INTERACTIONS BETWEEN HEDGEROW, SOIL AND HYDROLOGY
Zahra Thomas1,2, P. Merot1
1INRA,
UMR1069, Soil Agro and hydroSystem, F-35000 Rennes, France
e-mail: zthomas@agrocampus-rennes.fr
2Agrocampus Rennes, Soil Agro and hydroSystem, F-35000 Rennes, France
Hedgerows networks are a very common and ancient rural landscape especially in Europe. Usually
used for their positive effects such as windbreaks and soil erosion reduction, they constitute particular
field boundaries which make landscape unmonotonous. In many countries, hedgerows were removed
between the late 1960s and 1980 to increase field size and farming productivity. Several authors
reported positive impact of hedgerows both for runoff reduction [Merot, 1999], soil characteristics
improvement [Kiepe, 1995], and wildlife conservation [Marshall et al., 2006]. The benefits of
hedgerows are now largely admitted and European legislation promotes hedgerow restoration since
1997.
Contribution of linear vegetation structures such as hedgerows and shelterbelts, as well as that of
open woodlands and isolated trees as a tool for hydro ecosystems managing is an interesting
perspective. In many regions of the world, a positive effect of a combination of trees and arable crops
is expected. The role of Silvoarable Agroforestry (SAF) in increasing biomass production
[Droppelmann et al., 2000], retaining nitrate [Livesley et al., 2002] and efficiently minimize erosion
[Angima et al., 2002] have been narrowly studied. A method to predict the environmental effect of SAF
at the farm and landscape scale was applied in France, Spain, and the Netherlands SAF reduced soil
loss [Palma et al., 2007]. Comparing simulated and observed results, Ticehurst et al., (2005) identified
several dominant processes to include in the numerical model in order to take into account the effect
of tree belts on hillslopes (Evaporation, surface runoff, lateral flow, and vertical drainage). The large
impact of hedgerows on water balance is due to their high transpiration rate [Thomas et al., 2008].
Indeed transpiration of 8 mm.day-1 has been observed [Herbst et al., 2007]. Ghazavi et al., (2008)
studied the impact of hedgerow on rainfall distribution, soil-water potential gradient, lateral water
transfer and water balance. Their results show that the key processes involved are interception and
transpiration with a significant spatial and temporal variation during the hydrological year.
The aim of our study is to understand interactions between vegetation, soil, and hydrology in order to
evaluate benefits of hedgerows on hydrological functioning according to climatic context. We
compared field observation and numerical simulations to represent dominant processes on
hydrological functioning. Hydrodynamic models used shows that simulations accuracy are related to
soil hydrodynamic parameters representation, and initial and boundary conditions of the flow domain
considered. The hydrological functioning is controlled both by soil distribution which is highly related to
topography and by interactions between groundwater, stream flow and the atmospheric phase of the
water cycle (Rainfall, PET). The high transpiration amount related to hedgerow is a key process on
soil-water control and lateral flow in the unsaturated zone. Root-water uptake depends on soil
characteristics and hillslope position (upslope, downslope). We observed that the highest root density
corresponds to the area where the soil is thicker. Whereas, the root exploration is stopped by a high
bulk density observed downslope. Interaction between soil-water transfer and groundwater spatiotemporal dynamic shows a specific behaviour on hydrological functioning in the hillslope zone. In the
beginning of hydrological year (autumn and winter), shallow groundwater dynamic is controlled by
stream-flow showing stream and groundwater connexion. The second part of hydrological year (spring
and summer) is characterized by hedgerow control due to root uptake activation and a high
transpiration amount which is closely depending on climatic context. Transpiration estimated using sap
flow method indicates a highly variable amount depending on tree characteristics and soil-water
availability. Hedgerow functioning is well related to climatic conditions and soil characteristics.
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