Hocine Bourennane

Comparison of kriging with external drift and simple linear regression for predicting soil horizon thickness with different sample densities

Abstract This study examines two mapping methods sensitivity to the sampling density of the variable of interest, which is the thickness of a silty-clay–loam (TSCL) horizon. The two methods are simple linear regression (SLR) and universal kriging with external drift (UKEXD). As slope gradient ( β ) derived from a DEM, is available for the whole study area and linearly related to TSCL horizon, was used for TSCL prediction by SLR and by UKEXD. The accuracy performance of TSCL prediction using these methods was assessed by comparison with another group of 69 sample points (validation sample) where the TSCL is actually measured. In the validation procedure for the two methods, two indices were calculated from the validation sample (measured values) and predicted values. These two indices are the mean error (ME) and the root mean square error (RMSE). The results showed that UKEXD was more accurate than the SLR. The improvement of the accuracy of the prediction from SLR to UKEXD was about 38%. To examine the effect of sampling density of TSCL (variable of interest) on the performance of both mapping methods, five subsets of 40, 50, 75, 100 and 125 observation sites of TSCL were randomly selected from the 150 sites of the prediction sample. For each subset, a prediction of TSCL was performed over the study area by: (i) SLR; (ii) UKEXD. The validation sample was used to compare the performance of the two methods according to the sample size of the variable of interest. The results show that whatever the sample size may be, UKEXD performs on average more accurate predictions than SLR. Moreover, the results indicate that UKEXD performed better when the sample size of the variable of interest increases. On the contrary, the performances with the linear regression remain stable whatever the sample size may be.

Rill erosion on cultivated hillslopes during two extreme rainfall events in Normandy, France

In the Normandy region of France, two extreme runoff events took place during winter of 1999 and spring of 2000 that caused flooding and considerable on-site as well as off-site damages. After each event, erosion damage was mapped on an experimental cultivated catchment (94 ha). The location and extent of rill, ephemeral gullies and deposits were measured. For each field, information on land use and soil surface characteristics were also collected. Since 1991, when experimental work and survey campaigns were initiated on this catchment, interrill erosion dominated over rill or ephemeral gully erosion. Our objective was to link information on topography, soil surface characteristics, and land use with intensity and type of erosion that developed. Erosion features that were most related to topographic attributes and hence less affected by seasonal variations were ephemeral gullies and some predefined deposit types. Topographic attributes alone were not sufficient to determine development of rill erosion, due to seasonal differences in vegetative cover. At the catchment scale, total erosion varied from 10 t/ha in December with 93% of the catchment area with vegetation cover ≤20%; to 1.5 t/ha in May, with 73% of the catchment area with vegetation cover >60%. The relative importance of ephemeral gully erosion out of total linear erosion varied from ca. 24% for the rainfall events of December to more than ca. 83% for the rainfall events of May. These results also highlight the fact that average annual sediment delivery as well as the relative importance of different erosion forms at the catchment scale cannot be generalised. Erosion prediction and erosion assessment risks are strongly dependent on catchment land use, morphology and storm characteristics.

Modeling Vertical Distribution Of Carbon In Oxisols Of The Western Brazilian Amazon (rondonia)

Oxisols have great ecological significance in tropical soils because they are the dominant soil type of the Brazilian Amazon ecosystem, comprising more than 40% of its total. To estimate carbon (C) stocks and changes requires knowledge of the vertical distribution of C in profiles. The objective of this study was to determine if specific patterns occur in C profiles of Oxisols in the Western Brazilian Amazon so that total C storage can be assessed down to any given depth by simple models with low input data requirements. Two models, a power-based model and an exponential-based model, were tested using nonlinear regression analysis on a soil database made up of 129 Oxisol profiles corresponding to 519 soil horizons. These models, as judged by the coefficient of determination (R 2 ) value, explained more than 55% of the total variance for all of the horizons, whether or not segregated by taxonomic unit. The models were then tested with individual profiles. The power model exhibited a tendency to overestimate C stocks when integration was done for the 0-20-cm and the 0-100-cm layers. Results from the exponential model were better than those from the power model. The R 2 values were greater than 0.82, and the associated standard error was reduced. In a validation procedure, the mean error (ME) was close to zero for the exponential model, with a systematic ME of only 0.06 kg C.m -2 for the 0-100-cm layer.

Relationship of the presence of a non-calcareous clay–loam horizon to DEM attributes in a gently sloping area

Abstract The initial objective of this study was to identify relationships between terrain attributes and soil cover over a Loessic flat area covering a limestone plateau. The second objective was to determine those energy factors (flow path, solar radiation, wind intensity) which could improve the understanding of morphology and soil genesis. We describe 341 field observations taken over 1600 ha of an experimental area used for monitoring the water and nitrate supply in the Petite Beauce Region (100 km to the Southwest of Paris). For each observation, several soil variables were encoded. One was the presence of a non-calcareous clay–loam (NCCL) horizon. Relief is very smooth in this region (mean slope around 0.5%). The main terrain attributes were derived from a Digital Elevation Model (DEM) at 20×20 m and assigned to the pedological observations. A multiple logistic regression was used to analyse the relationship between the NCCL horizon and terrain attributes. Special statistics were used for aspect, because of the circular nature of this variable. The results show a strong relationship between the presence of NCCL horizon and slope gradient and slope aspect, while hydrological parameters are not correlated with this horizon. The mean angle of the aspect frequency of the NCCL horizon was calculated and compared to the mean angles of wind direction and solar radiation balance. There is a small difference between wind direction and aspect frequency of the NCCL horizon. This result confirms the role of wind in the spatial pattern of soils. Further data are required to better understand the combination of several factors (role of vegetation) and the age of the reshaping.

Using multiple external drifts to estimate a soil variable

Abstract Kriging with topographic variable (β) and electrical resistivity (ρ) measurements used as external drifts was compared with universal kriging and kriging either with a topographic variable or with electrical resistivity measurements taken as external drift, to predict the depth of a limestone bedrock upper boundary (LUB) in the “Petite Beauce” region, southwest of the Paris Basin. Two sets of limestone depth apparition (LUB) were recorded. A first set of 67 measurements was used for the prediction and another set of 50 measurements was kept for the validation. The slope gradient (β) and the resistivity (ρ), which are linearly related to the limestone upper boundary depth (LUB) and exhaustively sampled over the whole study area, were used as external drift variables for kriging. Two indices, (i) the mean error (ME) and (ii) the root mean square error (RMSE), as well as residuals analysis, were computed from the validation sample (observed data) and predicted values. On the 50 test data, the results showed that kriging using two external drifts proved to be less unbiased and more precise compared to kriging using only one external drift variable in predicting the target variable.

Scale of spatial dependence between chemical properties of topsoil and subsoil over a geologically contrasted area (Massif central, France)

This study investigates scale spatial specific dependence of some major and trace element contents in topsoil horizons and in alterite type horizons (subsoil horizons) over a weakly contaminated area. The study seeks also to compare the spatial distributions of chemical elements at both depths to the spatial variability of the alterite type in order to examine the effect of the parent material. The probability of occurrence of alterite type was estimated using indicator kriging (IK). Factorial kriging analysis (FKA) was used to analyze spatial variability in some soil chemical properties (Ca, Mg, K, Fe, Cr and Co) measured at two depths over a geologically contrasted area of 10 ha in central of France. The coefficients of the coregionalization matrix at different spatial scales reveal the dominant long-range autocorrelation and cross-correlation in all chemical elements in both depths except for trace elements (Co and Cr) where the short-range structure dominates the cross-correlation. The resulting structural correlation coefficients showed strong correlations between variables changing as a function of spatial scale. These relationships between chemical properties at different spatial scales were not revealed by the linear correlation coefficients. A principal component analysis was performed on the coregionalization matrices at each depth to summarize the relationships among the variables at the different spatial scales. Cokriging allowed mapping each spatial component for both depths. These maps were then compared with the probability map of alterite type estimated using indicator kriging. This comparison revealed that the spatial pattern of chemical elements in the subsoil horizons is almost certainly due to the alterite type effect, whereas the alterite type effect on the spatial pattern of chemical properties in the topsoil horizons was partly hidden by human activities and erosion.

Effect of Agricultural Practices on Trace‐Element Distribution in Soil

Abstract The impact of agricultural practices on the spatial distribution of trace elements (TEs) in the surface horizon of Albeluvisols was studied by comparing three types of situation with an increasing anthropogenic gradient: a forest soil, considered as a reference, and cultivated soils with and without spreading of sewage sludge. Total cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) were determined in soil surface horizons. The quantity of particles finer than 2 µm, organic carbon (OC), iron (Fe), manganese (Mn), and calcium (Ca) contents were also measured and used as tracers of soil components that may have contained TEs. Governing factors of the spatial variability of TEs were searched for through comparison of the descriptive statistics (mean and coefficient of variation) and the determination of the best multivariate model predicting TE contents. Nickel and Cr distributions seem to be mainly linked to variability of the parent material composition, whereas that of Co is related to redox pedological processes. In addition, agricultural practices play a role in TE distribution, through the input of TEs, mostly Cd, Cu, and Zn, via sewage sludge spreading and liming amendments, and lateral (homogenization) and vertical (mixing of horizons) redistribution through tilling the soil.

Exploring the Spatial Relationships Between Some Soil Properties and Wheat Yields in Two Soil Types

A field study was conducted to quantify spatial soil variability and to analyze correlations among soil properties at different spatial scales. Soil samples from 0 to 30 cm depth were collected from two adjacent fields in the southwestern Beauce Plain (France) which consisted of Haplic Calcisols and Rendzic Leptosols. Factorial kriging analysis (FKA) was used to describe the co-regionalization of nine soil properties. A linear model of co-regionalization including a nugget effect, and two spherical models were fitted to the experimental data direct and cross-variograms of the topsoil layer properties which were previously estimated. Co-kriged regionalized factors, related to short and long-range variation, were then mapped to characterize soil variation across the two fields. The potential value of ancillary sampled variables, such as yield data, to provide information on soil properties was tested. The relation between yield and measured soil properties appeared to be weak in general. However, the structures of the variation in yield appeared to be relatively stable for two years and showed similar patterns as the co-kriged soil factors. This suggests that information on the scale of variation of soil properties can be derived from yield maps, which can also be used as a guide to suitable sampling interval for soil properties and as a basis for managing fields in a precise way.

Short‐range spatial variability of metal contents in soil on a one hectare agricultural plot

Abstract As the soil varies spatially, monitoring of temporal changes might be biased by spatial variation. Hence, there is a need to develop tools to identify spatial structures and to optimize sampling designs. This study documents the short‐range variability of lead (Pb), copper (Cu), and chromium (Cr) in a 1‐ha agricultural plot. The aims of this work were to identify the spatial structures of these metals, to discuss their origin, and to optimize sampling design for monitoring the long‐term changes of these characteristics. We used geostatistics and sampling simulations on a set of 65 individual samples. Results demonstrate that small changes in metal contents might be hidden by spatial variation if the sampling design for monitoring is not appropriate and that former land‐use history and old management practices may have long‐term significant effects on the distribution of some metals.

A snapshot of soil water composition as an indicator of contrasted redox environments in a hedged farmland plot

While soil water composition has long been recognised as being related to soil type (characteristics of the horizons), the influence of structures resulting from agricultural activities (hedges, ditches, wheel ruts, etc) is still under discussion. This work was undertaken to show that a snapshot of spatial variability of the geochemical characteristics of soil water at the scale of a plot can improve our understanding of soil geochemistry in a farmland setting. We selected a 3 hectare hedged plot located on a hillside, limited by a stream and used as pasture where soils have developed in weathered gneiss. The water depth, electrical conductivity, major ions, temperature, pH, dissolved organic carbon (DOC) content, dissolved oxygen content, fluorescence, alkalinity, Fe(2+), Mn(2+), NO(2)(-), Fe(III) and F(-) contents were measured in 62 auger holes randomly drilled on the site. Four sectors were identified in order to describe the distribution of the main geochemical parameters. Electrical conductivity and some major ions, especially sulphate, had larger concentrations near hedges where oxic conditions prevailed. These features were attributed to the impact of the linear anthropogenic network on the circulation of subsurface soil waters and evapo-transpiration and represent sector I. Dissolved Mn was an indicator of well channelled runoff subsurfaces facilitating the circulation of more highly reducing water (sector III), while DOC probably marked areas drained less well, with a prolonged contact time between soil solutions and organic topsoil horizons (sector II). The presence of dissolved Mn and Fe(II) indicates bottomland anoxic conditions (sector IV). It is concluded that a survey of the chemical composition of soil water may be a direct approach to show the influence of permanent structures on current soil properties and dynamics.