Draw a labelled diagram of a hydrograph.

Explain how hydrographs are used to forecast floods.

“Dams and reservoirs create as many problems as they solve.” Discuss this statement with reference to multi-purpose schemes.

The hydrograph must include:

• Labelled axes showing time and discharge [1 mark]
• Line graph showing discharge [1 mark].

Any additional features or labels are worth 1 mark each, up to a maximum of 2 marks:

• Labelled rainfall
• Peak discharge
• Base flow
• Lag time
• Rising or falling limb.

There are many ways in which hydrographs are useful in forecasting floods. Answers could mention prediction of flood height and arrival time downstream using a study of lag times and stream response records. Hydrographs are also used to predict the return period of floods of different sizes through the use of hydrograph records over a long period of time. They may also be used to predict the extent of flooding and how long it will last.

At least two of these should be explained to gain 6 marks.

A strong account of one may compensate for a weaker account of another.

At least one example of a multi-purpose scheme should be named and located.

Solutions might include: the benefits of power generation, water supply, irrigation, transport, flood control, recreation and tourism.

These should be balanced against potential problems that could include: loss of land, displacement of population, loss of water through evaporation, silting, seismic problems, cost, diseases such as bilharzia, loss of nutrients due to flood control, salinization.

To access bands E and F, candidates should discuss both solutions and problems using at least one actual scheme.

Marks should be allocated according to the markbands.

Excellent at both levels, though a few higher level centres erroneously offered climate graphs rather than hydrographs.

Links to forecasting were often tenuous with few references to speed of flood onset, height, duration or return period.

The approach was usually sound, with some truly outstanding responses using detailed case studies and accurate supporting data.