The Unit Hydrograph theory assumes that the watershed is a linear system. This means that the outflow is proportional to the inflow regardless of the magnitude of the inflow. This is generally not the case however. When the flow in stream channels and on overland flow surfaces increases, the velocity also increases, causing a reduction in the time of travel to the outlet. Yet, for most natural streams, the velocity increases as the depth increases only until overbank flow begins. At this point, the velocity tends to remain constant, which satisfies the requirement of linearity. Therefore, unit hydrographs should be derived only from the larger floods for a particular watershed.

The Unit Hydrograph theory also assumes that the input rainfall excess is uniform over the watershed, and that the response to this input is invariable. Typically, the spatial variation of rainfall, and the difference in watershed characteristics can cause the rate of runoff to vary widely from place to place at any time. However, many watersheds do experience similar patterns of rainfall from event to event, and therefore the response to that rainfall excess can be effectively characterized by the unit hydrograph.

The unit hydrograph theory depends on the principle of superposition. This principle states that a flood hydrograph for a particular storm data can be built up from the unit hydrograph applied to the incremental rainfall excess during each period. In other words, the unit hydrograph can be applied to a series of inputs, and the resulting hydrographs can be added together to form the total hydrograph.

Related Topics:

• Generic Unit Hydrographs
• Soil Conservation Service (SCS)
• Unit Hydrograph Runoff Methods
• RTK Methods