Modeling the isotopic evolution of snowpack and snowmelt : Testing a spatially distributed parsimonious approach

This work was funded by the NERC/JPI SIWA project (NE/M019896/1) and the European Research Council ERC (project GA 335910 VeWa). The Krycklan part of this study was supported by grants from the Knut and Alice Wallenberg Foundation (Branch-points), Swedish Research Council (SITES), SKB and Kempe foundation. The data and model code is available upon request. Authors declare that they have no conflict of interest. We would like to thank the three anonymous reviewers for their constructive comments that improved the manuscript.Peer reviewedPublisher PD

Estimation of catchment-scale soil moisture patterns from topography and reconstruction of a preserved ash-flow paleotopography

2012 Spring.Includes bibliographical references.This dissertation consists of three parts, two of which examine methods for estimating spatial soil moisture patterns while the third investigates the reconstruction of a fluvially-eroded paleotopography. Part I of the dissertation evaluates unsupervised machine-learning techniques' effectiveness for estimating soil moisture patterns and compares them with linear regression. Physical processes that impact soil moisture are typically expressed as nonlinear functions, but most previous research on the estimation of soil moisture has relied on linear techniques. In the present work, two machine learning techniques, a spatial artificial neural network (SANN) and a mixture model (MM), that can infer nonlinear relationships are compared with multiple linear regression (MLR) for estimating soil moisture patterns using topographic attributes as predictor variables. The methods are applied to time-domain reflectometry (TDR) soil moisture data collected at three catchments with varying characteristics (Tarrawarra, Satellite Station, and Cache la Poudre) under different wetness conditions. The methods' performances with respect to the number of predictor attributes, the quantity of training data, and the attributes employed are compared using the Nash-Sutcliffe Coefficient of Efficiency (NSCE) as the performance measure. The performances of the methods are dependent on the site studied, the average soil moisture and the quantity of training data provided. Although the methods often perform similarly, the best performing method overall is the SANN, which incorporates additional predictor variables more effectively than the other methods. Next, Part II of the dissertation presents the development and testing of a new conceptually-based model for estimating soil moisture patterns and describes the investigation of the climatic, vegetation and soil characteristics that affect pattern organization and temporal stability with the model. Soil moisture is a key hydrologic state variable for the Earth's surface affecting both energy and precipitation partitioning. Additionally, the nonlinear dependence of hydrologic processes on soil moisture means that not only is the average moisture condition important for many applications, but the spatial patterns of soil moisture are also important. At the catchment scale, soil moisture patterns have been observed to exhibit different types of dependence on topography. Some catchments have their wettest locations in the valley bottoms, while others have their wettest locations on hillslopes that are oriented away from the sun. Additionally, some catchments have moisture patterns that maintain a similar organization at all times (time stability), while other catchments have soil moisture patterns that change through time (time instability). Although these tendencies are well known, the reasons for their occurrence at a particular catchment are not well understood. In this paper, we investigate the conditions under which the different types of topographic dependence and different degrees of time instability occur through the use of a new conceptual model. The type of topographic dependence and the degree of instability are quantified by two metrics that are also introduced in the paper, and the effects of soil, vegetation, and climatic parameters on these metrics are then evaluated. The evaluations indicate that saturated horizontal hydraulic conductivity, pore disconnectedness, vegetation evapotranspiration efficiency, and an exponent relating the horizontal hydraulic gradient to the topographic slope have the strongest effects on the organization and instability of the soil moisture patterns. In contrast, annual potential evapotranspiration alone does not strongly impact the organization or its stability. Finally, Part III of the dissertation describes the modification of a previously-developed interpolation scheme for fluvial topography and the reconstruction of a paleotopography that may be potentially important to groundwater movement by the modified method. Many applications in geology require estimation of the depth and thickness of lithologic layers based on limited observations. The boundaries of such layers are typically estimated using Kriging or other estimation methods that produce smooth surfaces. In some cases, however, smooth surfaces may be inappropriate. A boundary that is formed by a preserved hillslope and valley paleotopography, in particular, is expected to exhibit drainage characteristics and inherent roughness that are not consistent with standard estimation methods. This paper discusses the generalization of a technique originally designed to interpolate fluvially-eroded topography. The method incorporates a simple river basin evolution model to generate realistic topography and adjusts an erodability parameter in space to match observed elevations. The method is generalized to allow flow to enter from outside the interpolation region, which is a likely scenario when reconstructing paleotopography. The method is then applied to the lower boundary of the Tshirege Member of the Bandelier Tuff, which underlies Los Alamos National Laboratory and Bandelier National Monument in north-central New Mexico. The method produces surfaces with major valleys that are consistent with previous observations. The method is also applied in a framework that estimates the likelihood that any particular point within the interpolation region drains through a specified boundary. Although the surfaces vary between simulations, most portions of the interpolation domain drain through consistent boundaries

Representing past and future hydro-climatic variability over multi-decadal periods in poorly-gauged regions: the case of Ecuador

Cette thèse évalue des méthodes pour représenter la variabilité spatio-temporelle hydro-climatique passée et future dans les régions peu jaugées. Elle propose une procédure complète et reproductible appliquée à l'Équateur et s'appuyant sur des données hydro-climatiques observées et simulées en vue de représenter la variabilité passée et de projeter l'impact potentiel des changements climatiques sur les écoulements à la fin du 21ème siècle. Un état de l'art a permis d'identifier plusieurs techniques qui ont été intégrées dans une chaîne méthodologique pour obtenir des séries spatio-temporelles continues de température, de précipitation et de débit sur les périodes multi-décennales passées et futures. Trois chapitres centraux sont consacrés à cet objectif selon les thèmes suivants : (1) régionalisation de la température et des précipitations à partir de mesures in situ en comparant des techniques déterministes et géostatistiques avec une prise en compte de corrections orographiques; (2) reconstruction du débit dans différents bassins versants à l'aide de modèles hydrologiques conceptuels utilisés selon une approche multimodèle et multiparamétrique; et (3) projections hydro-climatiques basées sur des simulations de modèles climatiques sous contrainte d'un scénario marqué d'émission de gaz à effet de serre. La régionalisation du climat a révélé l'importance de caler les paramètres de spatialisation et d'évaluer les champs interpolés par rapport à des stations ponctuelles indépendantes et via des analyses de sensibilité hydrologique. La reconstruction des débits a été possible grâce aux simulations combinées de trois modèles hydrologiques évalués dans des conditions climatiques contrastées, et forcés par les variables climatiques régionalisées. Des simulations de changements hydro-climatiques à moyen terme (2040-2070) et à long terme (2070-2100) ont ensuite été analysées avec des intervalles de confiance de 95 %, en utilisant des scénarios de neuf modèles climatiques et en transférant les paramètres hydrologiques calibrés pour la reconstruction des débits. L'analyse de la variabilité hydro-climatique montre une légère augmentation des températures sur la période 1985-2015, tandis que la variabilité des précipitations est liée aux principaux modes des phases El Niño et La Niña à l'échelle inter-annuelle et au déplacement de la zone de convergence inter-tropicale (ZCIT) à l'échelle saisonnière. Une augmentation générale de la température (+4,4 °C) et des précipitations (+17 %) est attendue d'ici à la fin du 21ème siècle, ce qui pourrait entraîner une augmentation de +5 % à +71 % du débit annuel moyen selon les bassins versants. Ces résultats sont discutés en termes d'importance pour la gestion de l'eau, avant de suggérer de futures recherches hydrologiques telles que la régionalisation du débit des cours d'eau, une meilleure quantification des incertitudes et une évaluation de la capacité à satisfaire les futurs besoins en eau.This thesis investigates methods to represent the past and future hydro-climatic variability in space and over time in poorly-gauged regions. It proposes a complete and reproducible procedure applied to the continental Ecuador to deal with observed and simulated hydro-climatic data in order to represent past variability and project the potential impact of climate change on water resources by the end of the 21st century. Up-to-date techniques were identified in a literature review and were integrated in a chain protocol to obtain continuous space-time series of air temperature, precipitation and streamflow over past and future multi-decadal periods. Three central chapters are dedicated to this objective according to the following topics: (1) regionalization of air temperature and precipitation from in situ measurements by comparing deterministic and geostatistical techniques including orographic corrections; (2) streamflow reconstruction in various catchments using conceptual hydrological models in a multi-model, multi-parameter approach; and (3) hydro-climate projections using climate model simulations under a high range emission scenario. Climate regionalization revealed the importance of calibrating parameters and of assessing interpolated fields against independent gauges and via hydrological sensitivity analyses. Streamflow reconstruction was possible with the regionalized climate inputs and the combined simulations of three hydrological models evaluated in contrasting climate conditions. Future medium term (2040-2070) and long term (2070-2100) hydro-climatic changes were analysed with confidence intervals of 95% using scenarios from nine climate models and transferring the model parameters calibrated for streamflow reconstruction. Analysis of hydro-climatic variability over the period 1985-2015 showed a slight increase in temperature, while precipitation variability was linked to the main modes of El Niño and La Niña phases at inter-annual scale and to the displacement of the inter-tropical convergence zone (ITCZ) at seasonal scale. Under climate change, a general increase in temperature (+4.4 °C) and precipitation (+17%) is expected by the end of the 21st century, which could lead to between +5% and 71% increase in mean annual streamflow depending on the catchments. These results are discussed in terms of significance for water management before suggesting future hydrological research such as regionalizing streamflow, better quantifying uncertainties and assessing the capacity to meet future water requirements

Evaluating the long short-term memory (LSTM) network for discharge prediction under changing climate conditions

Better understanding the predictive capabilities of hydrological models under contrasting climate conditions will enable more robust decision-making. Here, we tested the ability of the long short-term memory (LSTM) for daily discharge prediction under changing conditions using six snow-influenced catchments in Switzerland. We benchmarked the LSTM using the Hydrologiska Byråns Vattenbalansavdelning (HBV) bucket-type model with two parameterizations. We compared the model performance under changing conditions against constant conditions and tested the impact of the time-series size used in calibration on the model performance. When calibrated, the LSTM resulted in a much better fit than the HBV. However, in validation, the performance of the LSTM dropped considerably, and the fit was as good or poorer than the HBV performance in validation. Using longer time series in calibration improved the robustness of the LSTM, whereas HBV needed fewer data to ensure a robust parameterization. When using the maximum number of years in calibration, the LSTM was considered robust to simulate discharges in a drier period than the one used in calibration. Overall, the HBV was found to be less sensitive for applications under contrasted climates than the data-driven model. However, other LSTM modeling setups might be able to improve the transferability between different conditions

Point process-based modeling of multiple debris flow landslides using INLA: an application to the 2009 Messina disaster

We develop a stochastic modeling approach based on spatial point processes of log-Gaussian Cox type for a collection of around 5000 landslide events provoked by a precipitation trigger in Sicily, Italy. Through the embedding into a hierarchical Bayesian estimation framework, we can use the Integrated Nested Laplace Approximation methodology to make inference and obtain the posterior estimates. Several mapping units are useful to partition a given study area in landslide prediction studies. These units hierarchically subdivide the geographic space from the highest grid-based resolution to the stronger morphodynamic-oriented slope units. Here we integrate both mapping units into a single hierarchical model, by treating the landslide triggering locations as a random point pattern. This approach diverges fundamentally from the unanimously used presence-absence structure for areal units since we focus on modeling the expected landslide count jointly within the two mapping units. Predicting this landslide intensity provides more detailed and complete information as compared to the classically used susceptibility mapping approach based on relative probabilities. To illustrate the model's versatility, we compute absolute probability maps of landslide occurrences and check its predictive power over space. While the landslide community typically produces spatial predictive models for landslides only in the sense that covariates are spatially distributed, no actual spatial dependence has been explicitly integrated so far for landslide susceptibility. Our novel approach features a spatial latent effect defined at the slope unit level, allowing us to assess the spatial influence that remains unexplained by the covariates in the model

Modeling the Isotopic Evolution of Snowpack and Snowmelt: Testing a Spatially Distributed Parsimonious Approach

Use of stable water isotopes has become increasingly popular in quantifying water flow paths and travel times in hydrological systems using tracer-aided modeling. In snow-influenced catchments, snowmelt produces a traceable isotopic signal, which differs from original snowfall isotopic composition because of isotopic fractionation in the snowpack. These fractionation processes in snow are relatively well understood, but representing their spatiotemporal variability in tracer-aided studies remains a challenge. We present a novel, parsimonious modeling method to account for the snowpack isotope fractionation and estimate isotope ratios in snowmelt water in a fully spatially distributed manner. Our model introduces two calibration parameters that alone account for the isotopic fractionation caused by sublimation from interception and ground snow storage, and snowmelt fractionation progressively enriching the snowmelt runoff. The isotope routines are linked to a generic process-based snow interception-accumulation-melt model facilitating simulation of spatially distributed snowmelt runoff. We use a synthetic modeling experiment to demonstrate the functionality of the model algorithms in different landscape locations and under different canopy characteristics. We also provide a proof-of-concept model test and successfully reproduce isotopic ratios in snowmelt runoff sampled with snowmelt lysimeters in two long-term experimental catchment with contrasting winter conditions. To our knowledge, the method is the first such tool to allow estimation of the spatially distributed nature of isotopic fractionation in snowpacks and the resulting isotope ratios in snowmelt runoff. The method can thus provide a useful tool for tracer-aided modeling to better understand the integrated nature of flow, mixing, and transport processes in snow-influenced catchments

Monitoring and analysis of soil-vegetation-atmosphere interactions at slope and catchment scale : Implications for mass-wasting in natural and man-made slopes

Slope mass-wasting (SMW) due to shallow landslides or debris flows is one of the most important erosional processes in many mountainous regions together with surficial erosion by rainfall runoff. These erosive processes cause great socioeconomic and environmental impacts. SMW is affected by soil-vegetation-atmosphere (SVA) interactions and is mainly triggered by climatic actions such as rainfall. SVA interactions involve many factors (e.g. soil type, slope topography, slope hydrological conditions and cover (e.g. bare or vegetated)) that are closely linked to slope thermo-hydro-mechanical response, and hence, to slope stability and erosion. The main aim of this work is to improve the knowledge on SVA interactions through two monitoring systems at slope and catchment scale. At slope scale, an embankment divided in four partitions with North and South orientations and bare/vegetated slopes in each orientation has been monitored. This work has analyzed four years (2017-2020) of soil hydro-thermal and atmospheric parameters related to SVA interactions. The results show that vegetation increases rainfall infiltration, which suggests a decrease in runoff. In addition, vegetation transpiration increases soil drying rates and states, which favors slope stability. Regarding the thermal response, orientation plays an important role with higher temperatures on the South slopes. However, vegetation diminishes the incidence of solar radiation, which reduces soil heat flux up to 75% on the South-facing slopes, and hence, reduces evaporation and daily temperature fluctuations. Nevertheless, the effect of vegetation transpiration is more important than orientation in developing drier soil conditions. This is clearly observed by comparing the drying rates and hydrologic conditions of the North-vegetated slope, which are higher and dryer, with those of the South-bare slope, which are lower and wetter. The results show that vegetation has effects on the hydrologic and thermal response of slopes that may be positive or negative for SMW. At catchment scale, a monitoring system for torrential flows (debris flows and debris floods) detection and characterization has been maintained and further developed in the Rebaixader catchment (Central Pyrenees). 12 years of rainfall and torrential flows occurrence (2009-2020) and 8 years of soil hydrological parameters (2013-2020) have been analyzed to characterize both hydrological soil conditions and rainfall characteristics necessary for torrential flows triggering. Most torrential flows are triggered by short duration (<3 hours) and high intensity (4-10 mm in 5 minutes) rainfalls. Furthermore, the rainfall intensity for torrential flow initiation must be higher when the soil is dryer, and vice versa. First, rainfall thresholds based on rainfall mean intensity (Imean) and maximum intensity (Imax) are defined by means of Receiver Operating Characteristics and Precision-Recall curves. The Imean threshold predicted 2 false negatives and 73 false positives for the 2013-2020 dataset, which includes 15 torrential flow events. This results in a low precision of 15%, since only 13 torrential flow out of 86 issued alarms are correctly predicted. Contrarily, the best Imax threshold reduced the false positives to 11 and predicted also 2 false negatives, increasing the precision to 54%. Then, the hydro-meteorological thresholds were defined by combining Imax and volumetric water content. The best hydro-meteorological threshold reduced the false positives to 8 and the false negatives to 1, increasing the precision to 63%. This confirms that soil hydrological conditions play an important role in torrential flows triggering and may improve early warning predictions. This work significantly contributed to improve our understanding on SVA interactions and its coupling with slope thermohydro-mechanical response, which is strongly related to SMW, by means of in-situ monitoring at slope and catchment scale.La pèrdua de sòl en ves s ants (PSV) per lliscaments de terra superficials o corrents d’arrossegalls és un dels processos erosius més importants a moltes regions muntanyoses junt amb l'erosió superficial per escorrentia de pluja. Aquests processos provoquen grans impactes socioeconòmics i ambientals. Les interaccions sòl-vegetació-atmosfera (SVA) afecten la PSV i es desencadena principalment per accions climàtiques com ara la pluja. Les interaccions SVA impliquen molts factors (tipus de sòl, topografia del talús, condicions hidrològiques i de coberta (nues o vegetades)del talús, etc.) estretament relacionats amb la resposta termo-hidro-mecànica del talús i, per tant, amb la seva estabilitat i erosió. L'objectiu principal és millorar el coneixement de les interaccions SVA amb dos sistemes de monitorització a escala de talús i de conca. A escala de talús s'ha instrumentat un terraplè dividit en 4 parts orientades al nord i al sud i amb un talús amb coberta vegetada i nua a cada orientació. S¿ han analitzat quatre anys (2017-2020) de paràmetres atmosfèrics i hidro-termals del sòl relacionats amb interaccions SVA. Es veu que la vegetació augmenta la infiltració de la pluja, suggerint una disminució de l'escorrentia. A més, la transpiració de la vegetació augmenta les taxes i l'estat d'assecat del sòl, el que afavoreix l’estabilitat del talús. Pel que fa a la resposta termal, l'orientació és important amb temperatures més elevades als talussos sud. Tanmateix, la vegetació disminueix la incidència de radiació solar reduint així el flux de calor al sòl fins un 75% als talussos sud i disminuint l'evaporació i les oscil·lacions de temperatura diàries. Tot i això, l'efecte de la transpiració de la vegetació és més important que el de l'orientació per desenvolupar condicions de sòl més seques. Això s'observa comparant les taxes d'assecatge i condicions hidrològiques del talús nord vegetat, més elevades i seques, amb les del sud no vegetat, més baixes i humides. Els resultats mostren que la vegetació té efectes en la resposta hidro-termal dels vessants que poden ser positius o negatius per la PSV. A escala de conca s'ha mantingut i millorat la instrumentació per detectar i caracteritzar fluxos torrencials a la conca del Rebaixader (Pirineu Central). S'han analitzat 12 anys d'ocurrència de fluxos torrencials i de pluja (2009-2020) i 8 anys de paràmetres hidrològics del sòl (2013-2020) per caracteritzar tant les condicions hidrològiques del sòl com les de pluja necessàries per desencadenar aquests fluxos. La majoria de fluxos torrencials es desencadenen per pluges curtes (< 3 hores ) i d’alta intensitat (4-10 mm en 5 minuts ). A més, la intensitat de pluja per iniciar un flux torrencial és més elevada quan el sòl està més sec, i viceversa. Primerament s’han definit llindars de pluja basats en intensitat mitja (Imean) i màxima (Imax) de pluja amb les corbes de rendiment diagnòstic (ROC) i de Precisió-Reclam. El llindar d’Imean prediu 2 fals os negatius i 73 falsos positius per al conjunt de dades 2013-2020, que inclou 15 fluxos torrencials. Això representa una precisió baixa del 15%, ja que només es preveuen correctament 13 fluxos torrencials de 86 alarmes emeses. En canvi, el millor llindar Imax redueix els falsos positius a 11 i també prediu 2 falsos negatius, augmentant la precisió al 54%. A continuació s’han definit llindars hidro-meteorològics combinant Imax i contingut volumètric d'aigua. El millor llindar hidrometeorològic redueix els falsos positius a 8 i els falsos negatius a 1, augmentant la precisió al 63%. Això confirma que les condicions hidrològiques del sòl juguen un paper important en el desencadenament de fluxos torrencials i poden millorar les prediccions d'alerta primerenca. Aquest treball ha contribuït significativament a millorar la comprensió de les interaccions SVA i el seu acoblament amb la resposta termo-hidro-mecànica del talús, fortament relacionada amb la PSV, mitjançant monitorització a escala de talús i de conca.La pérdida de suelo en laderas (PSL) por deslizamientos de tierra superficiales o corrientes de derrubios es uno de los procesos erosivos más importantes en muchas regiones montañosas junto con la erosión superficial por escorrentía de lluvia. Estos procesos provocan grandes impactos socioeconómicos y ambientales. Las interacciones suelo-vegetación-atmósfera (SVA) afectan a la PSL y se desencadena principalmente por acciones climáticas como la lluvia. Las interacciones SVA implican muchos factores (tipo de suelo, topografía del talud, condiciones hidrológicas y de cubierta (desnuda o vegetada) del talud, etc.) estrechamente relacionados con la respuesta termo-hidromecánica del talud y, por tanto, con su estabilidad y erosión. El objetivo principal es mejorar el conocimiento de las interacciones SVA con dos sistemas de monitorización a escala de talud y de cuenca. A escala de talud se ha instrumentado un terraplén dividido en 4 partes orientadas al norte y al sur y con un talud con cubierta vegetada y desnuda en cada orientación. Se han analizado cuatro años (2017-2020) de parámetros atmosféricos e hidrotermales del suelo relacionados con interacciones SVA. Los resultados muestran que la vegetación aumenta la infiltración de la lluvia, lo que sugiere una disminución de la escorrentía. Además, la transpiración de la vegetación aumenta las tasas y el estado de secado del suelo, lo que favorece la estabilidad del talud. En cuanto a la respuesta termal, la orientación es importante con temperaturas más elevadas en los taludes sur. Sin embargo, la vegetación disminuye la incidencia de radiación solar reduciendo así el flujo de calor en el suelo hasta un 75% en los taludes sur y disminuyendo la evaporación y las oscilaciones de temperatura diarias. Sin embargo, el efecto de la transpiración de la vegetación es más importante que el de la orientación en el desarrollo de condiciones de suelo más secas. Esto se observa claramente al comparar las tasas de secado y las condiciones hidrológicas del talud norte vegetado, más elevadas y secas, con las del sur no vegetado, más bajas y húmedas. Los resultados demuestran que la vegetación tiene efectos en la respuesta hidro-termal de la ladera que pueden ser positivos o negativos para la PSL. A escala de cuenca se ha mantenido y mejorado la instrumentación para detectar y caracterizar flujos torrenciales en la cuenca del Rebaixader (Pirineo Central). Se han analizado 12 años de ocurrencia de flujos torrenciales y de lluvia (2009-2020) y 8 años de parámetros hidrológicos del suelo (2013-2020) para definir tanto las condiciones hidrológicas del suelo como las de lluvia necesarias para desencadenar estos flujos. La mayoría de flujos torrenciales se desencadenan por lluvias de corta duración (< 3 horas) y de alta intensidad (4-10 mm en 5 minutos). Además, la intensidad de lluvia para iniciar un flujo torrencial es mayor cuando el suelo está más seco, y viceversa. Primeramente, se han definido umbrales de lluvia basados en intensidad media (Imean) y máxima (Imax) de lluvia con las curvas de Característica Operativa del Receptor (ROC) y de Precisión-Exhaustividad. El umbral de Imean predice 2 falsos negativos y 73 falsos positivos para el conjunto de datos 2013-2020, que incluye 15 flujos torrenciales. Esto representa una precisión baja del 15%, puesto que sólo se prevén correctamente 13 flujos torrenciales de las 86 alarmas emitidas. Por el contrario, el mejor umbral Imax reduce los falsos positivos a 11 y también predice 2 falsos negativos, aumentando la precisión al 54%. A continuación, se han definido umbrales hidro-meteorológicos combinando Imax y contenido volumétrico de agua. El mejor umbral hidro-meteorológico reduce los falsos positivos a 8 y los falsos negativos a 1, lo que aumenta la precisión al 63%. Esto confirma que las condiciones hidrológicas del suelo juegan un papel importante en el desencadenamiento de flujos torrenciales y pueden mejorar las predicciones de alerta temprana. Este trabajo ha contribuido significativamente a mejorar la comprensión de las interacciones SVA y su acoplamiento con la respuesta termo-hidro-mecánica del talud, fuertemente relacionada con la PSL, mediante la monitorización in-situ a escala de talud y a escala de cuenca.Postprint (published version