Hydrographs

Year 12 geography GCE

What is river discharge?

River discharge is defined as the volume of water passing a measuring point or gauging station in a river in a given time.

It is usually measure in cubic metres per second – (cumecs)

The following formula is usually used to calculate cumecs.

Discharge (m3 per sec) = cross – sections area (m2) X velocity (metres per second)

The average discharge of the Amazon is - 219,000 m3 / sec. Compare this to the River Nile at 2830 m3 / sec and the River Thames at 66 m3 / sec.

How can discharge be illustrated?

Discharge can be illustrated using hydrographs. There are two types of hydrograph.

The first show the annual patterns of flow (the river regime) in response to climate.

The second show the short term variations in discharge and are called flood or storm hydrographs.

What are the key terms to learn?

Peak rainfallPeak dischargeLag time

Base flowStorm flow

Rising limbReceding limb

Make sure that your know that overland flow and throughflow contibute to storm flow and that groundwater flow contributes to base flow.

How is overland flow caused?

Overland flow is happens in the following circumstances;

The soil is saturated (soil storage) and precipitation can no longer infiltrate into the soil.

The rate of precipitation (mm per hour) exceeds the infiltration capacity of the ground, (high intensity rainfall or compacted soil due to the use of machines).

The ground may be frozen or snow melts, but the ground below the melted snow is still frozen.

The surface is impermeable as in urban areas.

How do hydrographs differ in shape?

There are two main difference in the shape of hydrographs.

The first difference is that the level of peak discharge can be higher for some hydrographs.

The second difference is that the lag time can be shorter for some hydrographs.

If a hydrograph has high peak discharge and a short lag time this automatically leads to a rapid rise in the rising limb of the hydrograph.

The two hydrographs on the next slide illustrate this point.

The Factors that Affect Storm Hydrographs?

Intensity of precipitation (high or low)

Duration of precipitation (short or long)

Type of precipitation (snow)

Temperature (high or low - ET)

Type of soil(thin or deep – air space and grain size)

Type of rock(impermeable verses permeable or porous)

Size of drainage basin(small verses large)

Shape of drainage basin(round verses elongated)

Vegetation(different types – summer verses winter)

Land use(agricultural verses urban)

Urbanisation Human Intervention

How does precipitation Affect hydrographs?

How does soil types effect hydrographs?

How does size of the drainage basin affect hydrographs?

How does shape of the drainage basineffect hydrographs?

How does rock type effect hydrographs?

How does vegetation effect hydrographs?

How does drainage density effect hydrographs?

How do dams effect hydrographs?

Ox – Bow Lake