Changing bc in pdepe based on solution

Hi,

I am trying to solve a partial differential equation in matlab given at

http://www.mathworks.com.au/matlabcentral/fileexchange/15646-environmental-modeling/content/richards.m

The problem that I am facing is that whenever the value of "u" at the left boundary xl (or at the top if reference frame is considered vertical) approaches zero, the analysis quits and error message is obtained.

Now when the value of "u" approaches zero at xl I when to change the bc from that defined in the code to

p=ul(1)-s2; q=0;

Is it possible to change the bc depending on the solution?

Thanks, Abid