Do You Know the Difference Between Hydrology and Hydraulics?

Water is Earth’s most critical fluid in environmental processes and human life. When discussing stormwater management it is important to make the distinction between hydrology and hydraulics. Some aspects of hydrology involve hydraulics but, in general, the two are very different. Here’s how to understand these differences.

Hydraulic analysis can model stormwater flows.

Hydrology is defined as the circulation of water and its constituents through the hydrologic cycle, or the quantification of flows that are ultimately produced by precipitation. It deals with precipitation (rain, snow, sleet, hail, etc.), evaporation, infiltration, groundwater flow, surface runoff, streamflow, and the transport of substances dissolved or suspended in flowing water (David R. Maidment). In the field of stormwater engineering, hydrology typically refers to the rate of precipitation, quantity of water, rate of surface runoff, and timing of its arrival at a point of interest.

Alternatively, the term hydraulics is defined as the study of the mechanical behavior of water in physical systems (Henry M. Morris and James M. Wiggert). In engineering terms, hydraulics is the analysis of how surface, and/or subsurface flows move from one point to the next. Hydraulic analysis is used to evaluate flow in rivers, streams, storm drain networks, water aqueducts, water lines, sewers, etc.

As you can see, though hydrologic and hydraulic processes are interdependent, they are distinctly different.

Jonis Smith is a professional engineer with 21 years’ experience in all aspects of stormwater management, flood control engineering, and water resource system design. For more information, contact him at or 949.373.8334.