High Rates Of Soil Loss Research Articles

Assessing the effects of slope gradient and land use change on soil quality degradation through digital mapping of soil quality indices and soil loss rate

Slope gradient and land use change are known to influence soil quality and the assessment of soil quality is important in determining sustainable land-use and soil-management practices. In this study, soil quality indices (SQIs) were developed by quantifying several soil properties to discriminate the effects of slope gradient and land use change on soil quality in 480km2 of agricultural land in Kurdistan Province, Iran. Three soil quality indices (SQIs) were used. Each of the soil quality indices was calculated using two linear and non-linear scoring methods and two soil indicator selection approaches, a Total Data Set (TDS) and a Minimum Data Set (MDS). Nine soil quality indicators: pH, Electrical Conductivity (EC), Organic Carbon (OC), Cation Exchange Capacity (CEC), Total Naturalized Value (TNV), Soil Erodibility (K), Porosity (P), Mean Weight Diameter (MWD), and Bulk Density (BD) and soil loss rate were measured for 110 soil samples (0–30cm depth). Soil quality indices maps were developed using digital soil mapping methods. The >10% slope class had the highest soil loss rate and highest percentage of soils with very low quality (grade V) based on all SQIs. The results showed that soil quality was better estimated using the Weighted Additive Soil Quality Index (SQIw) (r2=0.78) compared to SQIa (the Additive Soil Quality Index) and SQIn (the Nemoro Soil Quality Index). The agreement values of all SQIs for the non-linear scoring method were higher than the linear scoring method. The mean values of all SQIs and the soil loss rate were higher and lower in rangeland than cropland, respectively, but they were not significantly different because of intensive grazing. Slopes with a large gradient and where land use was converted to agriculture were characterized by low values of SQIs, suggesting a recovery of soil quality through changing to sustainable practices and abandoning over grazing in these areas.

Estimation of soil erosion in a rain shadow river basin in the southern Western Ghats, India using RUSLE and transport limited sediment delivery function

Soil erosion and deposition in a tropical mountainous river basin, viz., Pambar River Basin (PRB), in a rain shadow region of the southern Western Ghats (India) were modelled using Revised Universal Soil Loss Equation (RUSLE) and transport limited sediment delivery (TLSD) function in GIS. Mean gross soil erosion in the basin is 11.70 t ha−1 yr−1, and is comparable with the results of previous soil erosion studies from the region. However, mean net soil erosion from the basin is 2.92 t ha−1 yr−1 only, which is roughly 25% of the gross soil erosion. Although natural vegetation belts show relatively higher gross- and net-soil erosion rates (mainly due to high LS and C factors), their sediment transport efficiency is remarkably less, compared to the land use/ land cover types with anthropogenic signatures (i.e., plantations and croplands). Despite the lesser amount of annual rainfall, the high rates of soil loss from the semi-arid areas of the basin might be the result of the poor protective vegetation cover as well as isolated high intensity rainfall events. The study highlights the significance of climate-specific plans for soil erosion management and conservation of the soil resources of the basins developed in rain shadow regions.

Runoff initiation, soil detachment and connectivity are enhanced as a consequence of vineyards plantations

Rainfall-induced soil erosion is a major threat, especially in agricultural soils. In the Mediterranean belt, vineyards are affected by high soil loss rates, leading to land degradation. Plantation of new vines is carried out after deep ploughing, use of heavy machinery, wheel traffic, and trampling. Those works result in soil physical properties changes and contribute to enhanced runoff rates and increased soil erosion rates. The objective of this paper is to assess the impact of the plantation of vineyards on soil hydrological and erosional response under low frequency – high magnitude rainfall events, the ones that under the Mediterranean climatic conditions trigger extreme soil erosion rates. We determined time to ponding, Tp; time to runoff, Tr; time to runoff outlet, Tro; runoff rate, and soil loss under simulated rainfall (55 mm h−1, 1 h) at plot scale (0.25 m2) to characterize the runoff initiation and sediment detachment. In recent vine plantations (<1 year since plantation; R) compared to old ones (>50 years; O). Slope gradient, rock fragment cover, soil surface roughness, bulk density, soil organic matter content, soil water content and plant cover were determined. Plantation of new vineyards largely impacted runoff rates and soil erosion risk at plot scale in the short term. Tp, Tr and Tro were much shorter in R plots. Tr-Tp and Tro-Tr periods were used as connectivity indexes of water flow, and decreased to 77.5 and 33.2% in R plots compared to O plots. Runoff coefficients increased significantly from O (42.94%) to R plots (71.92%) and soil losses were approximately one order of magnitude lower (1.8 and 12.6 Mg ha−1 h−1 for O and R plots respectively). Soil surface roughness and bulk density are two key factors that determine the increase in connectivity of flows and sediments in recently planted vineyards. Our results confirm that plantation of new vineyards strongly contributes to runoff initiation and sediment detachment, and those findings confirms that soil erosion control strategies should be applied immediately after or during the plantation of vines.

Estimating landscape susceptibility to soil erosion using a GIS-based approach in Northern Ethiopia

Soil erosion is a very critical form of land degradation resulting in the loss of soil nutrients and downstream sedimentation of water storages in the highlands of Ethiopia. As it is technically and financially impossible to conserve all landscapes affected by erosion, identification of priority areas of intervention is necessary. Spatially distributed erosion models can help map landscape susceptibility to erosion and identify high erosion risk areas. Integration of erosion models with geographic information systems (GIS) enables assessing evaluate the spatial variability of soil erosion and plan implementing conservation measures at landscape levels. In this study, the Revised Universal Soil Loss Equation adjusted for sediment delivery ratio was used in a GIS system to assess landscape sensitivity to erosion and identify hotspots. The approach was applied in three catchments with size being 10–20km2 and results were compared against quantitative and semi-quantitative data. The model estimated mean soil loss rates of about 45tha−1y−1 with an average variability of 30% between catchments. The estimated soil loss rate is above the tolerable limit of 10tha−1y−1. The model predicted high soil loss rates at steep slopes and shoulder positions as well as along gullies. The results of the study demonstrate that knowledge of spatial patterns of high soil loss risk areas can help deploy site-specific conservation measures.

Estimation of soil loss and identification of erosion risk zones in a forested region in Sarawak, Malaysia, Northern Borneo

Soil loss has been quantified and land area categorized for soil erosion vulnerability in a partially forested subwatershed of the Baram River basin (Sarawak, Malaysia) using Revised Universal Soil Loss Equation, which considers climatic and terrain variables. The quantification of soil loss was achieved by integrating the parameters related to rainfall (R), soils (K), terrain (LS) and land use practices (C). The resultant maps of soil erosion show soil losses ranging from 0 to 1190 t ha−1 year−1 with a mean of 28 t ha−1 year−1 in the 1029 km2 Sungai Patah subwatershed study area. The subwatershed was mapped using ArcGIS into five classes of soil erosion risk vulnerability. Among the five classes identified, very high and critically vulnerable zones show linear distribution in some areas which together constitute 13% of the total study area. High and medium erosion vulnerable zones cover 30 and 19%, respectively. Low erosion risk zones cover 36% of the total area. Mean soil loss assessed for each LULC (land use/land cover) class indicates that barren land with high slopes contributes comparatively high rates of soil loss (343 t ha−1 year−1). Field surveys in the study region have enabled identification of erosion hot spots, such as logging areas, shifting cultivation areas and road construction, which intensely modify the terrain, and explain the linearity of critical and severe erosion risk features. The output of the present study will help to frame appropriate management strategies to minimize erosion through implementation of alternative methods in logging activities and terrain management programs.

GIS-RUSLE Interphase Modelling of Soil Erosion Hazard and Estimation of Sediment Yield for River Nzoia Basin in Kenya

River Nzoia basin is predisposed to degradation attributed to poor anthropogenic land use practices, soil erosion and sedimentation. The objective of this study was to model soil erosion hazard and estimate sediment yield for river Nzoia basin. Database of the basin comprised of 90 m DEM, LandSat imagery, rainfall, and soil data. Simulated RUSLE model factors (R, K, LS, and C) were multiplied using the raster calculator in ArcGIS 10.1. This generated the soil erosion hazard map for river Nzoia basin with an average annual soil loss rate of 0.51 and a maximum of 8.84 Mton ha-1 yr-1. This translates into a mean annual soil loss of 6.579 × 105 Mtonyr-1. Sediment Delivery Ratio (SDR) of 0.121 revealed that 87.9% of the soil eroded by water in the basin is deposited before reaching the basin outlet. Average annual sediment yield estimated was 0.06 Mtonyr-1. Soil erosion modeling results showed that river Nzoia basin is experiencing varying erosion rates spatially. The interplay among the RUSLE factors strongly influence average annual soil loss rates. Areas experiencing high soil loss rates are closely linked to annual cropland, deforested and high elevation points. Low rates of soil loss are attributable to soil conservation practices and protected areas such as game parks. Thus, there is a close coupling between soil loss and land use category in river Nzoia basin. Sustainable land use practices should be embraced to support conservation programmes to mitigate soil erosion, prevent sedimentation and reduce sediment yield in the river channel.

Stages in the life of a magnetic grain: Sediment source discrimination, particle size effects and spatial variability in the South African Karoo

Mineral magnetic properties are commonly used to trace the sources of river and lake sediments with the aim of identifying areas of a catchment experiencing the high rates of soil loss. In some areas, however, there is poor understanding of how magnetic properties are able to discriminate between potential sediment sources. The basis for mineral magnetic source discrimination between topsoils and subsurface material, over different rock parent materials, was investigated in the South African Karoo. The effects of particle size, soil development/alteration and diagenesis processes on the source discrimination were investigated. Good discrimination (up to 100% correct discrimination) was found between the sedimentary sources (soils and subsurface) and dolerite soils due to much higher concentrations of magnetic minerals in the dolerite. There was little evidence of alterations to magnetic properties in subsurface sediments, resulting in only a small ability of properties to differentiate between surface and subsurface sources. It was found that during the development of topsoils there was a loss of high coercivity weakly magnetic grains and an enrichment in small superparamagnetic and stable single domain grains. Particle size was strongly related to magnetic properties with almost all of the secondary superparamagnetic and stable single domain grains created during soil formation being concentrated in the <32μm fraction and the loss of high coercivity grains taking place primarily in the >125μm fractions. The small ability of sediment to discriminate between surface and subsurface sources was based upon the dissolution of primarily χfd and χARM carrying grains in the <32μm fractions and to a lesser extent the 63–32μm fraction. As a result it was recommended that source tracing with the <32μm fraction of soils and sediment should be treated separately from the 63–32μm and larger particle size fractions. The relationships between particle size and magnetic properties were fairly consistent in all soil samples in each source group, raising the possibility that reliable correction factors could be developed for each source group.

The immediate effectiveness of barley straw mulch in reducing soil erodibility and surface runoff generation in Mediterranean vineyards

Soil and water loss in agriculture is a major problem throughout the world, and especially in Mediterranean areas. Non-conservation agricultural practices have further aggravated the situation, especially in vineyards, which are affected by one of the highest rates of soil loss among cultivated lands. Therefore, it is necessary to find the right soil practices for more sustainable viticulture. In this regard, straw mulching has proven to be effective in other crop and fire affected soils, but, nonetheless, little research has been carried out in vineyards. This research tests the effect of barley straw mulching on soil erosion and surface runoff on vineyards in Eastern Spain where the soil and water losses are non-sustainable. An experiment was setup using rainfall simulation tests at 55mmh−1 over 1h on forty paired plots of 0.24m2: twenty bare and twenty straw covered. Straw cover varied from 48 to 90% with a median value of 59% as a result of the application of 75g of straw per m2. The use of straw mulch resulted in delayed ponding and runoff generation and, as a consequence, the median water loss decreased from 52.59 to 39.27% of the total rainfall. The straw cover reduced the median sediment concentration in runoff from 9.8 to 3.0gL−1 and the median total sediment detached from 70.34 to 15.62g per experiment. The median soil erosion rate decreased from 2.81 to 0.63Mgha−1h−1 due to the straw mulch protection. Straw mulch is very effective in reducing soil erodibility and surface runoff, and this benefit was achieved immediately after the application of the straw.

Soil erosion under teak (Tectona grandis L.f.) plantations: General patterns, assumptions and controversies

High rates of soil loss are usually assumed under teak (Tectona grandis L.f.) plantations and some debate has been created about this during recent years. We analyzed the processes of soil loss and accumulation in a case study and performed a critical review of literature from other studies, due to the shortage of field experimental data from around the world to sustain this theory. The case study was established in Alfisols and slopes ranging from 30 to 60% in Guanacaste, Costa Rica, and compares secondary forests with mature teak plantations and young teak plantations under different management regimes. Over a period of 14months with higher rainfall than the regional average, very little soil loss was registered in the teak plantations of Guanacaste, although there were slight differences between treatments (1–4mm), while soil accumulation was measured in secondary forests (4mm). A number of other authors also report low levels of soil loss in teak plantations and those studies in which high erosion rates have been identified tend to be associated with teak plantations where fires are a common phenomenon. Hence, we conclude that poor forest management (prescribed fires, machinery, extremely steep slopes, previous land use, etc.) rather than the nature of the teak itself causes the high rates of erosion.

Modelización de la pérdida de suelo en una cuenca endorreica del Pirineo oscense

Erosion is a serious threat in Mediterranean regions, therefore assessment and quantification of soil losses at watershed scale is necessary. To promote soil conservation and assess the vulnerability of wetlands in the Estanque de arriba catchment, which is located in the province of Huesca (Spain), the semi-empirical soil erosion model SEMMED has been used. This model is based on the assessment of both the splash detachment and the runoff transport capacity rates per raster cell within the catchment to obtain a final map of annual soil erosion rate. Integration of the digital elevation model of the study area and the climatic and analytical database in the ArcGIS 9.0 has allowed generating maps for the different parameters and factors. In order to obtain the required values a field survey was carried out collecting a total of 80 samples of soil. The cumulative overland flow map has been calculated with the single flow, multiple flow and combined flow algorithms. The multiple flow algorithm has showed up as the best approach for modeling the drainage pattern of runoff, especially in flat areas. The mean splash detachment and transport capacity rates are 11.79 and 9641.28 kg m-2, respectively, whereas the mean rate of annual soil loss is 9.02 kg m-2 year-1. Crops of cereal and paths present the highest rates of soil loss, whereas the areas associated with sandy loam textures and high values of saturated hydrological conductivity present the lowest values. The external and internal boundaries of the catchment are characterized by no erosion processes.

MUSLE Evaluation of Soil Loss on a Construction Site by Using Gauging Weirs

During urbanization, large areas of soil are exposed to the risk of soil erosion due to extensive earthworks and construction activities. Runoff from construction sites is known by far the largest source of sediment clogging our waterways. Erosion occurred from the study area due to removal of vegetation, high rainfall intensity, alteration of existing topography, and the covering of previously vegetated surfaces with impervious cover such as roads, driveways and buildings. The main objective of this study is to evaluate the soil loss due to storm rainfall and runoff on a construction site located at Sungai Ara, Penang State of Malaysia. A purpose-built hydraulic structure namely Sharp Crested Rectangular Weir was constructed and installed on site for more reliable estimates of flow during storms. The soil loss was then evaluated by using an empirical modeling known as the Modified Universal Soil Loss Equation (MUSLE). Results showed that large amount of sediment has being eroded from the study area during these activities. The highest soil loss rate was estimated was 64 ton/ha during a storm event.

MUSLE Evaluation of Soil Loss on a Construction Site by Using Gauging Weirs

During urbanization, large areas of soil are exposed to the risk of soil erosion due to extensive earthworks and construction activities. Runoff from construction sites is known by far the largest source of sediment clogging our waterways. Erosion occurred from the study area due to removal of vegetation, high rainfall intensity, alteration of existing topography, and the covering of previously vegetated surfaces with impervious cover such as roads, driveways and buildings. The main objective of this study is to evaluate the soil loss due to storm rainfall and runoff on a construction site located at Sungai Ara, Penang State of Malaysia. A purpose-built hydraulic structure namely Sharp Crested Rectangular Weir was constructed and installed on site for more reliable estimates of flow during storms. The soil loss was then evaluated by using an empirical modeling known as the Modified Universal Soil Loss Equation (MUSLE). Results showed that large amount of sediment has being eroded from the study area during these activities. The highest soil loss rate was estimated was 64 ton/ha during a storm event.

Rainfall, runoff, and suspended sediment delivery relationships in a small agricultural watershed of the Three Gorges area, China

Following construction of the Three Gorges Dam, many farmers resettled in surrounding mountain areas and cultivated marginal lands, which are mostly on steep slopes with soil of poor structure. In general, high soil loss rates occur during intense storms. Therefore, soil erosion is a major environmental problem in the Three Gorges area (TGA) of China. Understanding and quantifying sediment load is important for sustainable agriculture and the environment in this region. This paper analyzes the relationships between rainfall, runoff, and sediment transport in the Wangjiaqiao watershed. Strong seasonal and monthly variability in sediment load was found. Sediment was strongly transported during summer months, a period when frequent flood events of high magnitude and intensity occurred. Analysis of the relationships between precipitation, discharge and sediment transport at an individual event scale showed significant correlations between total precipitation, peak discharge, total water yield, maximum 30 min rainfall intensity, and sediment-related variables. Stepwise multiple regression analysis revealed that rainfall amount is the major cause of runoff, while events producing a large discharge in a short time play an important role in inducing severe soil erosion. During 40 flood events, three different types of hysteretic loops were observed: clockwise (28 events, 70%), figure-eight (5 events, 12.5%), and complex (7 events, 17.5%). The results of this study confirm the complex and heterogeneous nature of sediment response in the Wangjiaqiao watershed.

Soil loss assessment in the Tasik Chini catchment, Pahang, Malaysia

The Universal Soil Loss Equation (USLE) model is used to estimate average soil loss generated from splash, sheet, and rill erosion in agricultural plots at the Tasik Chini catchment. Use of the USLE has recently been extended for predicting soil loss and plan control practices in agricultural catchment by the effective integration of Geographic Information Systems (GIS) based on procedures to estimate the factor values in a grid cell basis. This study was performed to predict the soil erosion risk by the USLE/GIS methodology for planning conservation measures in the site. Rainfall erosivity (R), topographic factor (LS) and land cover management factor (C) values for the model were calculated from rainfall data, topographic and land use maps. Soil was analyzed for the soil erodibility factor (K). Soil samples were selected from the eleven soil series around the Tasik Chini area. A total of 55 samples were collected from the eleven soil series namely Tebok, Lating, Serdang, Kuala Brang, Kedah, Bungor, Kekura, Malacca, Rasau, Prang and Gong Chenak. Physical properties such as particle size distribution, texture, hydraulic conductivity and organic matter content (OM) were analyzed in order to support the erosion rate analysis. Results shows that five soil series have low rates of soil loss, i.e. Tebok, Lating, Bungor, Kekura and Gong Chenak series, having an average soil loss of 0.65, 0.10, 1.61, 4.23 and 0.53 ton/ha/year, respectively. Two soil series have moderate rates of soil erosion namely Serdang and Prang series (average of 47.41 and 41.10 ton/ha/year, respectively). Two soil series with moderately high rates of soil loss are Kuala Brang and Rasau soil (average of 57.16 and 57.93 ton/ha/year, respectively). Malacca Series has high rate of soil loss, ranges from 21.44 to 348.7 or average of 130.26 ton/ha/year. Kedah soil series has very high soil loss, ranges from 79.99 to 319.75 or average of 180.49 ton/ha/year. This high erosion rate is expected to generate higher sedimentation rate into the Tasik Chini water body, elevating the lake fills and eventually forming an extinct lake.

Spatial soil loss risk and reservoir siltation in semi-arid Tunisia

Soil erosion vulnerability and extreme rainfall characteristics over the Mediterranean semi-arid region of Tunisia are crucial input for estimation of siltation rate in artificial reservoirs. A comprehensive high-resolution database on erosive rainfall, together with siltation records for 28 small reservoirs, were analysed for this region, the Tunisian Dorsal (the easternmost part of the Atlas Mountains). The general life-span of these reservoirs is only about 14 years. Depending on the soil degradation in the different catchments, the corresponding reservoirs display a wide range of soil erosion rates. The average soil loss was 14.5 t ha−1 year−1 but some catchments display values of up to 36.4 t ha−1 year−1. The maximum 15-min duration rainfall intensity was used to determine the spatial distribution of rainfall erosivity. The northwestern parts of the Tunisian Dorsal display the most extreme rainfall erosivity. Spatial erosion patterns are to some extent similar; however, they vary greatly according to their location in the “soil degradation cycle”. This cycle determines the soil particle delivery potential of the catchment. In general, the northwestern parts of the Dorsal display modest soil erosion patterns due to the already severely degraded soil structure. Here, the soil surface is often the original bedrock. However, the greatest soil erosion occurs in the mid-eastern parts of the Dorsal, which represents the “degradation front”. The latter corresponds to the area with highest erosion, which is continuously progressing westward in the Dorsal. The large variation between the erosive rainfall events and the annual soil loss rates was explained by two important factors. The first relates to the soil degradation cycle. The second factor corresponds to the degradation front with the highest soil loss rates. At present this front is located at 300 m altitude and appears to be moving along an 80-km westward path starting from the east coast. A better understanding of the above can be used to better manage soils and soil covers in the Tunisian Dorsal area and, eventually, to decrease the soil erosion and reservoir siltation risk. Citation Jebari, S., Berndtsson, R., Bahri, A. & Boufaroua, M. (2010) Spatial soil loss risk and reservoir siltation in semi-arid Tunisia. Hydrol. Sci. J. 55(1), 121–137.

Impact of different parts of unpaved forest roads on runoff and sediment yield in a Mediterranean area

Surface runoff and sediment production on unpaved forest roads in a humid Mediterranean mountainous area has been studied using a simple portable rainfall simulator at an intensity of 90 mm h − 1 . Thirty six rainfall simulations were carried out on road plots: on the roadbank (12), on the sidecast fill (12), and on the roadbed (12). On the roadbanks, the steady-state runoff coefficient was 85.9% and runoff flow appeared after 63 s on average. On the sidecast fills, the steady-state runoff coefficient was 58.6% and mean time to runoff was 48 s. Finally, on the roadbeds, the steady-state runoff coefficient was 21.5% and mean time to runoff was 41 s. The highest soil loss rate was found on the roadbanks (486.7 g m − 2 ), mainly due to low plant cover, soil texture and rock fragments. The total soil erosion on the roadbanks was 3 and 18 times higher than those from the roadbeds and the sidecast fills, respectively. As a consequence, roadbanks can be considered the main source of sediments on the studied sites, but the function of unpaved forest roads as source points for runoff generation is more important.

Soil loss and runoff rates on unpaved forest roads in southern Spain after simulated rainfall

We have studied the surface runoff and sediment production on unpaved forest roads in Los Alcornocales Natural Park (southern Spain) using a simple portable rainfall simulator at an intensity of 72 mm h −1. Thirty rainfall simulations were carried out on 10 road plots: on the roadbank (10), on the sidecast fill (10), and on the roadbed (10). On the roadbanks, the runoff coefficient was 58% and runoff flow appeared after 60–90 s. On the sidecast fills, the runoff coefficient was 27% and time to runoff was 5–108 s. Finally, on the roadbeds, the runoff coefficient was 51% and time to runoff was 25–89 s. The highest soil loss rate was found on the roadbanks (106 g m −2), due to slopes, the existence of loose colluviums, and a low plant cover. The total soil loss from the roadbank was 5 and 6 times higher than those from the roadbed and the sidecast fill, respectively. The sediment concentration increased during the first 6–8 min from the beginning of the simulations and then decreased steadily due to the loss of fine soil surface particles and porosity changes. Statistical analysis shows that plant cover on roadbanks, sidecast fills, and roadbeds has a significant effect on runoff. Rock fragment cover and slopes on the roadbank and the sidecast also influence runoff significantly. The behavior of roadbanks was similar to that of other cultivated soils and abandoned sloping fields. Sidecast fills had the same response as abandoned terraces or non-vegetated road embankments. Reducing the roadbank angle by approximately 40% and increasing the plant cover to 35–40% are necessary for keeping erosion below 18 g m −2, as it was on the sidecast fills.

A Survey of Soil Erosion in Australia using Caesium‐137

AbstractA survey of soil erosion was conducted in Australia using the fallout radioisotope caesium‐137 as an indicator of topsoil redistribution. Two hundred and six sites were sampled, 100 within rotational cropping and horticultural use, 52 within uncultivated permanent pasture and forest, and 54 in rangelands. Average net soil losses were approximately equal for cultivated cropping lands and rangelands (ca. 5.5 t ha−1 yr−1), and just over 1 t ha−1 yr−1 for pasture and forest. The Mann Whitney U Test revealed that losses under cropping and rangeland conditions were significantly higher (p &lt; 0.05) than under uncultivated pasture and forest. Soil loss was negatively correlated with mean annual rainfall and slope gradient, and positively correlated with slope length (Spearman's rank correlation). There was no correlation between rates of soil loss and a rainfall erosi‐vity index. An assessment of erosional events was provided by landholders for 104 sites, with their ranking being weakly but significantly correlated with soil loss estimates (r =+0.35). Sixty percent of sites had net soil losses greater than 1 t ha−1 yr−1, and 74% of sites had losses of more than 0.5 t ha−1 yr−1. This latter rate may be regarded as a limit for a tolerable level of soil loss. These high rates of soil loss have occurred since the mid‐1950s despite there being significant landholder awareness of the soil erosion hazard.

Performance of grass and rainforest riparian buffers in the wet tropics, Far North Queensland. 2. Water quality

Riparian lands have the potential to buffer streams from hillslope sediment and nutrient transport. Most research on buffers has been conducted under laboratory or manipulated field experimental conditions. Few quantitative data exist on buffer performance under natural field conditions. This study reports measured soil loss and evaluates the effectiveness of riparian buffers on planar and convergent slopes under field hydrological conditions in Far North Queensland. The conditions are extreme for testing the effectiveness of riparian buffers as the land is steep, intensely cropped and receives high intensity rainfall. Hillslopes cropped with bananas were monitored using paired flumes. Runoff, bedload, and suspended loads were measured leaving the crop (upper sites) and leaving the riparian buffers (lower sites). Highly variable hillslope soil losses of &lt;1 to &gt;70 t/ha per wet season were recorded. High rates of hillslope soil loss were from areas of steep gradient with little ground cover experiencing high rainfall intensity. On planar slopes, even with high soil loss, grass buffer strips were able to trap &gt;80% of the incoming bedload. Total N (TN), total P (TP) and suspended sediment (SS) loads were reduced between 25 and 65% by the planar slope grass buffer and within the first 15 m of the moderately convergent grass buffer. Loads leaving the moderately convergent buffer were often higher than those delivered from the crop, due to seepage after prolonged or high frequency rainfall. Under these conditions the buffer's main function is to prevent erosion rather than trap sediment and nutrients. Results from a highly convergent 5-ha hillslope, suggest that for buffers to be more effective in such topography, they should also be placed at the end of the crop rows, where contributing areas are smaller. Flow was able to concentrate within the crop and on at least one occasion was able to scour a 30-cm-wide channel through the entire width of the buffer releasing previously trapped material and making the buffer ineffective. A remnant rainforest buffer, receiving runoff from a planar slope, acted as a temporary store of sediment and nutrients that were reworked during subsequent events. This study demonstrates both a need for managed buffer strips on sloping tropical cropped land and identifies limitations on their potential effectiveness.

Multi-site assessment of cultivation-induced soil change using revised landform segmentation procedures

The range of soil properties associated with soil taxonomic class results from natural soil formation and soil-changing effects of human activity. My goal was to assess cultivation effects on soil formation and soil organic carbon (SOC) at four cultivated and two native hummocky till sites from the same climatic region of Saskatchewan. A revised landform segmentation procedure was used for comparisons across sites. Neither A horizon thickness nor SOC was related to topographical attributes at the native sites, but both had significant relationships (P = 0.001) with profile curvature (Pearson r= −0.53 and −0.57 for A horizon thickness and SOC, respectively), specific dispersal area (r = −0.40 and −0.54) and plan curvature (r = −0.24 and −0.34) at the cultivated sites. The greatest decreases in A horizon thickness (16 cm) and SOC (56 Mg ha-1) at the cultivated sites occurred in divergent shoulder elements, which have high rates of tillage-induced soil loss. Increases in soil thickness (including depth to calcium carbonate) occurred in concave footslope and depressional elements, but in situ losses of SOC outweighed the effects of SOC deposition. Soil thinning caused increases in the proportion of Orthic Regosols and decreases in the proportion of Calcareous Chernozems, but horizonation changes in the dominant Orthic Chernozems were minor. The self-terminating nature of the horizonation changes and improved conservation measures suggest that the future rate of soil change in these landscapes may be lower than in the past. Key words: organic carbon, topography, landscape-scale, erosion, tillage redistribution, digital elevation model, cesium