The scheme of the GR2M model is presented below.

Description of the GR2M model

In the GR2M model, the production function relies on a soil moisture store that is very similar to the one used in GR4J. Given a precipitation P, the level S in the store becomes S₁ and is defined by:

The parameter X₁, the maximum capacity of the store, is positive and is given in mm.
La pluie P₁ en excès est donnée par :
P₁ = P + S – S₁
Given the potential evapotranspiration E, the level S₁ becomes S₂: :

The production store then empties with a percolation P₂ and its level S, ready for the computations of the following month, given by:

and

The total precipitation P3 that reaches the routing store is given by:
P₃ = P₁ + P₂
The level R in the routing store then becomes R₁:
R₁ = R + P₃
A water exchange term is then calculated:
F = (X₂ – 1).R₁
The parameter X₂ is positive and has no dimension. The level in the store becomes:
R₂ = X₂.R₁
The store, with a fixed capacity equal to 60 mm, empties following a quadratic function.
The streamflow is given by:

 The model has two parameters to optimise during its calibration:

• X1 : the capacity of the production store (mm),
• X2 : the exchange coefficient (-).