Combined integrated hydrological modeling and particle tracking algorithms to infer the temporal variability of transit and residence time distributions within the Strengbach catchment (Vosges mountains, France)

<p>The temporal variability of transit-time distributions (TTDs) and residence-time distributions (RTDs) in hydrological systems has received particular attention recently because of their ability to inform on elementary processes impacting geochemical signatures and water fluxes in ecosystems. To date, these distributions and their temporal variability have been mainly investigated through concentration measurements of conservative geochemical or isotopic tracers. Even though physically-based and distributed hydrological models can render interpretations of TTDs/RTDs in terms of processes and physical controls, the variability of TTDs and RTDs has barely been studied using distributed hydrological modeling. In this study, an integrated hydrological model has been coupled with particle tracking algorithms and applied to the Strengbach Catchment – a small mountainous catchment belonging to the French network of critical zone observatories – to investigate the eventual link between water storage in the catchment and the temporal variability of TTDs and RTDs. The model calibration is performed relying upon both classical streamflow measurements and magnetic resonance sounding, a geophysical measure sensible to the water content in the subsurface. The model is then run over a 10-year period for which time distributions are calculated at various deadlines. The results show that the response of the Strengbach catchment is uncommon with short mean transit times (approximately 150-200 days) and a weak variability of TTDs and RTDs with the water storage. This specific behavior is mainly linked to the small size of the system and specific climatic and topographic conditions. Because the hydrological model was calibrated on the basis of unusual data (local water contents inferred via MRS measurements), ongoing investigations target the evaluation of the sensitivity of transit time distributions with respect to uncertainties plaguing calibrating data.</p>

Partial forest harvesting effects on erosion flux in a headwater catchment (Strengbach catchment, France)

<p>Due to the increasing global need for wood, forest management and especially tree harvesting have become increasingly challenging for the sustainability of forest ecosystems. Indeed, the natural dynamics of solid exports in rivers can be strongly disturbed by anthropogenic activities including forestry. The impact of forest management on erosion flux can be due to tree logging but also to forest roads, skid trails, stream crossings required for silvicultural operations.</p><p>The impact of forestry on solid exports in mountainous environment has been studied in a small granitic watershed (0.8 Km²) located in the Vosges massif. Between July and August 2014, the Strengbach catchment (Observatoire Hydro-Géochimique de l’Environnement) was concerned by clear-cutting on some plots located near the main stream. This small extended forestry operation (2.3% of the catchment) involved the logging of trees and the implementation of skid trail network including poorly designed stream crossings. The bedload flux was estimated since April 2009. The suspended sediment (SS) flux was evaluated on the basis of stream water samples collected every 16 hours and during high-flow events since December 2012.</p><p>Before the forestry operation, the mean bedload flux was 2.5 T/yr±8% for a mean outlet runoff of 730 mm/yr, although the SS flux was 7.7 T/yr±10% for an outlet runoff of 950 mm/yr.</p><p>The forestry operation occurring in 2014 has involved a significant and quasi-immediate impact on the SS concentration and flux. As an illustration, the mean SS concentration of the stream was 129 mg/L (outside high-flow periods) the fortnight after the forestry operation beginning, whereas it was only 6.2 mg/L just before. In addition, the forestry operation led to approximately 5 to 6 times larger SS flux than that expected for the July-August 2014 period. The impact on annual SS flux was significant during two hydrological years, with an increase of +100% and +50% for 2014 and 2015, respectively.  This relatively high disturbance is mainly due to the implementation of non-improved stream crossings and skid trails, responsible for the introduction of a huge amount of fine soil particles into the stream. At the opposite, no clear influence of the forestry operation on the bedload export could be observed in 2014 whereas it was 2 times higher than that expected the following year. This delay of the tree harvesting impact on coarse sediment export can be explained by the trapping of bedload upstream of the logs constituting stream crossings during the forestry operation. After the logs removal, the trapped sediments needed several flood events to reach outlet, explaining the delay. Overall, a post-logging recovery time of approximately 10 months can be assumed for the solid exports following the forestry operation.</p>

Chemical and U–Sr isotopic variations in stream and source waters of the Strengbach watershed (Vosges mountains, France)

This is the first comprehensive study dealing with major and trace element data as well as 87Sr/86Sr isotope and (234U/238U) activity ratios (AR) determined on the totality of springs and brooks of the Strengbach catchment. It shows that the small and more or less monolithic catchment drains different sources and streamlets with very different isotopic and geochemical signatures. Different parameters control the diversity of the source characteristics. Of importance is especially the hydrothermal overprint of the granitic bedrock, which was stronger for the granite from the northern slope; also significant are the different meteoric alteration processes of the bedrock causing the formation of 0.5 to 9 m thick saprolite and above the formation of an up to 1m thick soil system. These processes mainly account for springs and brooks from the northern slope having higher Ca / Na, Mg / Na, and Sr / Na ratios, but lower 87Sr/86Sr isotopic ratios than those from the southern slope. The chemical compositions of the source waters in the Strengbach catchment are only to a small extent the result of alteration of primary bedrock minerals, and rather reflect dissolution/precipitation processes of secondary mineral phases like clay minerals. The (234U/238U) AR, however, are decoupled from the 87Sr/86Sr isotope system, and reflect to some extent the level of altitude of the source and, thus, the degree of alteration of the bedrock. The sources emerging at high altitudes have circulated through already weathered materials (saprolite and fractured bedrock depleted in 234U), implying (234U/238U) AR below 1, which is uncommon for surface waters. Preferential flow paths along constant fractures in the bedrocks might explain the – over time – homogeneous U AR of the different spring waters. However, the geochemical and isotopic variations of stream waters at the outlet of the catchment are controlled by variable contributions of different springs, depending on the hydrological conditions. It appears that the (234U/238U) AR are a very appropriate, important tracer for studying and deciphering the contribution of the different source fluxes at the catchment scale, because this unique geochemical parameter is different for each individual spring and at the same time remains unchanged for each of the springs with changing discharge and fluctuating hydrological conditions. This study further highlights the important impact of different and independent water pathways on fractured granite controlling the different geochemical and isotopic signatures of the waters. Despite the fact that soils and vegetation cover have a great influence on the water cycle balance (evapotranspiration, drainage, runoff), the chemical compositions of waters are strongly modified by processes occurring in deep saprolite and bedrock rather than in soils along the specific water pathways.

Chemical and U-Sr isotopic variations of stream and source waters at a small catchment scale (the Strengbach case; Vosges mountains; France)

This is the first comprehensive study dealing with major and trace element data as well as 87Sr/86Sr isotope and (234U/238U) activity ratios (AR) determined on the totality of springs and brooks of the Strengbach catchment. It shows that the small and more or less monolithic catchment drains different sources and streamlets with very different isotopic and geochemical signatures. Different parameters control the diversity of the source characteristics. Of importance is especially the hydrothermal overprint of the granitic bedrock, which was stronger for the granite from the northern than from the southern slope; also significant are the different meteoric alteration processes of the bedrock causing the formation of 0.5 to 9 m thick saprolite and above the formation of an up to 1 m thick soil system. These processes mainly account for springs and brooks from the northern slope having higher Ca/Na, Mg/Na, Sr/Na ratios but lower 87Sr/86Sr isotopic ratios than those from the southern slope. The chemical compositions of the source waters in the Strengbach catchment are only to a small extent the result of alteration of primary bedrock minerals and rather reflect dissolution/precipitation processes of secondary mineral phases like clay minerals. The (234U/238U) AR, however, are decoupled from the 87Sr/86Sr isotope system and reflect to some extent the level of altitude of the source and, thus, the degree of alteration of the bedrock. The sources emerging at high altitudes have circulated through already weathered materials (saprolite and fractured rock depleted in 234U) implying (234U/238U) AR < 1, which is uncommon for surface waters. Preferential flow paths along constant fractures in the bedrocks might explain the over time homogeneous U AR of the different spring waters. However, the geochemical and isotopic variations of stream waters at the outlet of the catchment are controlled by variable contributions of different springs depending on the hydrological conditions. It appears that the (234U/238U) AR is an appropriate very important tracer for studying and deciphering the contribution of the different source fluxes at the catchment scale because this unique geochemical parameter is different for each individual spring and at the same time remains unchanged for each of the springs with changing discharge and fluctuating hydrological conditions. This study further highlights the important impact of different and independent water pathways in fractured granite controlling the different geochemical and isotopic signatures of the waters.