The Japan surface-flow-direction map is used to organize river location, river-bed elevation, and drainage structure on the model grid. |
| Terrain and water system | Lake topography | National Land Numerical Information, HydroLAKES, and GLOBathy are used to define lake-surface elevation, lake-bed elevation, and lake extent. |
| Geology and soils | Topsoil and cover layers | Japan Soil Inventory, soil maps, and forest-soil physical-property data are used to assign hydraulic properties such as water retention and hydraulic conductivity in shallow ground. |
| Geology and soils | Weathering and geomorphic surfaces | Thickness of Quaternary layers, plain classifications, geologic boundary surfaces, and weathering surfaces are combined to build the shallow-to-deep subsurface structure. |
| Geology and soils | Bedrock | The Seamless Digital Geological Map of Japan is reclassified into model geologic codes and used for deep hydraulic properties. |
| Geology and soils | Hydraulic properties | Initial values based on general and literature values are optimized using observed river discharge and dam inflow data from 101 sites. |
| Land use and vegetation | Land use | JAXA high-resolution land-use and land-cover maps and National Land Numerical Information are used to classify water bodies, built-up areas, paddy fields, cropland, forest, and grassland. |
| Land use and vegetation | Vegetation type | ALOS forest classes are used to identify forest areas and forest types for transient forest evapotranspiration analysis. |
| Land use and vegetation | Canopy height | ETH Global Canopy Height 2020 is used when setting evapotranspiration and vegetation parameters in forest areas. |
| Land use and vegetation | Tree density | Crowther global tree-density data is combined with canopy height to represent forest structure in transient analysis. |
| Land use and vegetation | Leaf seasonality | GSI is calculated from NARO Agro-Meteorological Grid Square Data and used to represent leaf-on and leaf-off dates in forest and vegetation areas. |
| Land use and vegetation | Crop coefficients | ISIMIP3 crop calendars and NARO meteorological data are used to build crop coefficients and soil evaporation coefficients for FAO-56 and evapotranspiration conditions over cropland. |
| Observations | Observed river discharge | Time-series data at Japanese river observation sites are compared with simulated discharge for model-performance diagnosis. |
| Observations | Dam observations | Dam inflow time series are compared with simulated discharge for model-performance diagnosis. |
3D Structure
Three-Dimensional Model Domain
This section covers the analysis domain, horizontal grid, surface conditions, and vertical layering.
Create the horizontal grid
Divide the target area at the specified horizontal resolution to create the computational grid.
Assign surface conditions
Organize elevation, rivers, lakes, ocean areas, land use, meteorology, and vegetation on a grid basis.
Build vertical layers
Divide the domain vertically from the ground surface to the lower analysis boundary.
AtmosphereAtmospheric boundary providing precipitation and potential evapotranspiration.
SurfaceSurface conditions including surface water, rivers, lakes, ocean areas, and land use.
Top SoilOne-meter topsoil layer reflecting soil properties, land use, and vegetation conditions.
1-5 mShallow surface ground divided into three layers.
5-50 mWeathered and cover layers divided into six layers.
50-200 mDeeper weathered and loosened zones divided into nine layers.
BedrockBedrock divided into four layers down to the lower boundary at elevation -3000 m.
Hydrogeology
Hydrogeologic Structure and Hydraulic Properties
Classification information from geologic and soil maps is converted into hydraulic properties used in the simulation. The model organizes topsoil, water-body sediment, cover layers, weathered and loosened zones, and bedrock, then reflects these elements in hydraulic conductivity, storage, and unsaturated-zone water-retention properties.
Hydraulic-property items
| Item | Description | Representation in the model |
|---|---|---|
| Hydraulic conductivity | A value describing how easily water passes through geologic layers or soils. | Affects groundwater flow, river discharge, and recharge. |
| Anisotropy kv/kh | Difference between vertical and horizontal permeability. | Controls whether groundwater tends to flow laterally or infiltrate downward. |
| Effective porosity | Fraction of pore space participating in water movement. | Controls groundwater storage and flow velocity. |
| Two-phase flow properties | Water-retention properties when water and air occupy the same pore space. | Used to represent infiltration, evaporation, and retention in the unsaturated zone. |
| Specific storage | Water released or stored per unit pressure change. | Relevant to groundwater-level change and storage change. |
Building the hydrogeologic framework
Topsoil, water-body sediment, cover layers, weathered and loosened zones, and bedrock have distinct hydraulic properties. The following tables summarize how each dataset is treated as a geologic or soil element and how it is reflected in model properties.
Topsoil and water-body sedimentSurface soil properties and water-body classification are converted into near-surface hydraulic conditions.
| Element | Content | Model property |
|---|---|---|
| Topsoil | Physical properties of agricultural soils, A- and B-horizon properties of forest soils, land use, and vegetation conditions. | Sets near-surface infiltration, water retention, effective porosity, and storage. |
| Water-body sediment | Classification of rivers, lakes, and ocean areas. | Represents locations where water exchanges between water bodies and the subsurface through infiltration and discharge. |
Cover layersThick aquifers, thin strata, and plain or coastal distributions are organized as shallow geology.
| Element | Content | Model property |
|---|---|---|
| Thick permeable layers | Surface-geology distribution and base-surface elevation of major plains are organized. | Placed as thick shallow permeable layers in plains with priority over bedrock. |
| Thin strata | Alluvial deposits and other thin strata are organized from surface geology and geomorphic classification. | Represents different shallow cover-layer thickness and permeability in mountainous and plain areas. |
| Bedrock below cover layers | Bedrock distribution beneath cover layers is estimated from surrounding bedrock distribution. | Treats cover layers and deeper bedrock as a continuous 3D geologic structure. |
Weathered and loosened zonesMultiple elevation surfaces generated from terrain data express depth-dependent hydraulic-property changes in bedrock.
| Element | Content | Model property |
|---|---|---|
| Classification-surface rasters | Multiple base surfaces from shallow to deep zones are prepared from terrain elevation, summit-level surfaces, valley-bottom surfaces, and related inputs. | Overlaid with bedrock distribution to determine the degree of weathering and loosening with depth. |
| Depth-dependent properties | The same bedrock type is configured to be relatively more permeable near the surface and less permeable at depth. | Adds vertical hydraulic-property variation to horizontally distributed geologic classes. |
| 3D integration | Bedrock plan-view distribution is combined with weathering and loosening classification surfaces. | Creates a 3D hydrogeologic structure combining geologic classes and depth-dependent property changes. |
BedrockBedrock used as deep material is organized from the 1:200,000 Seamless Digital Geological Map of Japan V2.
| Element | Content | Model property |
|---|---|---|
| Bedrock distribution | Bedrock exposed as surface geology is integrated with bedrock distribution estimated below cover layers. | Represents continuous deep geology beneath cover layers. |
| Sedimentary rocks | Age and lithology of sedimentary rocks are organized. | Differences among sandstone, mudstone, conglomerate, and related lithologies are reflected in deep permeability and storage. |
| Volcanic rocks | Lithology and age of volcanic rocks are organized. | Represents groundwater flow and spring characteristics in volcanic regions. |
| Plutonic and metamorphic rocks | Granitoids, metamorphic rocks, and other deep-basement rocks are organized. | Provides the deep structural background for regional groundwater flow. |
Transient Analysis
Forcing Conditions for Transient Analysis
In the steady analysis, precipitation from JMA Mesh Climatology 2020 and CHELSA V2.1 potential evapotranspiration are applied as year-round constant forcing. For discharge validation and seasonal diagnostics, the model uses daily meteorological data in transient analysis and sets evapotranspiration conditions for forest, cropland, and other land-use classes. In snowy regions, snow accumulation and snowmelt are treated separately, and model water input is given as rainfall plus snowmelt.
| Target | Main inputs and settings | Use in the model |
|---|---|---|
| Steady analysis | JMA Mesh Climatology 2020 precipitation and CHELSA v2.1 potential evapotranspiration × 0.65 | Average precipitation and corrected potential evapotranspiration are used as year-round constant forcing for the natural equilibrium state. |
| Transient forest analysis | ALOS forest class, canopy height, tree density, GSI-derived leaf-on and leaf-off dates estimated from NARO meteorological data, and daily meteorology | The forest-floor evaporation model and the canopy interception / transpiration model are combined to separately evaluate canopy interception, transpiration, and forest-floor evaporation. |
| Transient cropland and other analysis | Crop coefficients and soil evaporation coefficients for FAO-56, ISIMIP3 crop calendar, and NARO Agro-Meteorological Grid Square Data | Reference evapotranspiration is calculated with FAO-56 Penman-Monteith, and transpiration and evaporation components over cropland and other surfaces are evaluated. |
| Transient snow and snowmelt analysis | Daily meteorology, rainfall, and snowmelt | In snowy regions, snow accumulation and snowmelt are included following the approach of Fukazawa and Tada (2024), and water input is organized as rainfall plus snowmelt. |
| Conversion to actual evapotranspiration | Evapotranspiration efficiency based on topsoil properties | Potential evapotranspiration is reduced according to topsoil properties and water conditions. Forest areas use the Feddes et al. (1978) root water-uptake model, while non-forest areas use evaporation efficiency based on Lehmann et al. (2018). |
| Application to the model | Water input, potential evapotranspiration, and actual evapotranspiration | Rainfall, snowmelt, interception, and evapotranspiration efficiency are applied as water exchanges in the coupled surface-water and groundwater simulation. |
Validation
Discharge Validation at Nationwide Sites
The national model uses time-series data of observed river discharge and dam inflow to optimize part of the hydraulic-property set. This section interactively displays comparisons between observed and simulated discharge at 475 sites nationwide. Sites with lower reproducibility should be interpreted comprehensively, considering not only the 3D model and parameters but also observation accuracy, local geology, anthropogenic water use, and meteorological forcing.
Discharge comparison at 475 sites
Select a site on the map or from the list to display the corresponding discharge comparison graph.
475 sites
Loading station data.
Representative discharge comparison
A representative site is shown initially from the nationwide 475-site comparison.
The figure is generated from the discharge-comparison CSV for 475 sites across Japan. Site-specific reproducibility is affected by observation accuracy, local geology, anthropogenic water use, meteorological forcing, and other conditions in addition to the 3D model and parameterization.
Outputs
Main Water-Cycle Evaluation Metrics
Model outputs can be organized as map layers and regional diagnostic reports. By overlaying input data and model results, the effects of terrain, geology, land use, and meteorological conditions on water-cycle quantities can be evaluated.
Groundwater level
Evaluates groundwater-surface elevation relative to the ground surface and its spatial distribution.
G.L. mSurface-water depth
Evaluates spatial distribution of surface-water depth in rivers, lakes, and lowlands.
mTopsoil water saturation
Evaluates the degree to which the topsoil layer is filled with water.
-Surface-water flow
Quantifies water moving through rivers and over the land surface.
m³/dayGroundwater flow
Evaluates the direction and magnitude of water movement underground.
m³/dayRecharge
Evaluates water supplied to groundwater from rainfall and related processes.
mm/daySpring discharge
Shows the distribution and amount of water returning from the subsurface to the surface and rivers.
mm/dayActual evaporation, surface
Evaluates water returning to the atmosphere from the ground surface and water surfaces.
mm/dayActual evaporation, subsurface
Evaluates water returning to the atmosphere through the subsurface and unsaturated zone.
mm/dayEvaporation efficiency
Organizes the ratio of actual evaporation to potential evaporation.
-Effective precipitation
Organizes precipitation contributing to runoff and recharge as a water-cycle quantity.
mmFlow lines and flow conditions
Visualizes the flow direction and flow conditions of surface water and groundwater.
flow linesInput Conditions and Model Outputs in Webmap
Webmap lets users inspect terrain, land use, meteorological conditions, and computed water-cycle quantities incorporated in the National Water-Cycle Model. The Webmap top page provides links to the public viewers and technical overview pages for each model.