Most people understand what evaporation is. It is the process through which water is converted from a liquid state into a gaseous state and lost to the atmosphere. What is not so clearly understood are the volumes of water that evaporate from our land and water surfaces. When one considers the value of water in farm dams to the profitability of the farm enterprise it can come as a bit of a shock when the amount of water lost to evaporation is measured. Over many decades the Bureau of Meteorology and other scientific bodies have collected evaporation data and have produced a map of evaporation potential for all of Australia.

This map can be viewed on the BoM website www.bom.gov.au.  For example; all that area of Eyre Peninsula north of an east-west line from Port Neil to Point Drummond has an average evaporation of 2000+ mm per year. That is 2.0 metres and well over 3 times the annual rainfall.

Let’s put those facts in perspective and apply that information to a typical farm dam.

The dam measures 30 metres by 30m and is 4 metres deep, the full capacity is 2,300,000 litres or 2.3 Megalitres. The 2000mm evaporation rate means that every square metre of water surface would evaporate 2000 litres in a year. The dam (900 square metres) would lose 1,800,000 litres – nearly 80 percent of it’s total volume in 1 year to evaporation. That lost water would be enough for about 1800 sheep or would adequately irrigate 1.5 hectare of mature olive trees for a year. Quite a significant and valuable amount of the resource is being lost every year.

There are a number of actions a landholder can undertake to reduce evaporation.

Three main factors affect the rate of evaporation, higher wind speed increases evaporation as does higher temperatures and lower humidity. Vegetation and shading around a dam can have a fairly good impact in lowering the evaporative effect of these factors. Another useful action is to always try for the greatest volume to surface area ratio when constructing a dam i.e. deeper rather than wider. Alternatively, there are many types of dam covers available in the market and Rural Solutions SA is soon to begin monitoring the efficiency of a low cost cover about to be installed in the rangelands.

AUTHOR: Mark Sindicic, Land & Water Consultant, Rural Solutions SA.

CONTACT: Mark Sindicic, Pt Lincoln Rural Solutions SA office, Telephone 8688 3400 or visit the Rural Solutions SA website at www.ruralsolutions.sa.gov.au.