| Date | May 2019 | Marks available | 2 | Reference code | 19M.1.bp.1 |
| Level | SL and HL | Paper | 1 | Time zone | |
| Command term | Explain | Question number | 1 | Adapted from | N/A |
Question
Option A — Freshwater
The diagram shows the changing frequency of discharge* in a small drainage basin during two different time periods.
[Source: © International Baccalaureate Organization 2019]
* frequency of discharge: how often a river reaches or exceeds a particular discharge level
Estimate the highest discharge of the river during the 1950s.
State the number of times that river discharge reached 4 m3/s (cubic metres per second) during the 1990s.
Outline two possible land use changes that could account for the increase in river discharge over time shown in the diagram.
Explain how different channel modifications in a small drainage basin such as this can increase flood risk.
Explain how different channel modifications in a small drainage basin such as this can assist with flood mitigation.
Markscheme
Award [1] for 6.3 (m3/s) (allow 6.2–6.4).
Award [1] for 8 times.
In each case, award [1] for a valid land use change (eg, urbanization, deforestation) that might explain increased river discharge, and [1] for how this could account for the increase shown. For example:
- A change from rural to urban land use / urbanization [1] would result in an increase in impermeable surfaces and reduced infiltration, resulting in increased overland flow and thus river discharge [1]
- Change in vegetation cover by deforestation [1] would reduce interception and so increase surface run-off [1].
Award [1] for a valid modification and [1] for further development of how this might increase flood risk.
For example: Straightening the river channel [1] allows faster removal of an increased amount of water away from the area, increasing flood risk downstream [1]
Award [1] for a valid modification and [1] for further development of how this might decrease flood risk.
For example: Constructing man-made levees [1] increases channel capacity, allowing greater discharge without flooding [1].
As the question asks for "different modifications", do not accept the same modification twice.
Do not accept afforestation, changing agricultural practices or other land use changes.
Examiners report
There were no problems in identifying the correct figures.
There were no problems in identifying the correct figures.
A well answered question, with urbanization and deforestation the main land use changes considered for increasing river discharge.
Generally well answered. Weaker candidates did not focus on channel modifications and/or confused parts (i) and (ii) in their responses.
Generally well answered. Weaker candidates did not focus on channel modifications and/or confused parts (i) and (ii) in their responses.
