Why is State Forest So Important for Water Catchment

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Water, a Life-giving Resource from Forests

Water is one of the most important resources we obtain from forests. It is vital for all living things. Careful forest management is necessary to ensure that our present and future water needs can be met.  Much of Perth's water needs are supplied by four large reservoirs in the jarrah forest — Canning Dam, Serpentine Dam and the South and North Dandalup Dams.  A fifth dam, Helena Reservoir, provides some water to Perth, but the eastern hills, the Wheatbelt, and the Goldfields as far away as Kalgoorlie and Norseman, are its main recipients.

Where Does the Water Come from?

Moisture-bearing clouds from above the Indian Ocean are carried by winter winds and forced up and over the Darling Scarp, bringing rain to the coastal plain and the western edge of the Scarp. Further east, rainfall declines. The rain falls into catchment areas, with the runoff feeding the dams in State forests. The Helena Reservoir catchment is the area of land which collects rainfall, and drains by way of surface and underground streams into the Helena and Darkin Rivers and thence into the reservoir. This area of 1 470 square kilometres extends from farmlands as far east as York, south to the Brookton Highway and north to the Great Eastern Highway.

Do Forests and Their Users Affect Water Supplies?

Forests determine the quantity, rate and quality of water which flows into streams and hence into dams. Although the forests around the Helena Reservoir are managed primarily as a water catchment they have a number of other values: wildlife and nature conservation, timber production, mining, education, honey production, wildflower industry, tourism, recreation and research.

People and their vehicles in the catchment area may increase the risk of water pollution through soil erosion, vegetation loss from dieback disease, and wildfires. To prevent these there must be integrated catchment management, the coordinated planning, use and management of water, land, vegetation and other natural resources.

Dieback Disease

Parts of the jarrah forest are called Disease Risk Areas. These areas are managed to reduce the spread of Phytophthora cinnamomi, the fungus that causes dieback disease in plants. This microscopic fungus lives in the soil and attacks the roots of more than 1000 species of plants, including jarrah, starving them of water and food until they die. The fungus spreads rapidly in water and moist soil, infecting nearby plants. Infected soil, carried by vehicles, horses or people's shoes, spreads the disease still further.

Risk of infection is greatest during the wet winter months but the fungus spores (reproductive bodies) can survive long, dry periods and become active after rain.

Loss of trees and shrubs to dieback may leave soil unprotected and result in erosion. In addition the balance of species in the forest changes as dieback resistant species, such as marri (Eucalyptus calophylla), become dominant. A number of agencies are engaged in research to try to eradicate dieback. For example, the growth of seedlings from seeds of dieback-resistant jarrah trees is being tested in disease-affected areas. A chemical has been developed to protect small areas of jarrah forest. It is uneconomical, however, for large areas of forest to be treated.

There is currently no way of killing this fungus and the only management strategy is to regularly inject each individual infected tree and spray infected areas with phosphonate – the trees, plants and surrounding soil remain infected with die back, the phosphonate simply boosts the plant’s natural defences.

Die back is a very big problem in the Perth hills and indeed it is now spreading to parks and local gardens in Perth itself.  A significant cause of the spread of dieback is human activity and restricting such activity is imperative to halt the spread of this devastating fungus.  Construction, heavy machinery and clearing of forests can contribute significantly to the spread – all of which is necessary when building this sub-station.  Measures, such as spraying down machinery, can be used to assist in stopping the spread from one site to another; however all it takes is a pinch of infected soil to be transferred for the infection to spread.

Trees Fight Salinity

Trees draw huge quantities of water from underground through their roots to their leaves. Some of this water is transpired as water vapour into the atmosphere and some is used for plant growth. A 20-metre jarrah tree can draw an average of 50 litres of water out of the groundwater per day.

The soils over much of the south-west of Western Australia contain salt. It was originally deposited in the soil as dust, or introduced from rainfall which contains some salts picked up from air masses passing over the Indian Ocean. Salt has been accumulating in the soil like this for thousands of years.

Permanent clearing of forests removes the trees with deep roots which soak up the groundwater. As it rains, more water soaks into the soil, and the level of the groundwater rises and dissolves the salt in the soil. The salty groundwater may rise to the surface, forming salt scalds, or flow into streams and dams turning them salty. Salinity is the term used to measure the quantity of salt in streams, springs, dams, groundwater or the soil. It is a form of water pollution. Clearing of land for farming in catchment areas has, in the past, resulted in an increase in salinity of the dams. The broadscale cutting down of trees early this century caused the Helena Reservoir water to turn salty.  Careful management has enabled the reduction of salinity levels.

The Catchment and The Water Cycle

The catchment of the Helena Reservoir is the area of land from which water collects and runs into the reservoir. Less than 20 per cent of rainfall (precipitation) actually reaches the reservoir. There are a number of pathways that a rainwater drop falling on the jarrah forest catchment may follow and still be part of the water cycle:

Groundwater, the water that occupies the crevices in rock and soil, moves through the topsoil and eventually comes to the surface as a spring or a stream. It may take many years to reach the reservoir.  The amount of surface runoff depends on the density of the foliage and the weather conditions when the rain falls. After a long dry period the soil acts like a sponge, absorbing much of the water, and if rainfall is light and occasional there is less runoff.

If trees form a closed canopy, that is, the foliage of the trees almost touch, the amount of runoff is reduced. On the other hand, widely spaced trees leave the soil exposed to water and wind erosion.

By building the weir, humans have interrupted the natural water cycle. Water, which would otherwise make its way to the Helena River and eventually reach the ocean, is stored in the reservoir, then extracted and pumped to communities where it can be used.

Soil Erosion

Soil erosion occurs when soil is washed or blown away by water or wind. This happens when trees, ground-cover plants and ground litter are cleared. Splash erosion occurs when a raindrop hits bare ground, displacing soil.

More soil is eroded from steep slopes with loose soil because the speed of the water flow increases the movement of soil. Soil that has been eroded from catchments is carried into streams and rivers and then into dams or reservoirs. It turns them muddy (turbid), often killing local plant and animal life. Over the years, the soil settles and reduces the amount of water the dam can hold.

It is necessary to ensure that tree and ground cover plants are maintained to protect the topsoil and decrease the force with which raindrops hit the ground, thus preventing splash erosion. Vegetation also slows down runoff rates, particularly on the steeper slopes around the reservoir, and so reduces the loss of topsoil.

Badly built roads can cause turbidity in water collected from forested catchments.  Roads and pathways need to be built so that they slope gently, following the contour of the land. Roads also have drainage channels leading to adjacent vegetation, to slow water movement and therefore prevent erosion.

Fire and Water Quality

What has fire to do with water quality and quantity? The answer is—a great deal!

Wildfires are fires that burn very hot, are almost impossible to control and are dangerous to forests, towns, people's lives and farms. They occur in areas that have not been burnt for many years, and so have large amounts of leaves, twigs, bark and dead plants on the ground, known as ground fuel.  After a wildfire, the landscape is black and charred. If the fire is followed by rainfall, surface runoff may carry ash into streams and then into reservoirs.

If, during the course of a wildfire, the wind blows towards a reservoir, scorched vegetation is blown into the water. If heavy rainfall occurs before the plants have had time to recover, the unprotected topsoil can be eroded into streams and dams. In this way wildfires can result in considerable water pollution.

Source: Department of Conservation and Land Management – Catchment Carer’s Trail Years 5-7 Teachers and Students Notes

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