# 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.

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:

R= 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:

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