The Roubine catchment located in the experimental research station of Draix-Bléone (south French Alps) is situated in Callovo-Oxfordian black marls, a lithology particularly prone to erosion and weathering processes. For 30 years, this small watershed (0.13 ha) has been monitored for analysing hillslope processes on the scale of elementary gullies.
Since 2007, surface changes have been monitored by comparing high-resolution digital elevation models (HRDEMs) produced from terrestrial laser scanner (TLS). The objectives are (1) to detect and (2) to quantify the sediment production and the evolution of the gully morphology in terms of sediment availability/transport capacity vs. rainfall and runoff generation. Time series of TLS observations have been acquired periodically based on the seasonal runoff activity with a very high point cloud density ensuring a resolution of the digital elevation model (DEM) on the centimetre scale. The topographic changes over a time span of 2 years are analysed.
Quantitative analyses of the seasonal erosion activity and of the sediment
fluxes show and confirm that during winter, loose regolith is created by
mechanical weathering, and it is eroded and accumulates in the rills and
gullies. Because of limited rainfall intensity in spring, part of the
material is transported in the main gullies, which are assumed to be a
transport-limited erosion system. In the late spring and summer the rainfall
intensities increase, allowing the regolith, weathered and
accumulated in the gullies and rills during the earlier seasons, to be washed out. Later in
the year the catchment acts as a sediment-limited system because no more
loose regolith is available. One interesting result is the fact that in the
gullies the erosion–deposition processes are more active around the slope
angle value of 35
It is also observed that there exist thresholds for the rainfall events that
are able to trigger significant erosion; they are above 9 mm rainfall or of
an intensity of more than 1 mm min
This study improves knowledge of the spatial distribution of erosion seasonality in badlands and demonstrates the potential of careful 3-D high-resolution topography using TLS to improve the understanding of erosive processes.
This study is integrated into the cross-disciplinary research activities
conducted in the Draix-Bléone catchments (SOERE-RBV, i.e. network of catchments for
the study of the critical zone; see
The region of Draix, where the study was conducted, is located within
Jurassic, black marls of Callovo-Oxfordian age (also called
A terrestrial laser scanner (TLS) is a powerful tool to monitor erosion processes on the gully scale at a relatively low cost (Perroy et al., 2010; Jaboyedoff et al., 2012) where high spatial-resolution data on surface changes are needed (Jacome, 2009). Such high-resolution mapping of erosion rates on a fine (e.g. seasonal) temporal scale for an entire catchment is innovative and represents considerable progress in the field of erosion assessment (Lopez Saez et al., 2011). Preliminary studies show a great potential of TLS to measure and map surface erosion (Puech et al., 2009) since it can detect millimetre-scale changes at short-range distances (50 m; Abellán et al., 2009).
In this study, time series of intra-annual TLS observations are used to quantify surface erosion. The main objectives are (i) to create erosion and deposition maps for every season; (ii) to estimate the sediment budget and evaluate the accuracy of the volume calculation on the catchment scale by comparing it to sediment trap observation; and (iii) to propose a conceptual model describing the observed seasonal pattern of erosion and deposition. The results allow the identification and quantification of the topographic changes in the catchment in terms of regolith development, slope transfer processes, and transient storages of sediment within the rills and gullies. This is placed in the context of previous work on similar black marl slopes, confirming the high impact of season on erosion process cycles.
Location and picture of La Roubine.
The research has been conducted in the Draix-Bléone experimental
catchments (SOERE RBV network, Systèmes d'Observation et
d'Expérimentation pour la Recherche en Environnement Réseau de Bassins Versants) in south-east France, near the city of
Digne-les-Bains (Alpes-de-Haute-Provence). Draix-Bléone observatory is
composed of seven small mountain watersheds. It was created by IRSTEA
(Institut national de recherche en sciences et technologies pour
l'environnement et l'agriculture) in 1983 in order to better understand
erosion and sediment transfer processes, including hyperconcentrated floods,
and to improve the design of protections in response to erosion processes.
The experimental site selected for this work is the Roubine elementary
catchment (Fig. 1), which is the smallest (1330 m
This elementary watershed has a typical badland morphology, characterized by
v-shaped gullies, steep slopes (35 to 45
A sediment trap, a stream gauge and an automatic sampler are installed at the bottom of La Roubine in order to monitor the sediment yield and the water discharge (Mathys et al., 2003). Rainfall observations are collected by a rain gauge located 20 m from the Roubine outlet.
Large areas of south-east France are covered by black marls, which outcrop
over more than 10 000 km
The mean erosion rate of the black marls averaged over 3 years (1985–1987)
is 8 mm yr
The specific features of a Mediterranean mountain climate influence slope
erosion rates with strong seasonal and yearly differences in temperature and
rainfall. At Draix, the mean annual rainfall is 920 mm, with an interannual
variability of nearly 400 mm over the period 1970–2000. The summer is
relatively dry, but several heavy thunderstorms can occur, the intensity of
which sometimes exceeds 60 mm h
Observed height differences for the period 2007–2010 highlighting the soil surface changes of La Roubine catchment. The red outline indicates the boundary of the catchment (see large version in the Supplement).
The study site has been monitored using a TLS. This remote scanning device is a monochromatic laser pulse
transmitter/receiver. The laser beam pulses are oriented using mirrors or by
moving the laser source mechanically or both. The time of flight (TOF) is
the time for the pulses to travel the double distance (
Characteristics of the TLS alignment with, for each time period, the information on the number of scans used to create a scene. The table indicates standard deviation (SD) of the point from the surface matched (point-to-surface ICP), the mean difference between two scans (either used to align with period or for inter-period comparison), the average point density used, and the standard deviation of the average mean difference.
O: lidar Optech Ilris 3-D; L: lidar Leica TotalStation II.
Twelve TLS campaigns were performed from 9 May 2007 to 4 November 2010 (Table 1). The data of the years 2007 and 2008 are sparser as the methodology and protocol were being developed. In 2009 and 2010, TLS data were acquired more regularly throughout the year in order to take into account more precisely the influences of the seasonal rainfall.
For each TLS campaign, the measurements were performed from up to five different
scan positions in order to minimize shadow areas. However, some shadow areas
still remain because of the presence of foreground in the line of sight of
the scanner or of vegetation. All scan spatial resolutions are less than 10 mm at a 50 m distance range. The point cloud density is very high for all
the time series and ranges from 0.3 to 3 pts cm
The software Polyworks V. 9.1.8. (InnovMetric, 2010)
was used to process the TLS point clouds. First, the vegetated areas are
deleted from the raw point clouds in order to keep only the bare soil surface
for the analysis. The scans of each campaign are then aligned using the
iterative closest point (ICP) procedure (Chen and Medioni, 1992; Besl and
McKay, 1992) to obtain a point cloud of the entire catchment. The TLS
campaigns are aligned to a reference campaign (e.g. June 2009) using eight
white 180 mm diameter styrene spheres located around the catchment since
2008. Depending on the TLS distance of acquisition and the overlapping of the
different scenes, the final point cloud density is variable; thus, each TLS
point cloud has been interpolated into a homogeneous 0.02 m high-resolution
digital elevation model (HRDEM) using the Surfer 8 (GoldenSoftware) inverse
distance method (Shepard, 1968). A high-density point cloud produces an
over-defined problem during the interpolation due to too many points being
present in one grid cell (Schürch et al., 2011). If we do not consider
the systematic error (which will be discussed separately later), the law of
large numbers (Kromer et al., 2015) indicates that the accuracy will increase
when the average is taken, assuming that the surface is locally planar, on
the centimetre scale. As an example, considering that the lower accuracy for
one point is
We do not consider horizontal error for two main reasons: (1) the effect of the slope orientation on the measurement of point location is very similar for each scan (the average slope does not change significantly between scans); (2) when merging the scan with ICP, the errors between scans include their horizontal components. This error does not exceed the thresholds we used for detecting erosive processes (see Sects. 3.5 and 4.1). Assuming the systematic errors caused by slope orientation are similar (except for identified other problems) for all scans, we do not tackle this topic in more detail.
To quantify and map the erosion and deposition through time, the most recent HRDEM was subtracted from an earlier HRDEM (DeRose et al., 1998). The resulting elevation changes have negative pixel values representing erosion and positive pixel values representing deposition.
Eleven differences between successive HRDEMs providing digital elevation models (DEMs) of differences (DoDs) have been calculated from 9 May 2007 to 4 November 2010 (Fig. 2). Because the occurrence of processes is strongly related to each season, the HRDEMs are sorted by seasonal periods. The mapping of the different erosion and deposition areas is carried out for each season by taking into account the DoDs of the corresponding seasons. These comparisons allow the detection and mapping of the most erosion–deposition-prone areas in the catchment (Fig. 3) and the characterization of the annual pattern of erosion (Betts and DeRose, 1999). These maps represent a synthesis of the DoDs (Fig. 2) showing the erosion or deposition areas. On these maps (Fig. 3), the erosion areas are displayed in orange and the deposition areas are in blue (see also the video in the Supplement).
Typical pattern of slope activity (erosion in orange; deposition in blue) for each predefined season. The red outline indicates the boundary of the catchment (see also the video in the Supplement).
Measured volume of erosion and deposition. In order to perform a comparison with Table 3, 0.48 m
Measured volume in the sediment trap.
To quantify the volume of soil surface changes, the elevation differences
are summed and multiplied by the DEM squared cell resolution (0.004 m
In order to evaluate the role of topographic proxies (slope angle, slope
aspect, upstream contributing area (UCA), TOBIA index; Meentemeyer and Moody,
2000), the DoDs of each period are averaged on a 10 cm grid to reduce noisy
values. Several topographic parameters are computed on the June 2010
topography, using ArcGIS (slope angle, aspect, and upstream contributing
area) and Matlab (TOBIA). Gullies are identified as pixels with UCA values
over 10 m
The TOBIA index is computed assuming a uniform dip and dip direction of the
black marls at 25
The use of TLS observations resulted in several sources of errors that were quantified. First, TLS measurements are affected by instrumental errors as described by Abellán et al. (2009). These authors showed, in particular, that when averaging point clouds, the detection of changes improved significantly. Second, some scans are also affected by deformation due to atmospheric conditions. Third, some DoDs contain some misalignment characterized by slight tiling due to scan deformation and/or scan alignment inaccuracies; such a source of error is, for instance, clearly visible in Fig. 2f. The use of a high threshold value to validate DoD values to detect the process of erosion or deposition permits us to escape the problems of misalignment or atmospheric deformation (see below). In fact, the quality precision and precision of the two successive alignment procedures, i.e. the scan merging and the “georeferencing”, vary from one measurement campaign to another as the TLS instrument, the scan deformation, and the spatial resolution were different (Table 2).
A possible procedure to estimate the overall errors is to define the quality of (1) the alignments for each campaign and (2) the alignment between campaigns. This corresponds to the measurement of the dispersion of the points between two merged scans (1). This is performed by identifying the areas that have not changed (fixed surfaces like the spheres, wall, etc.). The average distance between two scans and the associated standard deviation is measured using the point-to-surface method. A similar procedure can be applied to successive campaigns (2). Note that the averaging of the HRDEM allows us to reduce the noise and minimize other measurement errors.
DoDs outline the slope surface changes on a centimetre scale (Fig. 2). All the DoDs are presented in a plan view on a hillshade of the watershed. The eastern limit of the catchment lacks data because of dense vegetation cover. The differences under 1.8 cm are not displayed. This limit has been chosen according to a trial and error procedure and assuming that it contains in most cases above 3 times the maximum error of alignment of 6 mm, except for the scans of 2007. A clustering of the height differences in three classes is proposed both for erosion and deposition: small height differences ranging from 1.8 to 4.0 cm; moderate height differences ranging from 4.0 to 10.0 cm; large height differences above 10.0 cm in absolute value. The erosion pattern is displayed in warm colours, while the deposition pattern is displayed in cold colours.
Vertical change over time vs. slope angle. Rates are calculated from a 10 cm grid average of original DoDs and divided by the time separating the two lidar acquisitions. Slope angle in degrees, calculated in ArcGIS on a 10 cm DEM of June 2010. Top graphs of the left column show rates for slopes and the right column for gullies and adjacent (20 cm or closer) pixels (see the Supplement for similar graphs showing vertical change in absolute value vs. slope angle).
Five erosion patterns are distinguished for La Roubine catchment: winter,
spring, late spring to early summer, summer, and autumn.
The winter seasons erosion pattern is illustrated by the DoDs between 3 November 2009 and 23 March 2010 (Fig. 2g). The spring seasons erosion pattern is represented by DoDs between 14 April to 1 June 2009 and 23 March to 28 May 2010 (respectively Fig. 2d
and h). The erosion pattern of late spring to early summer rainy season is
illustrated by the DoDs between 9 May to 24 June 2007 and 28 May to
23 June 2010 (respectively Fig. 2a and i). In 2009, the rainfall amount in this season was 30 % below average up to 20 December and 19 %
below average for the whole year, only thanks to some late precipitations in
the last days of the calendar year, most probably snow. Thus no DoD
corresponds to that period in 2009. The summer season erosion pattern is represented by DoDs between 1 June to 11 August 2009 and 23 June to 22 September 2010
(respectively Fig. 2e and j). The autumn season erosion pattern is represented by DoDs between 11 August to 3 November 2009 and 22 September
to 4 November 2010 (respectively Fig. 2f and k). During winter seasons (Fig. 3e), the alternating freezing–thawing cycles
favour regolith development. Gelifluction (and solifluction during
thawing) in the upper parts of the gullies and on the steep slopes
surrounding the gullies leads to an accumulation of material in the lower
parts of the gullies. The soil surface changes are mostly located on the
south-facing slopes of the watershed. A higher number of freezing–thawing
cycles results in a higher amount of regolith that can be mobilized (Maquaire
et al., 2002; Raclot et al., 2005) and therefore more production of sediment
along the slopes.
During the spring seasons, (Fig. 3a), the sediment accumulated during winter
in the main gullies is transported at the outlet of the catchment. This
transport is generally limited to the two main gullies. Erosion is
transport-limited and consequently only the gullies with the larger
contributing surface are drained.
During late spring to early summer rainy seasons (Fig. 3b), rainfall can be relatively intense (Mathys et al., 2005). Consequently, the secondary rills, gullies, and the steepest slopes can be strongly affected by Hortonian runoff and the weathered regolith developed during winter can be washed out. The south-facing slopes are more prone to such a type of erosion pattern as more weathered sediment is produced during winters. Often, small deposition levees are observed in the lower (flatter) parts of the gullies.
During the summer seasons (Fig. 3c), characterized by relative drought except for the occurrence of a few thunderstorms, there is very little erosion activity. The same small gullies and rills as those affected by the late spring erosion pattern continue to be eroded in their steepest parts, while deposits and small levees are formed in their lower parts. Most of the loose weathered regolith has already been washed away. The less frequent but more intense storms observed in this season do not impact the erosion pattern, now sediment-limited.
During the autumn seasons (Fig. 3d), the rainfall pattern is characterized by long and low-intensity events with some very short intense precipitation leading to slow soil wetting and consecutive increase in soil moisture. Progressively, a new layer of regolith is created, and in most of the rills and gullies, sediment transport is reactivated.
DoD value changes over time are plotted against topographic proxies. The DoD values are displayed for slopes (left) and gullies (right) for each period (Figs. 4–6; in the Supplement, similar figures provide DoD values plotted against topographic proxies).
The seasonal cycle is clearly visible on the slope angle vs. DoD rate of
change (Fig. 4), with the deposition–erosion cycle during the year and
regolith expansion. Autumn and winter periods see an accumulation of loose
sediments in the gullies, which are then eroded in late spring or early
summer. We may suspect that in winter, the cycle is a slow continuous process and in
late spring to early summer it was subject to an intensive short erosive event (this can be also an
artefact because of the unequal time between surveys). In those gullies,
both maximum deposition (November to March) and erosion (March to May) are
reached at a slope angle of 35
The slope aspect proxy (Fig. 5) also shows the annual cycle and different behaviour between slopes and gullies. South- and south-west-facing slopes are more susceptible to increased erosion and deposition and probably expansion (particularly in the November to March and June to September periods).
Vertical change over time vs. slope aspect. Rates are calculated from a 10 cm grid average of original DoDs and divided by the time separating the two lidar acquisitions. Slope aspect in degrees, calculated in ArcGIS on a 10 cm DEM of June 2010. Top graphs of the left column show rates for slopes and the right column for gullies and adjacent (20 cm or closer) pixels (see the Supplement for similar graphs showing vertical change in absolute value vs. slope aspect).
Vertical change over time vs. upslope contributing area. Rates are calculated from a 10 cm grid average of original DoDs and divided by the time separating the two lidar acquisitions. Upslope contributing area in square metres, calculated in ArcGIS on a 10 cm DEM of June 2010. Top graphs of the left column show rates for slopes and the right column for gullies and adjacent (20 cm or closer) pixels (see the Supplement for similar graphs showing vertical change in absolute value vs. contributing area).
The upstream contributing area shows the same seasonal pattern, with deposition (gullies) and expansion (slopes) in winter and erosion in early summer. From June to September, DoD values are inversely proportional to UCA in slopes, with gullies showing smaller changes (Fig. 6).
TOBIA index values show that the Roubine gully is mainly composed of orthoclinal slopes (52 % of area) and steepened escarpments (39 % of area), consistent with its east–west incision into east-dipping marls, but no link can be seen between erosion activity and structural outcropping conditions. The intersection between bedding and topography is very similar across the whole catchment.
For the quantification of the seasonal sediment budget changes, the TLS data of the years 2007 and 2008 have been ignored because TLS data did not cover the largest possible area of the catchment but others did. The eroded volume trend estimated with TLS (Table 2) is in agreement with the integrated measurement of coarse-sediment transfer at the outlet trap (Table 3; Fig. 7). The difference is in an acceptable range (5.5 % for the period from 14 April 2009 to 4 November 2010 on the total cumulated volumes).
Cumulated TLS-measured volumes (in bold black line) against
cumulated calibration on sediment-trap-measured volumes (thin black line) for
the years 2009 and 2010. In addition, the cumulative precipitation is shown
as well as the daily precipitation events (in grey) and the main daily
precipitation events that are considered as significant in red
(> 20 mm day
The maximum eroded volume is produced for the period March–June 2010 (Table 2) with a sediment transfer of ca. 8.7 m
With the TLS measurements, the balance value is negative when the erosion
sediment volume is larger than the deposited sediment volumes. The presence
of shadow areas in the TLS scans affects the TLS sediment budget as
erosion-prone areas can be hidden from the laser pulse. When these
erosion-prone areas are hidden, the deposited sediments can be considered to
be more important than the erosion volumes. The balance is therefore positive,
as for example for the period June–September 2010 (Table 2). The shadow
areas are usually located in the upper parts of the slopes and are often very steep
and close to the crests of the catchment; these areas are highly productive
sources of sediments. Therefore, it is hypothesized that the sediment budget
is underestimated for most of the periods. The value of eroded volumes based
on the sediment trap is increased by 20 % to include the suspended
sediments; otherwise the true erosion rates are underestimated. In addition, although
the swelling or inflation of the regolith surface (Bechet et al., 2015) can
have an influence on the georeferencing, it was not possible to quantify it
at the level of the Roubine catchment. Nevertheless, it has been shown that
the unweathered black marl density is 2650 kg m
Also, topographic height differences smaller than the TLS threshold of 1.8 cm are not integrated into the sediment budget although they could contribute to an important sediment volume because of their possible widespread occurrence, mainly during the summer storms that trigger important Hortonian runoff. This limitation also influences the sediment budget by underestimating the total volume of erosion. But because of the coherence of the results, we consider that it can be a base for an interpretation of the catchment erosional system.
Conceptual model describing the seasonal pattern of erosion and deposition and quantification of the volumes of sediment transfer at La Roubine catchment.
All the results can be synthesized in a conceptual model describing the seasonal pattern of erosion and deposition and quantifying the volumes of sediment transfer (Fig. 8). This seasonal pattern is controlled by the rainfall distribution and the availability of sediment during each period. The results may thus be different depending on the year, but the sequence will not change; only the time lapses between major erosive events will change.
During the spring and summer seasons, the sediment transfer consists of the erosion of the weathered loose regolith layer on the slopes and a mobilization of the transient storages of accumulated sediments in the rills and gullies (right graphs in Figs. 4, 5, and 6) if no exceptional rainfall event occurs. But during the late spring and early summer, intense rainfall events produce very high erosive events (Figs. 4, 5, and 6 and the Supplement). Most of the sediments exiting the Roubine catchment are a product of the winter weathering. It appears to be a slow process, with rates of change smaller than in the other periods, but it is longer, and as a consequence the total changes are important but can be an artefact due to the time period between surveys (see and compare Figs. 4–6 and the Supplement). The erosion progressively evolves from a transport-limited (at the beginning of spring) to a supply-limited (in summer) pattern. However, diffuse erosion may happen during intense summer storms as heavy drops may detach and displace small particles, creating sparse local erosion and deposition. Hortonian runoff may also be generated, but its effects could not be measured with the TLS technique as a higher accuracy is necessary. In the autumn seasons, a new layer of loose regolith is progressively created (Fig. 4, left graphs), and if an intense rainfall event has occurred before in summer, as is usually the case, then the quantity of available sediment is limited (Figs. 4, 5, 6, right graphs).
The main events summarized above all occurred after more than 9 mm rainfall,
which is also the limit for initiating runoff in a larger similar catchment
(Laval) after more than 5 antecedent dry days (Mathys et al., 2000). That
threshold is lower if the dry period is smaller than 5 days. These events
either possess an intensity that reaches 1 mm min
It is also clear that sediment transfer depends on the material available
(Bardou and Jaboyedoff, 2008). The winter period, because of frost–thaw
cycles and low rainfall intensity, permits the creation and accumulation of weathered material. The relatively low rainfall intensities of these winter
periods permit the mobilization of partial material that remains within the slope
and gullies. It is also interesting to note that the most active zones within the gullies, both
regarding erosion and deposition, are located around a slope
angle of 35
The difference between rills and inter-rill erosion depends on rainfall intensity and antecedent rainfall amounts. The accumulated material in the rills can be mobilized by moderate rainfall intensities for fine material, while inter-rills need intense precipitation and/or a well-developed upper part of the regolith. In winter, the upper regolith probably only moves a short distance on a centimetre scale by small mass movements (Bechet et al., 2015) and on a metre scale by MDFs. In spring, the material accumulated in the rill is washed away, and later the inter-rills and rills with small contributing areas can be eroded and the material transported outside the catchment. Autumn permits the material accumulated in the main rills during summer to be cleaned. This scheme may change depending on the future climate if less precipitation occurs and only intense rainfall events remain. The system could then concentrate the full erosion in one or a few events. A warmer climate may also reduce the number of frost–thaw cycles and thus also reduce the depth of the regolith layer generated every year. But in any case, the seasonality that leads to weathered material will remain in the cold period, which is the main producer of sediments that can be mobilized.
The observed high production of loose regolith is mainly caused by the
alternating of freezing–thawing and wetting–drying cycles, which is the key
process controlling the weathering of black marl slopes (Maquaire et al.,
2002; Brochot et Meunier, 1994). At the end of the winter season, a thick
layer of loose regolith can be accumulated in the areas of the slopes and
gullies with slope angles around 35
Another point is runoff seasonal changes. It is transport-limited in spring,
which is probably mainly caused by the limited amount of intense rainfall
during this season. Looking at the flow accumulation values, the gullies
that have a contributive area smaller than 100 m
By their strong and rapid responses to climate forcing, the black marls of Draix-Bléone are a good basis for the analysis of erosive processes. This article confirmed the results of previous works, i.e. the strong dependence on the seasons and the cycle of processes (Jacome, 2009), the issues linked to the sediment trap measurements, and the estimation of density of the regolith (Mathys, 2006). The prediction of the responses of small mountain watersheds to climatic events is improved.
TLS has proved to be an appropriate tool to monitor gully erosion while being easily reproducible and accurate all at once. It also allowed working on the centimetre scale with success. The method used to create and compare DEMs proved very effective to map and quantify topographic changes, but some difficulties have still to be solved to fully quantify the sediment transfer.
The TLS permits us mainly to locate the different processes. This first results show that the rainfall pattern, i.e. time series, intensity, and duration, controls the sediment delivery sequence, but the process of weathering (mainly freezing and thawing) is fundamental to providing material for either suspended load or bedload. The interplay of rainfall and weathering creates the seasonal pattern. The complete erosion processes seem to apply during winter, and they are slow transport-limited processes by weathering (swelling). However, they affect important volumes and later in summer the behaviour is sediment-limited, while from late spring to early summer the erosion is very intensely controlled by intense short erosive events induced by intense rainfalls. Further investigation could also focus on the dry granular transport and its role in the accumulation of sediments during winter periods.
Here we have shown the limits of the methods. Further investigations with HRDEM must be carefully set up in order to avoid errors from data acquisition. In addition, this surface monitoring must be coupled with more variables monitored simultaneously during the event, such as soil moisture, swelling, rain drop size, grain size distribution, and soil density. Furthermore, a density map of the regolith inside the catchment throughout the year (season by season) would be a great help to improve the TLS volume correction, but this will require physical intervention in the catchment to install sensors and collect samples.
Such procedures are not possible in the Roubine catchment, which must not be disturbed by human activity. A new small catchment (Roubinette) has thus been instrumented on the site of Draix, which will permit the installation of instruments inside the watershed itself. The next step will be to acquire TLS data during a storm event.
The data may be accessible on direct request to the corresponding author because no structured repository exists at present.
We would like to thank the IRSTEA who let us work on their field and who gave us valuable data. Thanks to the GIS Draix and particularly to S. Klotz (IRSTEA). We dedicate this paper to the first author, Jacques Bechet, who died in a snow avalanche on 28 March 2015. The content of this paper is an expression of his great ingenuity, curiosity and passion for research he shared with his co-worker Julien Duc. We will ever remember his enthusiasm. Edited by: G. Sofia Reviewed by: three anonymous referees