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.

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Glyphosate is the most applied herbicide for weed control in agriculture worldwide. Excessive application of glyphosate induces water pollution. The transfer of glyphosate to freshwater and groundwater is largely controlled by glyphosate sorption to soils and sediments. Sorption coefficients are therefore the most sensitive parameters in models used for risk assessment. However, the variations in glyphosate sorption among soils and sediments are poorly understood. Here we review glyphosate sorption parameters and their variation with selected soils and sediment. We use this knowledge to build pedotransfer functions that allow predicting sorption parameters, Kd, Kf and n, for a wide range of soils and sediments. We gathered glyphosate sorption parameters, 101 Kf, n and equivalent Kd, and associated soil properties. These data were then used to perform stepwise multiple regression analyses to build the pedotransfer functions. The linear (Kd) and Freundlich (Kf, n) pedotransfer functions were benchmarked against experimental data. We found the following major points: (1) Under current environmental conditions, sorption is best predicted by the Kd pedotransfer function. (2) The pedotransfer function is Kd = 7.20*CEC -1.31*Clay ? 24.82 (Kd in L kg -1 , CEC in cmol kg -1 and clay in %). (3) Cation exchange capacity (CEC) and clay content are the main drivers of Kd variability across soils and sediments. Freundlich parameters are additionally influenced by pH and organic carbon. This suggests that the formation of complexes between glyphosate phosphonate groups and soil-exchanged polyvalent cations dominates sorption across the range of analyzed soils.

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Pesticide sorption to ditch-bed materials can efficiently decrease pesticide concentrations in the flowing water. Pesticide sorption depends on flood characteristics and the nature and abundance of ditch-bed materials, such as soils, living and decaying vegetation and ash. However, the affinities of pesticides for various ditch-bed materials have rarely been investigated, and variations in the global sorption capacity of ditch beds resulting from their heterogeneous compositions and variable flood characteristics have not been determined. Thus, we studied the variability of sorption capacities of ditch-bed materials for glyphosate and diuron in three catchments in France and propose a method for calculating global sorption processes in heterogeneous ditch beds. The methodology consists in estimating a global sorption coefficient for the composite ditch-bed materials (Kd) and an indicator of the amount of pesticide potentially retained by sorption during a flood event (SPRI). Furthermore, we computed the Kd and SPRI of glyphosate and diuron for 8 ditches subjected to 3h flood events with water levels varying from 0.5 to 15cm. Our results show that increasing the water level from 0.5 to 15cm resulted in a 90% decrease in the sorption capacities of the ditch beds for both pesticides. At a medium water depth of 5cm, SPRI varied from 25 to 51% and from 7 to 35% among the ditches for glyphosate and diuron, respectively. The variabilities of the glyphosate and diuron sorption capacities among the ditches were mainly driven by the nature and abundance of soil and ash. As the management of farm ditches, performed to maintain their hydraulic performance, modifies the abundances of various ditch-bed materials, it constitutes a potential lever of action for water quality improvement. Thus, Kd and SPRI could serve as rapid and cost-effective tools for optimizing ditch network management strategies to improve water quality in cropped catchments.

International audienceDitch networks were traditionally designed to protect fields from soil erosion or control waterlogging. They are still frequently managed by either mowing, chemical weeding, dredging or burning to ensure their optimal hydraulic capacity. Ditches were recently reported also to improve water quality and sustain biodiversity. These ditch functions are related to specific ditch properties. By contrastingly modifying ditch properties, maintenance operations were supposed to regulate these functions. There is, therefore, a need to re-examine the design and maintenance strategies of ditches to optimize the whole range of ecosystem services that they provide. In this study, we address the innovator question of how maintenance operations affect the yearly evolution of ditch properties, and in turn, the panel of functions that ditches support. During one year, we monitored the vegetation, litter, soil properties, and ash cover of five ditches that were being unmanaged, dredged, mowed, burned, and chemically weeded, respectively, with timing and frequency as generally operated by farmers in the study area. We then used indicators to evaluate the effect that the evolution of these properties has on the ditch water conveyance, herbicide retention and biodiversity conservation functions. We found that the evolution of these properties significantly contrasted among the 5 maintenance strategies. All the maintenance operations cleared the vegetation, which improves the hydraulic capacity by up to 3 times. The optimal hydraulic capacity is maintained longer after chemical weeding and dredging, but these operations have negative impacts on the herbicide retention and biodiversity conservation functions. The litter and ash layers generated by mowing and burning, respectively, improve the herbicide retention by up to 45%. Our results confirm that maintenance can be an efficient tool for optimizing ditch functions. The choice of maintenance operation and timing are key to successfully optimizing most of the functions that ditches can support

The performance of buffer zones for removing pesticides from runoff water varies greatly according to landscape settings, hydraulic regime, and system design. Evaluating the performance of buffers for a range of pesticides and environmental conditions can be very expensive. Recent studies suggested that the fluorescent dyes uranine and sulforhodamine B could be used as cost-effective surrogates of herbicides to evaluate buffer performance. However, while transformation mechanisms in buffers have been extensively documented, sorption processes of both dyes have rarely been investigated. In this study, we measured the adsorption, desorption, and kinetic sorption coefficients of uranine and sulforhodamine B for a diverse range of buffer zone materials (soils, litters, plants) and compared the adsorption coefficients (Kd) to those of selected herbicides. We also compared the global sorption capacity of 6 ditches, characterized by varying proportions of the aforementioned materials, between both dyes and a set of four herbicides using the sorption-induced pesticide retention indicator (SPRI). We found that both the individual Kd of uranine for the diverse buffer materials and the global sorption capacity of the ditches are equivalent to those of the herbicides diuron, isoproturon, and metolachlor. The Kd of sulforhodamine B on plants and soils are equivalent to those of glyphosate, and the global sorption capacities of the ditches are equivalent for both molecules. Hence, we demonstrate for the first time that uranine can be used as a proxy of moderately hydrophobic herbicides to evaluate the performance of buffer systems, whereas sulforhodamine B can serve as a proxy for more strongly sorbing herbicides.