Two centuries of World deforestation have 
warmed the coastal lands they cleared and let their 
soils steal the rain once borrowed by their trees, to add
more to soil's moisture store and increase its run-off.
Sealed roads and roofs and runways now drain
more stolen rain into our rising seas.
Rain stolen from our lower falls
inland, so less runs off to
feed our inland rivers
as they dry.

 
Tony Mount has a sixty year interest in
forest fire ecology and is author of
Tasmania's Soil Dryness Index

Current global warming and rising sea levels are mostly blamed on carbon emissions from our burning of fossil fuels. But another, more certain man-caused climate change - two centuries of worldwide deforestation - seldom gets a mention. 

Fortunately, Tasmania's Soil Dryness Index has long measured interception - one of clearing forest's major climate-changers.         

Fifty years ago Forestry Tasmania combined four years of rainfall and river flow from Tasmania's Emu River, with two years of detailed research from Lidsdale, NSW, to create the Soil Dryness Index that still forecasts floods and fire weather today.

BORROWED RAIN         

Rain gauges are nearly always in forest clearings because any trees above them would intercept then evaporate (= borrow then return) some of the rain. So interception must first be estimated then deducted from each day's gauge reading, to tell how much rain wets forest soils. 

Tree canopy interception is zero at rain gauge level. Although vegetation below that level gets wet, its evaporation is effectively two dimensional - so the SDI incorporates it in evaporation from the soil surface.

 From zero at the gauge, intercepting increases with height - rapidly at first, as evaporation becomes three dimensions; then slowly as trunks  are added to the intercepting surface area; e.g. 2mm are intercepted at 2m; 4mm at 5m; 6mm at 10m and 7mm at 25m height. These 100% figures are multiplied by the percentage of intercepting surfaces per hectare.

Open eucalypt forests with 20% of intercepting surfaces borrow up to 5% of annual rainfall.  Dense, layered forests with 50% of intercepting surfaces borrow up to 10%. So the rain reaching their forest soils each year may be respectively 5% or 10% less than the rain gauge shows . 

This borrowed rain cools the canopy as it evaporates back into the system that provides rain further inland.

     Open Forest borrows 20% of each day's rain up to 1.3 mm 

- 0.5 mm is evaporated if next day wet and all if next day dry.  

DRYING SOIL

Evaporation from the soil surface, plus transpiration through plants dries the soil in five stages at decreasing rates - all driven by daily maximum temperature. 

Identical rain and temperature data was used to estimate soil-drying under three forest types at Lidsdale - open eucalypts, dense tall pines and scattered short pines.  All estimated two-year run-offs from these were within 3% of observed, showing all three types had similar evaporation plus transpiration - and that their big run-off differences were caused by big interception differences. 

                Dense Forest borrows 50% of each day's rain up to 3.6 mm -
                    1.4 mm evaporates if next day wet and all if next day dry.

        For Emu's four years of the constant mix of mostly dense forest types, the SDI modeled run-offs to within 1% of that observed. These seven Lidsdale and Emu annual comparisons each involved some nine hundred measurements and estimates. With so much data in each match, serious departures from observed run-off would have indicated systematic bias in estimated run-off. That the SDI was free of this problem shows the steps in the SDI model were both necessary and accurate.

STANDING FOREST CAPTURES CO2  

Every standing tree shades the soil, reduces both wind and daily maximum temperature, absorbs CO2 and turns it into wood - and the more CO2 there is, the faster this happens.

SOIL CLEARED STEALS BORROWED RAIN

But, once cleared, their soils now steal the 5% (or 10%) of annual rain their open (or dense) forests once borrowed. Cleared land gets more soil moisture, wind and sun and higher daily maxima. The system soils steal from has less rain to fall further inland. 

Half the World's forests were permanently cleared between 1800 and 2000 - so half the 1800's world's best CO2-fixers became CO2 gas in our atmosphere. Yet forest clearing continues at an alarming rate - as in the Amazon and in Queensland and NSW.

Where coastal open eucalypt forest has been mostly cleared for crops and cities, most of the annual 5% it once borrowed has now been subtracted from that past inland rainfall, so causing man-aggravated inland droughts and higher temperatures and seriously reduced inland run-off.       Coastal dense forest grows in higher rainfall areas so its widespread clearing adds 10% more rain to cleared soils, produces more run-off and raises sea levels - especially in tropical areas where there is less need to store water - all by stealing 10% from inland rains and more from inland run-offs.

LAND FOR FOOD AND FOREST PRODUCTION

Of course our crops need fertile land that's cleared of trees - except for agro-forest rows that slow the wind - because they need the extra rain trees once borrowed and held aloft, and the extra light and warmth each clearing causes, to grow more food for stock and people - but very little wood to harvest CO2.

Commercial harvesting of open forest is best kept selective. The need for intense fire to-regenerate dense forest, calls for minimum delays between felling and the dry-soil burn. But, for non-harvest-able forest land, why is there no global condemnation of clearing and so little promotion of reforestation?

PERMANENT FOREST CLEARING CHANGES CLIMATES

• Clearing heats the de-shaded land so adds to Global warming.
• It steals inland rains and dries up inland rivers
• It stops trees - the world's best CO2 fixers - making wood
• It increases cleared land flooding, soil salting, soil erosion, ocean pollution and coral bleaching.
• Roofs and driveways and roads and runways seal the cleared land
• More urban pipes drain the sealed land to the sea
• Forest clearing raises sea levels - wherever rains abound.

WHAT CAN WE DO?

We can find out and tell our friends about agro-forestry and urban tree planting and the fastest ecological ways of regenerating our commercial forests.      

Every tree we save; each seedling that we plant; and all tree seeds we sow, stem climate change and quickly grow from our high CO2. Their leafy tops hold rain aloft to cool and shade their land; and calm the wind, and make the wood that harvests CO2. Their borrowed rain as vapor is returned so inland rainfalls are restored.

All urban lands need trees to shade and cool where ere we walk and buildings clothed and roofed with plants all cooled and watered by what we today pour down the drain to raise our seas.

More canopies beat climate change - when non-crop land is fully stocked; and agro-forestry's fulfilled and parks full-treed and gardens blessed with fruit and nut trees - their cooling shade will slow ice-melt and rate of rising seas.

Very Tall Forest borrows 70% of each day's rain up to 5 mm
- 2 mm evaporates if next day wet and all if next day dry.