Due to the complexity of nitrogen dynamics in the soil, tracer techniques are employed to estimate the fate and transport of nitrate in agricultural fields. This study was conducted to examine effects of N fertilization and landscape position on two-dimensional redistribution of bromide in a hummocky landscape, and to identify the landscape position with the greatest potential for solute loss using a dual application of Br- and 15N. The field data on Br- transport was also simulated using the HYDRUS models. The study was carried out near Brandon, Manitoba in 2007 and 2008, using two separate plots denoted as Site-2007 and Site-2008, respectively. The field plot was delineated into three landscape positions as upper (UPP), middle (MID) and lower (LOW) slope. Each landscape position received labelled K15NO3 at the rates of 0, 90 and 135 kg N ha-1, and KBr at the rate of 200 kg Br- ha-1. Site-2007 was seeded to canola while Site-2008 had winter wheat. Soil samples were taken in the fall and the following spring and were analyzed for Br-, NO3-N, total N, and isotope N ratio. Nitrogen fertilization reduced the downward movement of Br- in the soil profile, resulting in a greater lateral movement of Br- compared to the unfertilized plots. The greatest vertical and lateral movement of Br- occurred at the LOW slope. In the dual-tracer experiment, the smallest amounts of Br-, 15N, and NO3-N were measured in the soil at the LOW slope, while the greatest amounts were at the MID slope; indicating that solute loss was: LOW > UPP > MID. In the absence of crop uptake, Br- transport was identical to that of 15N. The simulation study showed that HYDRUS-1D model was inadequate to describe solute transport in the landscape, as HYDRUS-2D/3D reproduced the field data better than HYDRUS-1D. However, the 2D model did not reflect effects of landscape position and N fertility on Br- transport. Overall, the study confirmed the “Campbell hypothesis” which states that proper N fertilization reduces nitrate leaching. The field experiment and model simulation both showed that downward movement is the main pathway of solute loss in the landscape.

Potassium is one of the three most important elements in fertiliser, but in comparison to the mobile anions nitrate and phosphate, there has been little study of its transport within drainage basins. Potassium is a monovalent cation that sorbs strongly to soil constituents; leaching losses are expected to be minimal. However, data from a variety of catchments indicate that concentrations of dissolved potassium increase up to ten-fold during storm events, suggesting that leaching does occur. Possible sources of potassium are vegetation, the soil, mobilised sediment on the hillslope and sediment within the river channel. Previous work has only considered vegetation as a source. Field data at three scales: the hillslope, first-order subcatchment and fourth-order catchment, are used to investigate the sources of potassium and the mechanisms involved during transport from the hillslope to the drainage basin outlet. The drainage basin studied is that of the River Gara, near Slapton in south west Devon, UK. A temporally and spatially intensive sampling programme of mobile and immobile soil waters at the hillslope scale was conducted during storm events in the winter of 1995-6. At this scale, old water that has been resident in the soil for some time is of higher potassium concentration than new water (precipitation inputs for that storm). Discharge of such water to the surface as return flow is particularly significant in potassium transport. The subcatchment response during storm events is consistent with these observations: maximum potassium concentrations occur at the same time as old water contributes to the storm hydrograph. Vegetation and mobilised sediment on the hillslope are demonstrated to increase the potassium concentration of overland flow, but the relative significance of these two mechanisms could not be assessed. Fertiliser applications of potassium are efficiently retained within the soil; no changes in the subcatchment response are apparent after a fertiliser application. At the catchment scale, the maximum potassium concentrations during storm events occurred at the same time as new water contributed to the storm hydrograph. This response is not consistent with the hillslope and subcatchment response. It is interpreted to indicate that the link between potassium transport from the hillslope to the basin outlet is complex, with storages and sinks of potassium reducing the significance of hillslope-scale processes in the catchment response. Suspended sediment within the river channel is demonstrated to contribute to the increase in potassium concentrations during storm events, although they do not account for all of the increase. An alternative explanation is that the land use of the subcatchment differs from that of the whole catchment. The percentage of land put to arable crops in the subcatchment was higher than in the whole catchment; this is likely to reduce the potassium concentration of overland flow (composed of new water), since organic material is more evenly distributed throughout the soil profile in this case. An autumn increase in potassium concentrations has previously been observed; this has been attributed to autumn leaf fall. Experimental data indicate that potassium lost from fallen leaves is efficiently retained within the soil. Weekly data from two gauging stations of Slapton Ley Field Centre (1987-1996) display a highly variable potassium response, and demonstrate no seasonal trend.

This study investigates the effects of fire on the chemistry and flux of precipitation diverted to the forest floor as stemflow and throughfall by observing the impact of the June 2013 Thompson Ridge Wildfire in the Jemez River Basin of New Mexico. The loss of canopy cover from wildfire drastically modifies landscapes and alters ecosystems as fire replaces leafy canopies with charred branches and trunks, changes soil composition and erosion processes, and affects hydrologic flow paths and water chemistry. In order to track these changes, throughfall and stemflow collectors were installed beneath burned and unburned canopies in two catchments impacted by the Thompson Ridge Fire. Throughfall, stemflow, and open precipitation samples were analyzed for major cations, anions, dissolved inorganic and organic carbon, trace metals, and rare earth elements to determine how fire affects the chemical composition of the precipitation that interacts with burned canopies. Precipitation samples collected from both burned and unburned sites during the 2014 summer monsoon season show variations across burn severity, specifically in calcium, strontium, phosphate, and dissolved inorganic carbon concentrations, and across collector type with stemflow concentrations generally higher than throughfall and open precipitation concentrations. A stem count model was used to determine tree density for individual plots and catchments from LiDAR images taken before the 2013 fire. The stem count model was used to upscale event and monsoon season solute fluxes from plot to catchment scale. Higher nutrient concentrations combined with higher volumes of precipitation diverted as stemflow in burned forests have a multiplicative effect resulting in greater nutrient fluxes via stemflow creating nutrient hot spots surrounding burned tree trunks. Upscaling these plot scale concentrations and solute fluxes allows this study to represent changes to an entire catchment and quantify effects of wildfire on chemical loads and water chemistry.

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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.

This thesis highlights the role of phase interfaces on phasetransformations in metallic materials. The deviation from localequilibrium at the moving phase interface has been analysed interms of solute drag theory and finite interface mobility. Inparticular the planar growth of proeutectoid ferrite fromaustenite in steel has been studied. The deviation from localequilibrium is caused by dissipation of Gibbs energy bydiffusion inside the phase interface and interface friction. Inthe analysis the interface is divided into three zones and thethermodynamic and kinetic properties are assumed to varycontinuously across the interface. A new model suitable formulticomponent alloys is developed. The model reduces to thefamiliar solute drag model by Cahn under simplifyingconditions. It was demonstrated how the interface model couldbe combined with a method for calculating the volume diffusionin both the growing and parent phases. With this combination ofprocedures the changes in local conditions at the interface, asthe growth rate changes due to long-range diffusion, could bedemonstrated for the case of continuous cooling in an Fe-Nialloy. The critical limit for massive transformation in the Fe-Niand Fe-C systems was calculated and found to lie well below theT0 line for both systems. The calculated limit for Fe-Ni wascompared with a recent experimental study and reasonableagreement was found. For the Fe-C system the limit calculatedwith the present model was compared with a phase-field model.The two approaches showed qualitatively the same behaviour andthe quantitative difference was due to different assumptions onhow properties vary across the interface. Finally, an attempt to simulate the partitionless growth offerrite in austenite in the Fe-Ni-C system was performed. Inthe applied model the dissipation of Gibbs energy inside theinterface and in the nickel spike ahead of the migratinginterface were accounted for. The long-range diffusion ofcarbon in austenite was treated with an approximate analyticalgrowth equation. A continuous change from paraequilibriumconditions and quasi-paraconditions could be shown in anisothermal section of the Fe-Ni-C phase diagram. Partitionlessgrowth starts in a parabolic fashion but slows down. For alloysoutside the limit for quasiparaconditions partitionless growthis predicted to stop abruptly while for alloys inside thatlimit growth approaches a second parabolic growth law. However,the latter case should not be expected in practise because ofimpingement effects.

<p>This thesis highlights the role of phase interfaces on phasetransformations in metallic materials. The deviation from localequilibrium at the moving phase interface has been analysed interms of solute drag theory and finite interface mobility. Inparticular the planar growth of proeutectoid ferrite fromaustenite in steel has been studied. The deviation from localequilibrium is caused by dissipation of Gibbs energy bydiffusion inside the phase interface and interface friction. Inthe analysis the interface is divided into three zones and thethermodynamic and kinetic properties are assumed to varycontinuously across the interface. A new model suitable formulticomponent alloys is developed. The model reduces to thefamiliar solute drag model by Cahn under simplifyingconditions. It was demonstrated how the interface model couldbe combined with a method for calculating the volume diffusionin both the growing and parent phases. With this combination ofprocedures the changes in local conditions at the interface, asthe growth rate changes due to long-range diffusion, could bedemonstrated for the case of continuous cooling in an Fe-Nialloy.</p><p>The critical limit for massive transformation in the Fe-Niand Fe-C systems was calculated and found to lie well below theT0 line for both systems. The calculated limit for Fe-Ni wascompared with a recent experimental study and reasonableagreement was found. For the Fe-C system the limit calculatedwith the present model was compared with a phase-field model.The two approaches showed qualitatively the same behaviour andthe quantitative difference was due to different assumptions onhow properties vary across the interface.</p><p>Finally, an attempt to simulate the partitionless growth offerrite in austenite in the Fe-Ni-C system was performed. Inthe applied model the dissipation of Gibbs energy inside theinterface and in the nickel spike ahead of the migratinginterface were accounted for. The long-range diffusion ofcarbon in austenite was treated with an approximate analyticalgrowth equation. A continuous change from paraequilibriumconditions and quasi-paraconditions could be shown in anisothermal section of the Fe-Ni-C phase diagram. Partitionlessgrowth starts in a parabolic fashion but slows down. For alloysoutside the limit for quasiparaconditions partitionless growthis predicted to stop abruptly while for alloys inside thatlimit growth approaches a second parabolic growth law. However,the latter case should not be expected in practise because ofimpingement effects.</p>