Vincent Chaplot

Improving soil hydromorphy prediction according to DEM resolution and available pedological data.

Abstract This study analyses the sensitivity of soil hydromorphy prediction methods with regard to the resolution of topographical information and additional soil data. Seven Digital Elevation Models (DEM) were computed and compared to topographic measurements, with different resolutions (10, 20, 30 and 50 m) and construction mode (inputting actual stream location in addition to contour lines). Prediction models of soil hydromorphy using linear regression and co-kriging were established from detailed descriptions of soil catenas and topographical investigations on a 2 ha site. These models were compared on a validation set. The DEMs with fine resolutions from 10 to 30 m estimated in a unbiased way the elevation ( E ), the elevation above the stream bank (ES), the downslope gradient (DG), and the upslope contributing area (Amu), whereas prediction errors increased for the lower resolution 50-m DEMs. Apparently, the location of the channel network had no systematic effect on the estimation errors. There was a strong relationship between soil hydromorphy index (HI) and ES ( r 2 =0.80) and the Compound Topographic Index (CTI)=ln(Amu/DG) ( r 2 =0.62). For DEM resolutions of less than 30 m, soil hydromorphy prediction models bound on a regression model with topographic attributes appeared efficient and even better than ordinary kriging (OR) with 10 or 60 point observations. Coarser DEM resolutions (30 and 50 m) highly deteriorated prediction quality. For these resolutions, quality of soil hydromorphy prediction was highly improved by co-kriging of 10 and especially 60 pedological data points with a topographical regression model.

Soil carbon storage prediction in temperate hydromorphic soils using A morphologic index and digital elevation model

Because soils are both a source and a sink for atmospheric CO2, there is an increasing need to characterize the spatial distribution of soil C pools. Large amounts of organic carbon (OC) accumulate in hydric bottom-lands soils. In the Armorican Massif (Western France) where these soils represent 20% of the total surface area, the spatial characterization of OC pools is difficult to assess due to methodological problems such as high spatial variability. Soil color indexes, which combine various characteristics of soil horizons or profiles, are an alternative approach for quantifying the differences in OC storage. In addition, terrain attributes derived from Digital Elevation Models (DEM) may be useful in characterizing the distribution of soil color indexes over large areas. Thus, the overall goal of this work was the development and application of a model for use in predicting the organic carbon (OC) content of soil areas. To accomplish this, extensive examination of soil morphology combined with selected terrain attributes measured in the field and calculated from a digital elevation model (DEM) were used. Soil samples were collected in Western France from a 2-ha agricultural parcel that forms the major part of a hillslope. The results indicate that OC stocks of the entire profile were correlated highly to a soil hydromorphic index (HI) (r2 = 0.80). HI is a function of the percent of the total soil profile depth constituted by horizons with some degree of hydromorphic feature development and the moist color of the surface A horizon. Using a stepwise regression technique, we constructed a prediction model of HI distribution by using the relations between HI and (i) the elevation above the stream bank (ES) (r2 = 0.80); (ii) the downslope gradient (DG) (r2 = 0.55); and (iii) the upslope contributing area (AMU) (r2 = 0.60). Validation of this model on a second site showed that topographical attributes explained up to 75% of the profile OC stock variability. These results confirmed that the integration of a soil index and topographical information is a useful tool for prediction of OC distribution. In addition, the use of soil morphologic indexes could significantly improved the construction and the validation of soil-landscape models because it would minimize laboratory measurements of OC reservoirs.

Nitrate depletion during within-stream transport: effects of exchange processes between streamwater, the hyporheic and riparian zones.

In regions with intensive agriculture and shallow hydrological systems, headstreams are often polluted with nitrate even at the springs. In North-West France, nitrate concentration was seen to decrease downstream during baseflow conditions when the stream flows on granite, but this does not occur on schist. In order to explain this difference in behaviour, we analysed the groundwaters and surveyed the redox conditions (using a field test for ferrous iron) in near-bank wet meadows as well as in the hyporheic zone. We show that the wet meadow groundwater was denitrified and that oxygen and nitrate were presentaround the stream channel in a wide zone on granite,compared with a very restricted zone on schist. Ongranite, exchanges between the stream and the hyporheic zone are favoured by sandy or peaty material having high hydraulic conductivity. This gives rise to two processes (1) lateral inflow of denitrified water from wet meadows, (2) in the opposite direction, supply of stream nitrate to denitrification sites in the hyporheic zone. In the second case, a high hydraulic conductivity also reduces the water residence time and limits denitrification, resulting in high levels of oxygen and nitrate. On schist, the low hydraulic conductivity prevents an efficientconnection between surface and subsurface waters.

Mapping field-scale hydromorphic horizons using Radio-MT electrical resistivity

Pedological soil surveys usually based on auger sampling encounter methodological and economic difficulties. Electrical resistivity (ER) techniques could be used as a simple and practical method to determine their spatial variability. However, attempts to map soils using ER techniques have very often limited success, especially in bottomland areas, due to large variations inherent in ground data. The aim of this study is to seek the interest of a geophysical method, the radio magnetotelluric-resistivity (Radio-MT), to map field-scale hydromorphic horizons for loamy pedological systems in bottomlands characterized by large variations of soil water content and depth to upper boundary of saprolite. The sampling survey was carried in the Armorican massif (western France). The electrical measurements were taken along transects on an agricultural field (80×150 m). The soil sampling was performed on a regular grid with a mesh of 10 m. On each point, some soil properties were measured (type and thickness of the loamy horizons, depth to the upper boundary of saprolite, soil water content at 10, 20, 40 and 60 cm depths). A direct relationship between apparent resistivity and horizon type distribution was not established. The best correlations were between the electrical conductivity and depth to the upper boundary of saprolite and topsoil water content. The correlation coefficients, r, are 0.51 and 0.34, respectively. To identify the soil types, we modeled the influence of these two soil properties by multiple regression technique. Deviations from the regression model were then interpreted by taking into account the succession of soil horizons. These results seem to indicate that the electrical method used in this study could not be directly used to evaluate spatial prediction of the hydromorphic soil distribution, but indirectly by taking into account soil properties such as the soil water content and the upper boundary of saprolite.

The use of auxiliary geophysical data to improve a soil-landscape model

Soil-landscape models have prediction errors that can be reduced by using auxiliary soil data. However, standard soil surveys using auger hole and laboratory analysis encounter both methodological and economical constraints because of, for example, the short-range variability of soils and the expens

Arguments cartographiques en faveur du rôle de la tectonique récente sur la distribution régionale des sols du Massif armoricain

Abstract The study of pedological maps from the Armorican Massif evidenced the effect of recent tectonics (500 000–700 000 years BP) on the regional hydromorphic soil distribution. Blocs in relative uplift were characterized by a low proportion of hydromorphic soils, whereas a higher proportion marked blocs in relative downlift. Such clear differences can be related to the denudation regime which affects topography and saprolite properties, two soil formation factors. Improvements in soil modelling may be achieved by taking into account the regional trends of soil waterlogging and hydromorphy.

Analyse de cartes pédologiques pour identifier le rôle du régime tectonique sur la répartition régionale de la perméabilité des altérites

Abstract Soil maps analysis to identify the role of the tectonic regime on saprolite permeability regional distribution. For applied geology, e.g., geotechnics or hydrogeology, it is of prime interest to know the spatial distribution of the saprolite permeability. This study focuses on the role of the tectonic regime on saprolite permeability regional distribution. Comparison of data concerning the uplift regime and soil organisation data from several pedological maps of the Armorican Massif (France) showed that blocks in relative uplift were characterized by a low proportion of hydromorphic soils, whereas a higher proportion marked blocks in relative downlift. Such differences can be related to the denudation regime, which affects the saprolite permeability.