“Ah earth you old extinguisher.”
from Happy Days by Samuel Beckett
On a hot summer day how much water does a tree use? How much water is available to use? How long can a tree survive without it raining?
How do Trees Use Water?
Trees use or lose water by two separate processes. Firstly, water is taken up by tree roots from the soil and evaporated through the pores or stomata on the surface of leaves; transpiration.
Secondly by the interception of water by the surfaces of leaves, branches and trunks during rainfall; evaporation. Taken together, these two processes are often referred to as evapotranspiration.
This note is a look at transpiration associated with trees in parks and gardens, fields and hedgerows.
What is Soil?
Soil is comprised of minerals, organic matter, air, water and multiple organisms. It covers the landscape and forms a thin layer of material over the superficial deposits and the bedrock geology. It is a medium for plant growth; a means of water storage, supply and purification; a modifier of the atmosphere and a habitat for organisms. From a geological point of view, most soils in the UK are less than 1.0m thick although they may be thicker where down slope accumulation has occurred.
What Factors affect Availability of Water in the Soil?
The amount of water available in the soil is determined by numerous factors, including;
- soil type – sandy soils have the least storage capacity, peat soils have the greatest storage capacity whilst clay, loam and silt based soils have a capacity somewhere in between.
- soil depth – thin soils have less capacity for water storage than thicker soils.
- soil structure – a typical good structure for a loam silt soil is 50% pore space, portioned at 25% air and 25% water at field capacity. Minerals, organic matter and organisms comprise the other 50%.
- physical barriers restricting root spread such as foundations, cultivation and iron pans.
- rainfall – the average rainfall in the UK is 885mm (33.7 inches) a year.
What Factors affect Water Demand?
Water demand is again determined by numerous factors, including
- tree species – oak has a high water demand, ash a medium water demand and birch a low water demand. Cypress has a high water demand and all other conifers have a moderate water demand.
- tree size – very simply, larger trees demand more water than smaller trees.
- temperature – trees demand more water in higher temperatures.
- humidity – high humidity reduces water demand.
- air movement – moving air increases water demand.
- daylight hours – the longer the day, the greater the demand.
How Much Water do Trees Use a Day?
It depends. Based on numerous studies, a high water demand mature tree on a hot day in the middle of summer is probably using somewhere between 50 and 200 gallons of water per day or between 225 and 900 litres per day.
How Much Water is Available?
Tree roots extend radially in every direction (assuming no physical barriers) to a distance equal to at least the height of the tree; 90% of all tree roots are typically found in the top 60cm of soil (Tree Root Systems). Assuming a tree 25.0m in height, the root system occupies some 1,178m3. Assuming a loam silt soil with 25% water, at field capacity, there is some 295 cubic meters or 295,000 litres of water available; the equivalent of at least 327 days water demand at 200 gallons or 900 litres per day for a single mature tree.
Trees of course have to share the available soil water with other plants. A mature tree in a woodland might have to share its rooting area with 5 or 6 other mature trees as well as smaller trees, shrubs and ground cover. A tree in a hedgerow might share its rooting environment pasture or arable crops, both of which use more water than a woodland in mid-summer.
Good soils hold considerable volumes of water so even a mature tree with a high water demand will be able to withstand a hot dry summer, but mature trees growing on sandy or thin soils will struggle if we get another exceptional long, hot, dry summer like 1976.
References: Water Tables and Trees – AAIS 110-93 By D R Helliwell; Soils Compaction – Causes, Concerns and Cures – University of Wisconsin – R. Wolkowski; Water Use by Trees – Forestry Commission – April 2005; Tree Root Systems – AAIS 130/95/ARB M Dobson.