Hydrologic phenomena are extremely complex, and may never be fully understood. However, in the absence of perfect knowledge, they may be represented in a simplified way by means of the systems concept. A system is a set of connected parts that form a whole. The hydrologic cycle may be treated as a system whose components are precipitation, evaporation, runoff, and other phases of the hydrologic cycle. These components can be grouped into subsystems of the overall cycle; to analyze the total system, the simpler subsystems can be treated separately and the results combined according to the interactions between the subsystems.