WHEN rain showers are few and far between, dam levels drop - and that is often a major concern to land owners.

That's why storages need to be large enough to handle these dry stretches.

Dams should be able to cope with drought periods of at least 18 months, including two summer seasons.

When a pump or windmill has to be used to supply dam water, it is advisable to:

Make sure the storage is large enough to meet anticipated needs.

Locate the dam in a position that allows supply to the whole farm over two days by gravity feed, even if at reduced pressure.

Evaporation is another key issue.

More than 40 per cent of your dam's water can be lost through evaporation in a 12-month period in certain locations.

This loss will reduce the amount of water available for irrigation, with potentially significant production losses.

Since evaporation is often the biggest consumer of water from a dam, it must be taken into account when choosing dam size.

Evaporation will vary according to climatic zone, time of year, dam size, dam shape and its specific location.

During summer, it is usually about twice that of either the spring or autumn months.

Hence the three summer months account for about half the yearly total.

Various measures to control evaporation have been suggested, including chemical films, floating or suspended material, windbreaks and water plants.

However, more information is needed in order to measure the relative efficiency of these methods.

A study by RMIT University into evaporation levels from off-waterway storage made the following comments:

Floating water plants do not achieve significant savings in evaporative losses.

Floating rings with reflective caps do significantly reduce evaporative losses. These rings are made from strips of buoyant plastic, covered with a cap of bubble plastic and painted with white reflective paint.

Initial tests showed windbreaks may reduce evaporation, but more work on this idea is needed.

Two simple methods for reducing losses are to have deeper storages and to split large storages into cells.