In order to accomplish the tasks of Water Management, it is of great importance to obtain hydrological information and quantify the processes in the catchment areas.

The types of hydrological information needed to achieve high level of flood protection, are mainly: determination of the peak flow and discharge hydrograph along the river, determination of flood stages, flood probability, inundation areas, and evaluation of groundwater recharge and the effect of flood control measures on the hydrological system.

For quantification, mathematical models are widely used for the simulation of hydrological processes in catchments and flow processes in rivers.

**Deterministic model (deductive)**

If the chance of occurrence of the variables involved in such a process is ignored and the model is considered to follow a definite law of certainty but not any law of probability, the process and its model are described as deterministic. For example, the conventional routing of flood flow through a reservoir is a deterministic process and the mathematical formulation of the unit-hydograph theory is a deterministic model.

**Stochastic model (inductive)**

If the chance of occurrence of the variables is taken into consideration and the concept of probability is introduced in formulating the model, the process and its model are described as stochastic or probabilistic.