Preferential flow and slow convective chloride transport through the soil of a forested landscape (Fougères, France)
Introduction
Assessing potential effects of modifications or disturbance on forest ecosystems, such as climate change or modifications of silvicultural practices, requires the correct modelling of physical, chemical and biological processes. Most of these processes are controlled by the soil water. The flow pathways and the velocities at which water is transferred through the soils also influence the biogeochemical processes that affect solute bioavailability for trees, thus controlling soil fertility and forest productivity. In the context of climate change, with a possible increase in growing season duration (Schär et al., 2004), modelling water flow is essential to compute precise input–output nutrient budgets (Ranger and Turpault, 1999), which are valuable diagnostic tools to determine the sustainability of forest ecosystems. In the nutrient budget approach, nutrient fluxes in soil and nutrient leaching are often obtained by coupling the water fluxes from a deterministic flow model to nutrient concentrations in soil water. Nutrient leaching, which is one of the terms of the budget, is particularly sensitive to water flow.
The transfer of water in soils is usually split into slow water movement through the soil matrix by slow convective flux, resulting from a piston-like flow (Jardine et al., 1990, McDonnell, 1990, Waddington et al., 1993, Rawlins et al., 1997, Sidle et al., 2000), and rapid or preferential flow via specific pathways which bypass a large proportion of the soil (Jardine et al., 1989, Kung, 1990, Tsuboyama et al., 1994, Köhne et al., 2009). As recalled by Deurer et al. (2003), the causes of preferential flow are related to hydraulic characteristics such as the occurrence of macropores (Beven and Germann, 1982, Jarvis, 2007), dual or multimodal pore-size distributions (Gerke and van Genuchten, 1993, Zurmühl and Durner, 1996), discontinuities of texture (Kung, 1990), the occurrence of biopores such as root channels (Bramley et al., 2003, Parsons et al., 2004) or the occurrence of glossic structures (tongues) as shown in Diab et al. (1988).
Forest soils require particular attention (Sidle et al., 2001, Uchida et al., 2001) mainly because their structure is rarely disturbed, hence the presence of well developed aggregates and pore systems, which are known to influence flow variability (Jarvis, 2007). Most agricultural soils are disturbed annually in the upper decimetres due to tilling operations, while the soil structure of forest soils and particularly the macropores and biopores (Beven and Germann, 1982, Hagedorn and Bundt, 2002) may last for decades or more, and may be continuous from the surface to the deepest soil horizons. For these reasons, the heterogeneity of forest soil structure is thought to have considerable effects on water flow.
Since it is always difficult to predict the spatial variability of the soil structure and thus to predict precisely what will occur on a specific forest site, the collection of information from field tracer experiments, such as the flow velocity or the porosity involved in transfer, is a crucial step for the precise calibration and setting-up of deterministic flow or transport models (Ptak et al., 2004). This study aims to assess the water flow and non-reactive solute transfers occurring in a glossic acidic soil under a beech forest in Brittany. The specific objectives were to study the water and chloride transfers in this soil, to understand the spatial and temporal variability of these transfers and to produce a data set from this forest site for future modelling.
Section snippets
Materials and methods
We carried out a field tracer experiment in an established experimental site located in Fougères forest, western France. For this, we sprinkled chloride enriched solution over two areas of soil (2 × 66 m2) in March, 2006 and monitored the composition of the soil solution collected by zero tension and tension lysimeters over a period of 18 months.
Hydrology
The cumulative precipitation was 876.4 mm for the hydrological year 2005–2006 and 1195.2 mm for 2006–2007 (Fig. 2a). The cumulative Penman evapotranspiration was 477.3 mm and 440.4 mm respectively for the above years.
In 2005 and 2006, the daily volumetric soil moisture values (Fig. 2b) followed a classic scheme with a period of stability before the growing season, a major decrease during the growing season (beginning around 1st June) and an increase from the beginning of autumn to reach soil
Discussion
In order to explain the chloride transfer through the soil, soil porosity needed to be divided into two components: i) the rapid-mobile porosity, responsible for the rapid transfer of water through distinct pathways travelling across the soil matrix and by-passing most of its volume and ii) the slow-mobile porosity, in which most of the solutes are transferred slowly by convective flux through the soil matrix.
The mixing phenomenon which may occur between old water and new water brought in from
Conclusion
The qualitative approach proves that preferential flows through rapid-mobile porosity and slow transfers by convective flux through slow-mobile porosity coexist in the soils of the experimental plot, and the time scales brought into play ranged from a few days to a yearly scale. The rapid transfer is mainly governed by the soil moisture combined with precipitation intensities and the slow transfer is mainly due to the cumulative percolation flux. Moreover, the rapid-mobile porosity involves
Acknowledgements
We would like to thank all the technicians without whom this project would not have been possible, in particular C. Antoine, S. Didier and L. Gelhaye from INRA Nancy.
This work was financed by the EFPA department (INRA), the GIP ECOFOR and by the Office National des Forêts in the context of one of the Environmental Research sites on “Lowland beech” part of F-ore-T network.
The English was corrected by Aldyth Nys.
References (44)
- et al.
Drainage networks in soils. A concept to describe bypass-flow pathways
Journal of Hydrology (Amsterdam)
(2003) - et al.
Water movement in a glossaqualf as measured by two tracers
Geoderma
(1988) - et al.
A lumped water balance model to evaluate duration and intensity of drought constraints in forest stands
Ecological Modelling
(1999) - et al.
The age of preferential flow paths
Geoderma
(2002) - et al.
Unsaturated solute transport through a forest soil during rain storm events
Geoderma
(1990) - et al.
A review of model applications for structured soils: a)Water flow and tracer transport
Journal of Contaminant Hydrology
(2009) Preferential flow in a sandy vadose zone: 1. Field observation
Geoderma
(1990)- et al.
Comparison of stable-isotopic composition of soil water collected from suction lysimeters, wick samplers, and cores in a sandy unsaturated zone
Journal of Hydrology
(1999) - et al.
Solute transfer in the unsaturated zone-groundwater continuum of a headwater catchment
Journal of Hydrology
(2007) - et al.
Physical and chemical controls of preferred path flow through a forested hillslope
Geoderma
(1990)
Numerical and experimental study of solute transport in unsaturated soils
Journal of Contaminant Hydrology
Tracer tests for the investigation of heterogeneous porous media and stochastic modelling of flow and transport – a review of some recent developments
Journal of Hydrology
Runoff mechanisms in a forested groundwater discharge wetland
Journal of Hydrology
Impact of afforestation on ground water resources and quality
Hydrogeology Journal
Comparison between suction and free-drainage soil solution samplers
Soil Science
Macropores and water flows in soil
Water Resources Research
Etude hydrochimique saisonnière dans trois sols acides: composition des eaux gravitaires et des solutions extraites à pF 4.4
Science du Sol
Floodwater infiltration through root channels on a sodic clay floodplain and the influence on a local tree species Eucalyptus largiflorens
Plant and Soil
Rainfall intensity affects transport of water and chemicals through macropores in no-till soil
Soil Science Society of America Journal
A dual-porosity model for simulating preferential movement of water and solutes in structured porous media
Water Resources Research
Soil solution nutrient concentrations sampled with tension and zero-tension lysimeters: report of discrepancies
Soil Science Society of America Journal
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