The clay content of the topsoil in two regions of contrasting physiography was predicted from sample data using four different procedures. The predictors were the means of mapped classes, the usual kriging estimator, a cubic spline interpolator and a kriging estimator within classes using a pooled within-class variogram. The performances of the procedures were evaluated and compared.In the first region, Sandford St Martin on Jurassic sediments where there were some abrupt changes in soil, the classification predicted best within those classes bounded by sharp change. Elsewhere the usual kriging performed somewhat better, and kriging within classes was still more precise. In the second region, Yenne on the alluvial plain of the Rh6ne where the soil varied gradually, kriging performed better than classification, though a small improvement resulted from combining kriging with classification. Both prediction by class means and kriging attempt to minimize the estimation variance, and their mean prediction variances were close to the theoretical values overall. Spline interpolation is more empirical, and though it followed the abrupt changes better than kriging, it fluctuated excessively elsewhere, and its overall performance was poorer than that of kriging.

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Abstract:Field limits, tillage practices and ditch networks constitute man-made hydrological discontinuities in farmed catchments, and are expected to influence hydrological response during flood events. The purpose of this study is to assess the role of human impact, especially the existence of tillage practices and ditch network, on flood events. The study area is the farmed catchment of Roujan (0Ð91 km 2 ) located in Southern France for which a spatially distributed hydrological model, MHYDAS, was developed and tested. The model considers the catchment as a series of interconnected field parts linked to the ditch network. Descriptions are provided for the main model procedures: computation of Hortonian excess rainfall on fields using the Green and Ampt approach, conversion of excess rainfall to surface runoff, interaction between ditch network and groundwater using a simple Darcian model and flood routing through the ditch network using the diffusive wave model. To analyse the role of both tillage practices and the ditch network, two sets of sensitivity analysis of the model were applied. The first set studied the role of tillage practices by comparing the actual spatial distribution of tillage practices on the catchment with three hypothetical scenarios. The second set studied the role of the ditch network by comparing the actual man-made ditch network with a hypothetical drainage network automatically extracted from a digital elevation model. Results show the importance of the role of tillage and the ditch network on the form of the hydrograph, the lag time, the runoff volume and the peak discharge. This technique could also be applied to study the impact of land use change on the hydrological behaviour of the catchment.

The contamination of soil and runoff water by two herbicides, diuron [N'-(3,4-dichlorphenyl)-N,N-dimethylurea] and simazine (6-chloro-N,N'-diethyl-1,3,5-triazine-2,4-diamine), were monitored on two fields, one no-till and one tilled. Experiments were carried out in a 91.4-ha watershed in southern France during the 1997 growing season in order to understand the patterns of pesticide transport from field to watershed. The persistence of the herbicides in soil was prolonged due to the climatic conditions. At the field scale, annual herbicide loads were due to overland flow and amounted to 65.6 and 6.3 g ha(-1) of diuron for the no-till and tilled field, respectively, and to 29.6 and 1.83 g ha(-1) of simazine. Maximum herbicide concentrations exceeded 580 microg L(-1) during the first storm event after application and decreased thereafter but remained for 8 mo above 0.1 microg L(-1). At the watershed outlet, estimated annual loads amounted to 4.12 g ha(-1) of diuron and 0.56 g ha(-1) of simazine. Among them, 96% of the losses in diuron and 83% of those in simazine were caused by the fast transmission through the network of ditches of the overland flow exiting the fields. For diuron, which was sprayed over most of the vineyards, its in-stream concentrations during storm flow were close to those at the outlet of the fields. The herbicide loads in baseflow were smaller than 0.2 g ha(-1). The patterns of the loads at the field and watershed scales suggested that a major part of the herbicides leaving the fields reinfiltrated to the ground water by seepage through the ditches, and was there degraded or adsorbed.

Agriculture must now feed the planet with the lowest environmental impact. Landscape management is a means to protect natural resources from the adverse impacts. In particular, the adequate management of ditches could improve crop quality. Here, we review ditch design and maintenance. We found the following major points: (1) ditch networks have been primarily designed for waterlogging control and erosion prevention. Nonetheless, when properly managed, farm ditches provide other important ecosystem services, namely groundwater recharge, flood attenuation, water purification, or biodiversity conservation. (2) All ditch ecosystem services depend on many geochemical, geophysical, and biological processes, whose occurrence and intensity vary largely with ditch characteristics. (3) The major ruling characteristics are vegetative cover; ditch morphology; slope orientation; reach connections such as piped sections and weirs, soil, sediment and litter properties, biota, and biofilms; and network topology. (4) Ditch maintenance is an efficient engineering tool to optimize ecosystem services because several ditch characteristics change widely with ditch maintenance. For instance, maintenance operations, dredging, chemical weeding, and burning improve waterlogging and soil erosion control, but they are negative for biodiversity conservation. Mowing has low adverse effects on biodiversity conservation and water purification when mowing is performed at an adequate season. The effects of burning have been poorly investigated.

[1] An infiltration test was performed from a ditch with the purpose of monitoring the evolution of the piezometric levels using self-potential measurements made at the ground surface. We used a set of 18 piezometers and a network of 41 nonpolarizable (Pb/PbCl 2 ) electrodes. The variations of the self-potential signals are linearly correlated to the piezometric level changes with an apparent voltage coupling coefficient of À5.5 ± 0.9 mV m À1 . We measured, independently of this infiltration test, the three material properties entering the macroscopic field equations. They are the resistivity distribution of the soil, its mean hydraulic conductivity, and its intrinsic streaming potential coupling coefficient (À5.8 ± 1.1 mV m À1 ). Then, we modeled numerically the infiltration test and the associated self-potential signals using a two-dimensional finite difference code. The numerical model reproduces fairly well the observed results. This investigation demonstrates the effectiveness of the self-potential method in field conditions to monitor small variations (<0.60 m) of the water table. It offers for the first time a test of the electrokinetic theory in the field with independent evaluation of the material properties entering the field equations.

Soil water status and its effect on plant water status are commonly evaluated for water stress diagnosis in annual crops. We investigated the application of this method to vineyards, using the fraction of transpirable soil water (FTSW) to characterise the soil water deficit experienced by the plant. The stability of the relationship between FTSW and predawn leaf water potential ( p) was analysed over two years (

The Mediterranean climate is characterized by a hot and dry summer where occasional storm events induce erosion and runoff. The high leaching potential of pesticides to surface waters under such climate conditions are not in relation to the main body of data that originated from summer‐rain row‐crop scenarios. In this 2‐yr study we monitored runoff discharge and concentrations of the two soil applied herbicides diuron [3‐(3,4‐dichlorphenyl)‐1,1‐dimethylurea] and simazine [6‐chloro‐N2,N4‐diethyl‐1,3,5‐triazine‐2,4‐diamine] from two field sites—one tilled and one no‐till—cropped with grapevine (Vitis vinifera L.; Cinsault and Aramon, respectively). Despite a time lag of 140 d in 1994 between chemical application and first runoff event, diuron concentrations in overland flow exceeded 200 µg L−1 at the no‐till site. In 1995 the first strong rainfall‐runoff event following application carried >87 and 60% of the respective seasonal simazine and diuron loss at both sites, although it accounted for <17 and 7% of the total runoff volume at the no‐till and tilled site, respectively. At the no‐till site, seasonal diuron loss during 1995 was 1.71% of applied; the corresponding value for simazine was 1.25%. Only 0.68 and 0.79% of the respective applied diuron and simazine mass were washed from the tilled field, reflecting differences in runoff volume between sites. Pesticide losses depended primarily on runoff volume and intensity. Event average herbicide concentrations in surface runoff followed an exponential decay over time. Estimated first order rate coefficients were at least twice as large as those derived from soil samples using the alcoholic solvent extraction technique. The decreasing water availability with time compared with the herbicide content at the soil surface indicated an increasing adsorption with time.