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 six-member U.S. delegation toured Tarbela Dam, on the Indus river, the largest earth-filled 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 signal-driven 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.


Spillway at Tarbela Dam, Pakistan.

Spillway at Tarbela Dam, Pakistan.