Is there any way to switch Boundary conditions based on time and solution variable in MATLAB pde?

I am using Matlab pde solver to solve Richards equation (parabolic pde) similar to heat equation. Now while defining the boundary I need to switch between neumann and dirichlet based on solution. So what I want to do is: if any state.u at the Face where BC is apllied is >0 use dirichlet else use neumann. So at each state.t the solver should check if the above condition is true. Based on that it will choose the BC at that instant. Is there any way to do that? To my understanding, the mixed BC works based on region, can it be modified for time. Any help will be aprreciated. Thanks