Control of Soil Erosion on Farmland

A summary of erosion’s impact on New Zealand agriculture, and farm management practices which counteract it

A report prepared for MAF Policy by D L Hicks

August 1995

ISBN: O-476-07403-4 ISSN: 1171-4662

MAF Policy Technical Paper 95/4





MAF’s Sustainable Agriculture Facilitation Programme aims to encourage the adoption of sustainable resource management practices so that the agricultural and horticultural sectors make a long term contribution to New Zealand’s economic, social and environmental well-being. As part of the programme, MAF is producing an information series under the banner "Towards Sustainable Agriculture", This report on the control of soil erosion on farmland is an addition to this series.

New Zealand has a long history of soil conservation and river control, starting with the River Boards of 1880 and the Land Drainage Boards of 1893. Although these Boards were consolidated in 1908, concerns continued to rise over the next 10 years about the deterioration of river channels, flooding, erosion damage alongside rivers, and inadequate drainage. In the 1930s flooding and erosion become so severe that following investigations, the Government introduced the Soil Conservation and Rivers Control Act 1941 to deal with the problem, and established Catchment Authorities.

For nearly 50 years since then, our use of the land has been influenced through regulation and subsidies for various conservation and protection practices. Times have, however, changed, and it is now over to land holders themselves to think carefully about how they can manage the erosion processes occurring on the land.

The legislative framework introduced under the Resource Management Act of 1991 encourages people to safeguard land for future generations.

"Control of Soil Erosion on Farmland" summarises the wealth of information that has been gathered over the years on managing soil erosion. It provides indicative valuations of the benefits and good practices that minimise the detrimental effects of land use. Many landholders are already using the sustainable practices presented here because they know that in the long run costs will be saved and the land resource maintained. Others have yet to do so. This publication is aimed at encouraging them to take greater responsibility for managing soil erosion on their property.

Control of soil erosion is one aspect of an integrated package to improve the use of New Zealand’s soi1 and water resources. I urge you to study this report and to become active in local initiatives to implement erosion control practices to enhance our collective well-being into the future.


"Control of Soil Erosion on Farmland" has been written as a contribution to MAF’s publications on Sustainable Agriculture. It is intended as a source of information for use by farmers, local government, farmer organisations, farm advisers and others who have an influence on rural land use. A copy has been sent to all secondary school, polytechnic and agricultural college libraries.

The publication summarises what is currently known about the extent of soil erosion on New Zealand’s farmland, how various agricultural practices can either exacerbate or mitigate erosion, and what effects they have on production. In the past, this information has only been available in the technical literature in a wide number of publications. It is hoped that this short, plain English summary of the facts and figures will help make the information more accessible to those concerned with the management of land resources.

Erosion is a natural process which occurs on all land. It becomes a problem for people only when it disrupts their activities. On farms, this happens in a number of ways: damage to improvements during storms, immediate loss of crop or livestock, and reduction in the soil’s productive capacity leading to depressed yields in the longer term. Erosion of farmland also causes disruptions elsewhere, notably by contributing sediment to waterways which reduces their capacity to pass floodwater. Erosion also lowers the quality of water available to downstream users. The good news is that simple changes in farm management can avoid most of the disruption. Arable land can be cultivated in ways that minimise damage to soil structure. Pasture can be grazed in ways which reduces depletion of ground cover in droughts and in cold or wet weather. Unstable hill country can be protected against slips and gullies by reinforcing the soil with tree roots at weak points. Siltation and pollution of waterways can be reduced by managing stock access to stream banks.

Particular emphasis in this report is given to those practices which have been used by many farmers for years. They may cost a little extra. mainly in time rather than money to implement, but their cost is outweighed by savings later on. It is hoped that by summarising what these farmers are already doing, and by explaining what some of the benefits are, this publication will stimulate others to adopt the same or similar practices.

There are good reasons why it is worthwhile for land managers to adopt practices to control soil erosion. Maintaining land production is one. Another is averting damage to farm improvements. A third is reducing downstream damage from siltation and pollution, therefore reducing the reasons for local authorities to impose planning restrictions on the use of land. And finally, but by no means least, is the fact that export markets are becoming increasingly concerned about the impact on the environment of the ways food and fibre products are produced. It is now important for New Zealand’s farmers not only to demonstrate that our produce is disease-free and uncontaminated by residues. but also to show that it comes from land that is being used in a way that does not cause environmental damage to waterways, wildlife or to the land itself.


Sustainable land use. Soil conservation, Erosion control. Effects on production. On-farm benefits. Off-farm impacts.


The publication has been compiled by Doug Hicks of Ecological Research Associates. Dr Hicks formerly worked for Ministry of Works and Development’s Water and Soil Division and DSIR Land Resources, and now advises a number of regional councils about soil conservation research. Don Bagnall of MAF Policy supervised the report’s preparation. A draft version of the publication was reviewed by Bill Garland (farmer and branch chairman, Federated Farmers Waikato), Don Ross (scientist, Landcare Research Alexandra), and Don Miller (soil conservator, Gisborne). Thanks are due to all four for their helpful comments and suggestions. A second draft was widely circulated amongst regional councils, Agriculture New Zealand, Ministry for the Environment and MAF Policy. Staff of these organisations also had many useful ideas about how it could be modified so that it reflects regional variations in the farm management practices which are used to control erosion. Their contributions are gratefully acknowledged.

Artwork is by Christine Tate and the report was formatted by Karen Thompson.

The Nature of Erosion

Erosion denotes all the processes which strip soil and weathered rock from the earth’s surface. making the fragments available for transport by wind. water or ice until they are deposited as fresh sediment somewhere else. New Zealand is a geologically unstable country with a variable climate. These factors make our landscape susceptible to high natural rates of erosion, particularly where :

In some respects, erosion is beneficial. Many of our best soils, for instance on alluvial river terraces or loess covered downlands, arc deposits of eroded material. People start to perceive erosion as a "problem" only if it disrupts their use of land. or if they are affected by the manner in which others use land. It can disrupt farming in a number of ways:

Some disruption by natural erosion has to be expected. given our geology and climate. Erosion went on for millions of years before man reached these shores. As Polynesian and European settlers cleared vegetation, cultivated soil, planted crops and grazed livestock, they sometimes speeded up its rate of occurrence, so the disruptions happened more often. Some people go so far as to claim that erosion has speeded up to a point where farming cannot be sustained on New Zealand’s erodible land for much longer. This is perhaps true for limited areas of particularly unstable terrain. Hut the reality is that roost erosion-prone land can be sustainably farmed by simple adaptations of agricultural management. Many farmers have done this for years. Others - sometimes farmers who have not been on a property long. or landowners without a farming background - do not realise the necessity for such adaptions until they see what a storm, a severe winter, or a drought can do to their land.

Types of Erosion and Their Extent

Clearly much erosion-suceptible land is unfarmed mountain. forest or coastline. The first column of Table I gives areas of erosion-susceptible land in agricultural use, for each region of New Zealand1. The percentage used for farming ranges from as low as 1% on the West Coast, where the erodible land is almost all forested or mountainous. to as high as 71% in Gisborne, where most of it is grassed hill country (second column, Table 1). In some regions erodible land accounts for most of the farmed area, for instance Canterbury’s plains, downs and high country: in others only a small proportion, like the limited areas farmed on steep hill country in Taranaki (third column of Table 1). The maps show where susceptible agricultural land is located nationwide.

The main erosion processes that affect farmed soils are:

Table 2: Percentage of Agricultural Land Susceptible to Different Erosion Processes
Region Windblow Sheetwash, Rilling Tunnel, Gully Earthflow Slump Soil Slip, Earth Slip
Northland 41 49 10 32
Auckland 18 18 2 32
Waikato 23 6 4 21
Bay of Plenty 29 8 <1 21
Gisborne 9 35 41 68
Hawkes Bay 30 13 17 45
Taranaki 6 1 5 24
Wanganui-Manawatu 25 10 20 39
Wellington 16 17 28 36
Nelson-Marlborough 77 9 <1 27
West Coast 6 <1 0 4
Canterbury 86 4 1 14
Otago 88 4 1 1
Southland 59 11 8 25

Erosion’s Impact on Farming

This has never been routinely surveyed nationwide. Fortunately enough data is available from site-specific field measurements3, to indicate the minimum and maximum impacts on farm production that can be expected (Table 3). If unchecked, surface erosion of topsoil (by windblow, sheetwash, rilling, tunnels and gullies) causes ongoing reductions of crop yield and pasture growth, amounting to at least 20% and in extreme cases more than 60%. Mass movements of subsoil (by slumps, earthflows, earthslips and soil slips) initially reduce pasture growth by 40 to 80%. After they have re-grassed, growth can completely recover, but more often remains depressed by 10 to 40%. Mass movement also damages fences, tracks and drains, and occasionally buildings. Gullies create long, narrow scars which do not usually cause much loss of crop or pasture (unless they become exceptionally deep and undermine adjacent slopes). However they disrupt farm improvements. stock movement and vehicle access just as badly as mass movement. Particularly on alluvial soils, streambank erosion can cause rather more loss of land than gullies, also disrupting farm improvements and access where these are close to weak points on banks. Table 4 summarises the lowest and highest farm repair costs (resulting from the cumulative impact of mass movement, gullying and streambank erosion) that have been recorded during several events in the 1980s-1990s4.

Table 3: Percentage Reductions in Crop amd Pasture Yield
Erosion Process Minimum Maximum
Surface erosion, cropland -18 -62
Surface erosion, pasture -40 -78
Surface erosion, tussock -28 -93
Deep mass movement, pasture (initial) -43 -77
Deep mass movement, pasture (regrassed) -7 -39
Shallow mass movement, pasture (initial) -65 -80
Shallow mass movement, pasture (regrassed) 0 -42

Just how severe any of these impacts become depends on the number of times land is eroded (frequency), and the proportion of land affected each time (magnitude). Incidents of mass movement and gully erosion occur at intervals ranging from about once every ten years in low-rainfall districts (less than 1000 mm.), to more than once a year where rainfall exceeds 3000 mm5.

District-wide, each event typically erodes at least 1%, but rarely more than 10% of the land6. Surface erosion occurs with annual or greater frequency. In low-rainfall districts where vegetation cover is often sparse, up to half the land can lose some of its topsoil each time, but in high-rainfall districts, which are usually well vegetated, topsoil loss is restricted to a tenth or less of the surface.

The frequency of streambank erosion depends on flood return periods, The small floods that recur annually are quite sufficient to cause minor bank collapse in any part of the country. Severe scour, along with extensive lengths of channel, is associated with floods. These occur several times a year in rivers with high-rainfall headwaters, e.g., on the West Coast. Elsewhere, they are typically spaced several years apart.

Table 4: Damage Repair Costs* Averaged Over Farm Area
Nature of repair Minimum $/ha Maximum $/ha
Fences 5.86 44.13
Tracks 1.35 25.62
Buildings 1.43 10.91
Pasture re-sowing 0.62 12.93
General clean-up 2.91 3.76
* Costs date 1988-1992

Source: compiled from data in reference 4


Why Planning Legislation Requires Farmers to Control Erosion

The Resource Management Act 1991 replaces the previous soil conservation legislation. The Act empowers local authorities to control the effects of land use as necessary to promote sustainable management, including minimising the harmful effects of soil erosion. It is largely implemented by regional councils through their regional policy statements and regional plans, and by district councils through their district plans.

Regional councils have a clear mandate for soil conservation under the Act, but considerable latitude as to how they use their powers. Auckland Regional Council has written specific policies into its draft policy statement. to protect productive soil from conversion to uses which would preclude agriculture. It has also prepared a land management plan which emphasises erosion and sediment control, restricting some of the agricultural practices which cause silting or pollution of waterways. Hawkes Bay Regional Council has erosion control policies in its policy statement, but presently has no land management plan. It expects farmers to comply with the Act voluntarily, and its land management staff offer them information and advice to help. Most regional councils are somewhere in between, with references to erosion control in their policy statements, and land management plans which are rule-based to varying degrees. For instance, Gisborne’s interim plan has rules requiring resource consents for vegetation clearance and tracking on erosion-prone hill country. Canterbury is preparing plans which have rules targeted at different kinds of land with different problems, from tussock high country (with low erosion rates hut prone to depletion of vegetation cover) to arable plains (prone to surface erosion. structural degradation of soil, and loss of nutrients into groundwater).

District councils control the effects of land use on the environment. This includes subdivision, natural hazards, removal of native vegetation. and aspects of landscape and amenity. Although they have no direct functions in relation to soil conservation, district councils may deal with aspects of erosion control when exercising their land use functions, For instance, the Waikato District Plan restricts subdivision of agricultural land on erosion-prone terraces formed from pumice alluvium. The Masterton District Plan (and a number of others) imposes rules on forest logging close to watercourses, to reduce sedimentation. The plan for Rodney District restricts clearance of hush and scrub on farmland, principally to conserve indigenous flora, but also because hush remnants are generally on steep, erosion-prone ground.

The Resource Management Act focuses not on regulating the activities themselves, but on controlling the effects of activities on the environment. The assumption in the Act is that those carrying out activities can use natural and physical resources as they see fit, hut they cannot go below certain standards without threatening their long term viability. So in principle. the Act allows people to do what they like on their own land so long as they do not harm the resource, neighbours or the environment. In practice many local authorities still write all sorts of restrictions on land use into policies and plans on the grounds that these are needed to prevent "adverse effects", whether actual or potential. There is still a tendency to prevent the effects by regulating the activities.

In many districts and regions, planners may eventually have to change their approach to one which is more in keeping with the Act’s intent, which is to allow any use of a resource as long as the use is sustainable and any adverse environmental effects are avoided, remedied or mitigated. Farmers can encourage them to do so, by demonstrating that agricultural management on their own properties is sufficiently sound to minimise any adverse effects of erosion. Doing this would remove one of the prime reasons for local authorities to impose restrictions on farmers’ use of land.

Now that the GATT round is leading to a reduction in restrictive trade practices such as quotas and tariffs on agricultural produce, countries seeking to protect their domestic producers from New Zealand’s exports may start to look around for other excuses to restrict access to their markets. Environmental criteria could come in very handy, and not just things like pesticide residues. A scenario whereby some export market discriminates against New Zealand farm produce because of claims that it comes from land where "rainforest" has been chopped down, "irreplaceable soil" is being eroded, and surrounding seas are being "polluted" is not far-fetched. One way New Zealand producers can avoid environmental barriers to trade is by being able to demonstrate that our farm management practices cause minimal environmental impact.

Susceptibility of Agricultural Land to Erosion

Susceptibility of Agriculture Land to Erosion - North Island Map





Compiled from the Landcare Research 1:3,000,000 Susceptibility to Erosion Map (Clough and Hicks 1992) and the MWD1:1,000,000 Vegetation Map (Newsome et al 1986) by D. Hicks (ecological Associates Auckland), July 1994









Susceptibility of Agricultural Land to Erosion

Susceptibility of Agricultural Land to Erosion - South Island

Compiled from the Landcare Research 1:3,000,000 Susceptibility to Erosion Map (Clough and Hicks 1992) and the MWD1:1,000,000 Vegetation Map (Newsome et al 1986) by D. Hicks (ecological Associates Auckland), July 1994










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