Using Trees to Control Groundwater Recharge: How Many are Enough?
Note number: LC0062
Published: November 1999
This note assumes a basic knowledge of dryland salinity and is relevant statewide.
Although native vegetation may consist of clumps of trees, understorey plants and/or grasses and herbs, this note refers only to the use of trees in salinity control.
The best way to control dryland salinity in Victoria is to prevent groundwater recharge by using rain where it falls instead of allowing it to move down through the soil beyond plant roots into the ground water system. the aim of recharge control is therefore to use as much of the annual rainfall as possible.
Trees are important in preventing groundwater recharge and are complementary to other methods, such as establishing perennial pasture, improving crop productivity and natural regeneration. Trees are recommended for land where either perennial pastures cannot b reliably established, will not persist or will be unable to provide adequate protection from groundwater recharge.
How much water could trees use?
The reduction of groundwater recharge by vegetation depends on its ability to use or evaporate water from the soil. the deep root systems and large, evergreen crowns of many native trees means they can use more water than other types of vegetation (e.g pastures, crops, shrubs). Nevertheless, trees do not have an unlimited capacity to use water.
Evaporation of water from trees or any vegetation, depends upon three things (See Figure 1):
- Water - a tree can only use as much water as it has access to in the soil (soil moisture). Soil moisture varies throughout the year and is lowest during late summer and early autumn. The maximum water use by a stand of trees growing in recharge areas will be the annual rainfall. Surface run-off and recharge that occurs between root systems, however, reduces the potential water use.
- Sunlight - the energy for evaporation comes mainly from the sun. In most parts of Victoria it has the potential to evaporate up to 1500 millimetres of water a year.
- Leaf area - most evaporation takes place through the leaves. As a tree grows its leaf area increases and so does its water use.
How much water do trees use?
The following information has come from recent research.
Table 1 Results of some Victorian investigations into water use by eucalypts growing on recharge
|Location (rainfall)||Age (years)||Tree water use-litres per day and range (average)|
|Burkes Flat (450mm)||6||10-100 (30)|
|Warrenbayne (800mm)||20||0-160 (50)|
|Warrenbayne (800mm)||>100||10-450 (140)|
Table 1 shows the variation in water use over a year by eucalypts on recharge zones. The lower figures are during rainy days in winter and the higher figures are in early summer before the soil moisture is depleted. Daily water use by a tree therefore can vary considerably throughout the year. The averages over the year are given in brackets.
Table 1 also shows that age (due to growing leaf area and root system) also has a significant effect on the ability of trees to use water.
Table 2 shows the importance of stand density and water use. In a paddock the amount of water any one tree uses has little significance in recharge control. What is most important is the water use by the whole stand.
Table 2 Stand density and water use
|Location||Tree age (years)||Average tree water use -from Table 1(litres per day)||Stand density(trees per hectare)||Stand water use(mm)|
Even though a 6 year old tree at Burkes Flat may use less than one quarter of the water compared to a 100 year old tree at Warrenbayne, the overall stand use is twice as much because the number of trees per hectare is ten times greater.
Opportunities for trees to control groundwater recharge
Since a tree's capacity to use water is limited, careful planning is required to ensure that trees are effective in controlling recharge. Planning must achieve a balance between:
- the amount of water to be evaporated;
- the density of trees in the plantation;
- what is an acceptable delay before the control of recharge is achieved.
To calculate the approximate number of litres per day each tree would have to average over the year, use this equation:
|No. of Litres||= water use required (mm a year) x 25|
|per day||number of trees a hectare|
This equation enables you to calculate whether or not you are 'asking too much' of your trees to achieve your required level of recharge control.
If the most water we can expect a mature tree to evaporate averages 140 litres per day (Table 1), it follows that to use a given amount of water a minimum number of trees per hectare are required. For example at least 100 trees per hectare are needed to use 500 nun of rain per year.
To reduce the amount of time to achieve a certain level of water use, more trees per hectare should be planted.
For example, imagine two farmers in the same district with an annual rainfall of 500 millimetres. Both have fenced out their high recharge on their farms want to plant them to trees for salinity control.
Farmer A's recharge is planted with 800 trees a hectare (3.5 x 3.5 metre spacing). It would take 5-6 years for these trees to use the required 500 millimetres of water a year.Each tree needs to use an average of 16 litres a day (obtained from equation above).
Farmer B's recharge is planted with 200 trees a hectare (7 x 7 metre spacing). Farmer B is asking each tree to evaporate a lot more water per day than farmer A. It may take 20 years for farmer B's trees to grow to a size where they can average the 62 litres a day (from equation) and achieve the 500 millimetres of water use a year.
If this farmer had chosen to plant the paddock at 100 trees a hectare (10 x 10 metres), then it may take 50-100 years for recharge control. Each tree is required to average 124 litres per day.
The average water use required by each tree at densities below 100 trees per hectare is so high that trees could only provide adequate recharge control by adding to water use by pasture. At low densities, even this contribution by the trees would only be achieved slowly.
How many trees are enough?
Dense plantations (at least 500 trees a hectare) are clearly the best means of achieving rapid and effective control of groundwater recharge. However, recharge control will not always be compatible with other land management objectives, such as maintaining grazing or growing trees commercially. Therefore the answer to the question of 'how many trees are enough to control recharge?' must be given for each general type of rural tree growing.
Protection and landscape plantings
Most tree growing in rural areas is for either protection (control of land degradation, stock shelter) or landscape (visual beauty, wildlife habitat) purposes. Dense plantations will generally be needed to meet these objectives. However establishment costs of these plantations can be expensive, since no direct commercial returns are expected.
Assuming a stand density of at least 400 trees a hectare (5 x 5 metre spacing) is necessary for protection and landscape plantings, it may take around 10 years for the trees to control groundwater recharge (depending upon rainfall and rate of recharge). Higher densities, can be relatively easily and cheaply achieved through direct seeding or fencing areas off to allow natural regeneration.These techniques can significantly reduce the delay before achieving recharge protection.
Trees in pasture
There are two forms of tree growing in pasture that may play a role in the management of groundwater recharge areas:
- agroforestry - where both the trees and pasture are managed to provide a commercial return, and;
- low density protection plantings - where trees are planted at a wide spacing to allow grazing to continue but close enough so that some, and perhaps eventually, complete recharge control is achieved.
Appropriate densities for tree growing over pasture are 20-200 trees a hectare. At the lower range, recharge control will only be very slowly achieved (50+ years) and will rely on the trees not significantly affecting the evaporation from the pastures below. At densities lower than 20 a hectare (23 x 23 metre spacing) the average daily water use to achieve a given level of annual evaporation is simply asking too much of the trees.
Competition between trees and pasture in the upper range of stand densities (say 200 trees a hectare) would not allow much grazing when the trees are fully grown. Gradually thinning as the trees develop would be necessary if grazing is to be sustained.
Final agroforestry crop tree densities are typically in the range 75-150 trees a hectare. Thinning to these densities should have no long-term detrimental effect on the level of recharge protection if it is properly managed. Pruning is common in agroforestry stands particularly with Pinus radiata to produce high quality timber in the butt log. Pruning will reduce tree water use and recharge control will take longer.
Commercial tree growing
Commercial woodlots are planted at such high densities (e.g. 1000+ trees a hectare; spacing less than 3 m) that there is unlikely to be any conflict with recharge control.Protection from recharge will be lost for up to 5 years after the stand is harvested.
Successful tree growing requires planning. Decisions need to be made about:
- objectives - what do you want from the trees and the land they occupy, in addition to reducing goundwater recharge?
- water - how much water has to be used and how long will it take?
- density - given the aims of tree growing and the amount of water to be used, how many trees should be established and what, if any,thinning will take place?
- establishment technique - what is the most appropriate establishment technique, given the number and future use of the trees?
These questions provide a basis for realistic expectations of tree growing for groundwater recharge control.
Clifton, C.A. (1992). Tree growth and crown development in plantations established to control groundwater recharge. Technical Report No. 4. Department of Conservation and Environment, Centre for Land Protection Research.
Clifton, C.A. (1992). Tree densities for recharge control -proceedings of a workshop. Research Note No. 1. Department of Conservation and Environment, Centre for Land Protection Research.
Department of Conservation, Forests and Lands (1989). Water use by trees. Research and Development Note No. 15. Department of Conservation, Forests and Lands, Lands and Forests Division.
This Information Note was developed by Craig Clifton, Bendigo and David Perry, Melbourne