Teacher background information

In this elaboration students have the opportunity to understand the methods that Aboriginal and Torres Strait Islander Peoples used prior to colonisation to reduce the evaporation of water resources. Students learn that evaporation is a type of vaporisation that occurs when a liquid changes into a gas. This process is responsible for accelerated rates of water loss in high temperature arid regions. While some parts of Australia receive reliable rainfall, other areas experience low rainfall and high rates of evaporation. In such environments water sources require conservation strategies. This elaboration provides students with the opportunity to learn that Aboriginal and Torres Strait Islander Peoples recognised and understood the phenomenon of evaporation and developed a variety of methods to mitigate its adverse effects.

Water is a necessity for life and knowing how and where to find water is a valuable skill that has been practised by Aboriginal and Torres Strait Islander Peoples for many thousands of years. Prior to colonisation Aboriginal and Torres Strait Islander Peoples used a variety of techniques to locate water including plant and animal indicators, maps, landscape references, and the oral transfer of knowledge. Preserving and managing water resources was, and remains today, an important skill to ensure the ready supply of drinking water for communities. Aboriginal and Torres Strait Islander Peoples have long and continuing practices for preserving precious water resources including evaporation prevention measures.

Within the natural environment evaporation of water is the change in state from liquid to gas that occurs when the water surface is exposed to heat from the sun and becomes water vapour. While some parts of Australia receive high annual rainfall rates, other areas are extremely aridFor example, in the desert regions of Australia rainfall is episodic and unreliable, with an average annual rainfall of less than 200 mm and evaporation rate exceeding 3,000 mm per annum in many areas. The rate of evaporation depends on factors such as air temperature, humidity, wind speed, and sunlight. Sources of water vary and depend on the bioregion of one’s Country/Place. In the desert regions of Australia there are generally no permanent rivers or freshwater flows. However, water can be found in rock-holes (often referred to as gnammas (Noongar term)), soaks and claypans. The maintenance and conservation of these water sources is important to ensure water availability. Prior to colonisation, methods of water conservation included using slabs of flat rock and branches over pools. Large rocks were placed over rock-holes containing water to provide a lid or cap, that slowed evaporation. Similarly, small soakages were always covered with branches, sticks or grasses after use, to reduce evaporation. The covers ensured that, as the water vapourised, the water vapour was contained within the water hole and not lost to the external environment.

The evaporation rate of water sources depends not only on the climatic conditions in the region, but also on the surface area of the water that is exposed to the sun. Aboriginal and Torres Strait Islander Peoples also understand that the greater the surface area of a water source exposed to the environment, the greater the rate of evaporation. Water sources in claypans that have a large surface area and little depth diminish quickly through evaporation. For this reason, after rainfall, claypan water resources were used first, before moving to more permanent water sources. Some gnammas were enlarged by gouging debris and loose rock from the bottom and sides of the rock-hole, demonstrating an understanding that evaporation of water occurs at the surface. Enlarging the rock-hole increases its water storage capacity without affecting water loss through evaporation, as the surface opening of the rock-hole remains the same.  This knowledge is also evidenced in the flask-shaped wells dug by Aboriginal Peoples, specifically designed to limit evaporation by creating a narrow entrance.

In the non-desert areas of Australia, Aboriginal and Torres Strait Islander Peoples also understand the importance of managing and conserving water resources. Apart from the wet seasons, freshwater is often scarce on the islands of the Torres Strait. On Damut Island in the Torres Strait, waterholes were covered using sticks and blocks of wood to protect the water within. The Baiyungu People of the north west Australian coast preserved water supplies after rainfall by covering the water that collected in rock cavities with lids, using flat pieces of limestone. In the Crystal Brook region of South Australia, the Nukunu People prevented evaporation of water from a deep spring by covering it with bushes and boughs. Early European explorers who observed these practices speculated that at times the spring may have dried up, and that the Nukunu Peoples found that this was the most efficient way to provide shade and prevent water evaporation. In Yankuntjatjara Country, around the Everard Range region of South Australia, there are many rock-holes capable of holding hundreds of litres of water. To prevent evaporation of the water in these rock-holes, the Yankuntjatjara Peoples placed sand in the holes to sequester water into the interstitial space that exists between each sand grain. This effectively covered the water and reduced exposure of the water surface to the atmosphere, thereby reducing evaporation. To access the water, a hole was dug in the sand, into which fresh water drained. This demonstrates deep scientific understanding of water evaporation, as the rate of evaporation from an open water surface exceeds that of water saturated sand.

Techniques used to cover waterholes to reduce evaporation are still applied today. For example, in drought affected areas, swimming pool owners are encouraged to purchase pool covers that reduce water loss by evaporation. This knowledge is also being applied on a much larger scale where biodegradable plastic spheres, known as shade balls, or other plastic coverings on large water reservoirs can reduce evaporation by up to 70 per cent.

This elaboration provides students with the opportunity to understand the chemical science content of changes in states of matter through Aboriginal and Torres Strait Islander Peoples’ knowledge of evaporation. Aboriginal and Torres Strait Islander Peoples’ understanding of how and why evaporation occurs has led to the development of practices to conserve water supplies, particularly in desert environments, dry seasons or where freshwater is scarce. Students have the opportunity to learn that Aboriginal and Torres Strait Islander Peoples’ practices of covering water sources, minimising water surface area openings, and prioritising access to different types of water sources, demonstrate a deep and long-held understanding of water evaporation and how to reduce its effects to conserve water.