The Moisture Motor is the simple cycle that reoccurs every 24 hours, driven by the difference in temperature between the ocean and the plains west of the Great Dividing Rang. The greater the inland day time temperatures cause the atmosphere to heat and expand upward, creating sufficient vacuum to draw inland the cool, moist breeze in the afternoons, increasing in pace until around midnight. Before dawn the inland is colder than the ocean, where moisture levels rise, until the cycle restarts during the afternoon. The bigger the difference in temperatures, the faster the wind blows and the more moisture it carries. This became very apparent during the seven years I spent living on top of the Bunya Mountains.
In this region, the moisture motor is often overrun by weather events like the movement of highs and lows across the continent from west to east, upper atmosphere moisture from the Indian Ocean, as well as the lows and cyclones generated in the Coral Sea. While most of our measurable rain comes in such events, it is the moisture motor that allows the rainforests that grow on our eastern escarpments, to harvest moisture. There a a number of reasons as to how and why this happens.
Firstly, the rainforest doesn’t allow the sun or dry winds to reach the forest floor to dry it out. Rainforests maintain elevated levels of local humidity due to the transpiration of the very, very large leaf area of the entire forest, and stored moisture in masses of spongy, decaying organic matter.
Secondly, there are some other quire interesting factors regarding the aero-dynamics of the eastern escarpments, whether bare, covered with plantations, eucalyptus or rainforests. On the bare slopes the wind accelerates in speed as it is compressed during its rush over the curve of the escarpment, creating low pressure and drawing moisture from the slopes. In an aero-dynamic sense, escarpments with plantations and eucalyptus forests are the same as they have regular sky lines, with all of the canopy trees in the forests being the same height. The escarpments with rainforests are quite different, as the have very uneven skylines, so that when the wind arrives, heavy with moisture, turbulence is created, causing the air to slow and release moisture. Bunya and hoop pines have the own design features, including dome tops, stiff curving leaflets and branches which enhance their moisture harvest.
Thirdly, the already elevated humidity enveloping the rainforest combines with this extra moisture to become wetter than the atmosphere. A cloud actually forms in and over the rainforest and is retained by static electricity, becoming a syphon for even more moisture. On the Bunya Mountains, 20 – 50 ml of rainfall can occur within the rainforest overnight, while on a glassy bald 50 metres away, 2 – 3 ml. With a strong easterly such a cloud can last up to 7 days, saturating the rainforest, while there has been little rain on the cleared, lower areas. Accumulated dust probably helps to attract moisture, as well as being a food source for these cloud forests.
Fourthly, as we know, atmospheric pressure assures the water table more or less follows the contours of the land. Most of our eastern escarpments are mineral-rich basalt formations. Basalt is described as fractious and porous in nature, meaning there are often considerable flows and reserves of water within these formations. The bunya-hoop rainforests’ root zone could extend 20 metres below the ground, with the enriched carbon zone extending much further. These zones can be considered as dry sponge, with the entire root zone acting as a suction pump.
This became apparent after extensive clearing of the bunya-hoop rainforests from the red soil ridges, following the word wars. During the wet 1950’s, the water table rose, dissolving the stored salts and other minerals. Because of this clearing, by 1970 the soil carbon content had reduced to the point that the red soils lost their ability to retain moisture, causing a partial collapse of the water table, bringing the salts to the surface at lower areas.
The restoration of the buya-hoop-silky-oak Cloud Forests to the red soil ridges of our region re-activates the moisture motor, restores the carbon cycle and creates a lot of oxygen, benefiting the entire ecosystem. Clearly, these projects are the carbon sinks of the future, with considerable economic benefit to the region, creating investment, employment and opportunity. Each replanted forest is managed as the single biomass with many trees living over 500 years. These new forests reach a maximum biomass at about 300 years, as the pioneering species are replaced by those that live much longer.
It is clear to me that if humans are responsible for the removal of the Cloud Forests from our ridge tops, then it is reasonable for humans to put them back. Re-establishing Bunya, Hoop and Silky Oak rainforests on ridge tops and in the heads of gullies will create moisture.
On lower area, hedgerows extending down the gullies to the creeks and along the fence lines will also create air turbulence and moisture, as was found in New Zealand, when a lot of hedgerows were planted to protect Kiwi Fruit. I have heart it sad ‘we changed the weather’.