ABSTRACT
Successful management of water resources requires a solid understanding of hydrology. Human activities directly and indirectly alter the natural hydrologic system. Environmental systems, including the hydrologic system, can be reduced to groups of nested and interconnected subsystems. A hydrologic system is uniform when there is no change in system variables over space, and non-uniform when there is spatial variation. The term dynamic equilibrium or steady state describes a certain condition of a system. Water quality problems include the migration of pollutants in groundwater or determining the behavior of chemical constituents in a stream. In either case hydrologic problems exhibit characteristics in both time and space. The use of abstract mathematical models dominates hydrologic modeling. This chapter introduces the concept of a system, with the hydrologic cycle being a premiere example of a physical environmental system. Even though a hydrologic system is understood in terms of the relationships between components, many hydrologic analyses are undertaken on individual components of the system.
