It is best that before you commence construction of a dam that youdDecide how much water you need. Water requirements vary considerably depending on crop type, geographic location, type of stock and stock numbers. You also need to decide whether to build one large dam or spread your risk amongst several smaller dams.
The estimated average annual water requirements for some crops and stock are listed here and can be used as a basis for calculating storage requirements. (for general guidance only)
NB 1 ML = 1 megalitre = one million litres
• pastures: allow 4–6 ML/ha a year
• vines: allow 2–3 ML/ha a year
• vegetables: allow 3–5 ML/ha a year
• stone fruit: allow 3–4 ML/ha a year
• sheep: allow 1,000 litres per animal per year
• horses: allow 14,000 litres per animal per year
• beef cattle: allow 15,000 litres per animal per year
• poultry: allow 100 litres per bird per year.
Calculating Dam Catchment Yield
Unless your dam fills from a spring or by pumping from a river or bore, the source of water will be rainfall that runs off the catchment. The catchment is the area that collects rainfall run-off and directs it to the dam. The amount of run-off also depends on several other factors, including ground slope, rainfall intensity, type of groundcover, soil type and existing drainage patterns.
If the catchment does not provide the amount of run-off water you need, then you can build catch drains to collect run-off water from outside the dam catchment area and direct it to the dam. The drains need to be built carefully to ensure a grade towards the dam; and they should be sealed with a covering of grass, to avoid erosion.
This Table will help you to estimate the likely yield of your catchment. These tabulated yields have been derived by NSW DPI using assumptions regarding slope, soil type, ground cover, catchment geometry and drainage paths. For example, if your average annual rainfall is around 750 mm and your catchment area is about 4 ha, then you could expect an average 3 ML yield from the catchment annually. This gives you some guidance as to the size of dam that you will need to build as well as the size of spillway to cope with overflows.
Estimating the capacity of your dam.
Here is a simple method devised by the Dept of Land and Water Conservation, NSW to estimate the capacity of your Dam.
- Decide on the nearest shape of your dam to the ones shown above.
- Measure the relevant width and length at top water level (in metres).
- Measure the maximum depth of the dam
- Use the formula relevant to the dam shape to calculate your surface area (in m2) for your dam.
- Calculate the volume (for all dam shapes) in cubic metres (m3) Volume = 0.4 X Surface Area X Depth NB. the conversion factor of 0.4 takes into account the slope of the sides of the dam.
- Divide the volume by 1000 to convert cubic metres into megalitres. This is your estimated Dam capacity in megalitres (ML).