A report by Ted Floyd
First Published in the Stormwater Industry Association Bulletin, no 108, Dec 2002.
Transpiration by trees returns water from soils back into the atmosphere and reduces stormwater problems.
In towns and cities many creeks and natural drainage lines are destroyed when houses, factories and roads are built. Rainwater is collected in concrete gutters, drains and pipes and then flows into rivers or the sea.
This article explains how green corridors can reduce flood peaks, reduce velocity of stormwater flows, spread biodiversity and reduces water pollution.
Growing plants absorb large amounts of water from soils, which travels up the roots and stems to the leaves. The water then evaporates through stomates, which are small pores in the leaf surface into the atmosphere. This process is called transpiration and the water travelling up the stems transports minerals from the soil up to the leaves.
The volume of water flowing up the stems is far greater than the amount of water needed for plant growth and the excess water evaporates into the atmosphere.
Plants transpire large volumes of water. In Sydney a large gum tree transpires about 200 litres of water a day. This volume of transpiration will occur from a tree with 6m diameter of foliage on a hot summer day when the soil is still moist after recent rain.
Direct sunlight on leaves is the driving force for transpiration. Trees with a large leaf area transpire water quickly. High up in trees, winds blow moisture away encouraging faster transpiration. Very little transpiration occurs in deciduous trees when they lose their leaves in winter.
Deep plant roots enhance transpiration. Many Australian native trees have very deep roots, up to 40 metres in favourable soils. In normal conditions tree roots are most active at 6-10 metres. During dry spells, surface soils dry out and native trees with deep roots continue to grow using water in the subsoil. This helps to make trees drought proof.
Grasses have shallow roots. Annual grasses have very shallow roots, often less than 0.5 metres deep. Permanent grasses have deeper roots and Kikuyu roots can be as deep as 2.4 metres.
Native trees are evergreens and transpire water in all seasons. Quick growing native trees have the ability to transpire 2-4 times more water in a year than the average annual rainfall.
Most urban streets have concrete gutters. Rainwater is collected from streets and roofs into gutters and flows in pipes and concrete drains to the sea, lakes or rivers. A hard surface prevents rainwater from penetrating into soils.
Soils have a permeable surface that absorbs rainwater. In green corridors, water is absorbed into permeable soils. Tree trunks and roots facilitate water entry, deep into subsoils. Vegetation and organic. matter improve soil structure and permeability Grass and trees transpire water back into the atmosphere and dry out soils.
Green corridors have a water cycle with a high percentage of rain entering the soil. Plants growing in the soil transpire water back into the atmosphere and complete the water cycle. Rainwater, which is absorbed by soils and transpired by plants, does not run off into creeks and rivers increasing flooding.
Water flow in a concrete gutter flows about three times faster than a grass waterway. A meandering, eco-designed waterway has a longer length, reduced slope, slower flow velocity and increased water storage in the waterway. In a meandering waterway there is a higher opportunity for water to be absorbed by the soil.
Native trees support native birds and animals, adding biodiversity to the built environment of developed suburbs where Australian native habitat is often very scarce. Many introduced species of trees only have a limited value in adding to native biodiversity.
Avenues of trees planted along streets will help birds and animals to travel between bushland areas. Streets lined with trees will help animals to move through the suburbs and will act as green corridors in highly developed urban bio-deserts.
Organic pollutants break down faster into harmless compounds in soils than in water or sediments. Healthy, fertile soils contain countless, numbers of bacteria and other microorganisms, which break down organic pollutants. Grass waterways trap pollution enabling their breakdown by soil microorganisms.
Rain washes pollutants off roads and roofs into gutters and into downstream waterways. Pollutants break down slowly in sediments found at the bottom of rivers and lakes because there are few microorganisms in these airless environments compared to soils.
In a grass waterway larger and heavier sediments settle to the bottom of the waterway. Some fine clay particles can become attached to grass leaves and other organic particles.
When it rains, water can become part of several different pathways in the water cycle. In an undisturbed catchment a high percentage of the rainwater enters the soil and is eventually returned to the atmosphere by evapotranspiration. In urban areas evapotranspiration is low.
In undisturbed catchments in the Sydney region 60% of the total rainfall is returned to the atmosphere by evapotranspiration transpiration (direct evaporation + transpiration). In developed suburbs evapotranspiration is less than 10% of the rainfall.
The water balance in a catchment greatly changes when suburbs grow. In urban areas runoff increases and flows faster compared to a natural, undisturbed catchment. Flash flooding is common in urban areas while in undisturbed catchments flooding is less severe and dry weather flows are more reliable.
Impermeable surfaces restrict water entry into soils, but evapotranspiration can occur from soils covered by impermeable surfaces if plant roots have spread under the impermeable surface., If a tree canopy spreads over an impermeable surface, the volume, of transpiration will depend on the area of the canopy and not on the area of impermeable surface. Tree canopies overlapping an impermeable surface could help to reduce the amount of runoff expected from an impermeable surface.
In a green corridor planted beside a street the tree canopy may spread over the impermeable surfaces of the street and footpath. The effective surface area for transpiration is the area of the tree canopy and this may be considerably larger than the permeable soil surface.
Cutting down trees and watering gardens may help water tables to rise. Rising water tables may bring salts to the surface in susceptible urban areas and may cause rising damp especially in old houses. Buildings are damaged by salt and weaken brick mortar. Plants are damaged by salt and are killed by severe salting. When rising water tables are causing problems, water should not be encouraged to enter soils. Planting trees can help to lower water tables.
During the development of urban areas most trees are cut down' Natural drainage lines are replaced by concrete drains and pipes. Most of the surface is covered by impermeable surfaces. Flash floods are common in highly developed urban areas.
The creation of green corridors with grass waterways and avenues of trees will help to reduce stormwater problems.