002 - The water balance

Syllabus Point

Explain (AO2) the concept of maximum sustainable yield of freshwater in terms of a balance between inputs and outputs.

Key Terminology

Define the terms below using your 'IB Geography. Course Book' by Cooke and Nagle, the command term posters in the classroom and the links below.

  1. Make a copy of the first graph on the video. 
  2. Explain what the graph illustrates.
  3. What is 'field capacity'?
  4. In which months is there a surplus of water?
  5. Has this graph been drawn to represent the Northern or Southern Hemisphere? Explain your answer.
  6. If you were asked to draw a water balance graph for the UK what data would you need?



Activity Two - Skill

The tables below give data for the climatic conditions of the UK. The first table gives data for the evaporation rates in the UK for 2010, 2011 and 2012. The second illustrates the average climatic conditions for the UK between the years 1981 and 2010. The data for the second table has been taken from the UK Met Office. 

  1. Your task is to construct a water balance graph using the data in the table.
        • First of all decide which data you are going to use. You do not need it all!
        • Then using graph paper draw out your axis - pencil and a ruler are essential.
        • CET = Corrective Effective Temperature - temperature taking into account the temperature, humidity and speed of the air.
https://unchartedterritory.wordpress.com/category/media/science-and-the-media/

Climatic Data for the UK 1981-2010

MonthMax. temp
(°C)
Min. temp
(°C)
Days of air frost (days)Sunshine (hours)Rainfall (mm)Days of rainfall >= 1 mm (days)
Jan6.40.911.347.2121.715.5
Feb6.60.711.369.888.612.3
Mar8.92.17.6101.895.113.9
Apr11.43.44.5148.172.711.7
May14.76.01.2185.970.011.2
Jun17.38.80.1169.573.411.0
Jul19.410.90.0172.478.111.4
Aug19.110.80.0163.089.512.0
Sep16.58.80.2124.796.412.1
Oct12.86.21.892.5127.115.0
Nov9.13.35.657.2121.215.2
Dec6.71.111.040.8120.214.7
Annual12.45.354.61372.81154.0156.2


  1. In which months would you be most likely to issue a flood warning? Explain your answer using your water balance graph, as well as your prior knowledge.



Exam Style Question


Suggest why an understanding of the water balance is important when discussing sustainable yields. (4 marks)