2.  The balance between the water inputs and the water
outputs
 Equation: P= Q + E + S
 Where: P= precipitation
Q= total streamflow (runoff)
E= Evapotranspiration
S= Storage (in soil & bedrock)

3.  used to help manage water supply and
predict where there may be water shortages.
 also used in:
 irrigation,
 runoff assessment
 flood control and
 pollution control.

4.  illustrated using a water balance graph
 plots levels of precipitation and evapotranspiration
 often on a monthly scale.
 The graph helps to highlight river-basin management
challenges, which include:
 Periods when there is likely to be water deficiency or drought
 Periods when flooding is most likely to occur
 The best time for irrigation
 Long-term changes in the storage capacity of the drainage basin
 The need for a water transfer system

6. 1. Soil moisture surplus:
soil moisture is full for plant use
and run off into streams
2 3
2 Soil moisture use:
plants must rely on stored 4
water which gradually used up 1 5
3. Soil moisture deficiency:
plants must have adaptation
for long periods or land must
be irrigated
4. Soil moisture recharge:
the soil water stored start to be
recharged
5. Field capacity:
the soil water stores are now full

7.  Annual precipitation always exceeds evapotranspiration
 Precipitation (Input) exceeds evt loss (output) = positive water balance
 soil moisture surplus
 However, 1974 &1975 as well as 1995 & 1996 – long dry summer
 Evapotranspiration exceeds precipitation  temporary negative water balance
 During winter, ppt > evt = soil moisture surplus
  considerable river run off & rise in river levels
 In summer, evt > ppt (plants & humans utilise the water from
the soil storage  depletion and fall in river levels
 By Autumn, ppt> evt = first water surplus is to recharge the until the
soil reach field capacity