design an effective drainage system, detailed soil profile
sampling and site analysis should be undertaken to assess
local variability and to support precision modelling and
mapping.
The New Zealand Soil Classification defines a ‘mottled
profile form’ which identifies imperfectly drained soils, and
a ‘gley profile form’ which identifies poorly and very poorly
drained soils. However, there are uncertainties when soil
colour patterns are interpreted, as inference of wet conditions
may be misleading where artificial drainage, or landscape
change, has caused water tables to recede leaving the gleyed
soil colours preserved. In cases where iron is still present in a
soil, upon drainage, and entry of oxygen into the soil
atmosphere, colours may quickly (within a few minutes or
days, depending on the rate of oxygen diffusion) return to
the ochreous brownish colours we most often see in soil.
5.7.3 Soil Drainage Design and Management
In the nineteenth century, as land was cleared of native
vegetation and developed for agriculture, the need for drainage on the more easily farmed flat lands became immediately evident in many regions. River mouths were cleared to
enhance flow to the sea, regional outlet drains or canals were
established, often along roadsides, and then surface or tile
drains were installed in paddocks. Because drainage cuts
across neighbourhoods, districts, and regions, people had to
work together to instal effective drainage systems. Drainage
boards were quickly established to support community
drainage efforts, and many have remained in action for a
long period. The 1876 Public Works Act formally empowered the government to control waterways and drainage in
that any natural watercourse of swamp could be declared a
‘drain’. The Land Drainage Act of 1908 (last amended in
2012) brought together a range of earlier legislation pertaining to land drainage. It provided for drainage boards to
be established to construct and maintain drains and water
courses with funding from rates. The 1908 Land Drainage
Act also set out rights and responsibilities of landowners.
However, according to Geoff Park (2013), application of the
1876 Public Works Act and the 1915 Swamp Drainage Act
by the government resulted in its compulsory acquisition of
large areas of wetlands (to enable drainage operations to be
completed), and the value or otherwise of the drainage has
thus been (and remains) controversial for multiple reasons.
Since implementation of the Resource Management Act in
1991 the work of drainage boards is managed by regional
and district councils. For example, the Waikato Region has
over 50 land drainage schemes, the largest of which is the
‘Thames Valley Drainage Area’ which includes about 5000
ratepayers.
The drained lands generally proved to be highly productive. G.R. Reid of the Thames Valley Drainage Board
noted in 1978 that ‘Probably there is no single factor which
gives such an early or quick economic response as artificial
drainage, nor any which, under attentive management, will
have such a permanent effect’. Drainage removes water from
the soil allowing oxygen to penetrate the soil, and drier soils
warm more quickly in spring. Thus drained soils are better
able to support microbial and plant growth as well as vehicle
and animal traffic.
In designing a drainage system the overall objective is to
keep soils from becoming saturated. There are various possible approaches depending on the site, the soil, and the
causes of water logging. Where water tables are high, lowering the water table using surface and/or subsurface drains
is likely to be the best option. Drain design has to take into
account the need for slope on drains and the location and
height of an outlet to the drainage system. In areas where
runoff accumulates at the foot of slopes then enhancing or
preventing water infiltration by diverting surface water flow
away with a surface, or subsurface drain is an option. Land
smoothing may be used to enhance runoff and remove hollows, thus preventing surface ponding in hollows. In some
cases techniques such as deep ripping may break up a slowly
permeable layer or iron pan within the soil, increasing the
infiltration rate and thus preventing prolonged surface saturation. Care needs to be taken to prevent over-drainage. It
may be useful to maintain a water table near the base of the
plant root zone to provide water in drier summer months.
Fencing off open drains will help support riparian vegetation and prevent stock access and damage to the drain.
A good grass cover provides the best filtering for sediments
and nutrients on drain margins. Planting of trees and shrubs
will provide shade but should be kept at least three metres
from the drain and constrained to one side to ensure digger
access for drains that will need to have ongoing maintenance. It is wise to consult with regional and district councils
and drainage engineers for advice—in many cases a resource
consent will be required for any new drainage development.
Due to the often slow permeability of heavy textured
Gley Soils, even when drainage has been installed, the soils
are frequently wet following periods of high rainfall, particularly in winter and spring. Thus a risk that must be
considered is the potential for soil compaction and damage
to topsoil structure by vehicle traffic, or animal treading,
when the soils are wet. Severe animal trampling, causing
pugging (Fig. 5.9), has been shown to result in a marked
decrease in pasture production over the following season.
During wet periods, options for farmers include using
stand-off pads so that grazing is restricted to just a couple of
hours and pugging is minimised, or keeping stock to paddocks on better drained soils.
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5 Gley Soils
sampling and site analysis should be undertaken to assess
local variability and to support precision modelling and
mapping.
The New Zealand Soil Classification defines a ‘mottled
profile form’ which identifies imperfectly drained soils, and
a ‘gley profile form’ which identifies poorly and very poorly
drained soils. However, there are uncertainties when soil
colour patterns are interpreted, as inference of wet conditions
may be misleading where artificial drainage, or landscape
change, has caused water tables to recede leaving the gleyed
soil colours preserved. In cases where iron is still present in a
soil, upon drainage, and entry of oxygen into the soil
atmosphere, colours may quickly (within a few minutes or
days, depending on the rate of oxygen diffusion) return to
the ochreous brownish colours we most often see in soil.
5.7.3 Soil Drainage Design and Management
In the nineteenth century, as land was cleared of native
vegetation and developed for agriculture, the need for drainage on the more easily farmed flat lands became immediately evident in many regions. River mouths were cleared to
enhance flow to the sea, regional outlet drains or canals were
established, often along roadsides, and then surface or tile
drains were installed in paddocks. Because drainage cuts
across neighbourhoods, districts, and regions, people had to
work together to instal effective drainage systems. Drainage
boards were quickly established to support community
drainage efforts, and many have remained in action for a
long period. The 1876 Public Works Act formally empowered the government to control waterways and drainage in
that any natural watercourse of swamp could be declared a
‘drain’. The Land Drainage Act of 1908 (last amended in
2012) brought together a range of earlier legislation pertaining to land drainage. It provided for drainage boards to
be established to construct and maintain drains and water
courses with funding from rates. The 1908 Land Drainage
Act also set out rights and responsibilities of landowners.
However, according to Geoff Park (2013), application of the
1876 Public Works Act and the 1915 Swamp Drainage Act
by the government resulted in its compulsory acquisition of
large areas of wetlands (to enable drainage operations to be
completed), and the value or otherwise of the drainage has
thus been (and remains) controversial for multiple reasons.
Since implementation of the Resource Management Act in
1991 the work of drainage boards is managed by regional
and district councils. For example, the Waikato Region has
over 50 land drainage schemes, the largest of which is the
‘Thames Valley Drainage Area’ which includes about 5000
ratepayers.
The drained lands generally proved to be highly productive. G.R. Reid of the Thames Valley Drainage Board
noted in 1978 that ‘Probably there is no single factor which
gives such an early or quick economic response as artificial
drainage, nor any which, under attentive management, will
have such a permanent effect’. Drainage removes water from
the soil allowing oxygen to penetrate the soil, and drier soils
warm more quickly in spring. Thus drained soils are better
able to support microbial and plant growth as well as vehicle
and animal traffic.
In designing a drainage system the overall objective is to
keep soils from becoming saturated. There are various possible approaches depending on the site, the soil, and the
causes of water logging. Where water tables are high, lowering the water table using surface and/or subsurface drains
is likely to be the best option. Drain design has to take into
account the need for slope on drains and the location and
height of an outlet to the drainage system. In areas where
runoff accumulates at the foot of slopes then enhancing or
preventing water infiltration by diverting surface water flow
away with a surface, or subsurface drain is an option. Land
smoothing may be used to enhance runoff and remove hollows, thus preventing surface ponding in hollows. In some
cases techniques such as deep ripping may break up a slowly
permeable layer or iron pan within the soil, increasing the
infiltration rate and thus preventing prolonged surface saturation. Care needs to be taken to prevent over-drainage. It
may be useful to maintain a water table near the base of the
plant root zone to provide water in drier summer months.
Fencing off open drains will help support riparian vegetation and prevent stock access and damage to the drain.
A good grass cover provides the best filtering for sediments
and nutrients on drain margins. Planting of trees and shrubs
will provide shade but should be kept at least three metres
from the drain and constrained to one side to ensure digger
access for drains that will need to have ongoing maintenance. It is wise to consult with regional and district councils
and drainage engineers for advice—in many cases a resource
consent will be required for any new drainage development.
Due to the often slow permeability of heavy textured
Gley Soils, even when drainage has been installed, the soils
are frequently wet following periods of high rainfall, particularly in winter and spring. Thus a risk that must be
considered is the potential for soil compaction and damage
to topsoil structure by vehicle traffic, or animal treading,
when the soils are wet. Severe animal trampling, causing
pugging (Fig. 5.9), has been shown to result in a marked
decrease in pasture production over the following season.
During wet periods, options for farmers include using
stand-off pads so that grazing is restricted to just a couple of
hours and pugging is minimised, or keeping stock to paddocks on better drained soils.
82
5 Gley Soils