TOPIC 3: EROSION MANAGEMENTINTRODUCTION

Management of erosion is one of the key objectives of the measures that come under the umbrella of Soil Conservation. Erosion has a direct influence on the productivity (in its widest sense of beneficial as well as economic use). It may deny access or destroy infrastructures associated with recreational uses of land. High sediment loadings may drastically impact on freshwater fisheries and therefore the tourist industry. In agricultural or horticultural terms, erosion may cause the following:

loss of soil, leading to shallow soils in which plants suffer poor root development and water stress

topsoil losses leading to low organic matter and deterioration of soil structure, manifesting in poor cultivation characteristics, loss of soil water holding capacity and infiltration of water

subsoils may be exposed that have poor plant growth medium characteristics lost topsoil carries with it much of the plant nutrients in soil (e.g. phosphorus

and nitrogen) that end up in waterways and lakes where they contribute to eutrophication, resulting in phenomena such as algal blooms

gullying may hinder being able to move stock or access areas of land in farming operations

farm infrastructure (buildings, fences tracks) may be damaged accelerated sedimentation raises the levels of affected river beds, causing

increased flooding risk in adjacent areas

The amounts of material transported in our river systems gives some indication of the rates of erosion today. For example, the Manawatu River at average flow moves a 5 tonne truckload of sediment under the bridge at Palmerston North every 13 seconds. This equates to a lowering of the catchment by an average of 1 mm every year. In an annual average flood this rate goes up to one truckload per second and in an 8-year flood, two truckloads per second. During cyclone Bola the Waipaoa River, near Gisborne, had a discharge rate of 5,300 m3s-1 and a suspended sediment discharge of about 400 m3s-1. Some 600,000 tonnes of sediment were discharged by the river in this storm event.

East Coast Hill and Steepland

The Hawkes Bay hill is part of a long belt of similar country extending from East Cape to eastern Marlborough. There are close similarities in climate, landscape and soil parent material through the entire region that contrast markedly with the hill country of Taranaki and Manawatu.

If you examine a map you will observe that the mountain ranges and chains of hills on the eastern side of North Island are aligned predominantly northeast-southwest, roughly parallel to the coast and the rocks and fault lines are

distributed along a similar trend. The explanation is found in a branch of geology called "plate tectonics" which recognises that the surface of the earth is made up of a number of semi-rigid plates moving relative to one another.

Hikurangi Trench (Figure 1) marks the position where the Pacific Plate collides with the Indian-Australian Plate. The Pacific Plate is composed of dense basaltic rocks so it slides beneath the lighter continental crust of the Australian Plate (whether we like it or not, the North Island and the west of the South Island are already part of Australia!).

Figure 1. A block diagram of the convergence of the Pacific Plate with the Australian Plate

The axial mountain ranges, inland valleys and coastal hills are produced by the collision of the plates like wrinkles in a rug. The Pacific Plate is speeding in at 40-50 mm/yr, but is also sliding south relative to the Australian Plate at 30 mm/yr. The result is that the eastern side is moving south relative to the western side.

Meanwhile, considerable uplift, rapid by world standards is occurring. The axial ranges are rising at up to 5 mm/yr (with a maximum of 22 mm/yr near Mt. Cook) while the coastal hills are rising at 0.5-3 mm/yr.

Rates of earth movement in this region are therefore rapid, often with disastrous consequences for humans. It is envisaged by scientists that earthquakes are generated by the friction between the descending Pacific Plate and the Australian Plate and by stresses accumulated in the Australian Plate above. Figure 1 shows that shallow (and potentially destructive) earthquakes may be expected quite frequently in eastern Hawkes Bay as strain builds up. Earthquake epicentres become deeper towards the west.

The nature of the parent rocks has also strongly influenced the development of landscapes and soils. The high axial ranges are composed of Mesozoic (a period of time - 60 million years ago), greywacke and argillite rocks. In contrast, the coastal ranges are a complex mixture of slivers of younger Cretaceous and Tertiary rocks (120 - 60 million and 60 - 2.5 million years ago respectively). These rocks vary from harder limestones and sandstones which tend to form ridges with steep faultbounded east facing slopes and more gentle west facing slopes, to soft mudstones and siltstones. Numerous active faults (Figure 1) have crushed, uplifted and strongly tilted these rocks rendering them very susceptible to erosion. The intervening plains and valleys receive the eroded material from the axial ranges and coastal hills, and are built up from aggradation gravels and sands to form terraces, fans and floodplains.

Climate is also a factor in predisposing the country to erosion. By consulting the New Zealand Atlas you will see that the region receives most of its rainfall in winter and that in summer droughts are to be expected more often than not. The result is that soils dry, shrink and crack in summer. In winter the soils become wet and clay particles are forced farther apart by water and may expand slightly. They then have reduced strength, and the weight of water is often enough to push the soil particles apart and send the whole soil layer cascading downslope in a muddy slurry, called a slip. In other situations the underlying rock may also be destabilised by the weight of water and a more deep seated slump may result. Erosion is usually particularly evident after the occasional high intensity rainstorms that affect the region, for example Cyclone Bola which struck in early March 1988.

Erosion in the hill country causes the streams and rivers struggling to remove the debris to aggrade. Since the original forests and regenerating shrubby forests were removed at the turn of the century and into the 1930s, some rivers and tributaries have built up their beds by more than 10 metres. All rivers have been affected by this aggradation which then predisposes the lower terraces on either side to flooding. Clearly what happens in the hill country affects the lowlands

Photograph 1 is of the enormous and much studied Tarndale slip in the headwaters of the Waipaoa River north of Gisborne. Faults have crushed the argillite (a form of hardened mudstone) rendering it prone to failure. The rocks also contain a clay mineral called smectite which has the ability to soak up water and swell. When dry this clay is stable, but it loses strength when wet and acts as a lubricant, allowing slope failure. The climate pattern of wet winters and dry summers accentuates the problem. Prior to the real commencement of the slide of 1932, it was possible for farmers to jump their horses across the stream in the gully bottom. Since then tens of metres of aggradation have taken place. Mangatu Forest has been planted in an effort to slow down the erosion.

(photograph by Noel Trustrum)

Photograph 2 clearly shows the ill effects of the erosion downstream, where flooding and rapid aggradation inundated this orchard near Gisborne during Cyclone Bola. In this case the whole enterprise has been ruined. The news is not all bad because the silt and sand deposited are extremely fertile. It will, however take many years of colonisation by plant roots and habitation by soil organisms to make a really productive soil from this new material.

(Photograph by Noel Trustrum)

Photograph 3 Lake Tuitira, northern Hawkes Bay following ca. 600 mm rainfall in 48 hours from Cyclone Bola, February 1988. Landcare research NZ Ltd have carried out intensive study in the Tuitira catchment and lake. Their work shows that major storms, greater than 200 mm rainfalls are responsible for the bulk of the sediment that enters the lake.

(Photograph by Noel Trustrum).

We should, however, always remember that the low terraces and plains were originally produced by deposition of silt and sand by huge floods long before men and women interfered with the natural vegetation. Beds of buried logs in the lowland soils right down the East Coast show that periodically, even with forest present, erosion took place. The evidence of Cyclone Bola and other serious floods this century does prove though that land currently protected by indigenous or exotic forests suffers much less damage than cleared land under pasture (Table 1). A small part of the soil (and therefore fertility) lost from the hill country is gained by the lowlands; the rest is washed out to sea.

The soils that form the flood-deposited silty or sandy alluvium are some of the most fertile in New Zealand. this is because the hill country rocks are usually calcareous (contain lime) and also micaceous (contain potassium). Because of the presence of abundant base cations (calcium, potassium, sodium, and magnesium), the pH of the soils is high which in turn stimulates an active and diverse array of soil organisms. Although many of the eroding parent rocks are fertile in terms of plant nutrients, when they lose the soil layer it takes some time to regenerate organic matter and become fully productive again. On some rock types pastoral production is back to 100% after 40-50 years but on other rock types it is over 100 years. The problem now is that erosion is of such magnitude that large areas of hill country in some districts are losing productivity at an alarming rate. Studies by Landcare Research NZ Ltd on some hill country in Wairarapa show that since forest clearance, over 40% of the

original soil has eroded. After 5 years the eroded areas were back to only 20% production. Even after 20 years the soils are not back to full production. One of the serious side affects is the reduced plant available moisture holding capacity of the new soil. Not only does this predispose the hillsides to drought, but also leads to increased runoff in winter storms, further increasing the propensity for erosion.

Serious flooding in the first half of the century led to massive flood protection works on the lower reaches of major rivers in Poverty Bay, Hawkes Bay, Wairarapa and Marlborough in the 1950's and 60's. For the most part these have been successful and now offer at least partial protection to productive pastoral and horticultural land and the towns that service them. Protection work carried out in the hill country has largely been through planting exotic forests and gully stabilisation. Some large forests have been planted on particularly susceptible land; for example Mangatu forest was established in the headwaters of the Waipaoa River to try and control the Tarndale slip and protect the surrounding land on similar crushed argillite rocks.

Geologists can follow the shifting paths of the rivers and the way they deposit alluvium at their mouths to create new land (a process of PROGRADATION) by studying he sediments, and datable volcanic ash and organic matter they contain. These studies show that since the last glacial period degradation and aggradation have occurred alternately in each major river. Periods of aggradation (and progradation at the coast) appear to be triggered by the following events either alone or in combination:

• high intensity storms• earthquakes• volcanic eruptions• sea level fluctuations• deforestation by men and women• drought, desiccation and fire• longer term fluctuations in climate• vegetational succession induced by prolonged weathering

and leaching of soils.

All these possibilities exist to produce periods of erosion and deposition that some scientists believe occurred at the same time down the entire East Coast. Soils on slightly higher and older terraces are flooded less often and develop deep dark topsoils. Soil mapping has identified these soils which then become most sought after for productive use. The slightly older soils also have the advantage of having weathered to develop brown earth colours and more stable

structures. Use of the soils on slightly lower and more flood prone areas is always going to be a risky venture, particularly where capital costs of development are high.

Manawatu-Taranaki Hill Country

During the Tertiary, the area from Manawatu to Taranaki and including the Rangitikei River valley lay beneath the sea in what has been called the Wanganui Basin. It was an area of rapid sedimentary accumulation. Pleistocene glaciation influenced global sea levels and the region was alternately a coastal plain undergoing erosion; and submerged, receiving further sediment. In deeper water, especially when the sea level was high during interglacials, mudstones were deposited. During colder glacial intervals, sea level was lower and sandstones and conglomerates were deposited offshore while erosion took place on land. Gradually, though, the Wanganui Basin was elevated at a rate of about 0.5 mmlyr and the coastline retreated to the south. The southern coastal strip is thus younger and dominated by marine terraces. Moving inland, we cannot fail to notice the progressive dissection with altitude (and age) until marine terraces are no longer recognisable.

As we drive through this inland hill country we notice that most of the hills are the same height i.e. had SUMMIT HEIGHT ACCORDANCE. Hills become gradually higher inland and lower toward the coast. Dissection increases inland, as steep-sloped V-shaped valleys gradually become dominant in area over flat to gently rolling interfluves. The landscape has an overall uniformity but local variations are caused by contrasting rock types. Unconsolidated sands, for example between the Pohangina and Oroua rivers, are deeply gullied by rivers and streams, while limestones, like those near Wanganui, form bluffs and cliffs.

Rainfall increases inland from 1000 mm for hill country near the coast to 3000 mm in the King Country. Polynesian people had little impact on the vegetation of the region. European settlers encountered dense lowland broad leaf-pod ocarp rainforest. The main species were tawa (Beilschmedia tawa), kamahi (Weinmannia racemosa) and rimu (Dacrydium cupressinum). Clearance began in about 1847 and was mostly complete by 1920. The method of felling and burning did not lend itself to preservation of forests for parkland or reserves. In the 1930s large areas of steeper and poorer land reverted to scrub and regenerating forest which was later cleared once more in the days of subsidies.

The hill country was obviously produced by erosion long before the intervention of men and women. If you look at your postcards of New Zealand you will see that the area was close to the boundary between grassland and cold temperate forest during the last glacial. It was probably during glacial times, when vegetation was destabilised, that the significant erosion took place. Clearance of forests has once again allowed processes of erosion to accelerate.

Following forest removal scientists are easily able to document increased soil erosion. The soil mantle that once existed under forest is removed by successive erosion events as the binding effect of the tree roots is gradually lost. The idea behind conservation planting of poplars, alders and willows is to replace some of these binding agents in the soil.

The valley sides are produced by soil creep, slips, slumps and debris flows. A characteristic of mudstone as it weathers is that the upper metre or so becomes loose and friable with a sharp distinct contact between soil and underlying rock. Soils become cracked and dry in summer, and supersaturated during storms in winter and spring when rapid runoff also occurs. These are ideal conditions for erosion. All the while, streams in the valley floors either aggrade when the supply of eroded detritus is high, or cut down, undercutting the valley sides and causing further erosion. Particularly severe periods of erosion probably relate to high intensity rainstorms such as Cyclone Bola and may affect whole regions. Some scientists believe that long term variations in storminess cause periods of erosion that may affect the whole country. Other causes of instability are: earthquakes, volcanic eruptions where vegetation has been adversely affected by ash-fall, and clearance and burning first by Polynesians (c. 600-800 years ago) and latterly by European foresters and farmers.

Erosion in this landscape has occurred in a relatively stable tectonic setting, where the rocks are only gently tilted, and uplift rates increase only slowly inland. The Quaternary rocks magnificently exposed in this region are one of the thickest and finest sequences of this age in the world. Consequently New Zealand and overseas scientists have made detailed studies of the rocks and fossils they contain to shed light on the climatic history of the world over the last 5 million years.

SPECIFIC EXAMPLES

Photograph 4 is taken from just north of Manawatu Gorge above the Ruahine Range looking northwest. In the foreground is the Pohangina River Catchment that drains south into the Manawatu River at the western end of Manawatu Gorge. The extensive terrace in the foreground is the Ohakean river terrace. Soils on the Ohakean terrace in this area belong to the soil group yellow-brown earths (now called Brown Soils). They are more stony than many soils within this group. Yellow-brown earths (Brown Soils) are widespread soils in New Zealand.

Beyond the Ohakean terrace is a vast area of hill country extending towards Mts. Ruapehu and Ngauruhoe in the distance. Throughout much of this country, downslope movement of soils is a common process rejuvenating land surfaces and leading to much soil erosion. Many different hill and steepland soils are mapped, but perhaps the most famous ones are Tafflape, Turakina, Whangaehu and Mangamahu steepland soils, all of which are steepland soils related to yellow-brown earths (Brown Soils).

Obviously, eruptions of Mts. Ruapehu and Ngauruhoe have contributed volcanic ash to many of the soils in the central and eastern North Island. As one moves progressively northwards up the Pohangina Valley one traverses the boundary from yellow-grey earths (Pellic Soils) and yellow-brown earths (Brown Soils) to yellow-brown loams (Allophanic Soils) on the high terraces in the middle distance. Two of these areas are visible in the middle distance at Apiti (left) and Table Flat (right).

(Photograph by V. E. Neall)

Photograph 5 is taken from hill country in eastern Taranaki near Whangamomona. The soil parent material is sandy mudstone; any volcanic ash deposited on these slopes has long since disappeared. The soils are shallow over easily erodible rocks. Numerous erosion scars of various ages are visible. The district has a cool climate and receives about 2400 mm rainfall. These factors combine to make farming in the district difficult, confined to store lamb and wool

production.The soils are difficult to characterise, because they vary considerably in drainage and soil thickness between components of the landscape. Four main categories can be recognized:

• deep, well drained soils showing more weathering and profile development that occur on broader ridges and spurs;

• thin soils showing weaker profile development that occur on hillside areas not recently eroded and on narrow ridges and spurs,

• very shallow soils with little development that occur on recent erosion scars. The soil generally consists of topsoil over mudstone, where thickness of the topsoil layer is controlled by age;

• deep, poorly drained soils on lower slopes where erosion debris has accumulated.

On most soil maps large areas are mapped with just one soil name. It is important to realise that different soils occur on components of the landscape. Our example comes from the middle of one of the slopes in the foreground.

(Photography by J.A. Pollok)

Photograph 6 is the Tahora steepland soil, a yellow-brown earth (Brown Soil). This particular site has not eroded recently and the soil has had time to develop a moderately deep profile, with an olive brown, altered B horizon that shows some structure. In the subsoil fragments of mudstone are common.

(Photograph by J.A. Pollok)

Erosion of the original forest soil results in thin soils with reduced water storage capacity. Not only are pastures then more susceptible to summer drought, but

winter rains quickly exceed the storage capacity of the soil allowing more runoff and erosion. It is not surprising that pasture production is adversely affected, and will continue to decline. More and more hill country will become uneconomic for farming, particularly when product prices are low. The problem is accentuated when farmers cannot afford to put fertiliser on the land, leading to reversion to poor grass species, scrub and fern.

Exotic forestry is becoming a common land use, and agroforestry (spaced planting of exotic trees to allow less intensive grazing between) is becoming a more attractive option.

SOIL CONSERVATION TECHNIQUES

Erosion is largely caused by the action of water and wind. Soil conservation measures are therefore based on practices which control those actions. These include; reducing the velocity of runoff and streamflow, improving soil stability, increasing soil infiltration and protecting the soil from raindrop impact. Management of erosion can be conveniently described under three headings; tillage techniques, vegetation techniques and construction of artificial structures.

TILLAGE TECHNIQUES

Conservation tillage is a method of cultivation that leaves a protective cover of crop residue, together with a roughened surface, both of which reduce raindrop impact and maintaining or improving the physical properties of the surface layer of the soil. Direct drilling, a technology developed at Massey University, is an aid to minimum tillage practice.

Sheet and rill erosion can be controlled by contour ploughing and strip cropping, where the furrows are aligned along the contour.

VEGETATION TECHNIQUES

These techniques involve the planting of trees to control erosion or its products. Trees have a twofold effect, they hold the soil together with their roots and the transpire water, maintaining the capacity of the soil to store water during rainfall events. They also intercept rainfall and lessen its impact on the ground and help maintain high rates of infiltration. All of this slows down the movement of water through the system, reducing flood risk in all but the more severe rainfall events. Poplars and willows are the most common species used because they can tolerate a wide range of conditions, transpire large amounts of water in their growing seasons and are easily established from poles or cuttings.

Agroforestry is one technique that can reduce erosion and at the same time add to land productivity. The more traditional techniques include:

1. Space planting. Trees are planted at wide intervals across erosion-prone surfaces where mass movement such as earthflows, soil slip and slumping occurs.

2. Pair planting A technique used to control gully erosion. Trees (willows) are planted in pairs at 3 – 5 m intervals along opposite sides of small gullies or eroding streambanks. The root systems of the pair meet in the streambed and reduce the scouring effects of streamflow.

3. Riparian zones These are established along streambanks to as buffer zones to filter out sediments and nutrients otherwise carried into waterways by runoff.

4. Gully planting Planting up of eroding gullies to stop headward erosion.5. Retirement Actively eroding areas are planted up and retired from

productive use.6. Shelter Belts These are often species other than poplar or willow and are

used to reduce the impact of high winds.

ARTIFICIAL STRUCTURES

These include a wide range of practices, from dam and weir construction to drainage techniques. In all cases the intent is either to remove water or to decrease its erosive power by lowering its velocity.

1.Drainage Contour banks, graded banks or pasture furrows may be constructed along contour to collect and slow down runoff before it builds up speed and erosive power downslope. Where earth flow or tunnel gullying has disrupted the land surface, land recontouring may be carried out. This involves using machinery to smooth the surface to improve runoff and cultivation. Horizontal bores to tap springs and drain subsurface water from mass movement structures are also used.

2.Detention Dams These are constructed in the upper reaches of streams where excessive runoff is anticipated. They are not designed to normally hold water but are meant to retain the water during floods and release it at a controlled rate governed by the size of a bypass pipe under the dam.

3.Debris Dams These are extensively used to control gully erosion and are constructed from a number of different materials, from concrete sputniks to poles, tyres and netting. They are designed to collect debris and cause it to flatten out the gradient of the river, thus reducing velocity and

erosivity. They are usually built in a series down the gully and have associated plantings to stabilise the adjacent banks.

4. Drop StructuresThese are used to allow water to abruptly change level without eroding the stream bed. They are constructed of concrete or rock resistant to the erosive power of the water. Sloping drop structures have long been used in irrigation canals to change canal level and in streams concrete structures called squash courts are used for a similar purpose in times of flood.

5. Flumes Structures made of Armco, corrugated iron or wood that carry water across the lip at the head of an eroding gully. Like debris dams, these should also have associated plantings of trees to insure their integrity.

6. Gabions Gabions are wire baskets filled with rocks and used to stabilise stream banks. They can be used to control stream direction as well as prevent streambank erosion.

7. Riprap A layer of rock placed along a stream bank to prevent erosion. It may also be used along the margins of a lake to prevent erosion from wave action.

8. Groynes Linear structures placed at strategic locations along stream or river banks to deflect the current away from critical areas to prevent their erosion.

9. Gravel Reserves Areas planted in trees through which a stream carrying a high sediment load discharges. The vegetation lowers the water velocity and the gravel load is dropped out in the reserve. Periodically the stream channel may be artificially redirected to evenly spread the gravel accumulation.

Examples of the various conservation techniques are on the STREAM site under extra resources for this module.

FOR EXTRA INFORMION ON THIS SEE THE SUPPLEMENTARY READINGS

1. Read the article reprinted from "STREAMLAND 62". This is a summary of some of the early work by Mr Noel Trustrum and his

group. Now part of the CRI Landcare Research NZ Ltd, the Group was then part of Soil Conservation Centre, Ministry ofWorks and Development.

Points to look for:

(i) Note the case of historical photographs to date landslide events (Fig 7).(ii) The relatively recent clearances of some of the land, felled by

subsidies.(iii) The nature of this erosion; soil slip that removes almost the entire

regolith.(iv) The slow development of a new soil mantle on the landslide scar.(v) The consequences of erosion for management.

2. Read this article reprinted from "Geomorphology" by Trustrum and De Rose. Concentrate on pages 154-155

Points to look for:

(i) The remnant forest soil has probably been developing for 14,000 years, since climate ameliorated at the end of last glaciation, and forest colonised the site. Prior to this, in the last glacial maximum, erosion would have been severe and soils thin.

(ii) Note how long it takes to form even 23 cm of new soil on the eroded site (82 years).

(iii) Think about the consequences of this for water storage and root exploration.

3. Read the paper by Blaschke et al.

Points to look for:

(i) Note the productivity decline data in Table 1 (p 534).(ii) Note the scenarios presented in Table 3 (p 538).

REFERENCES FOR SUPPLEMENTARY READINGS

Streamland 62 (1987): Farming the Hills - Mining or Sustaining the

Resource? National Water and Soil Conservation Authority, Wellington, New Zealand.

Trustrum, N. A. and De Rose, R. C. (1988): Soil Depth - AgeRelationship of Landslides on Deforested Hilislopes, Taranaki, New Zealand. Geomorphology 1: 143160.

Blaschke, P. W Gane, S. W., Gloyn, G., Hopkirk, D. L., Malcolm, J.,

Trustrum, N. A. and Van Der Weteringh, R. (1992): Implementing More Sustainable Land Use in New Zealand Steepland - A Case Study. In: The Proceedings of the 7th ISCO Conference Sydney People Protecting Their Land, Sydney, Australia, 27-30 September 1992 pp 533-540.

J. M. Soons and M. J. Selby (1992). Landforms of New Zealand(second edition) p 359.

To assist with your understanding answer the following review questions

These questions are not assessed. (Find answers to the following questions from the study guide and readings).

1. Which land use resulted in the least soil loss during Cyclone Bola and why?

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2. How was the chronology of landslides established at Makahu, Taranaki?

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3a. If the top 15 cm of a remnant forest soil (see the diagram in Trustrum & De Rose for the correct soil depth to use), contains 15% by volume plant available water, and the remainder of the profile 10% by volume plant available water, how many mm of water is available to plants?

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3b. How much water is available to plants in the soil on the site that eroded 44 years ago? Assume a plant available water content of 10%.

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3c. If the plant available moisture storage of the soils was filled, how many days in summer (evapotranspiration 4 mm/day) would grass continue to grow at each site?

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3d. If the total water the soil can store is estimated by adding 5% to each of the above figures (in a & b i.e., 20% by volume water in topsoil and 15% by volume in the subsoil), how much rain is needed to saturate each soil and begin runoff?

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4. If a slope at the Taranaki study site suffered 5% erosion every 10 years, how much would pasture production decline in 100 years (some mathematical skill needed here)?

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5. Express in your own opinion the environmental issue and sustainability of farming on hill and steepland of various slopes.

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