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Soil water content (SWC) is a key factor in numerous water flow and transport processes taking place in the earth surface layer. As these processes are nonlinearly related to SWC, characterizing and predicting SWC variability is essential to properly address hydrological issues. This study focuses on the understanding of within-field SWC variability and its control by maize plant and atmospheric conditions. An experimental plot within a maize field was set up and equipped with different SWC probes. Soil moisture was monitored during 3 years with Time Domain Reflectometry with high temporal resolution and Electrical Resistivity Tomography with 3D high spatial resolution. The standard deviation of the SWC (σ), estimated from the SWC data, was maximum (~0.05 cm3cm-3) during drying periods in the first soil horizon. This level of σ is quite close to observations made by other studies in much larger areas, illustrating the importance of local scale heterogeneity. The maize sowing pattern in rows (R) and inter-rows (IR) and its interactions with weather conditions had a major contribution to the SWC spatial variability. Increase of SWC during rainy periods can be promoted up to six times more in R than in IR. During dry periods, root water uptake created drying patterns with less water in R than in IR. The intra-plant variability, mainly induced by root distribution, is thus not negligible and should be inserted into hydrological models. Using a soil-plant detailed water flow model R-SWMS and simulation of three different maize plants, we properly represented 70% of the SWC variability during a drying period. (AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 2013

Constructed wetlands (CWs) are attractive system for wastewater treatment for small communities, but interactions between the removal processes are often badly understood. Modelling of CW systems is a way to unravel them and improve their design. A modelling study with the HYDRUS/CW2D model was carried out on a French first-stage vertical flow constructed wetland (VFCW) at Evieu (Ain). Characterizing the hydraulic properties of the filtering material is a prerequisite. However hydraulic characterization is difficult due to varying proportion of organic matter in the filter. I present a combined laboratory and in-situ approach to achieve it. The laboratory approach gives prior estimates that are later refined by in-situ measurements and inverse modelling. Thanks to a large experimental dataset, it led to a reliable hydraulic parameter set allowing accurate simulation of water content within the VFCW. Prior to biological modelling, i developed and validated an original method to measure nitrification rates at the different depths of the VFCW and consequently at different organic matter contents. It is specifically adapted for partially saturated matrix and is based on respirometric principles. The obtained maximum growth rate coefficients for the autotrophic biomass are critical inputs for the VFCW plant modelling. Once the hydraulic calibration carried out and some of the biological parameters assessed, we simulate reactive transport. Particular attention was devoted to the nitrogen fate. During a feeding period, influent fractionation was determined, influent and effluent pollutant concentrations continuously monitored as well as in-situ oxygen concentrations. These measurements have been used to fit the biological model by adjusting kinetic parameters highlighting the need to include adsorption of ammonium. Good agreement between model and experimental data is obtained based on 2-4 h average concentrations, while instantaneous measurements are badly fitted. (AGRO 3) -- UCL, 2012

The distribution and origin of silicon (Si) in the soil-tree system has not been thoroughly researched, despite the crucial role of the terrestrial Si cycle in major biogeochemical processes due to its close linkage with other cycles (i.e. N, P and C). The PhD thesis focuses on Si cycling in temperate forest ecosystems to investigate the effect of tree species on Si distribution in soil-tree systems and the relative role of biogenic and litho/pedogenic Si pools in the release of dissolved Si (DSi) in the aqueous phase. Our field location is a forest site in Central France (Morvan) with granitic bedrock combining various tree species in identical climate and soil conditions. Tree species clearly influence Si uptake rates and can affect Si transfer to hydrosphere since the DSi leaching from the soil profile is negatively correlated to the Si uptake by forest vegetation. In the top organic horizons, tree species impact (i) the amorphous Si (ASi) pool through contrasting Si uptake and restitution of biogenic ASi (phytoliths) to the topsoil and (ii) the litho/pedogenic Si distribution through different incorporation rates of silicate minerals by bioturbation. In underlying mineral soil horizons, pedogenic processes such as opaline sphere formation and ASi sorption onto Fe oxides can hide the impact of tree species on ASi distribution. In order to study the origin of DSi in shallow soil solutions, we measured Si isotope compositions and Ge/Si ratios in the different compartments of the soil-tree system. Relative to the granitic bedrock, clay-sized minerals are enriched in 28Si and have high Ge/Si ratio while biogenic Si from phytoliths is enriched in 28Si and has low Ge/Si ratio. The forest floor solutions are clearly enriched in 28Si and Ge relative to granite. This finding suggests that the release of DSi in shallow soil solutions is greatly influenced by the partial dissolution of secondary clay-sized minerals incorporated by bioturbation in organic-rich horizons. In the temperate soil-tree system studied, tree species thus influence the DSi content in forest floor solutions through various bioturbation rates and, to a lesser extent, through different ways of Si biocycling. Furthermore, this study opens up new horizons to assess the relative impact of biological and pedogenic processes on the carbon dynamic in forest soils. (AGRO 3) -- UCL, 2010

The spatial distribution and properties of rainfed orchards in semi-arid environments result from complex interactions between man and the physical and economical environment. This thesis investigates a number of these interactions in the context of the mechanisation of management practices since the 1950's. It is shown how the practice of clean sweeping (i.e. frequent shallow tillage) influences the orchard water balance and how the removal of traditional soil and water conservation structures affects the connectivity of overland flow to the river system. Although clean sweeping prevents transpiration and competition by weeds, it also constrains the root growth in the plough layer, so that the trees cannot access the water from small rain events. In addition, clean sweeping promotes accelerated soil erosion. It appears that the practice of clean sweeping limits the water availability in orchards on loamy soils with an annual rainfall in the order of 300 mm. It is demonstrated that the presence and properties of rainfed orchards are related to spatial patterns of soil characteristics and climate. The observed decline in conservation structures like terraces and check-dams leads to an increase in the connectivity of water and sediment to the river system. An alternative for these traditional techniques to retain the water and the soil is the application of cover crops. The advantage of cover crops is that they do not limit the field size. A drawback in dry areas is the competition for water and nutrients between the cover crop and the trees. Field evidence and water balance simulations suggest that cover crops are feasible in areas with an annual precipitation of 500 mm or more. (GEOG 3) -- UCL, 2008

During the last thirty years, new technologies have contributed to a drastic increase of the amount of data in environmental sciences. Monitoring networks, remote sensors, archived maps and large databases are just few examples of the possible information sources responsible for this growing amount of information. For obvious reasons, it might be interesting to account for all these information when dealing with a space-time prediction/estimation context. In environmental sciences, measurements are very often sampled scarcely over space and time. Geostatistics is the field that investigates variables in a space-time context. It includes a large number of methods and approaches that all aim at providing space-time predictions (or interpolations) for variables scarcely known in space and in time by accounting for space-time dependance between these variables. As a consequence, geostatistics methods are relevant when dealing with the processing and the analysis of environmental variables in which space and time play an important role. As direct consequence of the increasing amount of data, there is an important diversity in the information (e.g. different nature, different uncertainty). These issues have recently motivated the emergence of the concept of data fusion. Broadly speaking, the main objective of data fusion methods is to deal with various information sources in such a way that the final result is a single prediction that accounts for all the sources at once. This enables thus to conciliate several and potentially contradictory sources instead of having to select only one of them because of a lack of appropriate methodology. For most of existing geostatistics methods, it is quite difficult to account for a potentially large number of different information sources at once. As a consequence, one has often to opt for only one information source among all the available sources. This of course leads to a dramatic loss of information. In order to avoid such choices, it is thus relevant to get together the concepts of both data fusion and geostatistics in the context of environmental sciences. The objectives of this thesis are (i) to develop the theory of a Bayesian data fusion (BDF) framework in a space-time prediction context and (ii) to illustrate how the proposed BDF framework can account for a diversity of information sources in a space-time context. The method will thus be applied to a few environmental sciences applications for which (i) crucial available information sources are typically difficult to account for or (ii) the number of secondary information sources is a limitation when using existing methods. Reproduced by permission of Springer. P. Bogaert and D. Fasbender (2007). Bayesian data fusion in a spatial prediction context: a general formulation. Stoch. Env. Res. Risk. A., vol. 21, 695-709. (Chap. 1). © 2008 IEEE. Reprinted, with permission, from D. Fasbender, J. Radoux and P. Bogaert (2008). Bayesian data fusion for adaptable image pansharpening. IEEE Trans. Geosci. Rem. Sens., vol. 46, 1847-1857. (Chap. 3). © 2008 IEEE. Reprinted, with permission, from D. Fasbender, D. Tuia, P. Bogaert and M. Kanevski (2008). Support-based implementation of Bayesian data fusion for spatial enhancement: applications to ASTER thermal images. IEEE Geosci. Rem. Sens. Letters, vol. 6, 598-602. (Chap. 4). Reproduced by permission of American Geophysical Union. D. Fasbender, L. Peeters, P. Bogaert and A. Dassargues (2008). Bayesian data fusion applied to water table spatial mapping. Accepted for publication in Water Resour. Res. (Chap. 5) (AGRO 3) -- UCL, 2008

The classical Fickian model for solute transport in porous media cannot correctly predict the spreading (the dispersion) of contaminant plumes in a heterogeneous subsoil unless its structure is completely characterized. Although the required precision is outside the reach of current field characterization methods, the classical Fickian model remains the most widely used model among practitioners. Two approaches can be adopted to solve the effect of physical heterogeneity on transport. First, upscaling methods allow one to compute "apparent" scale-dependent parameters to be used in the classical Fickian model. In the second approach, upscaled (non-Fickian) transport equations with scale-independent parameters are used. This research aims at comparing upscaling methods for Fickian transport parameters with non-Fickian upscaled transport equations, and evaluate their capabilities to predict solute transport in heterogeneous media. The models were tested using simplified numerical examples (perfectly stratified aquifers and bidimensional heterogeneous media). Hypothetical lognormal permeability fields were investigated, for different values of variance, correlation length and anisotropy ratio. Examples exhibiting discrete and multimodal permeability distributions were also investigated using both numerical examples and a physical laboratory experiment. It was found that non-Fickian transport equations involving fractional derivatives have higher upscaling capabilities regarding the prediction of contaminant plume migration and spreading, although their key parameters can only be inferred from inverse modelling of test data. (FSA 3)--UCL, 2006

Small island developing states that wish to sustain their freshwater resources face two main challenges: intensification of agriculture and climate change and variability. These topics were studied on the main island of the Kingdom of Tonga, Tongatapu (272 km2 , 175°12'W, 21°08'S). In 1987, a squash (Cucurbita maxima L.) industry emerged on Tongatapu that solely exports to the Japanese market during an off-season niche period in November. At a national level, the squash industry has accounted for about 40% of the total export revenue. With the export of the squash, a ten-fold increase in the importation and use of agri-chemicals has occurred. At the same time an increased population (currently 100.000) puts pressure on the fresh groundwater resources of the island (there is no surface water on Tongatapu). These pressures include both pollution and an increase in pumping that leads to salt water intrusion. Coupled with expected sea level rise, and an increase in the frequency of El Niño episodes, this puts the water resources and the environment under pressure. Research was done on two scales, the so-called field scale and the island scale. At the field scale measurements and modelling was carried out of plant transpiration using heat-pulse and meteorological measurements made in this tropical maritime climate, water movement, and nitrate and ammonium transport using a hydraulic characterisation of the soil. The leaching of nitrates and ammonium from the field was quantified using fluxmeters and alternative agricultural strategies were proposed. The hydraulic transfer properties of the limestone aquifer were deduced from the time lag between the variations in rainfall and salinity measurements of the freshwater lens. Using an unique 10-year dataset of monitored salinity of pumped water and a model, it was shown that the rainfall variations related to the El Niño-Southern Oscillation determined the quality of the freshwater in the aquifer. It was also possible to predict the relative salinity of the freshwater 10 months in advance using the Southern Oscillation Index. The pressures on the water resources that result from agricultural and climatic impacts need to be resolved through a balanced management strategy. This management strategy should at one hand account for the benefits and impacts of agriculture to the long-term economical development of the coral atoll, and on the other hand use climatic information and predictability to harness the islands water resources against impacts of climate variability and change. Finally, the case-study of Tongatapu was used to sketch a framework for the sustainable development of a small island developing state. (AGRO 3) -- UCL, 2006

The overall objective of this thesis is to better understand and assess pesticide leaching at the regional scale, using both the analysis of monitoring data and spatially distributed modelling. Atrazine contamination of the Brusselian aquifer (central Belgium) is poorly understood. Considerable uncertainty surrounds whether the pollution is agricultural or non-agricultural in origin. The spatial and temporal covariance of atrazine concentrations was studied by fitting semivariogram models to monitoring data. Correlation ranges were found to be 600 metres and 600-700 days. A non-parametric one-way ANOVA found a strong relationship between mean concentrations and land use, whilst other environmental variables were found to be less important. Higher levels of pollution were detected in areas dominated by urban land use suggesting that atrazine residues in groundwater resulted from non-agricultural applications. Modelling pesticide leaching at the regional scale (Dyle catchment) was used to assess groundwater vulnerability. Different approaches to process soil information were tested with both a linear (modified Attenuation Factor) and a non-linear (GeoPEARL) leaching model. The CI (calculate first, interpolate later) and IC (interpolate first, calculate later) approaches were identical for the linear model, but differences in the amount of leaching were found for the non-linear model. The CI approach would be expected to give better results than IC, but the CA (calculate alone) approach is probably the best method if no spatial output is required. Finally, a methodology was developed to quantify the uncertainty arising from the spatial variability of non-georeferenced parameters (i.e. those assumed to be spatially constant in deterministic simulations). A Monte Carlo analysis of atrazine leaching was performed with six pesticide and soil properties as uncertain inputs. Spatial variability of non-georeferenced parameters had a significant influence on the amount of simulated leaching. In the stochastic simulation, concentrations exist above the regulatory level of 0.1 µg/L, while virtually no leaching occurred in the deterministic simulation. Including the spatial variability of substance parameters (half-life, sorption coefficient...) would have significant consequences for future registration policies, especially if risk assessments are implemented in a spatially distributed way. (GEOG 3)--UCL, 2006

The impact of the soil structure on flow and transport in partially water saturated soils is currently still a matter of scientific debate. The major aim of this thesis was to investigate the relation between heterogeneity and transport for a natural unsaturated heterogeneous Tertiary sand deposit. In the first part, we analyzed the flow and transport at the scale of an undisturbed monolith. Chloride breakthrough curve experiments were used to derive an apparent dispersion coefficient at the TDR sampling and monolith scale. Application of a Brilliant Blue pulse allowed further the visualization of flow distribution within the monolith. Small undisturbed soil cores were sampled throughout the monolith and the hydraulic characteristic curves at the scale of the cores were determined. Textural variability and structure as inferred from the inspection of the Brilliant Blue pattern and analysis of the small core sampling were subsequently implemented in a 3-D model and transport was simulated. The simulations clearly revealed the importance of the macro-structure on the transport behavior of the soil. We also showed that the micro-variability heterogeneity component was needed to assess the scaling of the effective and local scale dispersivity. In the second part, we studied in-situ chloride transport in the vadose formation separating the bottom of a lake and an unconfined aquifer. First the uncertainty generated by the undersampling of the lake chloride concentration time series were investigated. Subsequently, velocity and dispersivity profiles were assessed by inverse modeling of the soil chloride concentration time series. We observed that the clay layers induced an increase of the dispersivity below them. We hypothesize that fingering flow or convergence phenomena, occurring below sand-clay interfaces, lead to non-representative artificially high dispersivity values. Velocity and dispersivity values just above the clay layers however seem more reliable due to convergence phenomena and better lateral mixing induced by a larger water content. In this formation, the transport behavior could be characterized considering a hierarchical structure of the subsoil heterogeneity. In this model, the flow field micro-variability is influenced by pore structure (possibly characterized by scaling factors). The next complexity level is induced by the slight layering resulting from the sedimentation process (not investigated in this work). Then, the third hierarchical level is assessed by the macro-structure and the sequence of clay layers in the sand. Each of these levels is assumed to have an effect on the solute mixing process and effective macro-dispersivity. (AGRO 3)--UCL, 2004

The estimation of evapotranspiration (ET), a key process within the Earth's surface water and energy balance, remains an important challenge for a wide range of disciplines such as surface hydrology, irrigation management and meteorology. However, notwithstanding the considerable progress recently made in our understanding of the physical and biological processes governing ET, the accurate quantification of ET is very tricky to achieve, even at a limited spatial scale. In this study, we combine field measurements with numerical experiments to tackle issues related to the quantification of ET and the associated uncertainties for a maize cropped field using two different approaches, i.e. the agro-hydrological modelling and the soil water balance approach. For the agro-hydrological modelling, we mainly focus on the estimation of field-scale soil water content and on the identification of root water uptake parameters. With regard to the field-scale soil water content, we put forward that the within-field variability is large and that the maize crop plays a non-negligible role in the development of the soil water content patterns both at the field and at the maize row scale. For deriving root water uptake parameters (RWUP), we develop and test two different approaches, i.e. the simplified soil water balance and the inverse modelling approach. Using numerical experiments, we show that the simplified soil water balance approach produces quite accurate RWUP. On the other hand, the inverse modelling approach is only successful for some soils and for some conditions due to instability and nonuniqueness issues. For the soil water balance approach, we show that the accuracy of the local ET estimate is strongly dependent on the estimation method used to derive the bottom fluxes and that the use of pedotransfer functions is of little interest. For field-scale ET estimates, we show that the variability of ET is large both at the field scale (due to the variable crop growth) and at the maize row scale (due to the maize row layout). To produce accurate field-scale ET estimates, we suggest to scale up maize row scale ET estimates using the concept of temporal stability or using a covariant such as the Leaf Area Index. The results of our study suggest that the estimation of water fluxes or associated state variables for a row cropped field requires a two-step upscaling strategy, from the local scale to the row, then from the row to the field scale. (AGRO 3)--UCL, 2003

The knowledge of the dynamics of soil water is essential in agricultural, hydrological and environmental engineering as it controls plant growth, key hydrological processes, and the contamination of surface and subsurface water. Nearby remote sensing can be used for characterizing non-destructively the hydrogeophysical properties of the subsurface. In that respect, ground penetrating radar (GPR) constitutes a promising high resolution characterization tool. However, notwithstanding considerable research has been devoted to GPR, its use for assessing quantitatively the subsurface properties is constrained by the lack of appropriate GPR systems and signal analysis methods. In this study, a new integrated approach is developed to identify from GPR measurements the soil water content and hydraulic properties governing water transfer in the subsurface. It is based on hydrodynamic and electromagnetic inverse modeling. Research on GPR has focused on GPR design, forward modeling of GPR signal, and electromagnetic inversion to estimate simultaneously the depth dependent dielectric constant and electric conductivity of the shallow subsurface, which are correlated to water content and water quality. The method relies on an ultrawide band stepped frequency continuous wave radar combined with an off-ground monostatic TEM horn antenna. This radar configuration offers possibilities for real time mapping and allows for a more realistic forward modeling of the radar-antenna-subsurface system. Forward modeling is based on the exact solution of Maxwell's equations for a stratified medium. The forward model consists in elementary linear components which are linked in series and parallel. The GPR approach is validated for simple laboratory and outdoor conditions. GPR signal inversion enables the monitoring of the soil water dynamics, which can be subsequently inverted for estimating the soil hydraulic properties. A specifically designed hydrodynamic inverse modeling procedure which requires only water content data as input is further developed and validated to obtain the soil hydraulic properties under laboratory conditions. (AGRO 3)--UCL, 2003

Sahelian woody vegetation has been the topic of many researches since the great drought that struck the Sahel in the 1970s-1980s. However, there is currently no consensus about the long-term trends of these savannas as observations sometimes seem contradictory according to the indicators used. Moreover, the observed trends often lack an understanding of the consequences on ecosystem functioning and subsequently on the provision of ecosystem services (ES), even if a decrease in ES is often assumed. As the inhabitants of the sylvo-pastoral zone of Senegal rely on these ES in their daily life, it is crucial to be able to predict the woody vegetation response to the current pressures acting on these ecosystems. In this research, we investigate the long-term trends of woody vegetation in order to discuss its persistence and its effects on two ecosystem functions (primary productivity and biogeochemical cycles) when facing three types of disturbances (drought, fire and grazing). This vegetation is examined from 1965 to 2018 using different indicators of the vegetation state and persistence, computed at several spatial scales and derived from multiple datasets (field inventory, remote sensing and literature). We first studied the spatial differentiation of the current woody vegetation according to the topography. Second, we evaluated changes since 1965 onward in terms of specific composition, density and woody cover. Third, we transposed the different indicators from the first two parts into a functional level of the woody communities, focusing on the vegetation response to pressure and effects on ecosystem. Based on this functional approach, we suggest some avenues for a potential management of this zone, in order to maintain the sylvo-pastoralism as land use. (AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 2020

Coral populations have declined dramatically worldwide under the combined effects of climate change and local anthropogenic stressors. In addition to ocean warming and acidification, threats to coral reefs in the Caribbean include frequent and virulent disease outbreaks, and intensifying hurricanes. In particular, Florida's Coral Reef (FCR) is currently facing a multi-year outbreak of the stony coral tissue loss disease (SCTLD). First observed in 2014 during the monitoring of the PortMiami Deep Dredge Project (PMDDP), the disease has now spread through the entire FCR and has been reported in numerous territories of the Caribbean. However, the propagation of the disease through FCR seemed to slow down when it reached its southwestern end, between the Marquesas and the Dry Tortugas (DRTO). Although the causative agent of the disease remains unknown, studies showed that its transmission was likely waterborne and that sediments could act as SCTLD vectors. The hydrodynamics should therefore be highly explanatory of its spread. Furthermore, Florida is a prime landfall target for hurricanes during the reproduction period of corals. In addition to causing wholesale destruction of the reefs, hurricanes might thus also impact larval dispersal through wind-wave-induced currents. Here, we use the high resolution coastal ocean model SLIM to capture transport processes in the FCR, and study the impact of SCTLD and hurricanes to coral reefs. First, a coupled epidemio-hydrodynamic model is developed to reproduce and understand the observed spread of SCTLD in Florida. A sediment transport model is then used to evaluate the impact of the PMDDP on the observed onset of the disease in 2014. Finally, a coupled wave-current model is implemented to study the impact of Hurricane Irma (2017) on transport processes over reefs in the Florida Keys. Assuming the dispersal of disease agents within neutrally buoyant material, our model successfully reproduced the observed spread of SCTLD through the FCR and linked its apparent stalling before reaching the DRTO to eddy activity in the Loop Current/Florida Current system. Furthermore, the results of our sediment transport model suggest that the PMDDP might have triggered the onset of the disease. Finally, our coupled wave-current model showed the important impact of wave-induced currents on transport processes over reefs during hurricanes. This thesis highlights the potential of models informed by and confronted against field knowledge as powerful tools to inform the management of complex marine ecosystems such as Florida's Coral Reef. (FSA - Sciences de l'ingénieur) -- UCL, 2022