1. Learning objectives To understand the water balance
To understand how the water balance can change in different environments

2. Remember the drainage basin system
Remember the drainage basin system!! Can you remember the 3 outputs and the 1 input?
The water balance, or water budget is the balance between these inputs and outputs
River Runoff
Outputs = river runoff and evapotranspiration
Inputs = precipitation
Transpiration
Precipitation
Evaporation

3. P = O + E +/- S The Water Balance
Scientists want to know what happens to the water when it enters the drainage basin system.
This helps to plan for future water supply and also to help control flooding.
Precipitation = Total runoff (streamflow) + Evapotranspiration +/- Storage (soil or rock
P = O + E +/- S

4. Precipitation = Total runoff (streamflow) + Evapotranspiration +/- Storage (soil or rock
P = O + E +/- S
100% =
50%
30%
20%

5. The Water Budget Graph
The water budget (water balance) can be represented by an equation and by a graph…
The graph looks at how evaporation and precipitation levels affect soil moisture levels
Soil moisture recharge
Soil moisture utilisation
Soil moisture surplus
Soil moisture deficit

6. The Water Budget Graph Mean Precipitation (mm) J F M A M J J A S O N D
When evaporation exceeds precipitation in the summer the soil water is utilized…
… there is a soil moisture or water surplus. This water is now available to supply rivers
By the second half of summer there is a soil moisture deficit
Mean Evapotranspiration (mm)
Mean Precipitation (mm)
Precipitation increased and evapotranspiration falls in the autumn allowing soil moisture to be recharged once more 5
J F M A M J J A S O N D
In the winter precipitation exceeds evaporation and soil moisture is recharged until…
Soil moisture recharge
Soil moisture utilisation
Soil moisture deficit
Soil moisture surplus

7. What if precipitation exceeds evaporation in the winter?
What if GAME…..
What if precipitation exceeds evaporation in the winter?
What if evaporation exceeds precipitation in the summer?

8. TASK Use the table below to draw a water budget graph for England
Colour code the graph like the one on the left and annotate it to show what’s happening.
Mean Evapotranspiration (mm)
Mean Precipitation (mm)
J F M A M J J A S O N D
Month
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Precipitation (mm)
110
107
102 96 91 84 83 89 95
111
119
104
Evapotranspiration 10 22 41 69 90
114
124
126
115 70 11 15
Finished early? Can you explain your graph to the person next to you? Or do a sketch and explain it without looking?

9. How many of the new words can you remember in 60 seconds??
60 second challenge
How many of the new words can you remember in 60 seconds??