ABSTRACT
A system can be considered as deterministic or stochastic. A deterministic system defines a cause–effect relationship, and therefore predictions are possible if the cause is known. The cause is the driving function and effect is the response function. Deterministic systems can be considered as lumped parameter systems or as distributed parameter systems. In general, from a mathematical point of view, lumped parameter systems are represented by ordinary differential equations and distributed parameter systems are represented by partial differential equations. To make use of the principles of proportionality and superposition, the input functions to any linear system can be described as a linear summation of standard input functions. The hydrological cycle in a wider sense is a closed system as there is no net transfer of water from the earth. However, hydrologists are concerned about what happens within the confines of a certain spatial domain, such as, for example, a catchment.
