Hydrograph

The hydrograph is a graph that shows the variation over time of some hydrological information such as: water level, flow, sediment load, among others. For a river, stream, wadi, or canal, although it typically represents flow versus time; this is equivalent to saying that it is the graph of the discharge (L³/T) of a flow as a function of time. These can be storm hydrographs and annual hydrographs, which in turn are divided into perennial and intermittent.

Storm hydrogram due to rain received in the basin.

Allows you to observe:

• variations in the discharge through a storm, or through the hydrological year:
• the runoff peak (maximum channel of the avenue);
• the base flow or contribution of groundwater to flow; or
• the seasonal variations of the flows if a period of one or more years is graphed.

One mm of precipitation means that one liter of rainwater (1L/m²) has fallen on a surface of one m².

Hydrographs are useful, among other things, to compare the discharge times and peak flows of various streams or hydrographic basins, in order to know the differences between their response capacities to floods.

Unit Hydrograph: Basic response curve to a unit of precipitation that describes the way in which a basin returns a rainfall input distributed over time. It is based on the principle that said input-output relationship is linear, that is, that they can be added linearly. It is built based on an "Hydrogram in S" which in turn is built by breaking down various storms and their actual produced hydrographs.

Synthetic Hydrograph: Estimated unit hydrograph according to formulas that include physical parameters of the basin under study such as area, length of the main channel, average slope, and others. They are the best known synthetic hydrographs: the USDA Triangular, the Schneider, the Clark.

Determination of the discharge hydrograph of a basin

In some cases it is necessary to determine the total volume of surface runoff generated by rain in a given time. However, the case in which it is required to know the maximum instantaneous flow of a certain avenue is more frequent. Other times a complete knowledge of the hydrograph is required, that is to say the variation in time of the flow in a certain section in which it is intended to build a hydraulic work or protect an existing asset.

The methods used for these calculations are:

• The rational one.
• Unit hydrogram method.
• Mathematical models of watersheds.
• Hydraulic Transit through channels using the equations of Saint-Venant in 1D.