Introduction
Learn about hydrographs and their use in hyrdologic analysis. This includes how to carry out baseflow separation, how to create a unit hydrograph from a storm hydrograph, and how to predict the runoff from a given storm based on a unit hydrograph for the drainage area.
Hydrograph Definition
A hydrograph, as used in hydrology, is a graph or table of values showing the changes in flow rate over time at a point on a river or some other point of hydrologic interest. Such points of interest for a hydrograph, other than a point on a river or stream, include locations such as the entrance to a storm water detention system or a storm water drainage outlet from a drainage area. Hydrographs are commonly used to show flow patterns following a storm, thus providing information about the storm water runoff rate at the point of interest.
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A storm hydrograph for a point on a river will typically have a gradually decreasing flow rate prior to a storm. Then after the storm starts, the flow rate will gradually increase as storm water runoff from more and more of the drainage area reaches the river. The flow rate (discharge) will typically increase to a peak value and then gradually decrease to the pre-storm level again, as shown in sample hydrograph in the figure at the right. For the flow at an outlet from a drainage area, where there is no non-storm water flow, the hydrograph will start at zero flow prior to the storm and go back down to zero flow.
The Parts of a Hydrograph - Baseflow Separation
The portion of the hydrograph between the point where it starts to rise to the peak is called the rising limb and the portion to the right of the peak is called the receding limb.
The area under a hydrograph represents the volume of runoff. It can be considered to be made up of two parts: i) the normal flow that would have been present even if the storm hadn't occurred, called
baseflow, and ii) the flow due to the storm, called direct runoff. The direct runoff due to a storm is often of interest, so the baseflow component must be separated from it.
The diagram at the right illustrates three baseflow separation methods: i) the constant discharge method, ii) the constant slope method, and iii) the concave method. The constant discharge method consists of simply extending a horizontal line from the point where the hydrograph begins to rise to its intersection with the receding limb. The intersection with the receding limb for both the constant slope and concave methods is at the inflection point (where the receding limb changes from convex to concave). The time from the peak to the inflection point of the receding limb is often approximated by N = A^0.2, as shown on the diagram, where A is the watershed area in sq mi, and N is time in days.
Excel templates for baseflow separation by the concave method are available from the download page of EngineeringExcelTemplates.com.