preserve the remaining Solonetzic Semiarid Soils with cut
off drains to prevent up-slope irrigation water from reaching
such areas. Thus, it is not likely that widespread secondary
salinisation will become an issue for New Zealand.
15.7.4 Soil Erosion
It is evident that following the arrival of humans, first with
fire, then with grazing animals (including sheep, deer, and
thar, and notably rabbits that rapidly increased to plague
proportions), vegetation was rapidly depleted in many areas
of the Semiarid Soils. The loss of vegetation cover, including through spring burning to encourage new shoots but at
the expense of protective litter cover, led to a period of
extensive accelerated erosion. For instance, Leamy reported
that a period of aggradation of alluvial fans on the Hawea
flats commenced in the mid-eighteenth century but had
subsided by the mid-twentieth century. Leamy estimated that
a minimum volume of about 6 million cubic metres of debris
had been eroded from the mountains to the east of Hawea
Flat and deposited as alluvial fans, lowering the surface of
the eroded area by about 30 cm on average.
Semiarid Soils have a significant wind and water erosion
risk when not protected by good vegetation cover. Soil has
often been exposed, particularly during rabbit plagues when
areas of bare ground can be substantial, and also following
periods of prolonged drought and consequent overgrazing by
sheep. Loss of vegetation may be greater where soil water
storage and root depth are limited by stoniness, bedrock, or
very firm substrates. For example, in a prolonged drought in
the Hakataramea Valley in the early 1980s, severe wind
erosion of exposed soil left some paddocks with a desert
pavement-like stony surface cover. However, with careful
land management, vegetation was re-established and the
land appears to be recovered. Care needs to be taken to
destock or provide supplementary feeding during droughts
to prevent surface vegetation removal and reoccurrence of
erosion events. Given the potential for droughts, and long
periods of soil exposure if vegetation is removed, no-till
methods (and avoiding burning) are strongly recommended
for establishing arable crops or pasture improvement.
Surface erosion can also be slow and insidious with small
quantities of soil lost at any one time. In a study of soil
erosion over 40 years, using caesium-137 (
137 Cs, which
was deposited as a result of atmospheric nuclear weapon
testing in the 1950s and early 1960s) as a tracer, Allan
Hewitt estimated that about 34 mm of soil depth was lost
over the 40 years. This rate seems small but when it is
projected over the remaining (shallow) soil depth, the soil at
the study site had a life expectancy of only 44–72 years
before it is exhausted. Conversely, in a study in the
Mackenzie Basin in 2015, Hannah Leckie and Peter Almond
showed, using both
137 Cs and cryptotephrochronology
(analysis of fine tephra-derived glass shards), that the soils
had undergone negligible wind erosion since 1953. They
identified tiny concentrations of glass shards of the Kawakawa Tephra, blown to Otago during the Oruanui
super-eruption of Taupo volcano c. 25,400 years ago, distributed in soil profiles as a non-visible cryptotephra (i.e.
only as sparse glass shards, not as a layer), which helped in
dating the soils.
Fig. 15.11 Irrigation in orchards. Left: stone fruit trees with overhead sprinkler irrigation. Centre: frost fighting using overhead sprinklers. Right:
drip irrigation delivering water to individual trees with tensiometers to monitor the soil moisture content
15.7 Use and Management of Semiarid Soils
245
off drains to prevent up-slope irrigation water from reaching
such areas. Thus, it is not likely that widespread secondary
salinisation will become an issue for New Zealand.
15.7.4 Soil Erosion
It is evident that following the arrival of humans, first with
fire, then with grazing animals (including sheep, deer, and
thar, and notably rabbits that rapidly increased to plague
proportions), vegetation was rapidly depleted in many areas
of the Semiarid Soils. The loss of vegetation cover, including through spring burning to encourage new shoots but at
the expense of protective litter cover, led to a period of
extensive accelerated erosion. For instance, Leamy reported
that a period of aggradation of alluvial fans on the Hawea
flats commenced in the mid-eighteenth century but had
subsided by the mid-twentieth century. Leamy estimated that
a minimum volume of about 6 million cubic metres of debris
had been eroded from the mountains to the east of Hawea
Flat and deposited as alluvial fans, lowering the surface of
the eroded area by about 30 cm on average.
Semiarid Soils have a significant wind and water erosion
risk when not protected by good vegetation cover. Soil has
often been exposed, particularly during rabbit plagues when
areas of bare ground can be substantial, and also following
periods of prolonged drought and consequent overgrazing by
sheep. Loss of vegetation may be greater where soil water
storage and root depth are limited by stoniness, bedrock, or
very firm substrates. For example, in a prolonged drought in
the Hakataramea Valley in the early 1980s, severe wind
erosion of exposed soil left some paddocks with a desert
pavement-like stony surface cover. However, with careful
land management, vegetation was re-established and the
land appears to be recovered. Care needs to be taken to
destock or provide supplementary feeding during droughts
to prevent surface vegetation removal and reoccurrence of
erosion events. Given the potential for droughts, and long
periods of soil exposure if vegetation is removed, no-till
methods (and avoiding burning) are strongly recommended
for establishing arable crops or pasture improvement.
Surface erosion can also be slow and insidious with small
quantities of soil lost at any one time. In a study of soil
erosion over 40 years, using caesium-137 (
137 Cs, which
was deposited as a result of atmospheric nuclear weapon
testing in the 1950s and early 1960s) as a tracer, Allan
Hewitt estimated that about 34 mm of soil depth was lost
over the 40 years. This rate seems small but when it is
projected over the remaining (shallow) soil depth, the soil at
the study site had a life expectancy of only 44–72 years
before it is exhausted. Conversely, in a study in the
Mackenzie Basin in 2015, Hannah Leckie and Peter Almond
showed, using both
137 Cs and cryptotephrochronology
(analysis of fine tephra-derived glass shards), that the soils
had undergone negligible wind erosion since 1953. They
identified tiny concentrations of glass shards of the Kawakawa Tephra, blown to Otago during the Oruanui
super-eruption of Taupo volcano c. 25,400 years ago, distributed in soil profiles as a non-visible cryptotephra (i.e.
only as sparse glass shards, not as a layer), which helped in
dating the soils.
Fig. 15.11 Irrigation in orchards. Left: stone fruit trees with overhead sprinkler irrigation. Centre: frost fighting using overhead sprinklers. Right:
drip irrigation delivering water to individual trees with tensiometers to monitor the soil moisture content
15.7 Use and Management of Semiarid Soils
245