The last month's usage and inflow:|
Compare this to the same time last year.
What about inflow versus outflow?
A dam's level drops when more water is used (outflow) than flows in (inflow) obviously.
This diagram shows cumulative outflow versus cumulative inflow for the entire system,
using the SCA daily figures since November 2001. In other words we show for any moment how much water has
been used (or entered the dams) since November 2001.
Wince Nov 2001, the overall usage has been 5349.39
overall inflow has been 5613.89
GL (compare to the total capacity of Sydney's dams
is just under 2400 GL).
Note: when we discuss inflow, we refer to net inflow (i.e. after evaporation is taken into account).
This is why the inflow figure
can sometimes be below 0 (when evaporation is larger than actual inflows).
Although inflow has not equaled outflow for the past few years, it is still very significant. This is one of the reasons why rainwater
tanks in Sydney are still useful in drought conditions... it still does rain, just not so much.
The inflow to the dams is inconsistent, however. The following graph displays daily usage and
inflows since November 2001.
The usage graph shows us that the Summers of 2001/2 and 2002/3 had
higher usage, and otherwise there is a slight downward trend on water usage,
water saving devices and general water consciousness on the part of consumers. Water restrictions up until level 3 have supposedly
reduced water consumption by 12 percent. The large drop in usage in July 2005 corresponded to a fairly wet winter when the
(relatively unpopular) desalination plant location was announced. It looks like people take shorter showers when an environmentally
relatively unfriendly option is put forward.
A more smoothed out version of this (a one-year rolling average) follows:
Due to the smoothed nature of this graph, the trend is more clear. This is amplified when
examined on a year-by-year basis:
When inflow and usage are combined, a figure is arrived at which corresponds to the net gains (or losses) of the dams.
This is represented int he following graph (showing Warragamba dam, and the other dams combined). A result greater than
0 represents an increase over a 365 day period for the dams.
How low has Sydney's usage gone in the past few years?
These tables show the record minimum usage for a single day and the record minimum average usage
over an entire week.
|Date||Day Usage ML|
|Date||Week Usage ML/day|
How is usage affected by rain and temperature in Sydney?
We have already seen the effects of the 2001/2 and 2002/3 summers on water use, as well as the wet winter of 2005.
The following graph compares water usage versus Sydney rainfall on the day before.
In times of low rainfall, the usage is dependent on various factors, but when rainfall is high, the
water use is quite consistently low. The wettest day (130+mm) of average usage was in 2002
just before a new level of water restrictions came into force.
Temperature also has a large effect on water usage in Sydney.
The following diagram shows that low temperatures equate to lower water usage.
High temperatures can also match low water usage, however, as rain may occur to lower usage around such days.
The effect of a change in our water usage on Sydney's dam levels?
We have taken the data since November 2001 of Sydney's water usage and dam levels and calculated how full the dams
would be under several alternative scenarios:
If everyone had and used rainwater tanks for
their irrigation and hand watering of their gardens whenever possible, usage would probably drop by around 20%.
- 20% more usage of Sydney water
- 20% less usage of Sydney water
- 50% less usage of Sydney water
|Hypothetical usage since 11.2001||Available water||% full all dams||1 year change GL(%)|
|20% More|| 1430.6||55.41
|As is || 2500.5
|20% Less|| 2551.8||98.84
|50% Less|| 2553.2||98.89
These calculations are of course just on figures since November 2001.
When these figures are calculated back to 1998 when the dams were last full, a drop in usage would have produced more
dramatic differences in dam levels now.