The one FF would consist of a very long list of water levels measured every
15 minutes for the last 150 years or so. You then look which situation looks
most like what we have now. And if that situation is, for example, June 12 1957,
then you look up the situation at June 13 1957 and make that your prediction
for tomorrow.
The other FF would be based on the finite element method discussed in the seminar. You look in each river, canal, and ditch and look at the present water flow in liters/second and from that you calculate the water level differences some small time unit from now. That gives you new water levels from which you calculate new flow speed, that give you new water levels, etc.