Time-varying impacts of different management regimes on vegetation cover in agricultural ditches

doi:10.1016/j.agwat.2014.03.012

Agricultural Water Management

Volume 140, July 2014, Pages 14-19

https://doi.org/10.1016/j.agwat.2014.03.012 Get rights and content

Highlights

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We characterised frequent and diverse management regimes for ditches.
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We defined six main management regimes.
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Ditch vegetation covers exhibited strong temporal and spatial variability.
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The potential hydrological impacts of management were time-varying and contrasting.

Abstract

Man-made drainage networks consisting primarily of ditches have been constructed by farmers for centuries to remove excess water from fields. In addition to removing water, ditch networks are involved in other hydrological processes, such as redistributing soil or pesticides. To maintain the hydraulic capacity of ditches, farmers apply various management regimes to regulate ditch vegetation cover that can impact the aforementioned hydrological processes. The objective of this study was to characterise the time-varying impacts of ditch management regimes on vegetation cover and to discuss the resulting hydrological impacts. The vegetation associated with the surveyed ditches was observed and used to define the different ditch management regimes and to quantify their impacts on ditch vegetation cover. The management regimes were heterogeneous and involved a combination of dredging, chemical weeding, mowing and burning. The variety of management practices and the frequency with which they were applied to the ditches resulted in strong spatial variability in the vegetation cover and in strong temporal variability for a given ditch. Finally, it is probable that the surveyed management regimes had time-varying and contrasting impacts on the hydraulic capacity of ditches, the erosion of ditch banks, sedimentation and pesticide retention in the ditches, as indicated by the extensive literature on these topics.

Introduction

In the field of cultivated catchment hydrology, changes in landscape patterns resulting from different agricultural practices have been extensively studied to obtain information that would aim in reducing the negative impacts of agriculture on hydrological processes. Previous hydrological studies have considered the impact of spatial or temporal variability in the cultivated landscape resulting mainly from agricultural practices on plots, hedgerows and/or grass strips; for example, the effects of the spatial structure of hedgerow networks on evapotranspiration fluxes (Viaud et al., 2005) and the effects of agricultural practices on nitrate contamination in a small agricultural catchment (Ferrant et al., 2013) have been examined. To realistically describe differences in landscape patterns, these studies first attempted to quantify the variability in the anthropogenic events under study (e.g., hedgerow settlements and agricultural practices).

Despite the recognition that man-made drainage networks can have a strong impact on the hydrology of cultivated catchments (Krause et al., 2007, Stanchi et al., 2012, Dages et al., 2009), no previous studies have examined the impacts of the spatio-temporal variability of man-made drainage networks on the hydrology of these catchments areas. These impacts should be evaluated in terms of the vegetation cover, which can act on multiple hydrological processes such as flow resistance (Nepf, 2012), pesticide retention (Margoum et al., 2003), sedimentation and adsorption of pollutants (Lecce et al., 2006, Nguyen and Sukias, 2002).

Man-made drainage networks consisting primarily of ditches can have opposing effects on cultivated catchments; the magnitude of these effects can be greater than those of the initially anticipated effects. Most ditches must be managed to prevent them from being filled and invaded by vegetation, as the vegetation can affect their subsequent ability to remove excess water or can result in overflows. Farmers and local authorities can apply a variety of management operations, including dredging or mowing, that are organised within management regimes, as for crop management (Paré, 2011). These management regimes, which act on vegetation cover in the ditches, most likely impact the aforementioned hydrological processes.

Few studies in the literature have characterised ditch management regimes or evaluated their impacts on ditch vegetation. Several studies have considered the impact of ditch dredging on the hydrological processes occurring in peatland forest catchments (Koivusalo et al., 2008) and on the nutrient uptake capacity of ditches (Smith and Pappas, 2007); other studies have examined the impact of ditch mowing on temporal patterns of erosion and sedimentation in ditch networks (Lecce et al., 2006). These studies addressed a single aspect of ditch management at a single time point; however, management regimes may impact vegetation cover in multiple ways and may have time-varying effects on hydrological processes. To date, no studies have examined the variety of ditch management regimes currently in use or their consequences for vegetation cover. Such information is needed to subsequently quantify the impacts of ditch management operations on the hydrology of catchments.

The objective of this study was to identify ditch management regimes and to characterise the multiple, time-varying impacts of these regimes on the vegetation cover of ditches. This characterisation will provide a foundation that can be used to discuss the hydrological impacts of these regimes and the resulting vegetation cover. The study uses information obtained from observations made in a Mediterranean vineyard landscape in the south of France.

Section snippets

Study area

The study area is located in the downstream portion of the Peyne catchment in southern France (Fig. 1). The area is approximately 40 km² in extent, and approximately 60% of the area is covered with vineyards. The altitude ranges from 28 m to 128 m. The region has a Mediterranean climate, receives 600–800 mm of precipitation per year and is characterised by two short but intense rainy seasons in the autumn and spring. The vines are currently not irrigated.

The study area is prone to intense periods

Methods

To characterise the time-varying effects of the ditch management regimes in the study area on vegetation cover, we surveyed a sample of vine growers and characterised their ditch management regimes. We also visually assessed ditch vegetation to characterise the temporal evolution of the vegetation cover resulting from the management regimes.

Characterisation of ditch management regimes

The results of the survey showed that most of the ditches were actively managed. Four types of ditch management operations were identified: 58%, 49%, 54% and 28% of the ditches were dredged, mowed, burned and chemically weeded, respectively. The frequency of implementation of each type of ditch management operation was highly variable: dredging was undertaken at a median frequency of 0.1 times per year (or once per ten years), whereas the other operations were performed once every year.

Spatial and temporal variability in vegetation cover resulting from ditch management regimes

We found that the yearly management operations had a strong impact on the vegetation cover in the ditches (e.g, burning removed almost all of the living and dead vegetation). The senescence of living vegetation, the accumulation of dead vegetation and regrowth also impacted the vegetation cover.

The various yearly management operations were not mutually exclusive and occurred at different periods throughout the year. Different ditch management regimes (Fig. 3) that were characterised by a

Conclusion

We found that most of the observed ditches were managed at least once a year, which was surprisingly frequent for non-productive terrains. The observed management regimes were characterised by a combination of dredging, chemical weeding, mowing and burning. These management regimes have a drastic impact on ditch vegetation: strong spatial and temporal variability in the vegetation cover among ditches and within a given ditch, respectively, was observed. It is probable that the actual management

Acknowledgements

This work was partly supported by funds for a PhD thesis provided by the region of Languedoc-Roussillon. The authors thank the farmers and the local authorities for their hospitality during the survey period of this study. The authors also thank the long-term environmental research observatory (OMERE) for providing the meteorological data.

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The vegetation must be managed regularly by users (farmers and irrigation managers) to restore its hydraulic capacity for transport (Dollinger et al., 2015). In Mediterranean agricultural areas, one or two practices such as mowing, burning, clearing, and chemical weeding are conducted every year (Levavasseur et al., 2014a; Sänchez Martín et al., 2018). Dredging is carried out every 5 to 10 years to remove the deposited sediments.
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Farm ditch networks, infrastructures designed to regulate excess water in cropped landscapes, constitute pesticide dispersal pathways or buffer zones, depending on their soil properties. Despite the key role that ditch soils play in the regulation of water and pesticide fate, their properties, especially in intermittently flooded ditches, remain poorly characterized. Therefore, our aim was to evaluate the specificity of ditch material properties to determine whether ditches require an approach that differs from that of field soils when studying water and pesticide fate in farmed landscapes. We thus analysed the variations in the pedological, herbicide sorption and flow properties of soil materials along a 2D cross-section of an intermittently flooded ditch in the Roujan catchment of southern France. We found that the upper part of the ditch bed soil profile is composed of 3 horizons that formed after the original creation of the ditch, most likely via the deposition of field-eroded particles and the accumulation of organic matter. These specific horizons have greater porosity, mostly due to their dense root systems, and contain up to 2 times more organic carbon than the neighbouring banks or field soils. Consequently, the hydraulic conductivity is greater, and the sorption of hydrophobic herbicides is up to 2 times greater in ditch bed materials than it is in soils located farther away from the ditch surface. Moreover, significant macroporal flow was evidenced in both profiles but with different contribution to the global flow. The contrasts in the hydrodynamic and sorption properties between both the ditch bed and banks materials likely results in significantly different water and pesticide infiltration patterns in ditches compared to crop fields. Given these differences, we recommend investigating the specific properties of ditch beds when studying and modelling water and pesticide fate in croplands.

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Short communicationTime-varying impacts of different management regimes on vegetation cover in agricultural ditches

Highlights

Abstract

Introduction

Section snippets

Study area

Methods

Characterisation of ditch management regimes

Spatial and temporal variability in vegetation cover resulting from ditch management regimes

Conclusion

Acknowledgements

J. Nat. Conserv.

J. Hydrol.

Agric. Water Manage.

Ecol. Model.

Geomorphology

Agric. Ecosyst. Environ.

Agric. Ecosyst. Environ.

Agric. Ecosyst. Environ.

CATENA

Chemosphere

Quaternary Int.

Agric. Water Manage.

Short communication
Time-varying impacts of different management regimes on vegetation cover in agricultural ditches