In recent years, physically-based hydrological models provided a robust approach to better understand the cause-effect relationships of effective hydraulic properties in soil hydrology. These have increased the flexibility of studying the behavior of a soil system under various environmental conditions. One disadvantage of physical models is their inability to model the vertical and horizontal heterogeneity of hydraulic properties in a soil system at the regional scale. In order to overcome this limitation, inverse modeling may be used. Near surface soil moisture, which has been collected routinely by remote sensing (RS) platforms, and evapotranspiration, that is also a pivotal key for water balance near the land surface can be used as alternatives for quantifying the effective soil hydraulic parameters through inverse modeling. However, the new approach suffers from not only the scale discrepancy between RS pixel resolution and model grid resolution, but also its application in complex terrains. Furthermore, hydrological models require a number of required input parameters. Hence, this dissertation focuses on developing a methodology for addressing these problems. The field-scale Soil-Water-Atmosphere-Plant model (SWAP) was extended to regional application, and then coupled with a Genetic Algorithm (GA), to operate as the core of the developed decision support system at the regional level. Also, various stochastic processes were developed and applied to the GA for improving the searching ability of optimization algorithms. The computational simulation-optimization approach was tested and evaluated under various synthetic and field validation experiments demonstrating that the methodology provided satisfactory results. In this dissertation, the proposed methodologies analyzed the spatio-temporal root zone soil moisture with RS and in-situ soil moisture data at the multiple scales. Also, these approaches could provide better input parameters for hydro-climatic models, resulting in better understanding of the hydrologic cycle. Thus, a better understanding of water cycle would help us to be better prepared for efficient water resources management, agriculture, and devastating natural disasters in the real world.

No description available.

As land-cover conversion continues to expand into ever more remote areas in the humid tropics, montane rainforests are increasingly threatened. In the south Ecuadorian Andes, they are not only subject to man-made disturbances but also to naturally occurring landslides. I was interested in the impact of this ecosystem dynamics on a key parameter of the hydrologic cycle, the soil saturated hydraulic conductivity (synonym: permeability; Ks from here on), because it is a sensitive indicator for soil disturbances. My general objective was to quantify the effects of the regional natural and human disturbances on the saturated hydraulic conductivity and to describe the resulting spatial-temporal patterns. The main hypotheses were: 1) disturbances cause an apparent displacement of the less permeable soil layer towards the surface, either due to a loss of the permeable surface soil after land-sliding, or as a consequence of the surface soil compaction under cattle pastures; 2) ‘recovery’ from disturbance, either because of landslide re-vegetation or because of secondary succession after pasture abandonment, involves an apparent displacement of the less permeable layer back towards the original depth an 3) disturbances cause a simplification of the Ks spatial structure, i.e. the spatially dependent random variation diminishes; the subsequent recovery entails the re-establishment of the original structure. In my first study, I developed a synthesis of recent geostatistical research regarding its applicability to soil hydraulic data, including exploratory data analysis and variogram estimation techniques; I subsequently evaluated the results in terms of spatial prediction uncertainty. Concerning the exploratory data analysis, my main results were: 1) Gaussian uni- and bivariate distributions of the log-transformed data; 2) the existence of significant local trends; 3) no need for robust estimation; 4) no anisotropic variation. I found partly considerable differences in covariance parameters resulting from different variogram estimation techniques, which, in the framework of spatial prediction, were mainly reflected in the spatial connectivity of the Ks-field. Ignoring the trend component and an arbitrary use of robust estimators, however, would have the most severe consequences in this respect. Regarding variogram modeling, I encouraged restricted maximum likelihood estimation because of its accuracy and independence on the selected lags needed for experimental variograms. The second study dealt with the Ks spatial-temporal pattern in the sequences of natural and man-made disturbances characteristic for the montane rainforest study area. To investigate the disturbance effects both on global means and the spatial structure of Ks, a combined design-and model-based sampling approach was used for field-measurements at soil depths of 12.5, 20, and 50 cm (n=30-150/depth) under landslides of different ages (2 and 8 years), under actively grazed pasture, fallows following pasture abandonment (2 to 25 years of age), and under natural forest. Concerning global means, our main findings were 1) global means of the soil permeability generally decrease with increasing soil depth; 2) no significant Ks differences can be observed among landslides and compared to the natural forest; 3) a distinct permeability decrease of two orders of magnitude occurs after forest conversion to pasture at shallow soil depths, and 4) the slow regeneration process after pasture abandonment requires at least one decade. Regarding the Ks spatial structure, we found that 1) disturbances affect the Ks spatial structure in the topsoil, and 2) the largest differences in spatial patterns are associated with the subsoil permeability. In summary, the regional landslide activity seems to affect soil hydrology to a marginal extend only, which is in contrast to the pronounced drop of Ks after forest conversion. We used this spatial-temporal information combined with local rain intensities to assess the partitioning of rainfall into vertical and lateral flowpaths under undisturbed, disturbed, and regenerating land-cover types in the third study. It turned out that 1) the montane rainforest is characterized by prevailing vertical flowpaths in the topsoil, which can switch to lateral directions below 20 cm depth for a small number of rain events, which may, however, transport a high portion of the annual runoff; 2) similar hydrological flowpaths occur under the landslides except for a somewhat higher probability of impermeable layer formation in the topsoil of a young landslide, and 3) pronounced differences in runoff components can be observed for the human disturbance sequence involving the development of near-surface impeding layers for 24, 44, and 8 % of rain events for pasture, a two-year-old fallow, and a ten-year-old fallow, respectively. / Der tropische Bergregenwald in den Südecuadorianischen Anden unterliegt sowohl anthropogenen Eingriffen, d.h. der Umwandlung von Naturwald in Rinderweiden, als auch natürlichen Störungen in der Form von Hangrutschen. Ziel meiner Arbeit war es, die Auswirkungen dieser regionalen Störungsdynamik auf einen Schlüsselparameter des hydrologischen Kreislaufs, die gesättigte hydraulische Wasserleitfähigkeit (Ks), zu untersuchen und die resultierenden raum-zeitlichen Muster zu beschreiben. In der ersten Studie habe ich eine Synthese aktueller geostatistischer Forschung hinsichtlich ihrer Eignung für die Analyse bodenhydrologischer Daten entwickelt. Diese beinhaltet explorative Datenanalyse und verschiedene Techniken zur Schätzung der Kovarianzparameter; die Ergebnisse habe ich in Bezug auf die Ungenauigkeit räumlicher Vorhersagen bewertet. Es hat sich dabei herausgestellt, dass die Schätztechniken teilweise beachtliche Unterschiede in den Parametern hervorrufen, welche sich hauptsächlich in der räumlichen Konnektivität widergespiegeln. Die wichtigste Rolle im Zusammenhang mit der räumlichen Vorhersage kommt jedoch den vorgeordneten explorativen Analyseschritten zu. In der zweiten Studie habe ich mich mit der Beschreibung des raum-zeitlichen Muster der Wasserleitfähigkeit in den anthropogenen und natürlichen Störungsgradienten beschäftigt. Wichtigste Ergebnisse waren, dass es keine signifikanten Unterschiede der Wasserleitfähigkeit zwischen den verschieden alten Hangrutschen und dem Naturwald gibt. Daraus lässt sich schließen, dass die natürlichen Störungen im Untersuchungsgebiet lediglich marginale Auswirkungen auf die Bodenhydrology haben. Das steht in starkem Kontrast zum anthropogenen Störungskreislauf: die Wasserleitfähigkeit im Weideboden hat gegenüber dem Naturwald um zwei Größenordnungen abgenommen; eine „Erholung“ nach Nutzungsaufgabe scheint mindestens ein Jahrzehnt in Anspruch zu nehmen. Die räumlichen Abhängigkeit von Ks in den Oberböden von Wald und einer alten Brache ist stärker als in jenen der gestörten Flächen, was auf eine störungsbedingte Beeinträchtigung der räumlichen Struktur in geringer Bodentiefe schließen lässt. In der dritten Studie habe ich diese raum-zeitlichen Informationen mit dem örtlichen Niederschlagsregime in Verbindung gebracht, um Rückschlüsse auf die Auswirkungen der störungsbedingten Änderungen von Ks auf hydrologische Fließwege zu ziehen. Es hat sich gezeigt, dass im tropischen Bergregenwald und unter Hangrutschen ubiquitäre Tiefenversickerung dominiert, es allerdings zu einer Verschiebung in laterale Fließrichtungen für die seltenen intensiven Regenereignisse kommen kann. Anthropogene Störungen gehen mit einer um bis zu 50 Prozent erhöheren Wahrscheinlichkeit des Auftretens oberflächennaher Stauschichten einher, was die Bedeutung lateraler Fließwege erhöht. Dies trifft in vergleichbarer Größenordnung auch auf ein Vergleichsökosystem im Tieflandregenwald zu.

Conservation tillage systems are increasingly adapted replacing conventional turnover moldboard plowing practices worldwide. This is part of a sustainable intensification of agriculture to meet future global food demand while at the same time sustaining environmental resources. The choice of tillage system affects soil structure and thereby also soil hydraulic properties (SHP) such as the water retention characteristic (WRC) and the hydraulic conductivity characteristic (HCC). Effects of agricultural management on SHP have been widely studied in the past decades. Thereby, temporal variations were identified as a major source of variability in the quantification of soil pore space and SHP. Such variability is introduced by tillage creating a loose soil matrix that eventually settles due to gravity, wetting-drying cycles and temperature fluctuations but also variable soil organic matter distributions in the soil and biological activity. Past efforts to model soil water dynamics showed that consideration of time-variable SHP may significantly improve simulation results. This involves both the seasonal variability as well as long-term land-use changes from conventionally to untilled soil. A prerequisite for such an approach is the periodic quantification of the WRC and HCC in the field and laboratory. In addition to the direct provision of modeling parameters, the quantification of WRC and HCC over time yields information on soil structural changes in the shape of a soil pore size distribution (PSD). The evolution of derived PSDs can be modeled and with that, the evolution of SHP might be predicted. However, there is little data available and the processes happening over one cropping season or between land-use changes need to be better understood. The aim of this dissertation was to shed light on soil pore space and associated hydraulic property changes on a long-term (23 years) tillage experiment in Eastern Germany. Three treatments with varying tillage intensity were investigated: conventional tillage with a turnover moldboard plow (CT), reduced mulch tillage with a cultivator (RT) and no tillage with direct sowing (NT). The soil was a Haplic Luvisol with silt loam texture. Objectives were twofold: • Objective 1) was to quantify the temporal variability in PSD over one winter wheat cropping season by frequently measuring SHP. Soil physical quality of the three treatments was assessed using this data. • Objective 2) was to characterize the soil structural differences between the treatments by relating hydraulic conductivity over a wide soil moisture range to other soil physical and chemical properties. For Objective 1), undisturbed soil cores (250 cm3) were taken over one winter wheat cropping cycle on five occasions from December 2015 to after the harvest in August 2016. Those soil cores were used to determine the saturated hydraulic and the WRC as well as the HCC in the laboratory. The data was parametrized with the bimodal Kosugi and Mualem model. Soil physical quality was assessed by the relative field capacity and air capacity as suggested in recent literature. Results showed that tilled soil, i.e. CT and RT, exhibited a distinct bimodal PSD with a structural and a textural mode. However, this structural mode was temporally instable and diminished after the winter and throughout the early growing season. Likely processes behind those changes were wetting-drying cycles, rainfall impact and freeze-thaw cycles. Shortly before and after the harvest some of the structural mode was restored which was probably induced by decomposing organic matter mixed into the topsoil from the previous winter wheat harvest during stubble breaking. Described changes were evident in decreases of transmission pores (⌀ 50 - 500 µm) during winter and increases during summer. Untilled soil, i.e. NT, tended towards a unimodal PSD with less transmission but more storage (⌀ 0.5 - 50 µm) pores. Temporally this soil was rather inert. This was attributed to natural compaction in absence of annual tillage for more than 20 years. Soil physical quality varied with the changes in PSD. Water availability was not an issue. Overall, the soil physical quality indicators for soil aeration were outside of an optimal range for indicators for most of the time. For Objective 2), field infiltration measurements were conducted with a hood (tension) infiltrometer to obtain (near-) saturated hydraulic conductivity. Soil cores were taken to quantify unsaturated hydraulic conductivity. Other properties for correlation and multiple regression analysis were bulk density, the bubbling pressure, organic C, as well as macro- and mesoporosity. X-ray µCT imaging on undisturbed soil cores from CT and NT treatments gave additional information on soil pore metrics. Results pointed towards a distinctly different soil structure between tilled and untilled soil. Near-saturated hydraulic conductivity of tilled soil was negatively correlated with bulk density as well as macro- and mesoporosity. None of the properties was meaningful for untilled soil. Imaging results confirmed the hypothesis, that (near-) saturated hydraulic conductivity on NT is governed by few well-connected large pores, while the soil matrix is comparably dense conducting only small amounts of infiltrating water. On tilled soil, the overall porosity is relevant for water transmission. Large continuous pore systems, however, get destroyed by annual tillage. In summary, the study showed distinct differences in soil structure and inherently also SHP between conservation and conventional tillage treatments. Differences in SHP, both in (near-) saturated hydraulic conductivity as well as WRC and HCC were large between some occasions. Therefore, this study confirmed the notion that on arable soils one-off measurements of SHP are not enough for their proper quantification. This was especially true for tilled soil. Modeling tasks over one cropping period, i.e. for example for irrigation schedules, will make periodic measurements necessary, i.e. unless an accurate modeling of the PSD becomes feasible. Current restraints are that most PSD models only consider a short-term post-tillage loss of porosity while a restored macropore system is not accounted for. In contrast to CT and RT, NT soil was temporally stable. While water retention was improved, (near ) saturated hydraulic conductivity was overall lower than on tilled soil. Correlation and regression analysis in combination with X-ray µCT explained some of the differences observed by tension infiltration measurements. Results highlighted that for arable soil, tillage treatments and probably other agricultural management practices, need to be considered when developing pedotransfer functions for an accurate estimation of SHP.:Table of Contents Declaration of conformity I Acknowledgements II Table of Contents IV List of Figures VII List of Tables XI Nomenclature XIII Abstract XV Zusammenfassung XVIII 1 Introduction 1 1.1 The sustainable development agenda and conservation tillage 1 1.2 Soil structure and soil hydraulic properties 3 1.3 Effects of conservation tillage on soil hydraulic properties 5 1.4 Temporal variability of soil hydraulic properties 8 1.5 Objectives and hypotheses 10 1.6 Structure of the dissertation 12 2 Materials and methods 15 2.1 Study area 15 2.1.1 Tillage experiment Lüttewitz (‘Schlag Gasthof’) 15 2.1.2 Treatments and agricultural management 16 2.2 Sample design 20 2.3 Field measurements 22 2.3.1 Hood infiltrometer measurements 22 2.3.2 Analysis of hood infiltrometer measurements 24 2.3.3 Macropore stability indicator 24 2.3.4 Undisturbed and disturbed soil sampling 25 2.4 Laboratory measurements 26 2.4.1 Saturated hydraulic conductivity 26 2.4.2 Water retention and hydraulic conductivity characteristic 26 2.4.3 Other soil properties 27 2.5 Model fitting procedure 28 2.5.1 Bimodal models for the water retention and hydraulic conductivity characteristic 28 2.5.2 Parametrization to quantify changes in the pore size distributions and pore volume fractions 29 2.5.3 Parametrization to infer unsaturated hydraulic conductivity for variability analysis 31 2.6 Capacitive soil physical quality indicators 32 2.7 Relationship between imaged pore metrics and field near-saturated hydraulic conductivity 32 2.8 Statistical analysis 33 3 Results 35 3.1 Rainfall patterns 35 3.2 Field (near-) saturated hydraulic conductivity 36 3.3 Threshold pore radius 37 3.4 Laboratory saturated hydraulic conductivity 38 3.5 Unsaturated hydraulic conductivity 39 3.6 Soil pore size distributions and pore volume fractions over one cropping season 40 3.7 Capacitive soil physical quality indicators 45 3.8 Correlation and linear regression of hydraulic conductivity with other soil properties 46 3.9 Other soil properties 49 3.9.1 Bulk density 49 3.9.2 Soil organic carbon and nitrogen 50 3.10 Imaged soil structure and hydraulic conductivity 52 3.10.1 Comparison of hydraulic conductivity obtained through three methods in Spring 2018 52 3.10.2 Soil pore metrics 52 3.10.3 Correlation between hydraulic conductivity and pore metrics 53 4 Discussion 55 4.1 Soil pore size distributions over one cropping cycle 55 4.1.1 Soil pore size distribution is bimodal on tilled soil and varies with time 55 4.1.2 Summary Objective 1) Hypotheses A and B 58 4.2 The effects of a changing pore system on soil physical quality 59 4.2.1 Suboptimal soil physical quality indicators change with time 60 4.2.2 Summary Objective 1) Hypothesis C 62 4.3 Tillage effects on variability of hydraulic conductivity 62 4.3.1 (Near ) saturated hydraulic conductivity 62 4.3.2 Unsaturated hydraulic conductivity 64 4.3.3 Summary: Objective 2) Hypothesis D and E 65 4.4 Factors influencing water transmission and its temporal variation 65 4.4.1 Soil properties partly explain variability in hydraulic conductivity on CT 65 4.4.2 Imaged pore metrics explain differences in field hydraulic conductivity 67 4.4.3 Summary Objective 2) Hypothesis F 68 5 Summary and outlook 69 References 73 Appendix 93

Large areas in the humid tropics are currently undergoing land-use change. The decrease of tropical rainforest, which is felled for land clearing and timber production, is countered by increasing areas of tree plantations and secondary forests. These changes are known to affect the regional water cycle as a result of plant-specific water demand and by influencing key soil properties which determine hydrological flow paths. One of these key properties sensitive to land-use change is the saturated hydraulic conductivity (Ks) as it governs vertical percolation of water within the soil profile. Low values of Ks in a certain soil depth can form an impeding layer and lead to perched water tables and the development of predominantly lateral flow paths such as overland flow. These processes can induce nutrient redistribution, erosion and soil degradation and thus affect ecosystem services and human livelihoods. Due to its sensitivity to land-use change, Ks is commonly used to assess the associated changes in hydrological flow paths. The objective of this dissertation was to assess the effect of land-use change on hydrological flow paths by analysing Ks as indicator variable. Sources of Ks variability, their implications for Ks monitoring and the relationship between Ks and near-surface hydrological flow paths in the context of land-use change were studied. The research area was located in central Panama, a country widely experiencing the abovementioned changes in land use. Ks is dependent on both static, soil-inherent properties such as particle size and clay mineralogy and dynamic, land use-dependent properties such as organic carbon content. By conducting a pair of studies with one of these influences held constant in each, the importance of static and dynamic properties for Ks was assessed. Applying a space-for-time approach to sample Ks under secondary forests of different age classes on comparable soils, a recovery of Ks from the former pasture use was shown to require more than eight years. The process was limited to the 0−6 cm sampling depth and showed large variability among replicates. A wavelet analysis of a Ks transect crossing different soil map units under comparable land cover, old-growth tropical rainforest, showed large small-scale variability, which was attributed to biotic influences, as well as a possible but non-conclusive influence of soil types. The two results highlight the importance of dynamic, land use-dependent influences on Ks. Monitoring studies can help to quantify land use-induced change of Ks, but there is a variety of sampling designs which differ in efficiency of estimating mean Ks. A comparative study of four designs and their suitability for Ks monitoring is used to give recommendations about designing a Ks monitoring scheme. Quantifying changes in spatial means of Ks for small catchments with a rotational stratified sampling design did not prove to be more efficient than Simple Random Sampling. The lack of large-scale spatial structure prevented benefits of stratification, and large small-scale variability resulting from local biotic processes and artificial effects of destructive sampling caused a lack of temporal consistency in the re-sampling of locations, which is part of the rotational design. The relationship between Ks and near-surface hydrological flow paths is of critical importance when assessing the consequences of land-use change in the humid tropics. The last part of this dissertation aimed at disclosing spatial relationships between Ks and overland flow as influenced by different land cover types. The effects of Ks on overland-flow generation were spatially variable, different between planar plots and incised flowlines and strongly influenced by land-cover characteristics. A simple comparison of Ks values and rainfall intensities was insufficient to describe the observed pattern of overland flow. Likewise, event flow in the stream was apparently not directly related to overland flow response patterns within the catchments. The study emphasises the importance of combining pedological, hydrological, meteorological and botanical measurements to comprehensively understand the land use-driven change in hydrological flow paths. In summary, Ks proved to be a suitable parameter for assessing the influence of land-use change on soils and hydrological processes. The results illustrated the importance of land cover and spatial variability of Ks for decisions on sampling designs and for interpreting overland-flow generation. As relationships between Ks and overland flow were shown to be complex and dependent on land cover, an interdisciplinary approach is required to comprehensively understand the effects of land-use change on soils and near-surface hydrological flow paths in the humid tropics. / Tropische Regionen sind einem andauernden Landnutzungswandel unterworfen. Einerseits wird tropischer Regenwald für Holz- und Flächengewinnung abgeholzt, andererseits fallen Flächen im Zuge der Urbanisierung brach. Diese werden zum Teil mit Nutzholz-Plantagen aufgeforstet, zum Teil entwickelt sich auf ihnen natürlicher Sekundärwald. Änderungen in der Landnutzung beeinflussen Bodeneigenschaften und dadurch implizit den Wasserkreislauf der Region. Eine dieser wichtigen landnutzungsabhängigen Bodeneigenschaften ist die gesättigte hydraulische Leitfähigkeit oder Permeabilität, die maßgeblich die Versickerungsrate im Boden bestimmt. Eine niedrige Permeabilität hemmt die (vertikale) Versickerung und kann dazu führen, dass laterale hydrologische Fließpfade wie z.B. Oberflächenabfluss aktiviert werden. Dadurch wird sowohl die Bodenerosion und Nährstoffverlagerung verstärkt als auch die Auffüllung des Grundwasserspeichers verringert. In welchem Maße jedoch die Änderung der Permeabilität während des Landnutzungswandels eine Änderung der hydrologischen Fließpfade nach sich zieht, ist noch unzureichend erforscht. Die Zielstellung der vorliegenden Dissertation war, mit Hilfe der Permeabilität als Indikatorvariable die Auswirkungen des Landnutzungswandels auf bodennahe hydrologische Fließpfade zu beurteilen. Dabei sollten die Quellen der Variabilität der Permeabilität anhand des Einflusses von Bodentyp und Landnutzung eingeschätzt, diese Variabilität in das Design einer Probenahmestrategie für die Permeabilität integriert und die Auswirkungen dieser Einflüsse auf die Aktivierung lateraler Fließpfade untersucht werden. Die Studien wurden in Panama durchgeführt, da dieses Land stark vom Landnutzungswandel betroffen ist, eine gute Forschungsinfrastruktur aufweist und sich durch die hohen Niederschläge des tropischen Klimas Änderungen im Wasserkreislauf besonders stark auswirken. Zwei Teilstudien zu den Quellen der Variabilität der Permeabilität lieferten Hinweise auf einen möglichen Einfluss des Bodentyps, der jedoch durch den lokal stärkeren Einfluss der Landnutzung überprägt wurde. Auf regionaler Skala wurde die Erholung der Permeabilität unter Sekundärwald nach einer vorhergehenden Weidenutzung belegt. Beide Studien deuteten auf einen maßgeblichen Einfluss der Landnutzung und der räumlichen Struktur der Permeabilität auf die untersuchten Prozesse hin. Für die effiziente Abschätzung zeitlicher Veränderungen der Permeabilität, wie sie im Zuge des Landnutzungswandels auftritt, ist die Einbeziehung dieser räumlichen Strukturen in das Design einer Probenahmestrategie für die Permeabilität von großer Bedeutung, wie in einem Vergleich vier verschiedener Designs gezeigt wurde. Der Zusammenhang zwischen der Permeabilität und der Entstehung von Oberflächenabfluss konnte nicht durch einfache Ansätze, wie dem Vergleich der Permeabilität mit Regenintensitäten oder der Betrachtung des Gebietsabflusses, erklärt werden. Vielmehr scheint ein komplexes Zusammenspiel aus meteorologischen, biologischen, bodenkundlichen und hydrologischen Faktoren zu wirken. So wurde die räumliche Struktur des Oberflächenabflusses im Untersuchungsgebiet vermutlich durch eine Kombination aus Landnutzungs- und Permeabilitäts-Einflüssen bedingt, zu deren Aufklärung weitere notwendige Messungen vorgeschlagen werden. Zusammengefasst konnte anhand der Permeabilität der Einfluss des Landnutzungswandels auf die hydrologischen Fließpfade gut aufgezeigt werden. Eine besondere Bedeutung kommt hierbei der Betrachtung der landnutzungsabhängigen räumlichen Struktur der Permeabilität zu. Für die Prozessaufklärung der Aktivierung lateraler Fließpfade müssen jedoch Messungen aus verschiedenen Disziplinen kombiniert werden.

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Thesis (M.Sc. Agriculture (Soil Science)) -- University of Limpopo, 2021 / Woody plant encroachment results in the degradation of grasslands. It is defined here as the increase in density, cover and biomass of woody plants into formerly open grasslands, reducing grassland productivity. Globally, many arid and semi-arid savanna grasslands are affected by this land cover transformation which changes the vegetation structure by altering the ratio of woody plants relative to grass species and influences soil hydrology. In the existing literature there is limited information on the effects of woody plant encroachment on soil physical and hydrological properties, especially in savanna grasslands. This study quantified and compared the soil physical and hydrological properties in the topsoil and subsoil of open and woody plant encroached grassland sites located on two contrasting soil forms, namely Bainsvlei and Rensburg. To achieve this objective, the two soils were sampled at various depth intervals from dug soil profiles at both sites at Towoomba Research Station in Bela Bela, Limpopo Province, South Africa. Soil physical properties including bulk density, porosity and aggregate stability as well as hydrological properties (water retention and hydraulic conductivity) were determined from collected samples. Compared to open grassland, soil bulk density was 11% and 10% greater in the topsoil and subsoil, while porosity was respectively 6% and 9% lower in the topsoil and subsoil of woody plant encroached grassland for Rensburg soils. In Bainsvlei soil, there was a minimal increase and decrease in the soil bulk density and porosity, respectively. Soil aggregate stability increased by 38% in the subsoil of woody plant encroached grasslands in Rensburg soil, due to increasing clay content with depth. In Bainsvlei soil, the soil aggregate stability was 9% and 13% lower in the topsoil and subsoil of the woody plant encroached grasslands compared to open grassland. Furthermore, the results revealed that in both soils, there was lower soil water retention and hydraulic conductivity in the topsoil and subsoil layers of woody plant encroached grassland than in open grasslands. There were no significant differences observed for soil hydraulic conductivity in the Bainsvlei and Rensburg topsoil. The subsoil hydraulic conductivity decreased by 24% in Bainsvlei and 44% in Rensburg soils in the woody plant encroached grassland. The soil water retention (SWR) decreased with an increase in woody plants. Specifically, there was 25% and 42% decrease in SWR with woody plant encroachment in the topsoil and subsoil of Bainsvlei soil, respectively. The same trend was observed in the Rensburg soils with 50% and 19% decrease in SWR in the topsoil and subsoil, respectively. Overall, the results revealed that soil type and depth influenced soil physical and hydrological properties in the studied woody plant encroached savanna grassland. As such, interventions aimed at controlling woody plant encroachment need to factor in soil type and depth in the development of management practices tailored to improve the soil hydrology of savanna grasslands

L'ingénierie écologique, qui est décrite comme «la gestion de la nature», a d'abord été proposée par Odum en 1971. Dans les dernières décennies, l'ingénierie écologique a été largement consacrée à lutter contre l'érosion des sols et les mouvements de masse, tout en permettant d'assurer la durabilité des écosystèmes. L'objectif de cette thèse est d'évaluer l'impact de peuplements forestiers sur la stabilité de pentes de dimension finie, en considérant à la fois les effets mécaniques et hydrologiques des racines peu profondes contre les glissements de terrain. Deux sites forestiers monospécifiques et équiennes, plantés respectivement de Robinia pseudoacacia et Platycladus orientalis, ont été sélectionnés sur le Plateau du Loess en Chine et utilisés comme sites d'étude. Le Facteur de Sécurité (FoS) de ces pentes ont été calculées en utilisant un modèle éléments finis 2D qui prend en compte la distribution des racines dans les couches superficielles du sol.Des mesures de terrain et des tests de laboratoire ont été effectués afin d'estimer les principaux paramètres du modèle, à savoir la distribution des surfaces relatives de racines dans le sol (Root Area Ratio), la résistance à la traction des racines, ainsi que les propriétés mécaniques et hydrologiques du sol nu. La contribution des racines à la résistance au cisaillement du sol a été considérée par l'intermédiaire d'une « cohésion additionnelle » calculée à l'aide de modèles fournis par la littérature. Six modèles existants ont été testés. Cette thèse est composée de deux chapitres principaux portant sur: (1) l'effet mécanique de l'hétérogénéité spatiale de la distribution des racines à l'échelle de pente; (2) l'influence de la distribution des racines sur le couplage entre la diffusion de l'eau interstitiel et les contraintes mécaniques dans le sol et son impact sur la stabilité des pentes.Les simulations amènent aux conclusions principales suivantes: (1) les pentes en terrasse sont en théorie 20% plus stables que les pentes rectilignes, sans tenir compte des effets hydrologiques; (2) le FoS atteint une valeur asymptotique lorsque l'on augmente la cohésion des racines; (3) les variations de la cohésion des racines observées sur le terrain ont peu d'effet sur la stabilité des pentes. Toutefois le renforcement de la partie basse des pentes, où les racines ont un plus grand impact positif sur le FOS, peut permettre de diminuer le risque de glissement; (4) l'effet des fortes précipitations sur la stabilité de la pente pourrait probablement être atténué par la présence de racines, mais cet effet dépend des caractéristiques des racines et de leur influence sur le débit d'eau dans le sol. / Ecological engineering, which is described as ‘the management of nature', was first proposed by Odum in 1971. In the past few decades, ecological engineering has been largely devoted to combat soil erosion and mass movement all over the world, because of its benefit on sustainable ecosystems. The objective of this thesis is to evaluate the impact of forest stands on the stability of finite slopes, considering both the mechanical and hydrological effects of roots against shallow landslides. Two monospecific and even-aged forest sites planted with Robinia pseudoacacia and Platycladus orientalis respectively were selected on the Loess Plateau of China and used as study sites. Slope Factors of Safety were calculated using a 2D finite element model that takes into account the distribution of roots in the shallow layers of soil.Field site experiments and laboratory tests were performed in order to estimate the main parameters of the model, i.e. distribution of root area ratio within the soil, root tensile strength, as well as bare soil mechanical and hydrological properties. The contribution of roots to soil shear strength was considered through an additional cohesion calculated with models provided by the literature. Six existing models were tested. This thesis is composed of two main chapters that make the focus on : (1) the mechanical effect of the spatial heterogeneity of root distribution at the slope scale; (2) the influence of root distribution on the coupling between pore fluid diffusion and mechanical stress and its impact on slope stability. This study brings to the following main conclusions: (1) terraced slopes were 20% more stable than rectilinear slopes, disregarding the differences in hydrological regimes between the two sites; (2) FoS could reach an asymptotic value when increasing root additional cohesion; (3) variations of the actual root cohesion do not affect much slope stability. However more attention should be given to the reinforcement of the bottom part of the actual slopes, where roots have a larger positive impact on the FoS; (4) the effect of heavy precipitations on slope stability could probably be overcome or at least mitigated by root system network, but this depends on root characteristics and their resulting effect on soil water flow.

The present work is a contribution to description and understanding of the distribution and movement of water in swelling soils. In order to investigate the moisture distribution in swelling soils a detailed knowledge of volume change properties, flow characteristics and total potential of water in the soil is essential. Therefore, a possible volume change mechanism is first described by dividing the swelling soils into four categories and volume change of a swelling soil is measured under different overburden pressures. The measured and calculated (from volume change data) overburden potential components are used to check the validity of the derivation of a load factor, ∝. Moisture diffusivity in swelling soil under different overburden pressures is measured using Gardner's (1956) outflow method. Behaviour of equilibrium moisture profiles in swelling soils is theoretically explained, solving the differential equation by considering the physical variation of individual soil properties with moisture content and overburden pressure. Using the measured volume change data and moisture potentials under various overburden pressures, the behaviour of possible moisture profiles are described at equilibrium and under steady vertical flows in swelling soils. It is shown that high overburden pressures lead to soil water behaviour quite different from any previously reported.

Cette recherche doctorale porte sur la modélisation 3D de la dynamique des teneurs en eau dans le sol et le soussol lorsque les écoulements sont à saturation variable. La modélisation est basée sur une version généralisée de la loi de Darcy-Buckingham et de l’équation de Richards multidimensionnelle. Les recherches présentées dans cette thèse concernent différents volets, présentés ci-dessous, dont le fil conducteur est l’analyse de phénomènes d’écoulements en milieux poreux, contenant de fortes hétérogénéités et/ou perturbés par des fluctuations temporelles. Dans le cadre d’un partenariat de recherche entre l’IMFT et l'IRSN sur la problématique du stockage souterrain de déchets radioactifs, nous avons modélisé en 3D la dynamique du front de désaturation d’une couche argileuse autour d’une galerie souterraine ventilée, à l’aide du code volumes finis BIGFLOW 3D. Ce travail a permis de mettre au point une approche d’immersion pour la modélisation des écoulements en milieux composites. Un autre volet de cette thèse concerne une étude analytique et numérique des profils verticaux de succion et de teneur en eau lors d’une infiltration verticale, non-saturante, dans un sol hétérogène finement stratifié. Des solutions analytiques exactes et approchées sont obtenues en régime permanent, à l’aide de transformations de variables, et sont comparées avec des solutions numériques pour différents degrés d’hétérogénéité. De même, les phénomènes de « barrière capillaire » sont étudiés d’une part analytiquement et d’autre part, par expérimentations numériques transitoires d’infiltration sur des systèmes bicouches, en présence d’une nappe plus ou moins profonde. D’autre part, les écoulements nonsaturés sont étudiés, cette fois, en présence d’un forçage transitoire fortement oscillatoire, dans une colonne de sable fin homogène, sous l’effet de fluctuations périodiques du niveau de la surface libre. Une méthode multi-front mise au point lors d’une thèse précédente à l’IMFT est validée numériquement en montrant que cette méthode simule efficacement la dynamique oscillatoire des flux et des profils de pression avec un nombre limité de « fronts ». De plus, nous avons analysé la phénoménologie capillaire/gravitaire des écoulements oscillatoires dans la colonne grâce à un suivi dynamique du plan de flux nul. Enfin, nous présentons, comme extension aux travaux précédents, une étude préliminaire des phénomènes 3D d’infiltration et de redistribution d’eau, notamment lorsque l’hétérogénéité du sol est de type aléatoire. Les premiers essais d’infiltration permettent de tester, en réplique unique, des méthodes de prises de moyennes spatiales des champs de succion et teneur en eau et évaluer la taille minimum du domaine de calcul 3D permettant d’obtenir des profils verticaux moyens représentatifs de l’infiltration dans une réplique unique du sol aléatoire. / This doctoral research, defended at the Institut de Mécanique des Fluides de Toulouse, is devoted to modeling water pressure and water content in soils and in subsurface geologic formations, in the case of variably saturated flow. One of the main scientific objectives of this work is to analyze the response of unsaturated flow systems, and particularly capillary and gravitational effects, in the presence of material heterogeneities, discontinuities, and/or space-time fluctuations. Modeling is based on a generalized version of Darcy- Buckingham’s law, and of Richards’ flow equation. Overall, the topics developed in this PhD thesis focus on several related aspects of variably saturated water flow in the subsurface. These aspects all occur at once in most applications (drying/wetting,heterogeneity, temporal forcing), but they are "decoupled" here for convenience. A preliminary research (collaborative project IMFT / IRSN) was developed to study the 2D/3D drying process at the porous wall of a deep cylindrical excavation in response to hydrometeorological signals. This project has motivated the design and testing of a novel approach to include cavities in the modeling domain. A detailed study of steady state infiltration was developed for the case of finely stratified soils, with parameters that vary continuously and cyclically with depth. Exact and approximate analytical solutions are calculated based on variable transformation methods and on perturbation type approximations, and they are tested numerically using a finite volume code (BIGFLOW 3D). The sensitivity of suction fluctuations vs. stratification wavelength is investigated, as well as the effect of the degree of heterogeneity, and of water table depth. Capillary barrier effects are studied for the case of unsaturated infiltration in multilayer soil systems characterized by a discontinuity of soil properties at interfaces. Numerical experiments are developed for transient infiltration towards a water table through a two-layer system, the goal being to analyze possible capillary barrier effects under various scenarios. On the other hand, we study numerically the case of a partially saturated / unsaturated soil column submitted to highly variable oscillatory pressure at the bottom of the column: this leads to vertical flow oscillations in the unsaturated zone above the water table influenced by tides (coastal beach sand). We analyze the dynamics of this oscillatory flow, where capillary and gravitational effects compete; for this purpose we use a novel method that tracks the positions of the zero flux plane in the unsaturated column. Finally, we also present, as an extension to the previous studies, a preliminary investigation of multidimensional infiltration/redistribution phenomena, particularly for the case of fully 3D random-type soil heterogeneity. The first numerical experiments of 3D infiltration are undertaken based on the single realization approach to soil heterogeneity, and assuming a uniform distribution of wetting at soil surface.