The starting point for the Blatter-Pattyn model is the full Stokes equations
where P is the pressure and τ is the deviatoric stress tensor. The latter is given by
where σ is the full stress tensor.
There are a number of ways to argue that because of the "shallowness" of ice sheets - that is because the ratio of H/L, where H is the thickness and L is a relevant horizontal length scale, is small - the equations above can be reduced to the following "first-order approximation"
Unfortunately, the arguments that support this reduction are complex and we can't go into them in detail here. Two papers given in the references below (Schoof and Hindmarsh (in press) and Dukowicz et al. (submitted)) provide more details on the mathematical background that allows us to state that these equations are "first order accurate" approximations to the Stokes equations.
We continue by noting that the 3rd (vertical) balance equation above can be integrated through the depth of the ice sheet to give an expression for the pressure, P,
This expression can be substituted into the horizontal pressure gradient terms above to remove pressure from the equations. For example, for the 'x' component of velocity we have
Now we can use the incompressibility and stress-strain colinearity constraints to rewrite the vertical normal deviatoric stress in terms of horizontal normal deviatoric stresses. This allows us to write the horizontal balance equations in terms of horizontal stress gradients only. Again, for the x direction we have
Note that we have removed the vertical balance equation entirely; it is incorporated into the horizontal balance equations, which are
Next, we want to write these equations in terms of velocities, since that is what we are ultimately solving for. The link between stress gradients and velocities is through the constitutive law for ice (here we'll assume Glen's law) - relating strain rates to stresses - and the definition of the strain rate tensor - relating strain rates to velocity gradients.