Monitoring Hillslope Moisture Dynamics with Surface ERT and Hydrometricmeasurement: Point a Case Study Frommountains, Ore Germany R

Home , Ore Mountains

Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ects water bal- ff ected by seepage ff erent core samples. ff 1 0.9 m), and a high amount results in erent amounts of precipitation (small, < erent depths were a ff ff . Nevertheless, there is still a gap of knowl- ff 5860 5859 , and A. Kleber 2 erentiated. A period with a small amount causes a short ff , T. Günther 1 erent types of response to di ff , K. Heller 1 To improve the knowledge of hydrological responses on hillslopes withIn periglacial addition, surface ERT measurements of several profiles were applied to enhance This discussion paper is/has beenSciences under (HESS). review Please for refer the to journal the Hydrology corresponding and final Earth paper System in HESS if available. Institute of Geography, Dresden University of Technology, Helmholtzstr. 10, Leibniz Institute for Applied Geophysics (LIAG), Stilleweg 2, 30655 Hannover, Germany water. Three di medium, high), could beinterruption di of the drying patterninduces at a the distinctive surface reaction in at summer,a whereas shallow strong a depth response medium reaching ( amount down to 2 m. tivity, the parameters ofTo optimize Archie’s inversion law parameters were andter determined methods, content using the distribution was derived di comparedcoincidence. spatial to The and tensiometer measured temporal resistivity data wa- shows andpending a resulting on close in the correlation remarkable amount with and precipitation. intensity De- of rain, di tion and transferability. cover beds, hydrometricalcatchment measurements in have the eastern been Ore carried Mountains since out November 2007. onresolution a of small punctual spring hydrometricmonitoring data. in From nearly Maydynamics weekly to on intervals December the was hillslope 2008 scale. implemented geoelectrical To to obtain the trace link seasonal between moisture water content and resis- Hillslopes are one of theways basic units within that catchments. mainly control Theance, water structure movement e.g. and infiltration, of flow retention, path- theiredge and shallow of runo subsurface the a hydrological dynamics on hillslopes, notably due to the lack of generaliza- Abstract Hydrol. Earth Syst. Sci. Discuss.,www.hydrol-earth-syst-sci-discuss.net/11/5859/2014/ 11, 5859–5903, 2014 doi:10.5194/hessd-11-5859-2014 © Author(s) 2014. CC Attribution 3.0 License. Monitoring hillslope moisture dynamics with surface ERT and hydrometricmeasurement: point a case study fromMountains, Ore Germany R. Hübner 1 01069 Dresden, Germany 2 Received: 9 May 2014 – Accepted: 14Correspondence May to: 2014 R. – Hübner Published: ([email protected]) 5 June 2014 Published by Copernicus Publications on behalf of the European Geosciences Union. 5 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , ; , , , ; ; ; , , , ; et al. 2008 2006 2004 2012 2008 Sauer Kleber Brunet , , , , , ; ff ), cross Lesmes ; Be b McDonnell ). However, , ; 1998 2008 erent natural ff , , Ramirez et al. Di Baldassarre Massuel et al. b ; Robinson et al. 2013 , as well as for in- ; 1998 Descloitres et al. ; Tilch et al. , 2008a , ff ; ; , AD-hoc AG-Boden Moldenhauer et al. Kemna et al. 2007 ; , Bechtold et al. 2013 ), surface ERT ( 2009a 2008b 2008 , , 2003 , Uhlenbrook et al. , Uhlenbrook et al. , ). A major problem is that 2012 , 2004 components and residence , ff 2006 , ), or imaging of tracers or irriga- Heller Robinson et al. ; Popp et al. Leslie and Heinse b McDonnell Koestel et al. , ), time-lapse ERT has been frequently 2000 ; , and interflow ( Oldenborger et al. erent runo De Morais et al. ; ). Kleber and Schellenberger ff ff Descloitres et al. ; ). However, some research has been con- ; 2009 ; erent regional and local characteristics and 2010 2002a , ff Uchida et al. , , Kleber and Schellenberger 5862 5861 1992 2008 ; ; 2006 and pollutant-transport models ( 2012 French and Binley , , 2004 , 1999 , ff , , 2011 Slater et al. , 2004 1980 ; , , ), have di cult to generalize and to transfer results to ungauged ). Several studies have addressed hillslope hydrology ). Kleber ffi Völkel et al. ; 2005 Daily et al. ; Perri et al. 2013 Kirkby , ), imaging water flow on soil cores ( 2008 ; , ; 2007 , , 2008 Cassiani et al. ) or a combined surface cross-borehole ERT array ( ). Garré et al. ). Understanding the involved processes is of particular impor- 1999 generation in watersheds ( , ). ; , Uhlenbrook et al. 2005 2003 1990 b ff ) or preferential flow context ( ; , , , , ). These up to three layered cover beds (Upper Layer – LH, Inter- 2008 2001 ). Additional methods are needed to improve the understanding of 2004 , 2012 , , , 2009 2006 , 2013 , erent spatial and temporal scales is essential for the prediction of hy- ard et al. Benderitter and Schott 1996a 2005 Scholten Tromp-van Meerveld , ff , ; ffl , ; ; Chi McDonnell et al. Michot et al. Uhlenbrook et al. 2010 Singha and Gorelick 2001 Miller et al. 2001 Kleber and Terhorst Zhou et al. ; Tromp-van Meerveld and McDonnell , ; ). ; ; ; ; , , Beside hydrogeophysical methods such as EM ( Many studies show the potential of ERT for hydrological investigation by means Most of the implemented studies were based on invasive and expansive hydrometric The hillslopes are an important link between the atmosphere and the water input In catchments of Central European subdued mountain range, the shallow subsur- Anderson and Burt point measurements on the punctualentire scale catchments. or Due on to tracerlope) the investigation, scale which lack and integrates of considering directof the measurements near-surface existing on complexities processes an and andedge intermediate spatio-temporal groundwater regarding (hills- interlinking runo dynamics, there is still a lack of knowl- The knowledge of system-internalitation water on flow di pathwaysdrological and and the hydrochemical response dynamics to within precip- catchments ( 1 Introduction applied to hillslope investigationCassiani in et al. the runo 2006 et al. 2013 et al. 2013 2012 of synthetic case studiesVanderborght et for al. aquifer transportBinley et characterization al. ( ducted under natural conditions to characterizecharge water by content use change, infiltration of or cross dis- borehole ERT ( et al. these complex processes, especiallyods are at capable the of hillslope closingmethods and the scale. invasive gap Hydrogeophysical punctual between hydrometric meth- large-scaleand arrays Friedman depth-limited ( remote-sensing Uhlenbrook tions with surface ERT ( borehole imaging of tracers ( 1993 2008a ( into catchments, and theytimes mainly ( control di remarkable influence on water budgets astological well and as on substrate-specific water fluxes. properties,and Due texture, to e.g. they the grain are sedimen- of sizeterflow particular ( distribution, importance clast for content, near surface runo the spatial and temporal variabilitysettings of – the e.g. hydrological response geomorphological, due pedological,tial to lithological heterogeneity di characteristics – and make thebasins it spa- ( di face of hillslopes isand mostly Terhorst covered by Pleistocenemediate periglacial Layer – slope LM, deposits Basal2005 ( Layer – LB: classification according to Wenninger et al. tance for improving precipitation-runo and Uhlenbrook 5 5 25 25 20 10 10 15 20 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , ) ). 1 1 (1) Schön C. The alti- ◦ ). 2004 , Schön is the resistivity of the pore w erent amounts of precipitation is an empirical constant, which ρ ff Θ n ). The upper layer (LH) with a thickness . Bedrock is gneiss overlain by periglacial ◦ 2012 , 5864 5863 ). The exponent Heller ). Additionally, at the survey point H3a five ThetaProbes describes the increase of resistivity due to an isolating 2006 1 ) describes the connection between electrical resistivity , Θ F 1942 , components and the response to di ff Archie Θ n ). − 1 ). In the central part of the slope hollow, a silty-loamy intermediate layer Θ is the bulk resistivity of the soil probe and with: 1 w ff erent depths (cf. Fig. ρ e Θ ff Θ ρ Lesmes and Friedman erent runo F ; ff = ff This model disregards the surface conductivity, which may occur due to interactions The objective of this paper is to show the potential of non-invasive surface ERT for e Since November 2007 soil watersiometers (UMS tension – has T8, been 10 min5 measured intervals) to using arranged 7 76 in di 14 recording survey ten- points along the slope at 2.2 Hydrometrical equipment The study area covers the 6 hatains, of eastern a forested Germany, spring which catchment in is the located Eastern Ore in Moun- the Freiberger Mulde catchment (Fig. measurements (e.g. tensiometer and FDR-Sensors).we With this tried multi to method achieve approach, on a di better understanding ofon the hillslopes. influence of the layered subsurface 2 Material and methods 2.1 Study site dynamics on hillslopes is still uncommon. mapping the spatialwith heterogeneities periglacial of cover shallow beds.term subsurface soil Furthermore, structures moisture we dynamics on to used a improve time-tapse the hillslope spatial ERT resolution to of additional monitor hydrometric long- the use of ERT for mapping shallow subsurface structures and monitoring hydrological Quality, amount, and distribution of poreform water the exert link a between huge electrical influenceof and on Archie’s hydrological resistivity properties. law and The ( empirical relationship of pore water conductivity(VS-pro Vakuum and System Co. resistivity, UMS) soil atand four water depths cumulated at was as three a extracted locations weekly (S1, with mixed S2, suction sample. S3; Fig. cups 2.3 Laboratory work where fluid. The formation factor solid matrix and constitutes an intrinsic measure of material microgeometry ( (FDR-Sensors – Delta Tcontent. devices A – V-notch ML2x) weir were withRainfall installed a was to recorded pressure measure by meter volumetric 4 was water precipitation used gauges to with quantify tipping spring bucket. For discharge. determination and saturation incontent porous media. Instead of saturationρ we use the volumetric water between pore water andsizes. soil In matrix, our especially study withthis, the thus a curve it high fitting was percentage could not taken of be into small carried account. out grain very well without considering 2004 depends on the distribution of water within the pore space ( Annual precipitation averages 930 mm,tude ranges mean from annual 521 temperaturea to is mean 575 slope m 6.6 a.s.l. inclination The ofcover catchment beds approximately with 7 is up formed to as three layers a ( slope hollow with 3 m (cf. Fig. of 0.3 to 0.65 m(cf. consists of Table silty-loamy material with(LM) a follows low with bulk density higher andsandy-loamy bulk many basal density roots layer and (LB) a isclasts thickness characterized oriented of by parallel even up higher to to bulk 0.55 the density m. slope. and The Down coarse ubiquitous slope it may reach a thickness of at least 5 5 10 15 20 25 15 10 20 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | : ). ), β Φ 2004 ). This ◦ , , and mea- B90 as a disper- Θ according to 7 ∠ n L , O ◦ ). Interim values 2 2 P and A–G). A and C are 4 1 40 mm) taken at dif- Θ F = A102.5 Dahlin and Zhou ∠ ). 1989 36 mm, length , the resistivity of the pore water must , array was measured additionally. With Θ ects, which may cause inaccurate inter- = n ff , connecting inflection points of contour β ◦ ), a high vertical resolution for laterally bed- and Barker 5865 5866 K C). This corresponds to the mean conductivity ; Θ ◦ ect we used spring water with high conductivity F ff 25 1991 = , for each depth was used (cf. Table T w ρ for 1 , to calculate volumetric water content: − w Apparao ). ρ ), it is possible, with known parameters ; 1 1 ) is defined as the ratio between water content and porosity ( ) described remarkable conductivity increases of water with low S and 1971 ff , e , p. 393, 399–404, but with the sand-silt boundary at 0.063 mm). erent saturation conditions during the saturation process. A calibrat- 2010 ff 150 µS cm ρ . (3) ( S = . (2) = 1986 , w25 1 = Θ Φ δ 2 m). For all resistivity measurements, a Lippmann 4 Point light hp instrument Θ Θ 1 1 n n < − − Klute w w ff ff ρ ρ e e array was found to be the most suitable configuration for the study area. This is From May to December 2008, twenty seven time lapse measurements were carried After reforming Eq. ( Brunet et al. To investigate the petrophysical relationship between resistivity and water content, Aside from the invariant parameters Θ Θ ρ ρ F F α characterized by low geometric factorsded ( subsurface structures, and a good signal-to-noise ratio ( parallel to the slopelines. inclination B, D, of E, approx.arrangement F, allows 9 and identifying G potential are 3-D-e pretation perpendicular of to the subsurface these resistivity profiles distribution.by To ( improve hydrometric the mapping data, results the aided tance profiles were located closewith 50 to electrodes the was tensiometeror used. stations clasts) Because (dis- of and the the expected multiple heterogeneities layered (e.g. stratification by of roots periglacial cover beds, a Wenner- 2.4.1 Mapping In addition to conventional percussion drilling,7 at ERT the profiles end to of survey October the 2008 we subsurface measured resistivity distribution (Fig. 2.4 Field work sizes were determined by sievingsant and ( the pipette method, using Na 15 undisturbed soil coreferent specimens depths (diameter (0.3 to 1.4samples m) were were analyzed. saturated After successively. dehydrationmeasured Using in for a a di drying 4-point chamber, array,ing the electrical solution with resistivity known was resistivity was used to determine the geometric factor. Particle To improve the spatial resolution,an a electrode Wenner- spacing offor 1 each m, pseudo-section this with results adepth in maximum of a depth investigation of combined for investigationRoy dataset and radial of with Apparao 9.36 dipole m 784 in (Wenner- data homogeneous points ground 0.195 profile A and B (cf. Fig. out within almost weekly intervals. 2.4.2 Monitoring Time lapse measurements were performedand with spacing the used same equipment, for electrode the array mapping. The two time lapse profiles are congruent with As water saturation ( it is also possible to calculate the degree of saturation using: sured variables between the extraction depths were linearly interpolated. initial mineralization, due toresistivity contact with time. with To minimize the this(approx. soil e matrix. This mayof soil cause water in variation the of study area, which is influenced by long-termbe contact known with the to subsoil. calculatepossible to the extract water pore contentments water of from under pore resistivity water dry values. conductivityTo conditions calculate could Because in water be it content summer, carried was from out onlyover resistivity in not the a obtained late entire few by spring time measure- field and period surveys, early of the autumn. median value 5 5 10 15 25 20 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , ff ). (4) ). In is the 2003 Keller ( 2011 δ , 100). By 2006 , = λ ) was corrected Ma et al. T ; ) (0–0.2, 0.2–0.4, 0.4– ) as proposed in erent inversion param- 2004 1 4 Descloitres et al. ff Günther et al. , erent methodical approaches: being commonly used for geo- ff 1 Hayashi − ; C ). The empirical parameter ◦ ). With our data, each method gener- 150 mm), completely plunged into the 25 1966 ρ = ) at the temperature ( , t 2008 ρ 0.025 C( , ◦ = 5867 5868 δ 6 mm, length Miller et al. erent time steps. ) conclude that the model (Eq. = ff to the spring at most. Therefore, there is almost no runo ff 2011 ( )) ), is practicable within the temperature range of environmental C Keller and Frischknecht ◦ 25 1966 ( 30. The result was used as a reference model for the next time step. − Ma et al. T = ( . Median soil water tension of the study area, related to depth and time λ 1 δ − cient results with unsubstantiated artifacts. Changes have been calculated, + ), indicates the impact of soil moisture on spring discharge. During sum- ffi 1 2 ( t ) and installation depth of hydrometric devices (cf. Fig. ) showed with synthetic data that time lapse inversion may produce artifacts, 1 ρ = erences of the two data sets ( For inversion of the ERT data, we used the BERT CodeTo ( calculate changes in resistivity, there are three di In order to find representative resistivity values as a function of depth, which are The inversion had to be adjusted in order to minimize these artifacts. The expedient The installed tensiometers are able to measure soil temperature simultaneously. To compare time lapse measurements and to apply sophisticated inversion routines, Subsoil temperature, especially in the upper layers, is characterized by distinct an- ff 25 of 3 m intoTable seven layers according to the boundaries of the described layering (cf. tion strength The following model was calculateda with the chosen antecedent constraint time minimum stepusing length, as and this reference model, higher approach regularization withadapting strength each the ( model inversion as parameter,sive we reference results. could In model our reduce for case the the this artifacts provides next the and time best achieve step fitindependent conclu- to and on the small-scale hydrometric heterogeneities, data. we subdivide the model down to a depth mostly expected in our case. results were achieved byeters using for the the second initialfirst approach time reference with step model was di and calculated with each smoothness subsequent constraints of time 1st lapse order inversion. and regulariza- The especially due to changes caused by shallow infiltration (decrease of resistivity), as considered when comparing di These data have been used toture. correct Comparing resistivity several measurements existing to models a formeasurements, standard the tempera- correction of soil electrical conductivity the location of electrodes needs toless remain steel constant. For electrodes current (diameter injection weground, used thus stain- avoiding shifting of electrodes, except for natural soilnual creep. and daily variations. Therefore, temperature dependence of resistivity must be di ates insu which cannot be explained2008b or related to any natural process. (cf. Fig. mer increasing evapotranspiration causesonly the a slight drying-out reaction of todeficit soil. precipitation and events. causes The Rainfall no spring could runo only showed balance the soil water order to account for thecretization present of topography, we the applied surface. an unstructured triangular dis- either inverting the modelssion, for to each use point in the time initial separately model and as subtract after reference inver- model for the time step, or inverting the 2.5.1 Hydrometry During the period Mayand to 1.67 L December s 2008 the spring discharge varied between 0.07 0.9, 0.9–1.2, 1.2–1.5, 1.5–2.0resistivities and in the 2.0–3.0 m). layers from The theB, representative stations respectively. H1a values to are H4a and median H4b to H4a for2.5 profiles A and Results and Frischknecht monitoring. ρ With this equationto the a inverted resistivity resistivity at ( temperature a slope soil compensation, temperature with ofphysical applications 25 ( 5 5 25 25 20 20 10 15 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , ). 3 1 erent ff erence of ff ranges from and Table Θ 4 n . The formation fac- erentiation between ff Θ erentiated using ERT. n ff ). 3 ). 3 ranges from 1.8 to 2.3. The second Θ n ). With high saturation, the di 3 response with rain and the high discharges ff 5870 5869 0.852) (cf. Fig. − = 0.999). erences in soil or electrical properties between the r ff erent depths into two regions, it was possible to derive ff ) for each region (Table < r < 3 )–( 1 , 0.973 shows a positive correlation to small grain sizes, primarily medium 1 0.909), but in the same case a negative correlation to grain sizes ). Because of the anisotropic hydraulic properties, the percolation Θ 2 = n r shows the aggregation of the single samples into two regions with di erent “electrical” layers may be identified. This is due to the fact that the ex- 4 ff and causes strong interflow. response. Until the beginning of May and again from December the low tensions ff ff erentiate the basal layer from the overlying intermediate or upper layer (Table 90hPa) were measured. Less evapotranspiration results in a replenishment of the This relationship between water content and resistivity, shown in Fig. The fitted curves of both regions are quite similar, except for The first depth range comprises the upper and the intermediate layer. These two Furthermore, there is an influence of the layered subsurface on soil moisture and The exponent The amount of silt as well as the clast content are important distinctive attributes to In contrast, during winter season (starting in November) at all depths lower tensions 630 µm including clast content ( ff < Figure within the winter season. runo of the upper LBtensions indicate (cf. saturate Fig. conditions, in contrast to the deeper LB with higher charge is mainly caused by direct precipitation to the area( surrounding the spring. storage water reservoirs in the subsurface.tions Due predominate to and the cause moist a conditions, rapid high runo presatura- generation in the summer season. Primarily base flow dominates and decreasing dis- especially at low presaturations.larger Related changes in to resistivity this, than small in water the deeper content region. changes cause resistivity between the depthtivity ranges of is the small pore andhigher fluid, primarily exponent, but LH influenced increases and by with LM the decreasing react conduc- more water sensitively content. to As water a content result changes than of LB, the tors are very similar (0.577 vs. 0.587, Table is only a meancially value close for to each thesamples depth surface, even range. the at In di the the sameintense first depth biotic activity may depth near vary. range the Thisto surface, (0–0.9 higher enhancing m), deeper variation small-scale parts espe- may heterogeneity of be compared the explained soil. by the basal and intermediate layer, whoseBy boundary combining varies samples between from depths of di the 0.8 parameter to for 1 Eqs. m. ( 0.7 to 1.8. Within eachof of the these substrate, two depth similardepth ranges, electrical of we properties accept, 0.9 m analog with is totransition a not the zone, threshold developed properties because as at the an 0.9 samples m.shallow exact, right continuous or The from boundary. the threshold Rather this deeper it depth region, is may have similar a properties short to of the the geomorphological di depth ranges. periglacial layers are characterizedcomparatively by low a clast high content. amount Thedepth exponent of range is silt represented (mostly by theof medium basal coarse silt) layer. material This and at is the characterized expense by of a fine higher grain amount sizes. In this depth range ponent is strongly influencedupper by boundary grain of size, LB. which Onvery shows the similar a other between hand, remarkably LH grain change size and at distribution LM, the and so clast that content are these may not be di silt (6.3–20 µm, > Two di di into deeper parts ofand LB the is upper too LB. slow,runo and The the backwater seepage of water the is saturated accumulated depth in2.5.2 range the is LM mainly Laboratory involved in Within the separately analyzed samples,the non-linear method curve of fitting least was squares, carriedof the out. Archie’s data Using law could (Eq. be fitted using a power function in the form 5 5 25 15 20 10 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | er- ff ), which is 2 ). The water 1 m up to more ). Between the 7 Ω ). For profile A and 2003 indicates that this may be , 1 ). 8 erent hydrogeological conditions. ff and 7 ects. ff Zhou and Dahlin erent patterns are found. In the “inner” area ; 5872 5871 ff 1996 , 1.2 %) could be found between normal and recipro- ± ) and the porosity from Table 3 ). ). 5 6 ). 7 LaBrecque et al. m (cf. Fig. Ω erences become more notable. To exclude potential errors (e.g. electrode posi- ). ff 8 It is not feasible to relate a specific resistivity to the underlying gneiss or its regolith. ERT mapping of the spatial distribution of periglacial cover beds is associated with Percussion drilling confirmed that the thickness of LB downslopeThis exceeds zone 3.5 m. expands upslope, due to the fact that the depression lines diverge; the Due to the slope gradient, water from the hillsides and upper parts of the catch- At the intersection between the longitudinal and diagonal profiles, a good match of Since the laboratory results indicate similar electrical properties, remarkable di Resistivity decreases in greater depths (starting at 1 m). Thus, the basal layer is The resistivity distribution of the subsurface is characterized by large-scale and The hydrometric data show the driest conditions in 0.55–0.65 m (cf. Fig. the basal layer is lower than in the “outer” area (the hillsides) (cf. Fig. At our study sitethan the 4000 resistivity of the subsoil ranges from nearly 100 2.5.3 Mapping pore water, the main factorto driving improve resistivity. the On understanding the of other the hand moisture this conditions fact of may the be subsurface. used reached. If the maximum thicknessobtained of the by basal resistivity layer data, isset equal the to at change the a from saturated depth basal zone, around as layer 4.5 m. to underlying gneissseveral may restrictions. be In our study area, stratification is concealed by the influence of relief becomes smoother and doesmay not easier show spread such laterally. strong On recessand the (cf. more Fig. other hand constricted. towardtransferred According to to the the to subsurface spring, this, to itConvex identify the areas becomes regions indicate more of shape dryer di conditions of inor the the elongate basal parts surface layer of in may the comparison hillslope, to be which the partially may concave actPercussion as local drilling aquifers. was only realized down to 4 m depth where bedrock could not be resistivity in this zoneFig. as observed in all measured profiles at depthsTherefore, 1.5 we to assume 4.5 that mand the (cf. since entire it saturated is zone connected is to the located spring, within it the is basal also layer the source of the base flow. depression lines LB is characterizedtion as of saturation a using connected Eq. zoneinterpreted ( of as low a resistivity. connected A saturated calcula- zone (Figs. ment flows toward theand depression forms a lines, local where slope it groundwater reservoir. concentrates This within results the in a basal maximum layer decrease of measurements ( the calculated resistivity models may bethe found di at shallow depth. Withtioning increasing errors), depth, the data quality may be evaluated by comparing normal and reciprocal characterized by lower resistivitynot compared constant in to lateral thebetween direction. the overlying two Two layers. di depression However, lines this (approx. is between profile A and C), the resistivity of water content (cf. Fig. consistent with the high medianacquisition resistivity (cf. Fig. of the intermediate layer at the time of data sitivity, less spatial resolution or potential 3-D-e small-scale heterogeneities, butdepth also up distinct to patterns 0.9 m,the may upper the be and study the identified. area intermediate At layer. is characterized shallow by highences resistivity. between This upper comprises andThere are intermediate areas, layers where only the occur, intermediate layer if has water higher content resistivity, suggesting deviates. lower B repeated measurementsThereby, with no reciprocal large errors electrodecal (max configuration measurements. Because were of conducted. deviation with the depth absence may of be large only explained potential by errors, the the inversion process, increasing decreasing sen- 5 5 25 20 10 15 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , 4 er ff com- erent profile ff 9 b). Due 10 H3a ρ a). Θ 10 ). This correla- 4 ). 2 ) at time of mapping. Theta ect the resistivity of profile Θ ff 4.5 Vol% in comparison to the − 5874 5873 ), which cannot be rendered with the resistivity 2 erent trends could be identified. The initial condi- 0.9 m) show a strong negative correlation with the ff show the same depth profile, but the values di erences are marginal with only a continuous slight 1.5 m show no direct correlation with rainfall. Water < ff > Theta Θ erences both methods provide comparable results. ff and erent positions of the two probe locations could be one reason ff H3a ρ ). Θ 10 shows the trend of median resistivity for each depth range for the entire erent hydrological and electrical response. To better distinguish the results 10 ff cult to make reliable conclusions, which depth profile is more precise. However, ). Major changes occur at shallow depth and proceed to depth, though remark- ering pore water resistivity from the used median value (Table 2 ff ffi This issue is also evident when comparing the resistivity with the soil suction data. During the investigation period, di The variation of resistivity is significant influenced by rainfall. As shown in Table One problem is the temporal resolution. Because of the time intervals (usually During the measuring period, the upper layer (0–0.2 and 0.2–0.4 m) reacts with simi- The temporal changes in resistivity decrease with depth. Short time variations are Figure The resistivity of profile A clearly correlates with profile B (Table Because the data of the resistivity measurements and also the ThetaProbes may To check the equations obtained in the lab and also to compare directly with hydro- The values of 1 week), we are not able to resolve the entire temporal heterogeneity of the sub- limited down to 2 m.increase Below, during the the di investigated period. ture conditions can be expected in general. lar resistivity variations as the intermediatediate layer layer (0.4–0.9 may m). temporarily Resistivity exceed of the the upper interme- layer (e.g. profile A October–December). positions. Profile A is completely situated in one of the depression lines, in which mois- metric data, we used the water contents from the ThetaProbes at H3a. Figure survey (cf. Fig. tions in April and early May are characterized by a highly saturated subsurface. This is first and after that increasesto back the to missing the time initial step, this level of alteration is 3 not September tracedWith (cf. in Fig. the profile higher A (cf. temporaltime Fig. events resolution e.g. single of rain the events (Fig. tensiometer it is possible to resolve short or enduring dry periods. Depths cannot infiltrate straight to greater depthsevaporation, because storage, of or decreasing consumption hydraulic conductivity, of water by roots. ≥ surface, which may lead to16 misinterpretation. September, For the example, during amount theA. of However, period 32 33 from mm of 3 rain these to with 33 seems mm an not additional had measurement already to on been 9 a fallen September, resistivity until at 7 shallow September. depth At decreases profile B the upper and intermediate layers ( cumulated amount of precipitation (ppt).parts This of correlation the decreases basal layer with (0.9–1.5 depth. m) Upper respond slightly and delayed to intense rain events tion is more pronounced atthe shallow near-surface depths. The part absolute ofranges values have LH are higher similar, (0–0.2 amounts except m) at for and profile B parts than of A at LB all (1.5–2.0 points m). in These time, due two to depth the di and to deal withappropriate. the subsurface layered structure, a depth- or layer-basedtime analysis series is of profile Acomparison between with H1a daily accumulated and precipitation. H4a and profile B between H4b and H4a, in despite of these slight di 2.5.4 Monitoring As the hydrometric data show, theterized first by period drying from of May theFig. to subsurface. October After was that, mainly humid charac- conditionsably began attenuated. to Each dominate (cf. depthshows has di its own characteristics, its own variation in time and for this mismatch. Other reasonsor could di be the inversion process of the resistivitycontain data biased errors (e.g. causedis by di clast content or by the installation procedure), it pares water content,A calculated close with to H3A), temperature-corrected with resistivity water content ( from theslightly. ThetaProbes The ( resistivity depthThetaProbes. profile The shows di a shift of 5 5 25 20 10 15 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 23 mm) = ). However, 5 102.1mm) re- ected. So verti- = ff (Table ) and water content ) and soil suction at H3a 51.1 mm) causes a dis- Theta Theta ρ = Θ Θ shows already in late October ects LH, LM as well as parts of ff H3a ). As a result of drying, at shallow ρ erent response types could be iden- 2 ff 0.7). However, the signal is not traced Θ ∼ and compared to soil water tension. 11 5876 5875 12 correlates closely with erent depths over time at profile A 25 m near H3a ff H3a ρ 1.2 m), which remains completely una Θ > ected and dry continuously. Constant discharge indicates generation during this period. This amount of rain is only ff ff ). The resistivity of LH and LM fits well to the tensiometer data 0.85m), 5 erence is less than under humid conditions (e.g. May). ff ), is shown in Fig. ≤ 2 ) (cf. Table 0.9 m) resistivity quickly increases until July. Below, the increase proceeds < ). During the investigated period three di H3a 4 ) using Eq. ( Ψ interflow in the unsaturated subsoil. deeper layers. The shortin rise the of spring discharge bog. is caused byA saturation high overland amount flow ofsults precipitation in (cf. 22 a October–4 strongLB. November, response Such ppt down heavy to rain 2.0 periodLM m than does and the not a medium induce rainof larger period, magnitude resistivity but as changes influence above. in The deeperdeeper LH water regions basal and infiltrates in layer to the (2–3 m). the samecreasing The upper, order spring but vertical discharge does seepage may not only is be reach limited caused the and by therefore, lateral the subsurface flow, in- such as into the deeper ground ( cal seepage dominates in LHwater and is LM, predominantly which fixed leads to by recharge capillary of force; soil hence water. The it does not percolate into A small amountcauses a of short deferment precipitationmer of period. (cf. increasing The resistivity values 23 of ofnitude. initial LH September–7 state Within and and the LM October, time temporal during step ppt resolution,Deeper are the parts in only sum- the are a same not slight order a decreasethat of mag- could there be is recorded. noable runo to balance the deficit causedthe by temporal evaporation resolution at shallow of depths, measurements. at least within A medium amount of precipitation (cf.tinctive 1 July–15 reaction July, ppt at shallowcrease depth. by Resistivity comparatively at the these same depths ratio ( shows a sharp de- H3a ρ At shallow depth ( In deeper parts the variations are attenuated. At aIn depth 1.5 of m 1.2 m depth there the is almost response no of the ThetaProbes isThe same marginal holds until true for December, the but correlation between resistivity ( After the major rain event at the end of October, resistivity valuesA remain comparison of low. Due water content obtained by ThetaProbes ( A first annual trend covers the period between May and October. This period is As mentioned above, resistivity, especially of LH and LM (up to 0.9 m), shows a high 3. 1. 2. Θ calculated from resistivity data for( di to precipitation ofresistivity 102.1mm drops below during the the initial state period and from shows highly 19 saturated conditions. November to 16 December, tober is only 337a mm. In mainstream combination drying with of increasing the evapotranspiration, subsurface this (cf. causes Fig. to the humid conditions atshallow the subsurface beginning is of high thevariations. and measurements, the the observed conductivity resistivity of is the low relative tomainly the characterized seasonal by drying. The accumulated precipitation from 9 May to 21 Oc- indicated by low soil water tension, high spring discharge, and high water content. Due at the same depth, but in deeper parts it deviates. H3a ( July–October), the di response over the year, until the heavy rain period atthereafter the they end of show October. ana reaction increase. to In the heavy contrast, rain event, which is not reproducible with the ThetaProbes. there is a shift of theresistivity curves data during is the consequently whole smaller period. than The from volumetric water the content ThetaProbes. from In dry periods (e.g. (Table tified that are exemplarily illustrated in Fig. depths ( slightly, but continuously until October. short time variability and is strongly associated with the amount of precipitation (ppt) 5 5 25 20 10 15 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ects gen- ff ff ) demon- 2012 ( components, dif- ect the subsurface ff ff Heller . Runo ff ected, which leads after repeated ff erent depths take part in runo ff dynamics in catchments. According to this, ff 5878 5877 erent amounts of precipitation a ff erently and accordingly di erently with time. ff ff erentiate between LH and LM as one unit and LB as another. Like the ff erent moisture zones. On the hillsides water saturation of LB is less than ff erent causes. Both methods contain measuring errors, just as the labora- ff erent exponents. With the lower exponent, LB is characterized by lower resis- generation, and greater depths remain una ff ff erent or change di In summer the subsurface continuously dries, starting at the surface and proceed- Because pore water (amount and conductivity) is the main driver for resistivity, we During investigation time, temperature-corrected resistivity showed distinct seasonal Moreover, from the results of field measurements and pedophysical parameter deter- ERT lets us di On the contrary, LB has its own electrical characteristics. The pedophysical relation- These deviations between resistivity data and hydrometric measurements may ff sponse caused by a medium amount of precipitation includes LM and a small increase decline with increasing depth,LH, mainly LM, limited and to thealready a saturated upper depth and parts of changes are of 2 m. only LB, possible This due since primarily to it changes a ing may in to water be depth. conductivity. assumed Thisand that drying intensity deeper is of temporarily parts the interrupted are periods response by with strongly precipitation. a depend Penetration depth on smallruno the amount amount of of precipitation,occurrence precipitation. infiltration During to is drier limited conditions to within LH. There LM is compared no to LH. In contrast to this, a re- between the depression lines,implies where a local low slope resistivity groundwater shows reservoir. high water saturationvariations and due to changesand in moisture evapotranspiration. conditions, Close primarily to influenced the by precipitation surface, these variations are very evident and approaches, we improved our understandingment of of water the in allocation, the subsurface. distribution,moisture Di and conditions move- di eration. arrive at some comprehensiveThe interpretations high resistivities of of the LHinto subsurface and two LM moisture di indicate conditions. low water contents, whereas LB is divided with depth. mination in the laboratoryof we could seasonal derive changes a insoil noninvasive subsurface method moisture resistivity for on and direct the monitoring its hillslope relationship scale. to In precipitation combination and with commonly used hydrometric study area, which is further reinforced by the increasing mineralization of pore water have di tory and otherthe hydrometric ThetaProbes (e.g. and soil–water Tensiometers resistivity) measure measurements. punctual Furthermore, values. ship, with neglecting surface conductivity, showsbut equal di formation factors to LHtivity and LM, at the same water content. Therefore, the resistivity of LB is lower in the entire a hillslope with particular focus on mid-latitude slope depositsintrinsic (cover properties beds). (e.g. sedimentological), LH andacteristics. LM Therefore, have they very may only similar being electricaldi distinguished char- with ERT, if water contents are 3 Conclusions In drainage basins, hillslopes linkferent precipitation flow to pathways, and riverthe runo residence hillslope, times especially are the mainlyone shallow influenced subsurface. of by The the the knowledge keys of propertieswe to these of characterize used properties the ERT is runo for mapping the spatial heterogeneity of the subsurface structure on Therefore the data areor very layer. In limited, contrast, with ERTume, restricted has which validity the makes it for advantage more the towith suitable integrate entire for high over extensive depth resolution a depth-related range ERT interpretations. largerpreferential it Further, measuring flow is vol- pathways. possible to identify small-scale heterogeneities such as strated with dye infiltrationin experiments, our that study preferential area. Hence, flow hydrometricsoil is point moisture, an measurements depending may important over- on or process whether underestimate they are inside or outside a preferential pathway. 5 5 25 20 10 15 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Kuras , 2012. 5861 5887 5862 5862 5866 10.2136/vzj2011.0066 , 2013. is saturated overland flow from the ff 5864 5880 5879 5861 10.5194/hess-17-595-2013 ) pointed out that a combination of geophysical and hydromet- 5865 5866 , We acknowledge support by the German Research Foundation and the 2009 5862 ( generation, caused by lateral subsurface flow within LH, LM, and the ff ). e in Zusammenarb. mit den Staatl. Geol. Diensten, Hannover, 2005. 5862 ff 2009 ( , L., Günther, T., Vandoorne, B., Couvreur, V., and Javaux, M.: Three-dimensional moni- ff Cassiani et al. The results from ERT measurements show a strong correlation to the hydrometric One shortcoming is the temporal resolution. Some hydrological responses especially Another aim should be to improve the spatial resolution. Since preferential flow is an cal Resistivity Tomography (ERT) a case146–153, study 2010. in the Cevennes area, France, J. Hydrol., 48, tance tomography and1996b. dye staining image verification, Meas. Sci. Technol., 7, 384–390, West Sussex, England, New York, 1990. soil column using electrical resistance tomography, Water5862 Resour. Res., 32, 763–769, 1996a. Rohsto ductivity of the Vadose Zone, Soil Sci. Soc. Am. J., 53, 1356–1361, 1989. toring of soil waterEarth content Syst. in Sci., a 17, maize 595–609, doi: field using Electricalrelationship Resistivity with Tomography, Hydrol. rainfall, Eur. J. Environ. Eng. Geophys., 4, 37–49, 1999. tics, T. Am. I. Min. Met. 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Y., Shimada, J., and Sato, A.: Three-dimensional spatial and temporal monitoring of Zhou, B. and Dahlin, T.: Properties and e Scholten, T.: Periglaziäre Lagen in Mittelgebirgslandschaften – Verbreitungssystematik, Eigen- Völkel, J., Zepp, H., and Kleber, A.: Periglaziale Deckschichten in Mittelgebirgen – ein o Völkel, J., Leopold, M., Mahr, A., and Raab, T.: Zur Bedeutung kaltzeitlicher Hangsedimente Sauer, D., Scholten, T., and Felix-Henningsen, P.: Verbreitung und Eigenschaften periglaziärer Roy, A. and Apparao, A.: Depth of investigation in direct current methods, Geophysics, 36, Wenninger, J., Uhlenbrook, S., Tilch, N., and Leibundgut, C.: Experimental evidence of fast 5 5 30 25 20 10 15 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ∗ ) 1 − ) conductivity 3 − per depth. w ρ 5887 5888 and resistivity w σ (%) (%) (%) (cm d ] 72.4 107.8 111.6 114.7 135 156.7 1 − ). m] 138.1 92.8 89.6 87.2 74.1 63.8 2013 Ω [ ( [µS cm w w ρ Depth [m]σ 0.3 0.6 0.85 1.05 1.65 2.3 ). 1989 horizon (moist) Clay Silt Sand (%) (%) (g m , Properties of cover beds from the study site for an example profile – adapted from Median pore water conductivity Layer SoilLH ColorLM A/BwLB 2Bg Soil texture 10YR/5/8 3CBg 14 10YR/5/4 10YR/5/3 Clasts 12 52 bulk density 7 53 porosity 34 22 Hydraulic 35 36 71 43 56 1.2 1.5 1.7 0.55 0.43 0.36 27 9 52 Field-saturated hydraulic conductivity measured using the Compact Constant Head Permeameter (CCHP) method Amoozegar ∗ ( Table 2. Table 1. Moldenhauer et al. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ) and profile B ρ ) and B ( profile A a a a a b b b ∗ ∗ ρ ) and cumulative precipitation ,ppt) r initial 0.773 0.770 0.804 0.586 0.378 0.078 0.173 initial ρ timestep − − − − − − ρ /ρ ( erent depth ranges. r ff Θ n ) a a a a a a a timestep ρ profile B ρ , Θ F 5890 5889 profile A ρ ) of the two di ( 3 r )–( 1 0.01 0.01 0.01 0.9 m0.9 m 0.577 0.587 1.83 1.34 0.895 0.888 Depth range < ≥ p < p < p > Depth [m] 0–0.2 0.977 0.2–0.40.4–0.90.9–1.21.2–1.5 0.988 1.5–2.0 0.987 2.0–3.0 0.987 0.852 0.831 0.878 ∗ a b Correlation between median resistivity of profiles A ( Fitted parameters for Eqs. ( Table 4. Table 3. during the time step (ppt). between subsequent resistivity ratio of profile A ( Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ) and H3a

) and soil

g n i r p NE s [m] H3a ρ 350 D1 1 S D2 2 S yer D3 La r ate yer aye edi 1 La H r l L rm ) a a a b a ppe asa nte H3a H2 I 300 Ψ U B , 3 S LB LM LH 0.993 0.904 0.905 0.120 0.566 H3a H3 ρ ( r H4 s ct r ) a a a a a ups ete nse obe LB LM LH n c Pr Theta tra iom tio Θ T , eta R ens T Suc Th H3a E ρ 5892 5891 Θ 250

( r W 0 1 2 3 4 5 S ) and correlation of resistivity at H3a ( [m] Theta spring Θ D1b D2b D3b

H1b

0.01 0.01

H2b e

t a

g n i r p D1a s H3b D2a D3a H4b p < p > Depth [m] 0.300.550.851.201.50 0.863 0.957 0.885 0.136 0.619 C a b H1a H2a G H3a F E H4a A D B ). H3a Ψ Study site with locations of ERT profiles and hydrometric stations (left panel) and Correlation of volumetric water content calculated from resistivity values ( Study site with locations of ERT profiles and hydrometric stations (left) and profile section with instal- Fig. 1. lation depths of tensiometers, ThetaProbes and suction cups (right). Figure 1. profile section with installationpanel). depths of tensiometers, ThetaProbes and suction cups (right Table 5. water content from ThetaProbessuction at ( H3a ( Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ). 2012 ( Heller 23 erent grain sizes. 5894 5893 ff in dependence of di Θ n in dependence of different grain sizes. Θ n Exponent Spring discharge in comparison with daily precipitation (top panel), image of median Exponent erent depths (bottom panel) – adapted from ff Figure 3. Figure 2. soil water tension ofdi the shallow subsurface (middle panel) and median soil water tension for Fig. 3. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erent ff ) for two di w /ρ ff e ρ Ω ) for two different depth ranges. w /ρ eff ρ 25 24 5896 5895 Volumetric water content in dependence of resistivity ratio ( Resistivity results from ERT mapping (October 2008) of the study area: pseudo 3-D Resistivity results from ERT mapping (October 2008) of the study area: pseudo 3D view of the profiles Volumetric water content in dependence of resistivity ratio ( Fig. 5. A to G. Figure 5. Figure 4. depth ranges. view of the profiles A to G. Fig. 4. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 27 5898 5897 Median resistivity (left panel) and median water saturation (right panel) per depth for ERT section of profile A with plotted layer boundaries. Median resistivity (left) and median water saturation (right) per depth for the inner region (between the Figure 7. the inner region (between the depression lines) and outer region (hillslopes.) Figure 6. Fig. 7. depression lines) and outer region (hillslopes.) Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

2013 (

28 29 5900 5899

Moldenhauer et al.

Volumetric water content calculated from resistivity data close to H3a in comparison Calculated water saturation from resistivity data of the study site: pseudo 3-D view of Calculated water saturation from resistivity data of the study site: pseudo 3D view of the profiles A to Fig. 8. G - adapted from Moldenhauer et al. (2013). with ThetaProbes at the time of the mapping. Figure 8. the profiles A to G – adapted from Figure 9. Volumetric water content calculated from resistivity data close to H3a in comparison with ThetaProbes Fig. 9. at the time of the mapping. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | profile (b) profile A and (a) 31 30 erent depth ranges for 5902 5901 ff Trend of median resistivity for di Ratio between subsequent resistivity (first column), median resistivity as a function Trend of median resistivity for different depth ranges for (a) profile A and (b) profile B in comparison Ratio between subsequent resistivity (left), median resistivity as a function of depth (center) and median Fig. 11. soil water tension (right)and for (3) three high exemplary amount. precipitation responses: (1) small amount, (2) medium amount Fig. 10. with daily precipitation. Figure 11. of depth (second column)precipitation responses: and (1) median small amount, soil (2) water medium amount tension and (third (3) column) high amount. for three exemplary B in comparison with daily precipitation. Figure 10. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 32 5903 erent depths. ff Trend of volumetric water content, obtained by resistivity data and ThetaProbes with Trend of volumetric water content, obtained by resistivity data and ThetaProbes with daily precipitation Figure 12. daily precipitation for di Fig. 12. for different depths.