Curve and surface fitting

Hi,

I have several series of points (x, y) and I have fitted each series in an exponential curve using expfit function.

Each series of x and y points has the same z.

Now, what I am trying to do is getting a general equation (x, y, z)  to relate x, y and z in one equation.

I tried to create a surface with CreateMesh (using regress and interp with all the points, not the fitted curves) but it seems that I do not have access to the equation which links the points. Is it possible to get an equation of the surface?

I tried to do something with the fitted curves instead of the raw points but I have not found how to do it. s there a function in Mathcad 15 to do that?

I am attaching two files with the fitted curves and the surface created.

Could anyone help me?

Thanks in advance