influence of a hedge surrounding bottomland on seasonal soil-water movement
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HYDROLOGICAL PROCESSESHydrol. Process. 17, 18111821 (2003)Published online 2 April 2003 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.1214
Influence of a hedge surrounding bottomland on seasonalsoil-water movement
Virginie Caubel,1 Catherine Grimaldi,1 Philippe Merot1* and Michel Grimaldi21 INRA, Unite Sol et Agronomie de Rennes-Quimper, 65 rue de St Brieuc, 35042 Rennes cedex, France
2 IRD-INRA Unite Sol et Agronomie de Rennes-Quimper, 65 rue de St Brieuc, 35042 Rennes cedex, France
Abstract:The influence of a hedge surrounding bottomland on soil-water movement along the hillslope was studied on a plotscale for 28 months. The study was based on the comparison of two transects, one with a hedge, the other without, usingmainly a dense grid of tensiometers. The influence of the bottomland hedge was located in the area where tree rootswere developed, several metres upslope from the hedge, and could be observed both in the saturated and non-saturatedzone, from May to December. The hedge induced a high rate of soil drying, because of the high evaporative capacityof the trees. We evaluated that water uptake by the hedge during the growing season was at least 100 mm higher thanwithout a hedge. This increased drying rate led to a delayed rewetting of the soils upslope from the hedge in autumn,of about 1 month compared with the situation without a hedge. Several consequences of this delayed rewetting areexpected: a delay in the return of subsurface transfer from the hillslope to the riparian zone, a buffering effect ofhedges on floods, already observed at the catchment scale, and an increased residence time of pollutants. Copyright 2003 John Wiley & Sons, Ltd.
KEY WORDS hedge; oak; groundwater; soil water potential; evapotranspiration
Hedges are widely encountered in the world, but with different structures and roles. In western Europe, theyare mainly represented as bocage, a network of hedgerows enclosing fields (Baudry et al., 2000). This is avery old landscape structure, developed essentially during the eleventh and twelfth centuries, and then thenineteenth century. It was first considered as a border around properties and as a barrier for cattle. Since 1960,as many hedges were destroyed with agricultural intensification, concerns and studies about hedges developed,especially regarding their environmental role (social, economic and ecological). The role of hedges has beeninvestigated throughout the world, especially as windbreaks (De Jong and Kowalchuk, 1995), for limiting soiland water losses (Pihan, 1976; Kiepe, 1995; Alegre and Rao, 1996), and for protecting adjacent croplands(Buresh and Tian, 1998; Rao et al., 1998; Shively, 1998).
Only a few studies, however, have been conducted on the hydrological role of hedges in a temperate climate(Merot, 1999). On a catchment scale, comparison of watersheds differing in the intensity of bocage showedthat direct runoff volume and peak flow were one and a half to two times lower in the bocage catchment,and that the runoff coefficient was lower and more constant (Merot, 1978). Results based on hydrologicalmodelling, (Merot and Bruneau, 1993) demonstrated that the buffering effect of bocage on floods partiallyresulted from the bottomland hedge, which limited the extension of the saturated area generating overlandflow. Merot et al. (1999) demonstrated that bocage actually modified all catchment subsurface drainage andthat many hedges acted as a sink for water. They block the runoff and water can only infiltrate.
* Correspondence to: Philippe Merot, INRA, UMR Sol Agronomie, ENSA 65 route de saint Brieuc, CS 84215/35042 Rennes Cedex, France.E-mail: firstname.lastname@example.org
Received 2 August 2001Copyright 2003 John Wiley & Sons, Ltd. Accepted 8 August 2002
1812 V. CAUBEL ET AL.
On a local scale, measurements and observations emphasize the drying effect of hedges in summer, andthe vertical drainage capacity of the hedge and bank system (Carnet, 1978; Baffet, 1984), mainly owing tothe tree root system and to enrichment in organic matter. Ryszkowski and Kedziora (1987) underlined thehigh transpirative capacity of hedgerow trees owing to a well-developed root system and to turbulent airexchanges in the tree canopy. They calculated (Ryszkowski and Kedziora, 1993) that in some conditions,hedges perpendicular to the slope could take up all water passing through the root system by transpiration.There has, however, been no measurement of such a role at present.
Hydrological functioning of hedgerow systems moreover could be an important topic in water qualityrestoration in western Europe. Landscape structures such as wetlands and hedgerows could contribute to thereduction in transfer of pollutants to rivers (Peterjohn and Correll, 1984; Burel, 1996), and so complete actiontaken concerning low polluting agricultural practices. Whereas wetland efficiency in removing pollutantshas been thoroughly investigated (Gilliam, 1994), the potential role of hedges, assumed to result from thevegetation, is not really known (Merot and Reyne, 1995).
The bottomland hedge that surrounds riparian wetlands is a type of hedgerow frequently encountered inBrittany (Bazin, 1995). It is of particular interest because of its location between two different environments:the fertilized crop field on the hillslope and the hydromorphic unfertilized riparian zone (Merot and Reyne,1995). We assume that this hedge can modify the hydrological and geochemical features of fluxes from thehillslope, and affect the functioning of the riparian zone. In this context, our study aims at identifying thedirection and intensity of the water fluxes coming from the hillslope and passing through the bottomlandhedge to reach the riparian zone. This is a detailed study of one site, based on soil-water-potential monitoringalong two transects, one crossing the bottomland hedge, the other not crossing the bottomland hedge.
MATERIALS AND METHODS
Study siteThe study site is located in the Massif Armoricain (north-west France), 20 km west of Rennes, in Brittany.
The average annual precipitation for 30 years in the area is about 710 mm, and the average potentialevapotranspiration about 620 mm. The bottomland hedge, perpendicular to the slope, runs north-west tosouth-east, and is located about 60 m from a second-order stream (Figure 1a). It is planted with 100-year-old oaks (Quercus robur), 2 m apart, standing on a bank that induces a difference in height of about 04 mbetween the hillslope and the riparian zone. The hedge and bank were partially pulled out 40 years ago, whichenabled us to compare two transects, one crossing the other not crossing the hedge and bank (Figure 1a).
The upslope field was cultivated successively with wheat, rye-grass and maize, and was highly fertilized.The slope is steep and regular (5%) in the upper part, and gentle (
SEASONAL SOIL-WATER MOVEMENT 1813
Figure 1. Map of the study site: (a) general view; (b) the hedge and bank system of the transects, and the location of the water potentialmeasurements for the two transectscrosses stand for the monitored points
Tensiometers were placed following AFNOR recommendations (AFNOR, 1996), at depths of 02, 03, 04,05, 06, 08, 105, 13 and 155 m. They stop functioning when the matrix soil water potential reaches 8 m.Piezometers, made of 7-cm-wide PVC bottom sealed tubes, were placed in holes corresponding exactly totheir diameter, down to a depth of 18 m upslope and 12 m downslope. The deepest 40 cm of the tubeswere perforated. Although the water level measured in bottom perforated piezometers is formally equal tothe water table elevation only for vertical flow, the small horizontal water table gradient observed during thefield measurements allows this approximation.
Copyright 2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 18111821 (2003)
1814 V. CAUBEL ET AL.
No hedge transect
Figure 2. Lateral distribution of soil horizons for the two transects
Measurements were performed from February 1998 to June 2000, about every 15 days, or more frequentlywhen raining. Water table level was either measured in the piezometers or calculated from tensiometric data.Tensiometric data were collected with a calibrated pressure sensor. They were used to calculate the matrix soilwater potential, and the hydraulic head, which included the gravitational component of energy. The hydraulichead was calculated using the same elevation reference for all the tensiometers, i.e. the 10 m upslope pointon each transect. This enables the drawing of the soil water potential field along all the transects, and theanalysis of the direction and the intensity of the water fluxes. Interpolation was done by kriging, using Surfersoftware and a linear variogram calculated from the data set.
Precipitation and potential evapotranspirationPrecipitation P and potential evapotranspiration PET (Penman, 1948) were measured at the INRA
meteorological station of Le Rheu, 15 km east of the site (Figure 3). In 1998 and 1999, annual P was,respectively, 798 and 925 mm, and annual PET 609 and 751 mm. Precipitation was irregular during spring1998 (150 mm in April compared with 30 mm in May), conversely to spring 1999 and spring 2000. MonthlyPET was higher than monthly P from May to August, and summer 1999 was 60 mm dryer than summer1998. Autumns were wet, around 400 mm from September to December, although the rainy period began inAugust in 1999 but in September in 1998.
General hydrological dynamicsThe saturated zone. The depth from the soil surface to the water table, presented here for the monitored
points 10 m upslope and downslope (Figure 4), was very dependant on climatic conditions. Each year, weidentified three periods: