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Hydrological

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model

GloFAS v4 reanalysis, forecasts, and reforecasts renalysis and forecasts are generated using the open-source hydrological model LISFLOOD OS. LISFLOOD OS is a distributed, physically based hydrological model which has been developed by the Joint Research Centre (JRC) of the European Commission since 1997 (De Roo et al., 2000).  LISFLOOD OS is particularly suited for the modelling of rainfall-runoff processes in large catchments: starting from GloFAS v3.1 (operational release: 2021-05-26), LISFLOOD OS is the sole model at the core of GloFAS.

LISFLOOD OS is able to represent all the main hydrological processes: partition of precipitation into rain and snow, snow melting, canopy interception of rain, water infiltration into the soil, groundwater storage, surface runoff, lakes, dams, irrigation, and other human water uses, flow in the rivers and in the floodplains. The numerical simulation is driven by meteorological forcing data (precipitation, temperature, and evapotranspiration). A set of raster maps showing the terrain morphology, soil properties, land cover and land use features, water demand, enables the modelling of runoff processes in different climates and socio-economic contexts. LISFLOOD OS solves the water balance at each time step and for each grid cell.

, a physically based spatially distributed hydrological model. More detailed information about this hydrological model (e.g. modelled physical processes) are available here.

Specifically, GloFAS v4 makes use of LISFLOOD OS v4.1.3.  Compared to the LISFLOOD OS version used to produce GloFAS v3, LISFLOOD OS v4.1.3 features updates in the hydrological routines, such as pixel-by-pixel computation of water infiltration into the soil, and improvements in the modelling of water abstraction for anthropogenic use. Moreover, LISFLOOD v4.1.3 benefits of significant improvements in the management of large input datasets and in the computational performances.

Consistently with all the previous GloFAS versions, the computations for GloFAS v4 are completed with daily time steps for all the hydrological processes, while sub-daily (4 hours) time steps are used for the modelling of river routing. All the output variables are available with daily resolution.

Implementation 

Static maps

Accurate representation of the rainfall-runoff processes in different climatic and socio-economic contexts requires a set of maps showing the morphological, pedological, vegetation, land usecover, land cover and water use characteristics of the catchments. About 80 implementation maps are required as input to LISFLOOD OS. This set of implementation maps is traditionally called referred to as “static maps” or “surface fields dataset”.

The spatial resolution of the GloFAS the GloFAS v4 implementation maps is maps is 0.05 degrees, which is four times higher than the previous 0.1 degrees GloFAS v3. The much higher resolution of GloFAS v4 allows a more detailed representation of the river network and of the spatial variability of catchments’ characteristics. The benefits of using a higher spatial resolution can be appreciated in Figure 1: smaller tributaries can be more adequately represented when using 0.05 degrees resolution.

further information on the preparation, features, and availability of the dataset can be found here: CEMS-Flood surface field dataset - Copernicus Emergency Management Service - CEMS - ECMWF Confluence Wiki.

Meteorological forcings

 Figure 1 - River network at 0.1 degrees resolution (left) and 0.05 degrees resolution (right): drainage upstream area with increasing values from 50 km²  represented with colours from blue to yellow. The HydroRIVERS dataset is used as reference. 

Moreover, the latest research findings, remote sensing and in-situ datasets were used to prepare the 0.05 degrees resolution implementation maps. Examples include the latest land cover information and leaf area index layers from the Copernicus Global Land Service, global hydrology datasets based on MERIT Hydro, and global soil data from ISRIC.

The higher resolution and quality of the 0.05 degrees implementation set-up is expected to lead to a more realistic representation of the hydrological processes, and therefore to more accurate modelling of river discharge and hydrological state variables (e.g. soil moisture).

The GloFAS v4 implementation maps dataset is presented in Choulga et al. 2023[1] and the maps are available for download from the Joint Research Centre Data Catalogue [readme]. The complete list of source data and a detailed description of the methodology used to produce each map is available here. 

The meteorological variables used by GloFAS v4 (and GloFAS v3) are total precipitation, 2-metre temperature, 2-metre dew temperature, 10-metre U wind component, 10-metre V wind component, surface solar radiation downwards, surface thermal radiation. These variables are provided as input to GloFAS from different meteorological datasets, depending on the purpose of the model simulation: hydrological reanalysis, medium range forecast, seasonal forecast. The list of meteorological datasets used for each purpose is available here.from GloFAS meteorological forcings - Copernicus Emergency Management Service - CEMS - ECMWF Confluence Wiki. The meteorological variables are upsampled from their native resolution to 0.05 degrees resolution using bilinear interpolation: the interpolation algorithm is implemented in the open-source pre-processor pyg2p. It is here noted that nearest neighbor interpolation was used for 0.1 degrees GloFAS set up (v3 and preceding versions).

2-metre temperature, 2-metre dew temperature, 10-metre U wind component, 10-metre V wind component, surface solar radiation downwards, surface thermal radiation are used to compute  reference values of evapotranspiration according to the Penmann-Monteith equation, which is implemented in the open source pre-processor LISVAP.

LISFLOOD OS then takes as input total precipitation, 2-metre temperature, and reference evapotranspiration. Similarly to all the previous GloFAS versions, the computations for GloFAS v4 are completed with daily time steps for all the hydrological processes, and sub-daily (4 hours) time steps for the river routing. All the output variables are available with daily time steps.

References

[1] Choulga, Moschini, Mazzetti, Grimaldi, Disperati, Beck, Salamon, and Prudhomme. “ Technical note: Surface fields for global environmental modelling”. Submitted for publication to HESS, MS type: Technical notefrom each source are pre-processed as described in this page.