At the end of summer, if you can stand in the deepest part of your small dam without your hat floating, there may not be enough quality water to support additional stock. The amount of safe water left at the end of the dry season is effectively the amount of water available for any new animals.
The cost of increasing your water catchment or purchasing back-up scheme water needs to be weighed against expected returns from livestock sales.
The dry argument - how do I calculate available water?
On most small properties a single series of calculations will suffice. On larger properties it is recommended the calculations are done on a per paddock basis. The volume of water in your dams is then compared to your livestock requirements.
Dams filled from harvested rainwater, springs or soaks are the most common water source for livestock in south-west Western Australia.
Dam capacity can be calculated by using a number of formulas. The formula used will depend on the type and shape of dam to be measured. See Calculating dam capacity.
The surface dimensions and depth of the dam at various places are measured to calculate the volume of water.
Taking the measurements twice, once when the dam is full and again at the end of summer is preferable.
The second measurement will be a guide to the amount of water consumed by livestock over the season combined with evaporative losses.
Landholders need to understand how much water different livestock need and their tolerance to salt.
Evaporation will remove at least as much water from your dams as the livestock can drink. When calculating the amount of water available for your animals you should double the figure for water demand.
Evaporation also affects water quality by increasing the concentration of salts in the water.
Evaporation from dams ranges between 1–2.5m/yr. For example, at Donnybrook in Western Australia's south-west you could expect evaporation of 325mm in spring, 550mm in summer, 300mm in autumn and 150mm in winter, making a total of 1325mm.
Surface area and dam depth are important factors affecting evaporation losses.
Compare the dams below. Both contain the same volume of water.
The depth of water removed by evaporation will be much the same in both dams, but the total volume of water lost from the shallow dam will be significantly more.
The dam that is deeper and has steeper sides will retain water for longer and provide more water for livestock.
The water will also be cooler, helping to reduce evaporation.