Water resources and management

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Water resources and management

• Blackwood Valley Small Landholders Group

Tilwin Westrup Surface Water Management
Development Officer Department of Agriculture and
Food, Bunbury Adrian Williams Pendragon Dryland
Management Services Pty Ltd Bridgetown
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Water resources and management

• Morning
• Water Quantity current predictions
• Water Quality problems, causes and impacts
• Dams and catchment improvement
• Dealing with salinity in dams
• Bores and groundwater supplies
• Rainwater tank systems

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Water resources and management

• Afternoon
• Runoff control and runoff prediction
• Keeping nutrients out of dams
• Dissolved iron sources, causes and impacts
• Instructing the dam sinking contractor
• Fencing dams
• Dam evaporation control
• Identifying and treating dam leakage
• Water monitoring Kim Posavec

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1 A WATER QUANTITY
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Decrease of 90mm (13) in 118 years
Decrease of 30 mm (16) in 118 years
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Temperature
Predictions of Climate Change
1.5oC
4.5oC

• Projected mean annual temperature changes for
  2030 and 2070.Source IEA (1998).

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Rainfall
- 20 to 5
- 60 to 10
Projected rainfall changes for 2030 and
2070.Source CSIRO Atmospheric Research.
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Mean the numerical average of all the
numbers in a series Median the value of the
middle number in a series Modal the most
common number in the series Deciles rainfall
data ranked from smallest to largest and divided
into 10 equal parts
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Prediction Based on years thought to be
similar Indicates average rainfall for most of
the Blackwood for April June 2007
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Prediction Based on years thought to be
similar Indicates average rainfall for most of
the Blackwood for May October 2007
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Messages
1. Rainfall is declining in the Bridgetown area
2. Winter rainfall has declined more than summer
rainfall over the last 118 years in absolute
terms, but summer rainfall has decreased by a
larger percentage
3. Annual average temperatures are anticipated
to rise

• Increased temperatures are likely to lead to
  increases in evaporation and evapotranspiration

5. Drier, hotter conditions could lead to
decreased soil cover and therefore
increased rates of rainfall runoff, soil erosion
and salinity
6. Landholders should take these changes into
account when planning water







conservation and land use
enterprises
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What amount of rainfall can we rely on when
designing a dam or rainwater tank system?
Do you have alternative sources of water ?
What size of dams or tanks can you afford ?
Decile 1 98 confidence. Used for rainwater
tanks when there is no alternative source of
water available. Requires more tank storage
capacity than calculations based on Modal
rainfall.
Modal rainfall the rainfall most often
received. Normally used for calculating runoff
volumes when designing dams.
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4. SIGHTING A NEW BORE
The Archean rocks in the area around Bridgetown
contain only minor amounts of groundwater.
That limited groundwater is generally found in
faults containing fractured rock. Such faults
are called lineaments.
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LINEAMENTS
Fractured rock
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SIGHTING A NEW BORE
Streamline crossing minor lineament
Streamline crossing major lineament
Streamline
Where lineaments cross streamlines there are
opportunities for rapid recharge to the
groundwater that supplies nearby bores
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Lineament (Quartzite dyke) crossing streamline
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Locating bores near where streamlines cross
lineaments has been found to improve bore
yield and water quality. (Research from N.T.,
Canada)
Lineaments can be located by using geological
maps, aerial photographs, satellite imagery, or
field observation of linear rock outcrops or
streamlines controlled by fault lines and rock
jointing
HOWEVER.. Not every lineament is your
friend! In this area the water stored in
lineaments can range in quality from fresh
through brackish to saline. Also rock dykes can
impede through-drainage in the soil, and lead to
seeps which may produce saline water.
The effects of lineaments on water quality may
well depend on their location in the landscape.
Local knowledge is very important in recognising
problem areas.
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SIGHTING A NEW BORE
Knowing where to look for bore sites is a start.
But bore testing - particularly for water quality
- is essential before settling on which borehole
to equip.
Future water yield from a bore cannot be
predicted. Unlike a tank or dam you cannot see
the water in a bore running out! A decline in
water quality can be an early warning that the
bore water supply might be failing. Develop a
back-up to bore water supplies if possible
(rainwater tanks, dams, scheme water where
available).
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5. DOMESTIC WATER SUPPLY AND RAINWATER TANKS
Match objectives, rainfall patterns and roof
area(s) to rainwater tank sizes
1. Calculate water demand requirements 2.
Calculate roof catchment area and monthly water
yield from the roof 3. Calculate the required
size of storage 4. Calculate tank performance
through the year and estimate quantities of
overflow (available for environmental flows or
garden watering)
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Calculating water demand requirements
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Calculating roof catchment area and monthly water
yield from the roof
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Roof water yield per month
Using Decile 1 average monthly rainfall should
give a system that is 98 dependable. Use this
if you have no other source of water. Using
monthly modal rainfall will produce a less
reliable system, but one which will require
smaller tanks. Consider using these figures if
you have an alternative source of water in summer.
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Worked example
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Calculating water demand requirements
Litres/day x days in the month / 1000 KL used
per month
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WORKED EXAMPLE DECILE 1 OF MONTHLY RAINFALL
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WORKED EXAMPLE MODAL MONTHLY RAINFALL
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