Soil conservation is the management of land to maintain New Zealand's soil and water resources, to provide the widest range of sustainable benefits for the needs and aspirations of present and future generations. Soil conservation includes:
- maintenance of the productive potential of the nation's soil resources to retain sustainable land use options for present and future generations
- maintenance of catchments to provide high quality water resources for downstream users
- land management practices that further enhance the protection of waterways from suspended sediments, nutrients, harmful micro-organisms and other pollutants
- reduction of the effects of land-related hazards including flooding, subsidence and erosion
- maintenance of the aesthetic, scientific and cultural values of land and water.
Erosion is a natural phenomenon, which results in soil losses and water quality degradation. New Zealand is geologically young and active, and the natural level of erosion is high by international standards. Changes to the vegetative cover of the land from activities such as farming, introduction of pests, burning, forestry, road construction and urban development reduce protection against erosive forces and lead to accelerated erosion. Once erosion has occurred, the productivity of the soil rarely returns to its former level.
Some land types are more susceptible to accelerated erosion than others. These generally include hill country and the banks of rivers and lakes. In addition, some soils such as pumice soils are more prone to erosion than others.
In the Waikato Region, accelerated erosion occurs in different forms, depending on the locality. Erosion prone or ‘high risk erosion areas’ in the Waikato can be grouped as follows:
- The Central Volcanic Area - Includes the pumice land around Taupo. The soils are extremely fragile and prone to severe gully, sheet, rill, stream bank and wind erosion.
- The Western and Central Hill Country - Includes the hill country extending along the Waikato’s west coast and becoming more dissected in the north. Overlaid with volcanic ash, these hills are prone to sheet erosion, particularly where heavy stock are grazed on steep land.
- Stream banks - Including streams across the entire Region affected by erosion. Stream banks are subject to the erosive force of water, which can be exacerbated by land use activities that damage or disturb the banks and beds of rivers and lakes.
Water yields, and consequently, sediment yields from surface and stream bank erosion, have increased in catchments which have been largely cleared of forest. Land use also has a major effect on the sediment loads in rivers. Erosion prone areas yield higher levels of suspended sediment under agricultural use than under forest.
Accelerated erosion is generally caused by activities that disturb or expose the soil to the erosive forces of gravity and rain/water. Climatic or weather conditions combined with human activity can increase soil erosion. For example, severe and intense storms may increase the rate of accelerated erosion.
Different land uses have different effects. For example, forestry operations may have positive effects on land and water while trees are growing, followed by a period of adverse effects associated with land disturbance from harvesting. In contrast, erosion from pastoral activities may occur at reasonably constant rates for long periods, as there may be minimal protection against erosion.
Data from a wide variety of sources, including forestry industry research, shows the main land uses contributing to accelerated erosion are agriculture, earthworks, roading and tracking activities, establishment and harvesting of forests and mining. In catchments undergoing land use changes, sediment yields increase as vegetation cover is reduced, exposing the soil surface.
The adoption of unsuitable land uses and land management practices, are major contributors to erosion problems. For example, soil disturbance on steep slopes may lead to a high risk of accelerated erosion. Overgrazing by stock and uncontrolled browsing by pests may damage vegetation to the extent that it no longer adequately protects the soil from erosion.
Accelerated erosion may result in:
- loss of soil productivity, versatility or capability
- degradation of water quality and aquatic ecosystems
- increased adverse effects of flooding and instability hazards
- infilling of lakes, estuaries, artificial watercourses, rivers, wetlands and caves
- effects on the ecological values associated with land
- adverse effects on tangata whenua’s relationship with land and soil.
The soil conservation component of Project Watershed includes both existing and proposed soil conservation works. For existing works the level of service is detailed in each Asset Management Plan and has already been agreed with the communities, concerned following extensive consultation.
Under Project Watershed, Environment Waikato seeks to address the most severe erosion in the greater Waikato catchment. To predict the new works likely to be required, two categories of soil conservation works have been defined and included in this document. These are Hill Country/Upper Catchment works, and Riparian works. For Hill Country/Upper Catchment works, an assessment of Land Use Capability maps was carried out and an assumption made that 10 - 20 percent of class 6e, 7e and 8e pastoral land mapped as severely erodable would be treated. The exact proportion varies by management zone.
Estimates for Riparian Soil Conservation Works requirements are based on field observations made in each zone by Environment Waikato staff. An assessment was made of the percentage of stream banks actually eroding. The field survey indicated that across the greater Waikato catchment, between two percent and 13 percent of the streams surveyed in pastoral land were classified as eroding. For practical and farm management purposes, protection of a length of stream in excess of that actively eroding is generally required. In most cases, this has been assessed at an equivalent length to that mapped as eroding.
Environment Waikato’s soil conservation programme must be a balance between what is required for the maintenance and betterment of the greater Waikato catchment and what the affected landowner will support. Environment Waikato has consulted with the community, including affected landowners, via the liaison subcommittees in order to determine the appropriate balance. This document includes the proposed new work that has been agreed to through that consultation.
For the existing major schemes (largely Taupo based), the present annual cost comprises some or all of the following:
- Transitional maintenance work.
- Routine maintenance costs, including direct and indirect expenditure.
Transitional maintenance work is urgent work carried out in accordance with Land Improvement Agreements with landowners. This involves a separate grant to apply for a limited period only. This work is prioritised in order to bring existing schemes up to the required standard. Direct and indirect maintenance costs are the costs of maintaining the assets following the transitional period.
The types of additional works proposed within the soil conservation component of Project Watershed include fencing of stock from erodable land, planting of suitable vegetation to achieve soil stability, retirement of indigenous areas, provision of alternative water supplies where appropriate, and the installation of erosion control structures.
Council has developed a complementary but distinct riparian strategy to address non- point-source discharge pollution via surface runoff and ground water. Further discussion of the relationship between Project Watershed and the riparian strategy can be found in Appendix 1 of this document.