How does dialogue between pedologists and hydrologist improve the knowledge on functional landscape entities? A case study in the Attert River basin, Luxembourg.
Abstract:
How does dialogue between pedologists and hydrologist improve the knowledge on functional landscape entities? A case study in the Attert River basin, Luxembourg.
Jérôme Juilleret, Jean François Iffly, Christophe Hissler and Laurent Pfister.
Department of Environment and Agro-Biotechnologies, Centre de Recherche Public Gabriel Lippmann, 41, rue du Brill, L-4422 Belvaux, Grand-Duchy of Luxembourg.
- 1. Introduction
The Weierbach catchment is characterized by a unique geological substratum of schist and phyllites. The winter streamflow response to a rainfall show two distinct peaks with markedly different time scales (Van den Bos et al., 2006b), whereas the summer response present only the first one. The hydrograph show that the first peak is near concomitant with rainfall. Chemical analyses have suggested that water collected during the two peaks has distinct chemical compositions and is likely to originate in different catchment compartments (Pfister et al., 2006). For Kavetski et al. (2011), the first peak is the result of direct runoff over the near-stream riparian zone according to the rise of dissolved organic carbon and the high velocity of reaction of the hydrograph. Untilnow, this assumption was never proved.
We hypothesized that the first peak is the result of direct runoff over the near-stream riparian zone according to the rise of dissolved organic carbon and the high velocity of reaction of the hydrograph. The second peak disappears in the summer season and is characterized by a delayed response of the basin, which usually reaches its maximum after 48 to 72 h. In this study, we investigate the potential origin of the fast component of water observed in the first peak based on combination of field expertise from an experimental soil scientist and a hydrologist. We highlighted that the internal structure of the Bedrock (, i.e. cracks density, tectonic fractures and their orientation) will control the time, pathway and volume fast bedrock subsurface flow.
- 2. Methodological approaches
2.1.Study area and physiography
The Weierbach catchment with an area of 0.44 km2, is located in the Luxembourgish Ardennes north Attert and is fully forested. Devonian schist and phyllite dominate deep bedrock geology. Whereas subsurface regolite is made of Periglacial Slope Deposits (PPSD) from which soils developed (Semmel and Terhorst, 2010; Juilleret et al, 2011).
From the plateau to the bottom of the hillslopes, soils are quite similar in their characteristics showing an important amount of coarse elements and silty matrix related to the different layers of the PPSD. On the plateau part of the basin, the occurrence of redoximorphic features shows that a nonpermanent water level occurs during winter. Contrariwise hillslopes soils do not present redoximorphic features and previous measures of saturated hydraulic conductivity indicates a high capacity of the soil to drain water.
In a classification point of view, soils of the catchment are generally not very deep, the bedrock occurs between 50 and 100 cm from the surface. They are classified according to the World Reference Base (WRB) soil classification (IUSS Working group WRB, 2007) as Stagnic Leptic Cambisol (Humic, Ruptic, Dystric, Endoskeletic, Siltic) in the plateau and as Leptic Cambisol (Humic, Ruptic, Dystric, Endoskeletic, Siltic) in the hillslopes. Along the riparian zone and in swampy areas, soil are more superficial and highly organic, they are classified as Histic Stagnic Leptosol (Dystric, Skeletic).
2.2.Hydrological monitoring
To support our research hypotheses, the Weierbach experimental catchment was equipped to quantify the contribution of the riparian areas during a single and quick flood event: streamgauge (ISCO© 4120) at the outlet of the catchment, raingauge (Young tipping bucket on a Cambell CR200X) in an open area of the catchment, three piezometers (instrumented with Ott CTD Orpheus mini) and 6 soil moisture monitoring stations (Campbell CR800 with CS650 sensors, 2 profiles at 10, 20, 40 and 60 cm depths). Streamgauge, raingauge and piezometers run at a 15-min time-step, whereas the soil sensors recorded the moisture at a 30-min time-step.
During the selected flood event, the volume of the water exported to the catchment and the related runoff coefficient were calculated at the outlet. The averaged interception was estimated to 1.5 mm, which correspond to previous studies made for similar land cover in our xperimental catchments. The combined behaviour of the piezograph and the soil moisture profile at the foot of the hillslope were analysed at the catchment scale. The effective quantity of rain that fell directly on the whole riparian area was estimated and compared to the water volume exported at the catchment outlet.
- 3. Results
3.1 What does the pedological survey tell?
The parent material of the first meter of the soil before the bedrock is made of different layers of the PPSD. The first material called “Upper Layer”, represent a silty-stony material made of loess and rock debris where horizon A & B developed. Where not eroded, it covers the land surface with a thickness of about 50 ± 20cm in all relief positions. Under it, and lying on the top of the bedrock, a “Basal Layer” arises and represents the C layer of the soil. The “Basal Layer” consists of debris of schist orientated parallel to the slope and is free of loess. Its thickness varies from 20 to100 cm. Finally, Bedrock (2R layer) occurs consisting of either phylittes, or schist. The latter rocks are characterized by their foliation The Bedrock is made of fractured and foliated rock. Foliation measurement shows a constant strike of 70° to the North in the entire basin whereas as the dip is considered as vertical. A consequence of the foliation is the occurrence of a distinct cleavage parallel to the foliation at the near surface bedrock. The spacing of the fractures varies from a few millimetres to 2 cm. This “geogenic macropores” are the consequence of the mechanical weakening of the bedrock by weathering and creeping.
From these observations, we can draw a runoff processes scheme of the Weierbach catchment. Measurements of saturated hydraulic conductivity on the A and B horizons have shown that the “Upper Layer” is extremely permeable. Consequently, water drains vertically through it into the “Basal Layer” where the amount and orientation of stones will act as a first hydraulic “brake” engendering “Subsurface Flow”. Despite an impermeable material in its internal properties, water does not move on top of the bedrock but is able to penetrate the Bedrock in the “geogenic macropores” created between the cleavage plan at the Near Surface bedrock. Water will move deeper until it hits the fractured bedrock. In fractured rocks like schist, water flow and storage occur mainly in the fractures, whereas the matrix presents low permeability (Bruel et al., 1994). This water flowpath is reported as bedrock flow in the literature (Oda et al, 2013; Uchida et al, 2010; Montgomery & Dietrich, 2002; Anderson et al, 1997). The bedrock flowpath is extremely important for recharge of lower slopes, groundwater levels and generating baseflow in some catchments (Fanning & Fanning 1989; Ticehurst et al., 2007). Bedrock flow can be split in other subcomponent processes like (1) near surface bedrock flow ( Montgomery et al, 1997) occurring top the bedrock, (2) bedrock fracture flow which penetrate the bedrock (Hughes, 2010). In our case the geogenic macropores of the near surface bedrock made from the opening cracks (fig 1) due to combination of weathering processes and creeping constitute a material able to generate a combination of processes. The main are near surface bedrock flow and bedrock fracture flow according to the depth of the geogenic macropores, density, connection and the essentially the saturation state of the material. The latter saturation state is seasonally driven, hence for the same hydropedological compartment (reservoir), processes will changes according to the saturation.
3.2. What does the hydrological monitoring show?
The event occurred during summer conditions (08/06/13), when the potential contributive reservoirs to the runoff are not connected anymore in the catchment. Rainfall, runoff, groundwater level and soil moisture dynamics were analyzed during the 3 hours of the flood event. 15.8 mm of rainfall were recorded in 1 hour, with more than 14 mm in half an hour. The rising limb of the hydrograph was completely synchronous with the rainfall pattern, indicating a very fast transfer from the catchment surface to the stream outlet. At a 15-min time step registering, no lag-time was observed and the baseflow was recovered 3 hours later.
The volume of water that fell on the riparian area was calculated and compared to the volume of the total peakflow. Due to natural daily fluctuations during low flows of the previous days, an average baseflow value of 5.2 l/s was used. Considering equilibrated water mass balanced at the catchment scale during the rainfall event, the riparian areas only generated 45% of the runoff. Therefore, other contributive reservoirs were activated in the catchment during this very fast rainfall event.
Soil moisture monitoring is made from the soil surface (10 cm depth) to the contact between the soil and the saprock (60 cm depth). In the entire catchment, the upper sensors (10 to 40 cm depth) indicated a maximum soil moisture 1 hour after the peakflow, whereas the deeper sensors (60 cm depth) exhibit a very little increase in soil moisture 4 to 6 hours after the peak flow.
Piezograph GW 3 presented a small and similar reaction like the soil superficial sensors with a maximum arising more than 1.5 hour after the peakflow. The depth between soil surface and groundwater table is close to 1.7 m.
Therefore, we can conclude that the intensive hydrological monitoring used during the flood event could not identify the very fast flowpath that contributes to the sharp peakflow recorded. We could hypothesize that the time step of the data collected do not capture the instantaneous reaction to rainfall.
- 4. Conclusion
Hydrological analyse have shown that only 40 to 50 % of the peakflow is generated by riparian zone and implies other contributing areas. This conclusion is consistent with the pedologist point of view, which highlighted the occurence of fast subsurface flow at the Soil-Bedrock interface and in the “geogenic macropores”of the near bedrock interface ass well than in the fractures. It is clear that our lack of knowledge on the internal structure of the bedrock allowing to better identify water pathway should be solve in the future. Hence, we propose to focuses on the characterization of the structure, and connectivity of the “geogenic macropores”by intergrating observations and data (density, orientation) ass well than their spatial relation with the river. Indeed, those macropores resulting of the initial foliation of the bedrock present a more or less constant orientation. Analysing the effect of the orientation of those preferential water pathways to the river is a challenging issue for the Weierbach and for similar basins in similar fractured geology. This issue can only be solved by real interdisciplinary approaches combining field geology, structural geology, geophysics, geomorphology and hydrology. Such interdisciplinary studies will help to better characterised the hydrological functioning of the entire “critical zone” of catchments and to improve our knowledge on functional landscape entities.
Acknowledgements
This study was part of the SOWAT project (Soil-water bypass and water connectivity at the headwater catchment scale) in the framework of the CORE research program (Contract no. C10/SR/799842). It also took place in the framework of the CAOS project (INTER/DFG-FNR).
References
Anderson, S. P., Dietrich, W. E., Montgomery, D. R., Torres, R., Conrad, M. E.& Loague, K. (1997) Subsurface flow paths in a steep unchanneled catchment, Water Resour. Res., 33, 2637– 2653.
Bruel, D., Cacas, M.C., Ledoux, E.& de Marsily, V. (1994), Modelling storage behaviour in a fractured rock mass, Journal of Hydrology, Volume 162, Issues 3–4, November 1994, Pages 267-278, ISSN 0022-1694.
IUSS Working group WRB. (2007). World Reference Base for Soil Resources 2006, First update 2007. World Soil Resources Report No. 103, FAO, Rome.
Juilleret, J. Iffly, J.F., Pfister, L.& Hissler, C. (2011) Remarkable Pleistocene periglacial slope deposits in Luxembourg (Oesling): pedological implication and geosite potential. Bulletin de la Société des naturalistes luxembourgeois 112: 125-130.
Hughes, D. A. (2010). Unsaturated zone fracture flow contributions to stream flow: evidence for the process in South Africa and its importance. Hydrol. Process., 24: 767–774.
Kavetski, D., Fenicia, F.& Clark, M.P. (2011). Impact of temporal data resolution on parameter inference and model identification in conceptual hydrological modelling: insights from an experimental catchment. Water Resour. Res. 47, W05501.
McGuire, K. J., & J. J. McDonnell. (2010). Hydrological connectivity of hillslopes and streams: Characteristic timescales and nonlinearities, Water Resour. Res., 46, W10543.
Montgomery, D. R., Dietrich, W. E., Torres, R., Anderson, S. P., Heffner, J. T. & K. Loague. (1997), Hydrologic response of a steep, unchanneled valley to natural and applied rainfall, Water Resour. Res., 33(1), 91–109.
Montgomery, D. R. & Dietrich, W. E. (2002)., Runoff generation in a steep, soil-mantled landscape, Water Resour. Res., 38(9), 1168.
Oda, T., Suzuki, M., Egusa, T. & Uchiyama, Y. (2013), Effect of bedrock flow on catchment rainfall-runoff characteristics and the water balance in forested catchments in Tanzawa Mountains, Japan. Hydrol. Process., 27: 3864–3872. doi: 10.1002/hyp.9497
Pfister, L., et al. (2006), Study of the water cycle components in the Attert River Basin (CYCLEAU), report, Fonds Natl. de la Rech., Luxembourg.
Semmel, A. & Terhorst, B. (2010). The concept of the Pleistocene periglacial cover beds in central Europe: A review. Quaternary International 222: 120-128.
Ticehurst, J. L., Cresswell, H. P., McKenzie, N. J. & Clover, M. R. (2007). Interpreting soil and topographic properties to conceptualise hillslope hydrology. Geoderma 137, pp. (279 – 292).
Uchida T, Asano Y, Ohte N. & Mizuyama T. (2003). Analysis of flowpath dynamics in a steep unchannelled hollow in the Tanakami Mountains of Japan. Hydrological Processes 17: 417–430.