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How much water do I need?

Note Number: AG1406
Published: March 2010
Updated: March 2010

 

This Note is to assist farmers to estimate water storage needs based on likely water requirements. The figures are broad guides only and should be tweaked for local conditions.

Use this Note in conjunction with the following:

AG1401: Finding a dam site
AG1397: Soil materials for farm dam construction.
LC0085: How to avoid dam construction failures

Initial considerations

Few rural properties are serviced by public water supplies. In most cases, the only option is to use water resources available on the farm. Careful planing is essential before embarking on costly constructions and other commitments.

There are three preliminary planning steps:

1. Determine the specific water uses of the farm.
2. Determine water needed for each proposed use, and in which seasons it will be needed most.
3. Determine what options are available to provide the required supplies at the level and timing wanted.

Factors affecting these differ considerably between properties. Look carefully at your own circumstances. It is important that all estimates be as accurate as possible.

It is convienent to work with average daily or average annual requirements for ease of planning, but be aware that specific daily requirements will vary considerably over the year and from year to year.

Any water storage not used for valid stock and domestic use now requires a license from your Rural Water Authority. Contact them to find out appropriate guidelines and requirements. It is especially important to get their interpretation of reasonable stock and domestic allowances for your area.

All figures given in this Note are based on general experience within Victoria. They will benefit from local adjustment to meet specific circumstances, and have not yet taken climate change into account.

Annual water requirements are subdivided into household, garden, stock, and fire fighting uses. Irrigation needs are not dealt with in this Note.

Household requirements

Melbourne residents are encouraged to contain their daily water use to a maximum of 155 litre per day per person. However this does include an allowance for garden watering. If we assume 10 % of this is for the garden, then this becomes a working figure of 140 litre per day per person.

A reasonable working annual figure on this basis is in the order of 50,000 litre per person

Note however that evaporative air conditioners on hot dry days can use large quantities of water (up to 25 litre per hour).

Roof runoff into tanks provides high quality water for drinking, hot water services, household cleaning and various veterinary and crop spraying purposes.

Earthern dams however provide lower quality water, with turbidity as the most usual quality problem. Often dam water is settled, clarified and even disinfected (in a tank) and then transferred to dedicated house supply tanks. In other cases, a house may be plumbed to keep the two supplies separate.

Garden requirements

Table 1: Typical supplementary water requirement (summer period) for gardens

Type	Watering allowance in litre per m2 (averaged over the year)
	Zone RF>800 mm	Zone RF 500 – 800 mm	Zone RF< 500 mm
Native garden (no lawn)	50	100	150
Lawn with shrubs	150	300	450
Vegetable garden	300	600	900

 

This amount of water only supplements existing rainfall. It is not sufficient to meet all plant requirements, even for just the summer.

Stock requirements

Drinking

Drinking water requirements for a grazing animal will vary according to weather, water quality, nature and quality of feed, age of animal, condition of animal and even social behaviour. Summer water requirement is usually 125% of the average daily requirement. Winter requirement is usually 75% of average daily requirements.

The following table is as a starting point for estimating stock drinking requirements.

Table 2: Annual average drinking water requirement of grazing stock in central northern parts of Victoria

Stock	Annual average drinking water requirement (litres/animal/year)
Sheep:
nursing ewes on dry feed	3,650
mature sheep - dry pasture	2,555
mature sheep - irrigated pasture	1,280
prime lambs - dry pasture	1460
prime lambs - irrigated pasture	400
Cattle:
dairy cows, dry	29,200
dairy cows, milking	54,750
beef cattle	25,550
Weaners (250-300kg)	18,250

 

These figures are generalized. Stock will drink more with greater heat load, lower quality water, dryer food, and lower feed quality. They will drink less with the converse.

Additional water should be allowed for miscellaneous stock management activities, as exampled below. The figures will be dependent on equipment used and practices deployed.

Miscellaneous stock management

Dairies, piggeries, and sheep dips all have specific water requirements over and above stock drinking requirements. They are quite specific for each circumstance but need to be covered in a farm water plan.
The DPI publication (2009) Dairy shed water: How much do you use? provides a sound guide for dairy water use.

Fire fighting reserve

Rule-of-thumb figures are as follows

Table 3: Typical reserve volumes for fire fighting

	litres per sq m
Buildings	120
Grass areas	75

 

However, chase specific recommendations from your local government or CFA office.

Water supply from roofs

The amount of runoff from a roof may be calculated from the following relationship

YR = R x Ax 0.9

Where YR = Annual yield from roof (litre)
R = Annual rainfall (millimetre)
A = Area of roof (square metre)

If the average annual rainfall is 600 millimetres and the roof area is 250 square metres, then

YR = 600 x 250 x 0.9
=135,000 L

Evaporation directly from the roof surface at the start of rain , will reduce the potential yield, as will splash and overtopping of the collection plumbing. The factor of 0.9 assumes that approximately 90% of rain falling on the roof can be channeled into a storage tank.

Rainfall replenishment on a regular basis make it possible for more water to be used over the year than actual tank capacity. Therefore, it is advantageous to calculate inputs and outputs on a monthly basis. Total storage capacity should be able to cover periods of low rainfall identified from local rainfall records.

The adequacy of a particular sized tank for household need is determined from a monthly assessment of the expected use compared to expected runoff from the roof. Assume the tank is full at the start of September. Calculate the difference between use and inflow for each month and the amount of water in the tank at the end of the month. Carry this figure over to the start of the next month, but remember that excess water will be lost as overflow. Complete for one whole year and assess whether that sized tank is likely to be suitable.

Although monthly average rainfalls are published for Victoria, the actual rainfall for each month can vary widely from year to year. Too much reliance on the published average figures is therefore not advisable when assessing annual inflows.

Water supply from field runoff

Earth dams intercept and store field runoff. The catchment to the dam has to be large enough to regularly fill the dam.

It is not an easy task to estimate runoff. Rainfall amount, rainfall type, soil type, vegetative cover and land slope all influence it. The more variable the rainfall, the less reliable the runoff will be.

Choosing dam size

The following need consideration:

• characteristics of dam site
• likely yield of runoff from the catchment
• evaporation
• seepage
• dam health reserve

contingencies
length of replenishment period
See Figure 2: Water partitioning for considering dam size

Characteristics of dam site

See Notes
AG1401: Finding a dam site
AG1397: Soil materials for farm dam construction

Yield of runoff

There is no point in building a dam which will not be filled on a predictable basis. See Note AG00XY: Estimating runoff from field catchments (in production)

Evaporation

Evaporation can be the biggest consumer of water from a poorly designed dam. It needs to be allowed for when determining dam size. The loss from evaporation depends on climatic zone, time of the year, dam size, dam shape and specific location of the dam.
The long-standing approach to estimating annual evaporation from a dam is by using the following relationship:

LE = 0.67 E x AF

Where LE = Evaporative loss (litre)
E = Local annual evaporation (millimetre)
AF = Surface area of the dam at full supply level
(square metre)

Evaporation however varies considerably through the year. During the summer months it is usually about twice that of either spring or autumn months. Hence the 3 summer months account for about one half of the yearly total. This variation will need to be allowed for if estimating evaporations for other than full year periods.

 

Figure 1. Average annual evaporation rates (in mm from sunken pan). (Based on BOM web site map)

Seepage

Seepage loss will depend on the material on which a dam is located and built. It will also depend on the quality of construction. Some dams “hold like a bottle” as is common where soil materials are slightly dispersive. Others can steadily leak into the excavation as is common in ‘red mountain soils’

It may be advisable to use about one tenth of the dam volume as a seepage allowance.

Dam health reserve

When designing a dam it is important to allow a significant proportion of the dam volume as a reserve for the health of the dam itself. This store is often called the minimum drawdown.

In dams with earthen walls this reserve water is important to keep the dam material moist and avoid the problems associated with the dam drying out completely.

This reserve water also provides an important habitat for fish, plants and other organisms that contribute to good water quality.

There are no absolute values that can be applied to all storage’s but if the last 1.5 - 2 metre depth of your storage is kept as a minimum drawdown then most of the issues will have been dealt with.

Contingencies

Farm and living requirements for water are changing all the time. Consider what these might be when calculating water requirements.

It is also important also in the planning stages to consider potential waste and loss of water due to accident or inefficiency associated with water distribution. What is the chance of a broken pipe or a sticking valve? What would be the consequence of this? What allowance should be made for it?

Replenishment period

What length of “no-runoff” period should be planned for in designing the size of water storage?

The term replenishment period is use to define required time that a dam water must last between expected fillings. It is also refered to as the “security factor”

Replenishment periods in Australia have been related to average annual rainfall. The lower the rainfall, the lower the reliability of runoff will be.

The following table has been used as a rule-of-thumb for replenishment periods.

Table 4: Rule of thumb replenishment periods

Annual rainfall (mm)	Replenishment period (months)
Over 800	12 - 18
500-800	18 - 24
Under 500	24 - 36

 

Water quality

Water quality will determine just what the water may be used for. Cool and clean water of low salt content is best for stock health and for household use.

Stock may require more water where salt levels increase.

Further information on water quality is contained in the Note: AG1402: Water quality needs for farm water supplies.

Drought reserve dams

The preceding information indicates that there are considerable advantages in utilizing large and deep earth storages to meet the intended uses while at the same time minimizing evaporation and seepage. See Note: AG1400: Drought Reserves Dams.
The following table provides rule-of-thumb guidelines for minimum depths of water for farm dams, assuming normal seepage and evaporation losses.

Table 5: Rule of thumb guide for minimum dam depth

Annual rainfall (mm)	Minimum water depth over 25% of the surface area (meter)
> 1250	2.5
1,000-1,250	3.0
800-1,000	3.5
500-800	4.0
300-500	4.5
< 300	5.0

 

Summary

Roof tank supplies

Look at the maximum amount of water reliably available from the roof area and then relate to the range of domestic, stock and other farm requirements to be satisfied. Tank storage capacity required is determined by the expected replenishment period – but as the roof has about a 90% yield, the replenishment period will be considerably less than for field runoff

Earth dam supplies

• Decide on proposed water usages.
• Determine replenishment period for your location and drought strategy.
• Calculate (for the length of the replenishment period) the estimated requirements for the full range of water uses.
• Sum the figures.
• Consider possible locations for dam. Seek advice.
• Make first approximation of dam design.
• Calculate volume required to satisfy evaporation for the appropriate replenishment period.
• Calculate volume required to act as reserve for dam health.
• Consider possible allowance for seepage losses and other contingencies for the replenishment period.
• Make grand total.
• Adjust required uses if appropriate and recalculate.
• Finalize dam design.

Further advice

Local consultants and experienced contractors should be able to provide on-site advice
See also DSE publication Your dam your responsibility 2007

Acknowledgements

Compiled from the combined experience of many Departmental Officers

This Note was originally developed by David Cummings, September 2002.

It was reviewed and re-assigned to AG1406 by Farm Services Victoria, Sustainable Landscapes branch. March 2010.

 

Figure 2: Water partitioning for considering dam size