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7 Summary and Conclusions

  1. This review, commissioned by the Ministry for the Environment is aimed at:

  • determining whether, with existing information and analysis, it is possible to make credible, ‘order of magnitude’, quantitative estimates of any flooding and erosion reduction benefits (or costs) associated with measures to sequester carbon dioxide or reduce emissions of greenhouse gases in New Zealand

  • providing the initial estimates of the benefits (or costs), or where information is not available, to provide advice on an alternative methodology for measuring such benefits.
  1. This report present detailed information about estimates of benefits and costs, in three sections:

  • physical effects of afforestation and reversion on flood reduction

  • physical effects of afforestation and reversion on sediment yield

  • physical effects of afforestation and reversion on erosion reduction.
  1. The issue of what part of a catchment is being afforested or reverted is crucial to understanding the above responses to land use change. Vegetation cover is just one among many factors affecting response. We consider the likely consequences of increased afforestation under three scenarios of different intensity:

  • incremental afforestation: diffuse afforestation by individual owners on their own properties that continue to also support farming

  • wide-scale afforestation: whole properties changed in land use and afforested, affecting a large proportion of catchment or sub-catchment

  • whole-catchment afforestation: Crown intervenes to afforest or promote reversion of whole catchment or sub-catchment.
  1. We then discuss studies that estimate the monetary values of these effects, and comment on the types of studies that could be undertaken to provide more reliable estimates; and some of the methodological and other issues in extending the work on quantification of benefits and cost of climatic change mitigation measures in New Zealand.
  2. There are many sources of information relevant to flooding, sediment yield and erosion occurrence under different land uses in New Zealand, but most are of low direct value for the purposes of this review. In particular, flood frequency / magnitude studies are either short-term, or undertaken in catchments where land use is mixed; this makes regional extrapolation and interpretation very difficult.
  3. The reported studies for sediment yields, flood flows and erosion under different land uses have high variability, even within similar environments. Few studies provide margins of error for results. As a result it is very difficult to provide predictions of likely physical responses to land use changes, even before the monetary value of such changes is considered.

Effects of afforestation and reversion on floods, low flows and water yield

  1. Reductions in flood peak under forest compared to pasture of 30% to 90% have been measured during small (up to annual) floods, but are lower (20% to 50%) in large floods, and cannot be expected during extreme floods.
  2. Reduced flood peaks do not necessarily indicate reduced flood volumes. Afforestation results in somewhat broader, lower flood waves, that discharge much the same volume of floodwater downstream but over a longer period of time.
  3. Planted or regenerating forest reduces annual water yield (typically by 20% to 60%) compared with pasture. The amount of reduction depends on local rainfall and evapo-transpiration.
  4. Measured reductions in low flows under forest compared to pasture are in the range 0% to 50%. If these lower flows result in reduced water availability for downstream uses in dry seasons, then this is a cost imposed by afforestation, not a benefit.
  5. Large changes in flood peaks, water yields or low flows have been observed only in small catchments, where close to 100% of catchment area has been afforested or retained in native cover. The few published studies of partial afforestation in large catchments, all report much smaller changes in river flow.
  6. What happens when measurable flood reductions in small sub-catchments are added together in a large catchment, is complex, depending on many factors in the catchment topology. Vegetation change in only some parts of a large catchment does not cause a substantial drop in main-channel flood peak. Widespread afforestation would be needed across most of a large catchment area, for any measurable reduction in main-channel flood peak to occur.
  7. A marginal reduction in flood peak during a large or extreme flood, would not ensure that water level will not reach over a stopbank, nor that property damage or production loss will not be caused by other forms of flooding.
  8. Absolute flows or percentage reductions in flows, from small research catchments entirely under forest or scrub, should not be extrapolated to the entire area of medium or large catchments, where afforestation / reversion is patchy and occupies a small percentage of catchment area. Under a ‘diffuse afforestation’ scenario, reduction in runoff from small blocks of afforested or reverting land is overwhelmed by ‘normal’ runoff from much larger areas of surrounding farmland. Only under a ‘whole catchment afforestation’ scenario would it be valid to extrapolate flow reductions from small research catchments.
  9. It should therefore not be assumed that afforestation for carbon sequestration may reduce floods enough for damage reduction benefits in any catchment, whether small or large, so long as the pattern of afforestation / reversion remains diffuse.
  10. A flood reduction large enough for damage reduction benefits to accrue might be expected if a large percentage of a catchment is afforested or allowed to revert. However, a changed flow distribution curve does not avert many of the most common causes of flood damage, such as breach of stopbanks.
  11. We do not recommend any attempt to quantify nationwide carbon sequestration-related flood reductions. Should MfE wish to verify that flood reduction effects are indeed small in medium to large catchments, two hydrological models are currently available: Watyield or Topnet. We do not recommend the use of other models unless there is prior evidence of their calibration for use in the New Zealand landscape.

Summary of impact of afforestation on flooding under different scenarios

  1. Diffuse reversion / afforestation by individual owners on parts of their properties (on which farming use also continues), will create a fragmented pattern of tree cover on a relatively small percentage of catchment area. This scenario will not appreciably alter even small flood waves passing down the main channel of a catchment.
  2. Widespread reversion / afforestation by owners changing land use on entire properties will create contiguous blocks of tree cover on a significant percentage of catchment area. This scenario will alter the magnitude and duration of small and medium flood waves passing down the main channel of a catchment. However there will be little or no reduction in overbank flooding and associated damage, because these impacts become substantial only during large events.
  3. Whole catchment reversion / afforestation by a public agency intervening to change land use on all properties, will ensure tree cover on almost entire catchment area. This scenario will substantially alter the magnitude and duration of small to medium flood waves, but will only slightly alter large flood waves (>10-year frequency) passing down the main channel of a catchment. It will not reduce large flood waves enough to avoid overbank flooding and associated damage.

Monetary value of flood reduction

  1. Early estimates of flood reduction benefits and costs pre-date the 1980s economic reforms, and the differences in the economic environment between then and now make their use invalid. A few regional and national studies since 1987 provide a more useful framework for analysis of flood reduction values but their costs and benefits do not directly correspond to afforestation / reversion effects.
  2. Given that flood reduction effects attributable to afforestation or reversion are too small to avert large floods overtopping stopbanks, and unlikely to affect other large-flood thresholds, consequential flood damage reductions are unlikely. There seems little point in proceeding with an attempt to quantify the monetary value of any associated benefits.

Effects of afforestation and reversion on sediment yield

  1. Sediment yield investigations show that substantial reductions in annual sediment yield can be expected where small catchments are retained in bush or scrub, compared to being clearfelled or converted to pasture. The same can be expected where small catchments are planted in exotic forest on former pasture and tussock.
  2. Where terrain is highly erodible, sediment yields in watercourses remain high during initial afforestation with exotic species or initial reversion to scrub.
  3. The high yields persist for several decades due to ‘lag effect’, until sediment stored in and near the channels is transported downstream. Sediment yields gradually decline, to levels below those in watercourses on equivalent terrain still used for farming.
  4. These research findings are either from small catchments, or from medium-sized catchments where unstable headwaters have been substantially afforested. It would be erroneous to assume from them, that forestry or reversion will substantially reduce sediment yield in all situations.
  5. Other evidence about variations in sediment yield is available on a NIWA database, from longer-term measurements of sediment transport in medium to large rivers. These data indicate that high sediment yields are associated with either unstable geological terrains or high rainfall zones; and particularly with areas within catchments where the two coincide.
  6. On a national scale, sediment yield is controlled by geology and climate, more than by vegetation cover. The highest sediment yields come from catchments vegetated by what is generally perceived as ‘intact’ native forest and scrub, because catchments in the most erosion-prone climates and geologies are more likely to remain in native vegetation, than to be developed for commercial land use.
  7. Afforested or reverting catchments may still have high residual sediment yields. Nevertheless the NIWA dataset confirms that, where an area within a catchment has the same erosion-prone geology, sediment yield is generally lower from parts that are forested or scrub-covered, than from parts in pasture or tussock.
  8. Necessary data for nationwide land identification are already to hand, stored in several agencies’ geographic information systems. With this data it would be possible to overlay cadastral information for land parcels, aggregated by ownership type.
  9. We recommend that MfE proceed with at least one study to identify land in catchments nationwide, where sediment yield reduction could be a significant benefit of carbon-sequestration-related afforestation and reversion.
  10. This procedure would provide reasonably accurate figures for the percentage of each catchment presently occupied by privately owned land under woody vegetation (existing contributions to carbon sequestration), as well as the percentage occupied by farmland or tussock grassland (potentially available for carbon sequestration initiatives); both on terrains identified as having high sediment yields, and on terrains identified as not.

Summary of impact of afforestation on sediment yield under different scenarios

  1. Diffuse reversion / afforestation will only reduce a catchment’s sediment yield if located on geologically unstable land. The limited percentage of such land that would be likely to be converted to tree cover, precludes a large absolute yield reduction for an entire catchment.
  2. Widespread reversion / afforestation will only reduce a catchment’s sediment yield if located on geologically unstable land; but if targeted, could result in tree cover on most such land. Whether absolute yield reduction will be large or small, depends on whether the geologically unstable areas are in high-rainfall or low-rainfall zones of a catchment.
  3. Whole-catchment reversion / afforestation will reduce a catchment’s total sediment yield, because all geologically unstable land would be covered by trees. Absolute yield (tonnes per square kilometre per year) reduction will be large, provided some of the geologically unstable areas are in high-rainfall zones.

Monetary value of sediment reduction

  1. We are less optimistic about recommending an attempt to quantify the nationwide value of sediment yield reductions. Although past attempts to quantify these monetary values have produced a range of estimates, their authors generally acknowledge a high margin of error, due to scarcity of sound field-collected information about sedimentation’s impacts.
  2. Should MfE wish to proceed with a new attempt, we recommend that it be done in a single region, perhaps in just one large catchment within that region. It would be better to have reliable values from a single catchment, than unreliable ones from a study that attempts valuations over a wide area where supporting data do not exist. A key element in calculating reliable monetary values would be to identify a catchment where local government agencies already have good records of sedimentation, as well as historical costs of repair.

Effects of afforestation and reversion on terrestrial erosion

  1. Storm damage surveys since 1970 show that the area of soil eroded by storms is consistently less where forest is planted, scrub is allowed to revert, or bush is retained, than under pasture. Reductions are mostly in the range of 50% to 90%.
  2. Point samples for recent state of the environment reports provide regional estimates of eroded area under planted forest, natural forest and scrub. These are anything from 10% to 100% lower than the area eroded under agricultural land uses.
  3. Soil conservation effectiveness surveys since 1988 confirm the reduced levels of terrestrial erosion under planted forest, natural forest and scrub. They also show that erosion can be reduced by space-planted trees in grazed pasture; though not as much as by close afforestation.
  4. Investigations of afforestation and reversion have been carried out widely enough in the New Zealand landscape to allow confidence that the measured reductions are real and substantial.
  5. Because the reductions occur on-site and are proportionate to the extra area of soil protected from erosion, benefits to the individual landowner can become significant, additional to off-site and public good benefits.
  6. The nationwide extent of carbon sequestration-related reductions in terrestrial erosion is worth estimating, if MfE wishes to do so. The effects and associated benefits are not limited by either the fragmented nature of afforestation and reversion for carbon sequestration (unlike flood effects), or by location of the fragmented land parcels (unlike sediment yield).
  7. As with sediment yield, the identification of land potentially available for carbon sequestration, and whether it is on erodible or stable terrain, are essential first steps to take before attempting any estimates.
  8. Once this is done, the most promising way to estimate nationwide reductions in terrestrial erosion, if specific land types and percentages of land were to be afforested / reverted, appears to us to be the New Zealand Erosion Estimation Model (NZEEM).

Summary of impact of afforestation on erosion under different scenarios

  1. Diffuse reversion / afforestation will only reduce terrestrial erosion if located on erosion-prone land. This scenario would ensure tree cover on some but not all such land in a catchment. Nevertheless it could have large benefits (saved soil, land retained in production, reduced land management costs), because the benefits occur on-site, in direct proportion to area of erodible land covered.
  2. Widespread reversion / afforestation will also only reduce terrestrial erosion if located on erosion-prone land; but if targeted, would ensure tree cover on most such land in a catchment. Whether the benefits will be small or large, depends on how much of the catchment is erodible.
  3. Whole-catchment afforestation and reversion will reduce terrestrial erosion, because it would ensure tree cover on all erosion-prone land in a catchment. Its benefits will be large, provided a substantial part of the catchment is erodible. However this scenario also entails tree cover on other parts where land is not subject to erosion (and where no benefits from erosion control could be expected). For areas already in forest, further effects on the incidence of erosion would require major changes in the condition of canopy and understorey vegetation, through sustained control of the animal pests that affect quantity or quality of vegetation.

Monetary value of terrestrial erosion reduction

  1. Attaching monetary values to reductions in terrestrial erosion is a very difficult proposition. Some data from North Island hill-country regions are available, but there are not enough such studies for the data to be nationally extrapolated.
  2. Recent regional and national estimates of soil erosion costs and soil conservation benefits provide a useful framework for analysis, but costs given are imprecise and not particularly useful for estimating benefits and costs specifically associated with afforestation / reversion.
  3. As with sediment yield, it may be better to rely on a single indicative well-done catchment study, rather than to commission regional or nationwide estimates when satisfactory data to support them do not exist. Potentially usable studies include benefit-cost analyses carried out for Environment Waikato’s Project Watershed and Lake Taupo retirement schemes, and an economic analysis of afforestation and poplar planting compared with farming on the East Coast of the North Island.

Targeting benefits of afforestation and reversion for sediment yield and erosion reduction

  1. The greatest benefits in terms of sediment yield and erosion reduction will be obtained from land with the following characteristics:

  • erodible lowlands and hills (but not mountains or catchment headwaters with naturally very high erosion rates)

  • areas where there are frequent high-intensity rainfalls

  • areas in grass or tussock cover prior to afforestation

  • areas of planted forest or scrub reversion or bush regeneration established more than 5–10 years (threshold date varies)

  • areas of spaced tree planting on erodible hill country that is still farmed (applies only to reduction of terrestrial erosion effects)

  • areas upstream of urban or rural infrastructure or intensive agriculture on lowland floodplains or other flood-prone landforms (applies only to reduction of sediment yield effects).
  1. Afforestation and reversion targeted in this way will provide a full spectrum of ecosystem services, including greenhouse gas reduction, biodiversity protection, and soil and nutrient retention; as well as the erosion and sediment yield reduction functions described in this report.

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