What Is Evaporation? Causes, Rates and Costs for Australian Dams

Every dam, channel and open water storage in Australia loses water every day through evaporation. For farmers, water managers and property owners, understanding what evaporation is and what drives it is the first step to protecting a supply that costs real money to capture and store.

What is evaporation?

Evaporation is a phase change: water molecules at the surface absorb enough heat energy to break free of the liquid and enter the air as vapour.

Water molecules are always moving. Those near the surface that gain enough energy, mostly from sunlight or warm air, escape as gas. Water does not need to boil for this to happen. Evaporation occurs at any temperature above freezing and speeds up as conditions get warmer.

The vapour rises, condenses into cloud and eventually falls as rain, completing the water cycle. For a dam owner, though, the water that leaves the surface is gone from storage.

What drives the rate of evaporation?

The rate of evaporation is not constant. Five factors control how fast water leaves a surface:

• Temperature: warmer water and air give molecules more energy, so a dam in outback Queensland loses water far faster on a 40°C day than in winter.
• Surface area: a wide, shallow dam exposes far more water to the air than a narrow, deep one, which is why surface area is central to estimating dam evaporation.
• Humidity: dry air can absorb more vapour, while humid air slows evaporation. This is why losses are worst in Australia’s inland and arid regions.
• Wind speed: wind clears moist air from just above the water and replaces it with drier air.
• Salinity: salty water evaporates more slowly than fresh water, which matters for mine water and some saline farm storages.

Why does Australia have such high evaporation rates?

Australia is the driest inhabited continent. High summer temperatures, strong sunshine, low inland humidity and persistent dry winds combine to pull moisture from open water quickly.

Average annual pan evaporation exceeds 2,000 mm across much of inland Australia, far more than the rainfall those areas receive, as shown on the Bureau of Meteorology evaporation maps.

Many Australian farm dams are also shallow and wide, which gives evaporation a large surface to work on. Most Queensland farm storages are less than 7 metres deep and under 2 hectares in area, per the UniSQ farm dam evaporation fact sheet.

Everyday examples of evaporation

• Drying clothes: wet washing on a line dries as sun and breeze drive moisture into the air.
• Puddles disappearing: after rain, puddles shrink over hours or days.
• Weather: evaporation from oceans, lakes and land feeds the cloud and rainfall cycle.
• Farm dams and reservoirs: evaporation steadily lowers stored water, often unnoticed until the dam is low.

The cost of evaporation for Australian water storage

Australia had about 1.77 million farm dams in 2018 to 2019, storing roughly 10,990 GL of water, according to a continental survey published in Remote Sensing.

Up to 40% of storage volume can be lost to evaporation each year in the irrigation areas of northern NSW and Queensland, according to University of Southern Queensland research.

Shallow dams in low rainfall zones can lose up to 50% of their total storage volume to evaporation each year, per Agriculture Victoria.

For farmers in dry regions, that loss raises irrigation demand, weakens stock water security and can force water carting. In arid areas, evaporation can outpace inflow in dry months and empty a dam without a full drought.

Evaporation also concentrates dissolved salts in the water left behind, which lowers its quality for stock and irrigation. In mining, it affects process water supply and water balance reporting. Our guides on farm dam water quality and evaporation in mining and industrial storage cover those effects in more depth.

Evaporation and energy

Evaporating water absorbs a lot of heat. At everyday temperatures, about 2,450 kilojoules of energy are taken up for every kilogram of water that evaporates.

That is why evaporation cools, whether it is sweat on skin or an evaporative air conditioner. It also means every litre lost from a storage carries the energy already spent pumping, treating or moving it.

Signs evaporation is costing you water

• Steady level decline: a dam that drops steadily in hot, dry or windy weather with no sign of a leak.
• Water line stains: a tide mark above the current level shows where the surface used to sit.
• White crusting: salt build-up on banks or irrigation gear shows minerals concentrating as water evaporates.
• Plant stress: crops or pasture wilting in hot spells because irrigation water is running short.
• Pan readings: a Class A evaporation pan near the dam gives daily figures to put into a water budget.

A falling level is not always evaporation. If losses are higher than the weather explains, read the difference between a leaking dam and a seeping dam. For measurement methods, see our guide to estimating evaporation from a dam.

How to reduce evaporation from dams

Does planting around a dam reduce evaporation?

Windbreaks on the prevailing wind side slow the air moving across the water, which cuts the wind-driven part of evaporation. Choose species and spacing carefully, because tree roots on or near a dam wall can cause leaks.

Can dam design reduce evaporation?

Yes. A deeper, narrower dam holds the same volume under a smaller surface. When Lindsey Hughson and the Big Ditch team design a new farm dam, depth relative to surface area is one of the main levers for keeping evaporation down.

How well do floating covers and shade balls work?

Continuous floating covers cut evaporation by 70 to over 90%. Shade balls save roughly in proportion to the share of the surface they cover and suit larger or irregular dams. Our shade balls vs floating covers guide compares cost and performance, and Evap.co shade balls are supplied through Big Ditch.

For the full list of options, see 8 strategies to cut evaporation losses in dams.

Frequently Asked Questions

What causes evaporation to increase?

Higher temperatures, lower humidity, stronger wind and a larger exposed surface all increase evaporation. Australian summers combine all four, so dam evaporation usually peaks between November and March.

How much water do farm dams lose to evaporation?

In northern NSW and Queensland irrigation areas, up to 40% of storage volume can be lost each year. Shallow dams in low rainfall zones can lose up to half. Your own figure depends on local pan evaporation and your dam’s surface area.

What is the best way to reduce evaporation from a dam?

Covering the surface gives the biggest saving: floating covers on smaller dams, shade balls on larger or irregular ones. Good dam design and windbreaks help, and the right mix depends on the dam’s size, use and water value.

Watching your dam level fall faster than it should? Book a site inspection with Big Ditch and we will tell you whether it is evaporation or a leak.