In the Fall of 1987, I was invited to participate in the International Seminar on Physical and Mathematical Modeling of Hydraulic Structures, in Lahore, Pakistan. As part of the visit, the sixmember U.S. delegation toured Tarbela Dam, on the Indus river, the largest earthfilled dam in the world. One evening, after dinner at the guest house, one of the members of the team posed the following question to me: "Could you tell us what the difference is between deterministic and stochastic modeling?" I said: "It is simple: If you have a signal, the process should be modeled deterministically; on the other hand, if all you have is noise, the process should be modeled stochastically." In fact, it would be a mistake to handle a signaldriven process such as flood wave routing with a stochastic model. On the other hand, daily flows in upland watersheds usually lack enough diffusion to develop a signal; therefore, they are better handled with a stochastic approach. Both deterministic and stochastic models are useful tools in applied hydrology.
 
