ÉROS functionalities 

The ÉROS software offers the following features: 

• Calculating the balance of rainfall, actual evapotranspiration, run-off and infiltration into the underlying water table, 
• Generating long series of flow rates or piezometric levels from historical rainfall data, after calibration for a relatively short period, 
• Analysing the influence of climate change on flow rates and levels, 
• Analysing consistency between climate observations and observations of flow rates or piezometric levels. 

In practice, ÉROS enables analysis of the hydrological functioning of a heterogeneous catchment basin or one that is subjected to pumping from rivers and the extrapolation of data flow rates and/or levels. It can therefore support the design of engineering structures such as dams, or amenities such as car parks, water abstraction points in rivers, micro hydro-electric plants, etc. 

Once calibrated, the model can: 

• for a given catchment basin, recreate the flow rates of rivers and/or the piezometric levels at a given point in the underlying aquifers during periods when such measurements are not available, 
• simulate: 
  • flow rates resulting from dry periods (due to low water periods or dam design) or exceptional precipitation sequences (flood risks), 
  • groundwater piezometric levels at a given moment based on observed precipitation and extrapolated to produce precipitation scenarios (drought or high water periods). 

The ÉROS computing program is a highly suitable tool for regional syntheses that require coherent modelling of a set of nested catchment basins. 

• predict the influence of climate change on a large basin based on scenarios of rainfall and potential  evapotranspiration. 

ÉROS also offers the following features: 

• Simultaneous calibration of modelling parameters for series of flow rates measured at the outflow of catchment basins and sub-basins, and for series of piezometric levels at a point that is representative of certain sub-basins. This function significantly improves the reliability of the calibration. 
• Time series of abstraction (or injection) data of flow rates in rivers in certain sub-catchment basins can be included in the calculations. 

The user may choose to calculate a nitrate balance and transfer rate, based on farming practices and the needs of the crops in each sub-basin. 

ÉROS can then also calculate for each sub-basin: 

• The nitrate concentration in the river at the outlet of the sub-basin, 
• The nitrate concentration in the groundwater table below the lower part of the sub-basin. 

The nitrate transfer is calculated using the BRGM BICHE calculation scheme (Thiéry & Seguin, 1985c; Thiéry, 1990).

Working principle of the ÉROS software 

In each sub-basin, ÉROS uses the GARDENIA lumped hydrological model with reservoirs. Each global model simulates the main water cycle mechanisms in a catchment sub-basin (rainfall, evapotranspiration, infiltration, run-off) by applying simplified laws of physics. These simplified physical laws correspond to flows through successive reservoirs. 

In a sub-basin, transfers from one reservoir to another are governed by simple laws described by the model's design parameters (retention capacity of the ground, transfer time, flooding thresholds, etc.). 

The global hydrological model provides the elemental flow at the outlet of the sub-basin and the piezometric level at a point representative of the underlying water table. 

The elemental flows of the sub-basins are then aggregated from upstream to downstream, taking into account the transfer time from one basin to another. 

Because the process produces an overall schematic picture at a sub-basin scale and because of the complexity of the actual hydrological system, these parameters, although physically meaningful, cannot easily be deduced a priori from the physiographic characteristics of a catchment basin at a given point (geology, plant cover, etc.). 

The modelling parameters must therefore be determined by simultaneous adjustment (calibration) of: 

• series of flow rate observations at the outlet of sub-basins, or some of them; 
• series of observations of piezometric levels in the underlying water table of certain sub-basins. 

Data required for using ÉROS 

The data required to calibrate the parameters are time series with constant time steps: 

• Series of model "input" data for each sub-basin: 
  • Continuous time series of climate data used as input: rainfall and potential evapotranspiration (and air temperature if snowmelt needs to be taken into account); 
• Possibly, series of model "input" data in some sub-basins: 
  • Possibly a time series of river abstraction flow rates (or injection) in some sub-basins; 
• Model control data series in some sub-basins: 
  • Time series of flow rate observations at the outlet of sub-basins. These series may have gaps in observations at certain time steps. 
  • Time series of observations of piezometric levels in the underlying water table of sub-basins. These series may have gaps in observations at certain time steps. 

These control series will be compared with the corresponding model data. 

ÉROS software 

ÉROS is a generalised application of GARDENIA. It is used to produce models of heterogeneous catchment basins as ranked clusters of GARDENIA models, where each GARDENIA model is applied to a sub-basin. 

Free licensing available 

ÉROS is available for downloading for Windows 8 or10 ( for 64-bit computers).