|This type of hydrograph is known as a storm or flood hydrograph and it is generally drawn with two vertical axes. One is used to plot a line graph showing the discharge of a river in cumecs (cubic metres per second) at a given point over a period of time.
The second is used to plot a bar graph of the rainfall event which precedes the changes in discharge.
The scale on the horizontal axis is usually in hours/days and this allows both the rain event to be recorded and the subsequent changes in river discharge to be plotted.
The shape of the hydrograph varies according to a number of controlling factors in the drainage basin but it will generally include the following features.
The baseflow of the river represents the normal day to day discharge of the river and is the consequence of groundwater seeping into the river channel. The rising limb of the hydrograph represents the rapid increase in resulting from rainfall causing surface runoff and then later throughflow. Peak discharge occurs when the river reaches its highest level. The time difference between the peak of the rain event and the peak discharge is known as the lag time or basin lag. The falling limb (or recession limb as it is sometimes known) is when discharge decreases and the river’s level falls. It has a gentler gradient than the rising limb as most overland flow has now been discharged and it is mainly throughflow which is making up the river water.
A number of factors (known as drainage basin controls) influence the way in which a river responds to precipitation and have an effect on the shape of the hydrograph.
The size, shape and relief of the basin are important controls. Water takes longer to reach the trunk stream in a large, round basin than in does in a small, narrow one.
Where gradients are steep, water runs off faster, reaches the river more quickly and causes a steep rising limb. Prolonged heavy rain causes more overland flow than light drizzly rain.
Areas of permeable rocks and soil allow more infiltration and so less surface run off.
The way in which the land is used will also have an influence on the hydrograph – vegetation intercepts precipitation and allows evaporation to take place directly into the atmosphere so reducing the amount of water available for overland flow while the large number of impermeable surfaces in urban areas encourages run off into gutters and drains carrying water quickly to the nearest river.