Quantitative evaluation of groundwater potential is possible only after a set of basic parameters is properly defined and physically related to some aquifer properties. Reliable interpretations and conclusions about the whole aquifer performance in an area are possible, provided that the accurate determination of basic parameter values is achieved. In addition to temporal variability of groundwater occurrence, distribution, and movement, there is distinctive spatial variability due to the geological composition. Consequently, the basic parameters will also be functions of space and time. Fortunately, the parameters relating to the aquifer medium are independent of time compared to human life but they are certainly changing with geological time scales which is not significant from groundwater abstraction point of view. Hence, medium-dependent parameters do change with space and give rise to the questions of anisotropy and heterogeneity which will be described later.