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.
The excess precipitation P1 is given by:
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 (-).