Thom Bogaard

Feasibility of soil moisture estimation using passive distributed temperature sensing

Through its role in the energy and water balances at the land surface, soil moisture is a key state variable in surface hydrology and land?atmosphere interactions. Point observations of soil moisture are easy to make using established methods such as time domain reflectometry and gravimetric sampling. However, monitoring large?scale variability with these techniques is logistically and economically infeasible. Here passive soil distributed temperature sensing (DTS) will be introduced as an experimental method of measuring soil moisture on the basis of DTS. Several fiber?optic cables in a vertical profile are used as thermal sensors, measuring propagation of temperature changes due to the diurnal cycle. Current technology allows these cables to be in excess of 10 km in length, and DTS equipment allows measurement of temperatures every 1 m. The passive soil DTS concept is based on the fact that soil moisture influences soil thermal properties. Therefore, observing temperature dynamics can yield information on changes in soil moisture content. Results from this preliminary study demonstrate that passive soil DTS can detect changes in thermal properties. Deriving soil moisture is complicated by the uncertainty and nonuniqueness in the relationship between thermal conductivity and soil moisture. A numerical simulation indicates that the accuracy could be improved if the depth of the cables was known with greater certainty.

How vegetation reinforces soil on slopes

Once the instability process e.g. erosion or landslides has been identified on a slope, the type of vegetation to best reinforce the soil can then be determined. Plants improve slope stability through changes in mechanical and hydrological properties of the root-soil matrix. The architecture of a plants root system will influence strongly these reinforcing properties. We explain how root morphology and biomechanics changes between species. An overview of vegetation effects on slope hydrology is given, along with an update on the use of models to predict the influence of vegetation on mechanical and hydrological properties of soil on slopes. In conclusion, the optimal root system types for improving slope stability are suggested.

Quantifying hyporheic exchange at high spatial resolution using natural temperature variations along a first?order stream

Hyporheic exchange is an important process that underpins stream ecosystem function, and there have been numerous ways to characterize and quantify exchange flow rates and hyporheic zone size. The most common approach, using conservative stream tracer experiments and 1?D solute transport modeling, results in oversimplified representations of the system. Here we present a new approach to quantify hyporheic exchange and the size of the hyporheic zone (HZ) using high?resolution temperature measurements and a coupled 1?D transient storage and energy balance model to simulate in?stream water temperatures. Distributed temperature sensing was used to observe in?stream water temperatures with a spatial and temporal resolution of 2 and 3 min, respectively. The hyporheic exchange coefficient (which describes the rate of exchange) and the volume of the HZ were determined to range between 0 and 2.7 × 10?3 s?1 and 0 and 0.032 m3 m?1, respectively, at a spatial resolution of 1–10 m, by simulating a time series of in?stream water temperatures along a 565 m long stretch of a small first?order stream in central Luxembourg. As opposed to conventional stream tracer tests, two advantages of this approach are that exchange parameters can be determined for any stream segment over which data have been collected and that the depth of the HZ can be estimated as well. Although the presented method was tested on a small stream, it has potential for any stream where rapid (in regard to time) temperature change of a few degrees can be obtained.

The water footprint of bioenergy from Jatropha curcas L

The water footprint (WF) of energy crops should be calculated by relating the energy yield of a crop to its actual water use under actual climatic conditions during the growing season. Recently, average precipitation and additional irrigation to satisfy potential crop water requirements were used as input to the WF computation (1). We disagree with this approach because it relates optimized crop water use to actual yields, thereby introducing bias towards inefficient WF for low-yielding crops under suboptimal rain-fed conditions.

The slope movements within the Mondorès graben (Drôme, France); the interaction between geology, hydrology and typology

The Mondores graben in the south-eastern French Alps is an uncommon structural feature, which originates in a complex polyphasic tectonic evolution. In contrast with its immediate surroundings, with hardly any huge landslides, the Mondores graben is characterised by various types of landslides. A huge sagging caused part of the limestone cliff to subside some 50 m within 50 years. Two recent mud flows that occurred were considered a potential threat to some inhabited places downstream. The hydrologic aspects of the Boulc-Mondores landslide resemble the geology: infiltration (and karst input?) in the rock-sliding zone and exfiltration in the marls resulting in slumps and mud flows. The mass movement interactions could be explained by a structural geology analysis with geodetic monitoring using different techniques. It is also shown that hydrochemistry as well as geophysical surveys are of importance in unravelling the hydrologic systems and the geological subsurface structures. The present paper aims at explaining the geological control of the different slides in view of estimating their potential danger. Understanding the geological structure and its evolution therefore is a necessary prerequisite.

Contribution of time-related environmental tracing combined with tracer tests for characterization of a groundwater conceptual model: a case study at the Séchilienne landslide, western Alps (France)

Groundwater-level rise plays an important role in the activation or reactivation of deep-seated landslides and so hydromechanical studies require a good knowledge of groundwater flows. Anisotropic and heterogeneous media combined with landslide deformation make classical hydrogeological investigations difficult. Hydrogeological investigations have recently focused on indirect hydrochemistry methods. This study aims at determining the groundwater conceptual model of the Séchilienne landslide and its hosting massif in the western Alps (France). The hydrogeological investigation is streamlined by combining three approaches: a one-time multi-tracer test survey during high-flow periods, a seasonal monitoring of the water stable-isotope content and electrical conductivity, and a hydrochemical survey during low-flow periods. The complexity of the hydrogeological setting of the Séchilienne massif leads to development of an original method to estimate the elevations of the spring recharge areas, based on topographical analyses and water stable-isotope contents of springs and precipitation. This study shows that the massif supporting the Séchilienne landslide is characterized by a dual-permeability behaviour typical of fractured-rock aquifers where conductive fractures play a major role in the drainage. There is a permeability contrast between the unstable zone and the intact rock mass supporting the landslide. This contrast leads to the definition of a shallow perched aquifer in the unstable zone and a deep aquifer in the intact massif hosting the landslide. The perched aquifer in the landslide is temporary, mainly discontinuous, and its extent and connectivity fluctuate according to the seasonal recharge.RésuméL’élévation du niveau de l’eau souterraine joue un rôle important dans l’activation ou la réactivation des glissements de terrain profonds et ainsi des études hydromécaniques nécessitent une bonne connaissance des écoulements d’eaux souterraines. Les milieux anisotropes et hétérogènes combinés à une déformation des glissements de terrain rendent les études hydrogéologiques classiques difficiles. Des études hydrogéologiques ont récemment mis l’accent sur les méthodes hydrochimiques indirectes. Cette étude vise à déterminer le modèle conceptuel hydrogéologique du glissement de terrain de la Séchilienne et son massif dans les Alpes occidentales (France). L’étude hydrogéologique est rationalisée en combinant trois approches: une campagne unique d’essais de traçage multi-traceur au cours des périodes de hautes eaux, un suivi saisonnier de la teneur en isotope stable de l’eau et de la conductivité électrique, et une campagne hydrochimique au cours des périodes de bases eaux. La complexité du contexte hydrogéologique du massif de la Séchilienne mène à développer une méthode originale pour estimer les altitudes des zones de recharge des sources, basée sur des analyses topographiques et sur la teneur des isotopes stables de l’eau des sources et des précipitations. Cette étude montre que le massif siège du glissement de terrain de la Séchilienne est caractérisé par comportement à double perméabilité typique des aquifères fissurés où les fractures conductrices jouent un rôle majeur dans le drainage. Il y a un contraste de conductivité hydraulique entre la zone instable et la masse rocheuse intacte sous-jacent du glissement de terrain. Ce contraste conduit à la définition d’un aquifère perché peu profond dans la zone instable et un aquifère profond dans le massif intact siège du glissement de terrain. L’aquifère perché au sein du glissement de terrain est temporaire, essentiellement discontinu, et son extension et la connectivité fluctuent en fonction de la recharge saisonnière.ResumenEl ascenso del nivel de agua subterránea juega un papel importante en la activación o reactivación de deslizamientos de tierra profundos y por lo tanto los estudios hidromecánicos requieren de un buen conocimiento de los flujos de agua subterránea. Los medios anisotrópicos y heterogéneos asociados a la deformación por el deslizamiento de tierra hacen dificultosas las investigaciones hidrogeológicas clásicas. Las investigaciones hidrogeológicas recientemente se han centrado en métodos hidroquímicos indirectos. Este estudio tiene como objetivo determinar el modelo conceptual del agua subterránea en el deslizamiento de tierra en Séchilienne y en el macizo hospedante en los Alpes occidentales (Francia). La investigación hidrogeológica se hace más eficiente mediante la combinación de tres enfoques: una relevamiento de pruebas de multitrazadores por única vez durante los períodos de alto flujo, un monitoreo estacional del contenido de isótopos estables y de la conductividad eléctrica en el agua, y un relevamiento hidroquímico durante los períodos de bajo flujo. La complejidad del entorno hidrogeológico del macizo de Séchilienne conduce al desarrollo de un método original para calcular las elevaciones de las áreas de recarga de los manantiales, basada en análisis topográficos y contenidos de isótopos estable del agua de los manantiales y de la precipitación. Este estudio muestra que el macizo hospedante del deslizamiento de tierra de Séchilienne se caracteriza por una doble permeabilidad, comportamiento típico de los acuíferos en rocas fracturadas, donde los conductos de las fracturas desempeñan un papel importante en el drenaje. Existe un contraste de permeabilidad entre la zona inestable y el macizo de roca intacta donde se apoya el deslizamiento de tierra. Este contraste conduce a la definición de un acuífero superficial colgado en la zona inestable y un acuífero profundo en el macizo de roca intacta que aloja el deslizamiento de tierra. El acuífero colgado en el deslizamiento de tierra es temporario, mayormente discontinuo, y su alcance y conectividad fluctúa de acuerdo a la recarga estacional.摘要地下水位上升在深层滑坡的激活或者再激活过程中发挥着重要作用,因此,流体力学研究需要彻底了解地下水水流。各向异性和非均质介质加上滑坡变形使传统的水文地质调查非常困难。最近的水文地质调查主要集中在间接的水文化学方法上。这项研究目的就是确定(法国)阿尔卑斯山脉西部Séchilienne滑坡及其主要地块的地下水概念模型。通过三种方法结合起来使水文地质调查更加合理:高水流期一次性的多示踪剂实验调查,水中稳定同位素含量和电导率季节性监测及低水流期的水化学调查。Séchilienne地块水文地质背景的复杂性致使原来的方法得到进一步开发,根据地形分析和泉水和降水中的稳定同位素含量估算泉补给区的海拔高度。这项研究显示,支撑Séchilienne滑坡的地块呈现出双重渗透性行为特征,这种行为是断裂岩层含水层的典型特征,在这种含水层中,传导断裂在排水中发挥着主要作用。非稳定带和支撑滑坡的完整岩石块之间有渗透性差异。依据这种差异可以区分出非稳定带中的浅的表层含水层和支撑滑坡的完整地块中深层含水层。滑坡中的表层含水层是暂时的、大体上不连续的,其范围和连通性根据季节补给量波动。ResumoA elevação do nível da água subterrânea desempenha um papel importante na ativação ou reativação de deslizamentos em profundidade, de modo que estudos hidromecânicos exigem um bom entendimento sobre os fluxos da água subterrânea. A combinação de meios heterogêneos e anisotrópicos com a deformação gerada por deslizamentos dificulta as investigações ambientais convencionais. Recentemente, investigações hidrogeológicas têm focado em métodos hidrogeoquímicos indiretos. O presente estudo visa à determinação do modelo conceitual de águas subterrâneas do deslizamento de Séchilienne e seu maciço hospedeiro no oeste alpino (França). A investigação hidrogeológica é organizada por meio da combinação de três métodos: teste de multi-traçador em único evento durante os períodos de alto fluxo; monitoramento sazonal do conteúdo de isótopos e condutividade elétrica da água; e levantamento hidroquímico durante períodos de baixo fluxo. A complexidade do contexto hidrogeológico do maciço Séchilienne motiva o desenvolvimento de um novo método para estimar as elevações das áreas de recarga das nascentes, baseada na análise topográfica e conteúdo de isótopos estáveis nas águas das nascentes e das chuvas. O presente estudo mostra que o maciço hospedeiro do deslizamento de Séchilienne é caracterizado pelo comportamento de dupla permeabilidade, típico de aquíferos em rocha fraturada onde as fraturas desempenham papel principal na drenagem. Entre a zona instável e a rocha sã do maciço subjacente ao deslizamento há um contraste de permeabilidade. Tal contraste motiva a definição de um aquífero suspenso raso na zona instável e a um aquífero mais profundo na rocha sã do maciço hospedeiro do deslizamento. O aquífero suspenso no deslizamento é temporário, predominantemente descontínuo, e sua extensão e conectividade varia conforme a sazonalidade da recarga.

How the Stabilizing Effect of Vegetation on a Slope Changes Over Time: A Review

The principles of slope stabilization through vegetation are well known but substantial uncertainty remains about its transient effects, for example that of a forest stand throughout its life cycle. This comprises direct impacts but also more indirect ones that influence soil development that can be important but also difficult to observe and quantify. Often these effects are ambiguous, having potentially a stabilizing or destabilizing influence on a slope under particular conditions (e.g., more structured soils leading to both rapid infiltration and drainage).

Evaluating data quality collected by volunteers for first-level inspection of hydraulic structures in mountain catchments

Volunteers have been trained to perform first-level inspections of hydraulic structures within campaigns promoted by civil protection of Friuli Venezia Giulia (Italy). Two inspection forms and a learning session were prepared to standardize data collection on the functional status of bridges and check dams. In all, 11 technicians and 25 volunteers inspected a maximum of six structures in Pontebba, a mountain community within the Fella Basin. Volunteers included civil-protection volunteers, geosciences and social sciences students. Some participants carried out the inspection without attending the learning session. Thus, we used the mode of technicians in the learning group to distinguish accuracy levels between volunteers and technicians. Data quality was assessed by their accuracy, precision and completeness. We assigned ordinal scores to the rating scales in order to get an indication of the structure status. We also considered performance and feedback of participants to identify corrective actions in survey procedures. Results showed that volunteers could perform comparably to technicians, but only with a given range in precision. However, a completeness ratio (question / parameter) was still needed any time volunteers used unspecified options. Then, volunteers’ ratings could be considered as preliminary assessments without replacing other procedures. Future research should consider advantages of mobile applications for data-collection methods.

Identification of Hydro-Meteorological Triggers for Villerville Coastal Landslide

The Villerville–Cricqueboeuf landslide (Normandy, France) is an example of a very well monitored coastal landslide. The long time series goes back to the 1980s initiated after the January 1982 major re-activation of the landslide complex. The combination of translational movement of large blocks of chalk and sandstone and the rotational movement at the cliff’s toe results in a very complex hydrological system. Earlier research showed a qualitative connection of displacement or re-activation with meteorological forcing and inland hydrological circumstances.

Identifying hydrological pre-conditions and rainfall triggers of slope failures at catchment scale for 2014 storm events in the Ialomita Subcarpathians, Romania

This paper addresses a regional-scale analysis of the rainfall-induced landslides for 2014 storm events based on detailed hydro-meteorological data set in the Ialomita Subcarpathians. This area is located in the western part of the Curvature Subcarpathians, a complex geological and geomorphic unit in Romania. The high temporal frequency of landslide events in the last decades (1997, 1998, 2005, 2006, 2010, 2012 and 2014) leads us to considerer that these processes play a major role in the evolution of this area’s landscape where the most frequent landforms are rotational slides, translational slides, mudslides and complex movements. The rainy period between April and August 2014 induced numerous flash floods and landslides in this specific area that resulted to severe economic losses estimated to 8 million euros in Dambovita County. Spatially distributed rainfall during the main storm events estimated from adjusted radar-based precipitation was used to investigate the hydro-meteorological conditions that triggered or not landslides in the Ialomita Subcarpathians. Hydrological pre-conditions were assessed by hourly in situ soil moisture measurements at local scale and hydrological modelling at regional scale. ModClark semi distributed model implemented in HEC HMS software that integrates radar data was used to analyse catchment response to the main rainfall event that resulted to landslides in 2014. Analysis between rainfall, soil moisture conditions and direct runoff was performed for identifying the contribution of the hydro-meteorologic conditions to landsliding process in the Ialomita Subcarpathians. A detailed landslide inventory based on field mapping and visual interpretation of satellite and aerial images was completed with information from local authorities and mass media. Despite the limited number of landslide events, this study allows a detailed insight of understanding the influence of rainfall in landslide occurrence in this specific area.