Models of the Hydrologic Cycle

The study of hydrologie processes or systems uses historical data collected from natural phenomena which are observed once, and only once, and probably will not be observed again under exactly the same conditions. All hydrologie processes are more or less stochastic, and as such are subject to the vagaries of probability. Furthermore, these stochastic processes are generally nonpure-random and also nonstationary; i.e., the probability distribution changes during the course of the process making them time-dependent. Chow (1964a) outlined a dichotomous scheme for classifying hydrologie phenomena by mathematical definition (Fig. 1). This rational classification emphasizes the mathematical complexity of analyzing and modeling hydrologie data.