Tips for putting in a new dam
By Monica Ley
Senior Land Services Officer
Mixed Farming Systems
Thinking of putting in a new dam on your farm? Setting up water storage on-farm might not be as simple as digging a hole and waiting for it to fill with water. Farm dams should be planned for and built with the same care you would for a shed, a set of yards, or any other infrastructure. Rural landholders in NSW are entitled to build and maintain dams up to a certain size without a licence. Those not requiring any licence include:
- Harvestable right dams (i.e. small dams to collect rainfall runoff)
- Dams built before 1999 that are used only for stock and domestic purposes
- Dams up to one megalitre on small properties where the property was approved for subdivision before 1 January 1999.
Any other types of dams may require a licence. The type of licence required depends on whether the water comes from a source governed by a water-sharing plan. Water-sharing plans have rules for sharing water between environmental needs and the users. The Department of Industry Water issues licences and has a range of useful information about building dams in NSW
There are several things that need to be considered when planning a farm dam, including:
- government regulations that apply to your dam, including the maximum allowable size
- the size of your dam, which would allow for farm water needs and evaporation losses
- the location.
Other things to consider include:
- who should build your dam?
- what site preparation needs to be conducted?
- how will the dam be built and maintained.
Lack of attention to the catchment runoff, water supply requirements, site selection, and standards for design and construction is likely to result in dam failure.
The dam site
The dam site you choose will determine its success. To successfully construct a farm dam that sufficiently fills with water, it must be in a location suitable for its required size and within an adequate catchment area. The catchment area is the total area of land that contributes to runoff into the dam. Catchments can be measured from farm plans, aerial photos, on-ground measurement, GPS or through mapping programs such as Google Earth Pro.
Dams are often built in a gully as it can be cheaper due to a smaller amount of earthworks required. While not everyone may have the use of a gully site, many farm dams are successfully built on hillsides or within topographic depressions.
How much water can your catchment catch for your dam?
For dams fed entirely on runoff, calculating the catchment yield is essential to determining whether the dam size is appropriate. Careful consideration of the potential yield of a catchment is essential to determine the appropriate size dam to build and ensure that it fills regularly and reliably.
To calculate catchment yield:
Catchment yield (megalitres) = A x R x YC / 10,000
Where: A = Catchment area (hectares)
R = Average annual rainfall (mm)
YC = Yield coefficient (from graph 1)
For example, the catchment area is 50ha and located within a 570mm rainfall zone. The yield coefficient is 6 (from graph 1).
Catchment yield = 50 x 570 x 6 / 10000
= 17.1 megalitres.
Graph 1, Source: Agriculture Victoria
Note: 1 megalitre = 1,000 kilolitres = 1,000 cubic metres = 1,000,000 litres
An accurate assessment of your catchment yield can only be made after investigation of the characteristics of your catchment, including a range of factors, including the soil infiltration and water-holding capacity, the type of groundcover and vegetation present, and where the catchment is located in regard to evaporation rates and rainfall.
Adjusted Catchment Yield (megalitres) = SF x GF x LF
Where: SF = Soil factors
High permeable soils x 0.3; permeable soils x 0.6; medium-textured soils x 1.0; heavy textured soils x 1.3.
GF = Groundcover factors;
Annual pasture cover x 1.0; perennial pasture cover x 0.5; timbered or forested cover x 0.5.
LF = Location factors; catchment area is located:
South of the divide x 1.1; on the divide x 1.1; north of the divide x 0.9.
Working with the above example of a catchment yield of 17.1 megalitres, if the catchment was to have a loamy soil with a perennial pasture base, and is located on the Great Dividing Range:
SF = 0.6 (loamy, permeable); GF = 0.5 for groundcover of perennial pasture; LF = 1.0 located on the divide.
Adjusted Catchment Yield will be = 17.1 x 0.6 x 0.5 x 1.0
= 5.1 megalitres.