R. C. Derose’s research while affiliated with Manaaki Whenua - Landcare Research and other places

This paper describes the use of Digital Elevation Models (DEMs), constructed from sequential aerial photographs, as a tool for measuring gully erosion in a geomorphologically unstable environment. The technique is applied to a case study that examines erosion in 26 gullies in two study areas in the upper Waipaoa catchment, eastern North Island, New Zealand. Changes over two consecutive time periods, ranging in length from 14.0 years to 33.2 years, were studied at each site, drawing on available historical aerial photography. Several key aspects of the method used are described and discussed in detail, and recommendations are made for future application of DEMs for assessment of landscape change. DEM-measured gully degradation rates are directly proportional to the square root of the gully area. From this relationship it should be possible to rapidly estimate gully erosion at a catchment scale on the basis of gully areas alone. DEM-based measurement techniques, together with appropriate consideration for the sensitivity of the method, have significant cost and efficiency advantages over manual approaches to erosion measurements.

Pastoral land use in New Zealand's North Island hill terrain has led to high rates of rainstorm-induced landslide erosion higher than existed under the indigenous forest regime, with consequent soil productivity declines in the long term. To assist extrapolation of research results to other areas, and to shed light on long-term erosion risks, a simple model was developed that simulates the evolution of hillslope soil productivity, taking into account the effect of slope, rainstorm magnitude–frequency relations and soil recovery rates. Risks are evaluated by Monte Carlo simulation, and reflect parameter uncertainty as well as the natural randomness associated with climatic events. A sensitivity analysis showed that landslide risk was most affected by the rainfall threshold for landsliding, the mean of the extreme value distribution for annual maximum storm rainfall, and the maximum degree of recovery of pasture productivity following landsliding. Simulations suggest productivity stabilizes at a reduced level well before all steep terrain is affected by landsliding, and that subsequent expected landslide-induced productivity declines are too slow to provide sufficient economic motivation for measures to prevent landslide damage. A refined model showed that long-term average rates of productivity decline are sensitive to changes in recovery rates resulting from progressive removal of the soil resource. Charts summarizing simulation results can be used to estimate long-term productivity declines. Copyright © 1999 John Wiley & Sons, Ltd.

The methodology and errors involved in determining the amount of sediment produced during two (19·5 and 33·2 year) periods by 11 (c. 0·01 − >0·20 km2) gullies within a 4 km2 area in the headwaters of the Waipaoa River basin, New Zealand, using sequential digital elevation models are described. Sediment production from all gullies within the study area was 0·99 ± 0·03 × 106 t a−1 (2480 ± 80 t ha−1 a−1) during the period from 1939 to 1958. It declined to 0·62 ± 0·02 × 106 t a−1 (1550 ± 50 t ha−1 a−1) during the period from 1958 to 1992, when many of the smaller gullies were stabilized by a programme of afforestation, which commenced in 1960. Both figures are very high by global standards. The two largest (the Tarndale and Mangatu) gully complexes together generated 73 and 95 per cent of the sediment in the specified time periods, but the latter amount is equivalent to only c. 5 per cent of the total annual sediment load of the Waipaoa River. © 1998 John Wiley & Sons, Ltd.

Research is being conducted into the inter-relationships between slope morphology, soil properties, and the incidence of shallow landslides in a steepland landscape in the North Island of New Zealand. Eastern Taranaki hill country is characterised by a dendritic drainage pattern which has formed in consolidated Tertiary sandstone over a period of more than 600,000 years. Hillslope evolution is by the gradual colluvial infilling and periodic evacuation of 0-order basins. Residence times of soil and regolith decrease from greater than 26,000 years on gentle hillslopes to less than 1000 years on the steepest hillslopes. On slopes less than 31°, airfall deposits of andesitic and rhyolitic tephra within soil profiles indicate stability throughout the Holocene. On steeper hillslopes recurrent landsliding has generally removed these tephra and soils have developed in weathered bedrock or colluvial slope deposits. Empirical relationships between the area of landslide scars, regolith depth, and slope angle provide a way for determining hillslopes sensitive to deforestation. Long term erosion rates on forested hillslopes were calculated from soil residence times and average regolith depths, by assuming an equilibrium between sediment production on hillslopes and sediment yield. Erosion rates increase from 0.1 mm yr-1 on modal slopes of 28-32°, to 0.5-1.0 mm yr-1 on steeper hillslopes. Landslide displacement volumes were used to calculate erosion rates over a 50 year period for hillslopes under pasture. Erosion rates increase from about 1.4 mm yr-1 on modal slopes, to 2.3 mm yr-1 on the steepest hillslopes of 40°, and are greater when compared to forested hillslopes because of increased landslide occurrence. Hillslopes above 28° are considered sensitive to deforestation. Application of regolith depth and landslide density relationships to maps of slope angle derived from a high resolution digital elevation model (DEM), demonstrates a technique for extrapolating and mapping soil properties, erosion hazard, and areas sensitive to land use change over larger areas of hill country.

Herbage accumulation, botanical composition, and selected soil properties were measured on hillslope pastures at three localities within eastern Taranaki hill country over 4 years, beginning in 1984. Measurement sites were either on uneroded soils representing top, middle, and bottom slope positions, or on landslide scars with ages ranging from 12 to 80 years. On uneroded sites, net annual herbage accumulation decreased with increasing slope angle from the bottom to the top of hillslopes. Net herbage accumulation was lower on landslide scars when compared with uneroded sites of similar slope. This was attributed to the presence of bare ground, lower soil water‐holding capacities, and lower ryegrass content when compared with uneroded sites. Results confirmed previous findings from Wairarapa and Wairoa hill country, but showed that pasture recovery on landslide scars in Taranaki hill country was slower. Pasture recovery on landslide scars was greatest during the first 40 years after slipping, followed by a more gradual increase. Annual herbage accumulation on 12‐ and 40‐year‐old scars, was 24 and 74%, respectively, of uneroded levels. Further recovery was related to soil moisture status. On scars where soil moisture conditions did not limit pasture growth, net herbage accumulation recovered to levels of uneroded soils after 80 years. In comparison, where soil moisture conditions limited pasture growth during late summer and autumn months, herbage accumulation was similar to 40‐year‐old scars. Results indicate that landslide erosion causes permanent reductions in mean herbage accumulation on hillslopes. These reductions increase from about 1 to 3% per decade with increasing slope angle from 28 to 42°, mainly because of increased landslide densities. Model simulations suggest that the rate of reduction will decrease over longer periods, corresponding to fewer fresh landslides being produced on hillsides.

An algorithm for automating the mapping of land components from digital elevation data is described. Land components are areas of relatively uniform slope and aspect and often correspond with ridge crests, shoulders, head slopes, back slopes or foot slopes. Aspect regions, which generally span from stream to ridge, are first identified by generalizing an aspect map derived from digital elevation data. The aspect regions are then split successively into land components by grouping pixels above or below an automatically determined contour of elevation or ‘distance from stream’. The contour approximates a slope break. The land components mapped in this way give a complete polygonization of a hilly landscape and are a reasonable approximation of manually mapped land components.

The sustainable management of erodible pastoral hill country is a major focus of land use research in New Zealand. A multi-disciplinary study, using a high resolution lake sedimentation record, is being conducted to determine the role that cyclonic storms and natural and human-induced vegetation changes play in the erosion history of a landslide-prone hill country watershed. Sediment cores from Lakes Tutira and Waikopiro in northern Hawke's Bay were analysed to construct the magnitude-frequency history of storm-induced erosion since European settlement. Pulses of sediment representing individual storms can be clearly identified and are correlated to a storm history derived from analysis of a 93 year daily rainfall record. Correlation and dating are confirmed by pollen and diatom analysis,137Cs distribution, tephrochronology and reference to a well documented land use history. Annually laminated, organic rich deposits, which occur in the uppermost sediments and represent the annual decomposition of biogenic material associated with eutrophication, are also used to confirm the chronology. A high correlation was found between storm sediment thickness and total storm rainfall (R2=0.8). Although sediment producing storms (>150 mm) occur on a near annual basis, the two largest storms (>600 mm) contributed 54% of the total sediment thickness. The presence of well defined ‘storm sediment pulses’ has enabled the lake storage component of a sediment budget to be calculated for Cyclone Bola (1988), the most recent and largest rainstorm on record. The integration of this budget with the storm-magnitude-frequency history will be used to develop watershed-based models to predict the impacts of land use changes and the erosion response to climate scenarios.

Soil erosion on steepland hillslopes in Taranaki, New Zealand, where landsliding is the dominant erosion form, was investigated by comparing mean regolith depths between first‐order basins that have had their forest cover removed for different periods of time. Regolith depth and slope angle data were collected along 19 profile lines and 30 profile lines from steepland basins that had been deforested for 10 and 85 years, respectively. These profile lines were subdivided into a total of 236 profile segments of relatively linear slope angle and uniform regolith depth, that averaged 17·5 m in length. The depth of pre‐existing regolith on post‐deforestation landslide sites is estimated from a regression of regolith depth on slope angle for undisturbed (non‐landslide) profile segments. Regolith depletion on landslide sites is in turn estimated by subtracting the depth of regolith on landslide sites from the estimate of pre‐existing regolith depth. Regolith depletion by post‐deforestation landslides, averaged over the entire length of profile lines, gives an estimate of average surface lowering. For the area deforested for 85 years, average surface lowering by post‐deforestation landslides is 0·15 ± 0·04 m, and is the same as the difference in mean depth of 0·15 ± 0·11 m between this area and the area deforested for 10 years. Erosion of regolith from hillslopes by processes other than landsliding appears to be minimal. The 0·15 m average surface lowering represents a regolith depletion rate of 1·8 ± 0±5 mm yr ⁻¹ . For hillslopes steeper than 28°, where all post‐deforestation landslides occur, average surface lowering is 0·20 ± 0·05 m, and the regolith depletion rate is 2±4 · 0±6 mm yr ⁻¹ . Average surface lowering is greatest at 0·23 ± 0·07 m on hillslopes steeper than 32° where most post‐deforestation landslides occur. Here, the regolith depletion rate is 2·7 ± 0·8 mm yr ⁻¹ . A large‐magnitude, low‐frequency storm in March 1990, produced an average surface lowering of 0·041 m. There were proportionately more landslides in the area deforested for 10 years, illustrating the importance of previous erosion history of hillslopes on the spatial distribution of landslides. There were also comparatively few landslides on steeper hillslopes because previous lower magnitude storms had already removed much of the deeper regolith.

Lowland steeplands cover more than 40% of New Zealand's area. Occurring in a tectonically active environment and subject to recurrent erosion-inducing storms, they were largely forest-covered until about 130 years ago, when they were rapidly deforested and converted to pastoral land use. We describe the geographical and historical background to this deforestation and some of the profound ecosystem process changes that have accompanied it, particularly those related to mass movement erosion. We then relate research findings on the ecosystem changes to the sustainability of current land use.Our own research in three contrasting regions of the North Island, New Zealand has examined disturbance regimes, vegetation succession, soil properties and soil erosion rates on pastoral hillslopes and in forest ecosystems. Under both forest and pasture, almost all ecosystem processes were highly dependent on topographical unit and slope steepness. A comparison of ecological process and erosion rates in pasture and forest, and analysis of pastoral production data, indicated that although significant erosion occurred under forest and was one of its principal regeneration mechanisms, rapid net soil depletion was occurring on steep hillslopes under pasture and that pastoral use was not sustainable on these areas.A more sustainable pattern of land use can be achieved by intensification of pastoral use on gentler hillslopes and better spatial integration of pastoral use with timber plantation, agroforestry, and conservation land uses. Strategies for the implementation of more sustainable land management are also discussed. Establishment of critical parameters for the development of land management guidelines has been dependent on a good prior understanding of landscape ecological processes.

Many physical processes operating in the landscape are locally dependent on landscape geometry. The decription of surface geometry contained in a digital terrain model (DTM) offers the opportunity to objectively evaluate these processes using computer technology. Examples of DTM application, shown in this paper, demonstrate the high potential that DTMS have in terrain evaluation. As the cost of DTM capture continues to decline, it is likely that in the near future, DTMs will become more routinely used.