The area of the triangle formed by a cross section of the trough is

By similar triangles

The volume at some height h is then

The full volume of the trough is

Therefore, when it is 1/4 full by volume:

and and

Now, can you take it from there by differentiating and finding dh/dt.

The surface area of the water at some height h would be given by the length of the trough times b (the width of the water surface at some height).