Evaporation in Agriculture: How Water Is Lost and How to Stop It

Evaporation in agriculture is one of the steadiest drains on an Australian farm’s water supply. From storage dams to irrigation channels to the soil surface, water is lost to the atmosphere at every step between catchment and crop. Knowing where it goes is the first step to keeping more of it.

Evaporation is the change of liquid water into water vapour at a surface, driven by heat, dry air and wind. For a plain explanation of the process, see what evaporation is and what drives it.

Where does agricultural water actually go?

Agriculture is Australia’s largest water user, yet a share of that water never reaches the plant root zone. It is lost in transit, through evaporation at several points in the supply chain.

• Open storage dams and reservoirs: usually the largest single source of farm evaporation loss.
• Open irrigation channels: unlined or exposed channels lose water to both seepage and evaporation.
• Overhead irrigation: sprinklers running in hot, dry or windy weather lose part of the applied water before it reaches the ground.
• Bare soil: water sitting on hot, bare soil after irrigation evaporates before plants can take it up.

Evaporation from on-farm water storage

On-farm storage dams are the backbone of water security for Australian properties. They hold water for irrigation, stock, domestic use and fire protection, and an open water surface is exposed to evaporation every day of the year.

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 for the Queensland Government.

Evaporation from storages in Queensland’s Granite Belt is typically 1.6 to 1.8 metres a year, per the UniSQ farm dam evaporation fact sheet.

For a 10 megalitre dam with a generous surface area, that rate can mean 1.5 to 3 megalitres gone each year. That is water that could have irrigated a paddock of vegetables, carried stock through a dry spell or stayed in reserve for fire. To work out your own figure, see how much water your dam is losing to evaporation.

How much water does overhead irrigation lose?

Centre pivots, lateral moves, travelling guns and hand lines are widely used across Australian broadacre and horticulture. They save labour and cover ground, but they throw water through hot, moving air, and some of it never lands where it is needed.

Wind drift and evaporation losses from sprinklers range from close to zero to as high as 40 to 50% of applied water, depending on sprinkler type, height, pressure and weather, according to Utah State University Extension.

Irrigation system	Typical wind drift and evaporation loss (% of applied water)
Travelling big guns and pivot end guns	35 to 40
Impact sprinklers on hand lines or wheel lines	25 to 30
Centre pivot, mid elevation spray	15 to 20
Centre pivot, low elevation (LESA or LEPA)	5 or less
Drip (surface or subsurface)	Negligible in the air

Every litre lost in the air is a litre you paid to pump that never reached the crop. Ways to cut sprinkler losses include:

• Irrigating at night or early morning: cooler air and calmer wind reduce losses.
• Lowering the nozzles: low elevation applicators shorten the droplet’s time in the air.
• Converting to drip: surface or subsurface drip puts water at the root zone and avoids losses in the air.
• Scheduling to crop demand: irrigating to replace measured crop water use, not by the calendar, cuts the total applied and the total lost.

Soil evaporation: the hidden loss

Water that reaches the soil is still exposed. In hot conditions, bare or sparsely covered soil can lose a large share of an irrigation through surface evaporation within hours.

Mulch is the most direct fix. A mulch layer sits between the soil and the sun and wind, keeping moisture below the surface for the roots. Cover crops and minimum tillage help too, by holding soil structure and shrinking the area of bare ground.

What is evapotranspiration?

Evapotranspiration (ET) is the total water lost from a crop area through evaporation from soil and water surfaces plus transpiration from plant leaves. The Bureau of Meteorology and state agriculture departments publish ET data, and it is the basis of science-based irrigation scheduling.

By tracking ET for their location and crop, farmers can irrigate to replace what has actually been used rather than applying a fixed volume on a schedule. ET-based scheduling is one of the most effective ways to lift overall water use efficiency.

The case for covering your farm dam

Of all the evaporation losses in the farm water chain, dam evaporation is often the cheapest to address. Unlike rebuilding irrigation infrastructure or changing practices across a whole farm, a cover works on one existing asset.

Shade balls are floating plastic spheres that cover a water surface to block sunlight and wind. Continuous floating covers cut evaporation by 70 to over 90%, while shade balls and other floating modules save roughly in proportion to how much of the surface they cover. Our comparison of shade balls and floating covers sets out the costs and trade-offs.

Lindsey Hughson, who runs Big Ditch, designs and builds farm dams with this in mind. Where a new dam is planned, a deeper, narrower profile exposes less surface for the same storage. On an existing dam, the shade ball cost calculator gives a quick estimate of what covering it would cost.

Frequently Asked Questions

How much water is lost to evaporation in Australian agriculture?

It depends on where you look. Open storages in northern NSW and Queensland can lose up to 40% of their volume each year. Sprinklers can lose anywhere from almost nothing to 40% or more of applied water in hot, windy weather, depending on the system.

What irrigation method loses the least water to evaporation?

Subsurface drip loses the least, because water goes straight to the root zone below the soil surface. Surface drip and low elevation pivot applicators are the next most efficient. High pressure guns and impact sprinklers lose the most.

Does evaporation from irrigation dams affect crop yields?

Yes. When a storage loses water to evaporation over summer, less is left for irrigation at peak crop demand. That can cut yields directly or force a farmer to buy water at high market prices during drought.

Losing more of your irrigation water than you should? Book a site inspection with Big Ditch and we will look at your storage, its evaporation and the fix that pays back.