Forests are key components of the earth's water cycle. Forests do not just respond to rainfall, they actively generate their own. They recycle water from the soil back into the atmosphere by transpiration, create the updrafts that facilitate condensation as the warm air rises and cools, create pressure gradients that draw moist air in from afar, and, just to be sure, release the atmospheric particles which are the nuclei around which raindrops form.
Forests have been described as 'biotic pumps' driving regional rainfall because their high rates of transpiration return large volumes of moisture to the atmosphere and suck in moisture laden air from afar.
While most of our rain originates from evaporation of the oceans, it is estimated that 40% of the rain that falls on land comes from evaporation from the land and, most importantly, from transpiration by vegetation. Recycled water vapour becomes increasingly important for inland rainfall.
Having created and attracted the water vapour, the plants then make it rain. Plants emit volatile organic compounds (VOCs), such as plant scents and the blue haze characteristic of eucalypt forests. They play an important role in communication between plants, and messages from plants to animals, and also between plants and moisture-laden air. They oxidise in the air to form the cloud condensation nuclei around which waterdrops form.
The transpiration of vegetation also results in evaporative cooling whereby the surface heat is transferred to the atmosphere in water vapour. The resultant clouds also help shade and cool the surface.
Forests store water in their tissues, in the soil amongst their roots and in the protected microclimate beneath their canopies, releasing it over time to the atmosphere by evapotranspiration and to streams through the groundwater system. Forests are a vital component of our hydrological cycle and due to their roles in attracting and recycling rainfall, reducing temperatures and regulating runoff they provide immense economic benefits to human societies. Their importance will become increasingly significant as climate change results in more erratic rainfalls and intense dry periods.
There is abundant scientific evidence that deforestation and degradation of vegetation causes significant reductions in rainfall by:
reducing the recycling of rainfall to the atmosphere by transpiration
reducing the drawing in of moist coastal air
reducing updrafts of moist air
reducing rooting depth and the recycling of deep soil moisture
increasing loss of water from the land by runoff
reducing the organic aerosols necessary for the condensation of rain drops.
The overwhelming evidence from around the world is that land-clearing has directly caused a significant reduction in regional rainfalls and an increase in land temperatures (See: Clearing Our Rainfall Away). These impacts have been compounded through the release of stored CO2, with land clearing contributing around a third of our CO2 emissions in the past two centuries.
Across drier areas of Australia the removal of deep rooted forests and woodlands has caused water tables to rise, allowing long-buried saline ground-waters to rise towards the surface, with the resultant dryland salinity affecting millions of hectares.
Logging has the opposite effect, with increased transpiration by the regrowth lowering water tables. The generalised pattern following logging of an oldgrowth forest is for there to be an initial increase in runoff peaking after 1 or 2 years and persisting for a few years. Water yields then begin to decline below that of the oldgrowth as the regrowth can consume 50% more water than oldgrowth. Water yields are likely to reach a minimum after 20-30 years before slowly increasing towards pre-logging levels in line with forest maturity. It can take over 150 years to restore the original water yields. (See: How Forests Regulate Streamflows)
Reductions in water yields are dependent upon the rainfall and the amount of water used by the vegetation. When rainfall is low the regrowth can consume most of the water, leaving little surplus for streams. In areas experiencing high rainfalls regrowth has been found to depress annual water yields by some 50%, though with low rainfalls there may be little water left for streams surplus to the requirements of the regrowth.
With declining rainfalls due to clearing, the increased demand for water by the regrowth can dry catchments, cause water stress and kill trees.
Allowing regrowth forests to mature results in significantly increased water yields to surrounding streams and dams. Water yields will go on increasing for many decades. The increase in water yields from maturing forests represents a significant economic benefit to all downstream users, particularly during dry periods.
Fig. 1 from Speer et. al. (2011): Map of Australia highlighting the decline in annual rainfall (mm/10 years) around Australia from, 1950–2007.