Specification

Factors affecting river regimes: precipitation including storm hydrographs, temperature, vegetation, land-use, water abstraction, dams.

What is a "river regime"?

The regime of a river is the way in which the discharge of the river changes over time or in the course of the year.

How would the regime of a river be affected by each of the following:

1. A sudden intense period of rainfall
2. Snow melting on mountains at the end of summer
3. A long period of drought

Sketch a set of axes like the ones shown below on a mini-whiteboard and work in pairs to draw and try to explain how you think the river regime would look for each of the rivers listed:

• A river on Java, Indonesia
• A river in northern Nigeria, on the edge of the Sahel. Rain falls only between June and August
• A river in NW Europe where precipitation falls year round. During the winter months precipitation falls as snow which does not melt until March.

Storm hydrographs (also known as Flood Hydrographs)

A storm hydrograph is a special kind of graph which shows how the discharge of a stream or river changes after a period of rainfall.

You need to :

• Know how to draw and interpret storm hydrographs 
• Be able to use the key terms associated with storm hydrographs (discharge, rising limb, recession limb, peak flow, peak rainfall, lag-time)
• Understand how factors like vegetation, land use, water abstraction and dams can affect the hydrograph of a river.

a) Key features of a hydrograph

Use the slides to fully annotate your own copy of the storm hydrograph. Make sure you have explained each of the following:

• base flow
• peak flow
• peak discharge
• peak rainfall
• rising limb
• recession limb

b) Drawing a hydrograph

Use the data you have been given to practise drawing your own storm hydrograph. Check that you have included/done all of the following:

• Graph has a clear title
• All axes are clearly labelled
• Each of the following are labelled:
  • Peak flow (label includes the time and the discharge)
  • Peak rainfall (label includes time and amount of rainfall)
  • Lag time (this is shown diagramatically and has been calculated)
  • Rising limb
  • Recession limb

c) Hydrographs in action

Watch the video clip below carefully. Can you write a commentary which explains and links what you see to the storm hydrograph?!

Now work in 2s or 3s and discuss what effect each of the following would have on the lag time and the peak discharge:

• planting trees in a drainage basin
• people in a city deciding to pave over their gardens to provide extra parking
• planting grass on bare soil in the catchment area
• A city growing within the drainage basin of a river

For each, try to sketch the storm hydrograph before and after and make sure you can justify what you think.