How to include constraint for pdepe solution (e.g., u>0)

Hello, I want to solve a parabolic partial differential equation (i.e., mixed Richards' Eq): du/dt=d/dz(A(z)*du/dz+B(z)) I want to put a constraint so that all solution are greater than a constant value, c (u>c). I would be grateful if you help me.

Thanks, Abedeh

Answer 1

[Gareth Thomas]

Piecewise function

In R2016b one can use the Piecewise function. I hope that this helps you.

[Abedeh Abd]

Sorry! I did not get your hint. Would you please explain more. Assume I use pdepe and pdepe solve it (i.e., u). some elements of solution (u) is less than 0.05. However, I know u never is less than 0.05 in nature (physically basis). How should I define this constraint to pdepe? I do not want to manually change it using u(find(u<0.05))=0.05 after pdepe solve equation.