TRANSPIRATION AND URBAN CATCHMENTS

TRANSPIRATION and URBAN CATCHMENTS

Transpiration Benefits For Urban Catchment Management

An analysis by Ted Floyd
First published in the Stormwater Industry Association Bulletin, no 92, Aug 2001

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.

Urbanisation of a catchment increases the area of impervious surfaces. Roads and buildings seal the surface which prevents water absorption by the soil. Impervious surfaces also prevent evaporation from soils.


Water Balance


 

1. Typical Undisturbed Catchment in Sydney Region

    Interception and evaporation  
    from plant surfaces 15%
    Runoff 15%
    Seepage to groundwater and rivers 10%
    Evapotranspiration 60%

 

2. High Density Urban Catchment

    Runoff up to 90%
    Evapotranspiration and  
    seepage to ground water 10%

Evapotranspiration

Falling rain is intercepted by vegetation. A small proportion of the rain is evaporated directly from the plants surfaces. During rain, water is stored on the surfaces of leaves and stems. When it ceases to rain, water will continue to drip from a tree. This can help to even out a rainstorm and to reduce flood peaks.

Table 1 presents rainfall and evaporation data for Sydney. During the summer months evaporation is greater than rainfall. The difference between evaporation and rainfall is demonstrated by the P/E ratio. Yearly P/E for Sydney is 0.67 which demonstrates that evaporation is greater than rainfall.

During the growth of vegetation, water is removed from the soil by transpiration. The rate of transpiration is proportional to the leaf area. Trees have a large leaf area and deep roots encouraging a high transpiration rate. Transpiration by plants helps to dry out soils. During rain, water will infiltrate more readily into a dry soil. The removal of water by transpiration allows more water to enter soils during rain and this will reduce water runoff and lower flooding.


Table 1 RAINFALL AND EVAPORATION DATA SYDNEY


 

Monthly Rainfall – means (mm)

  (Observatory Hill)
  Jan Feb Mar Apr May June Jul Aug Sep Oct Nov Dec

  102 113 135 124 121 131 100 77 69 78 81 78
  Yearly mean 1213mm

 

Monthly Evaporation (mm/month)

  (Sydney Airport)
  220 176 164 126 90 78 90 115 141 171 192 239
  Yearly mean 1802mm

 

Daily Evaporation (mm/day)

  7 6 5 4 3 3 3 4 5 6 6 8

 

P/E (precipitation/evaporation)

  0.46 0.64 0.82 0.98 1.34 1.68 1.11 0.67 0.49 0.46 0.42 0.33
  Yearly P/E 0.67

The rate of evapotranspiration is regulated by the total evaporation. Different plant species transpire at different rates. The ratio between evapotranspiration/total evaporation is often called the crop factor and can be as high as 0.95 for Lucerne in Jan. Deciduous trees vary from 0.75 in Jan down to 0.1 in June. When deciduous trees lose their leaves in winter they have a very low evapotranspiration rate. Table 2 lists values of crop evaporation factors for several crops.


Table 2 EVAPOTRANSPIRATION RATES OF AGRICULTURAL CROPS


 

Crops

Crop Evaporation Factors


    Jan July

  Lucerne 0.95 0.55
  Citrus 0.55 0.50
  Grapevines 0.60 0.15
  Deciduous orchard 0.75 0.15
  Pasture 0.70 0.40

  Crop factor= Evapotranspiration Total evaporation
  Reid, R. L. Ed. (1981). Manual of Australian Agriculture Heineman, Melbourne

Deep rooting plants have the ability to utilise water at greater depths in the soil. This enables these plants to grow and transpire when shallow rooting plants have wilted and ceased to transpire. Deep rooting plants generally have high crop factors, e.g. Lucerne develops very deep roots and has a high crop factor.

The depth roots penetrate into a soil depends on the availability of water. If no available water is in the subsoil, plants develop a shallow root system. Frequent light watering encourages shallow roots while less frequent heavy watering ensures water penetrates deep into the subsoil and encourages growth of deep roots. Plants with deep roots are more drought tolerant. Some Australian native trees have been observed with roots as deep as 30 metres. Table 3 lists the root depths of several common grasses.


Table 3. ROOT DEPTH OF COMMON TURF GRASSES


 

Species

Root depth in metres


  Kikuyu 2.40  
  Paspalum and Couch 1.50
  Buffalo 1.00
  Kentucky bluegrass 0.40
  Bent grasses 0.35
  Poa annua 0.15
  Lucerne 6.00

Handreck, K.A. and Black, N. D. (1994)
Growing Media-for ornamental plants and turf.
University of NSW Press, Sydney.
  


In certain situations the removal of trees has resulted in the rise of the water table. This has caused great problems when there is salt in the subsoil. A rising water table brings the salt to the surface and this can kill vegetation and in urban areas salt can cause damage to buildings. In areas susceptible to salting, water infiltration into soils should not be encouraged and gardens and lawns should not be over watered.

The vegetation covering the land is a very important component of the water cycle. When vegetation is removed or reduced the cheeks and balances in the water cycle are disrupted. Trees are large, with deep roots having a major role in maintaining a balanced water cycle.

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