Or you can build the seed into the structure you pass to the shaders.
It depends what you want to do, when I do my cloud simulations I use the old trick of using floating point textures and swapping buffers.
So you populate a texture with variables, run the shader on it and write the results to another texture. Then swap the textures.
You are limited in that you can only have 4 variables per pixel, so your equations have to be broken down into separate passes, but it's effective.