Quantifying evaporation drying front dynamics using neutron radiography

Soil evaporation plays an important role in the hydrologic cycle and affects near surface energy balance. Changes in evaporative drying rates reflect complex interplay between soil pore space and transport properties and atmospheric conditions. Typically, initial drying rate from the wet soil surface is high and is limited by atmospheric conditions. Subsequently, when hydraulic connection between the receding drying front and atmosphere can no longer be maintained, the drying rate becomes dominated by vapor diffusion through the air-filled pores. We show that the depth of drying front where a transition from atmospheric (high) to diffusion controlled (slow) evaporation takes place, is related to air entry value of the medium (or to a critical pore size). Fine textured soils with small pores sustain higher drying rates than coarser ones. We delineated drying front position and morphology of sand-filled Hele-Shaw cells using neutron transmission technique. Insights on water content distribution and dynamics of drying front in sands of different textures will be presented and compared with theoretical predictions.

Drying front and water content dynamics during evaporation from sand delineated by neutron radiography

Dani Or, Nima Shokri

Evaporative drying of porous media is jointly controlled by external ( atmospheric) conditions and by media internal transport properties. Effects of different atmospheric potential evaporative demand on observed drying rates were studied in a series of laboratory experiments using sand-filled Hele-Shaw cells. We examined two potential evaporation rates of about 8 and 40 mm per day. The evolution and geometry of the drying front (marking the interface between saturated and partially dry regions) and water content distribution above the drying front were measured every 5 min at 0.1 mm spatial resolution using neutron radiography. Water loss rates decreased with time for both rates, but the decrease was more pronounced for high evaporative demand. External evaporative demand had no effect on drying front geometry or spatial water content distribution for depths below 2 mm. Cycles of roughening and smoothing of drying fronts due to interfacial pinning and unpinning were observed. The water content above the front showed irregular patterns due to formation of isolated liquid clusters with the general profile showing a decrease in mean water content with deepening drying front. Measured water content profiles support the hypothesis that liquid flow supply surface evaporation during stage 1 and water content distribution were not affected by external drying rates. Additionally, observed saturation profiles indicate that the corresponding hydraulic conductivity supports fluxes larger than the highest drying rate measured for sand, suggesting that decreasing drying rate was limited by vapor exchange between progressively drying surface and the viscous boundary layer above.

2008

Moisture and energy dynamics in fields soils

Francesco Ciocca

Soil moisture is crucial to water-cycle as it affects infiltration, runoff and land-atmosphere interactions. In dry soils the role of water vapor fluxes becomes essential and causes a tight coupling between soil moisture dynamics and heat transfer, which makes measurements and modeling extremely difficult. Since advances in both theory and experimental techniques are required, we have focused on both aspects: we have performed theoretical-numerical investigations of soil moisture and energy dynamics at the land surface; tested novel measurements techniques, in laboratory and at field scale, to provide new insights into soil moisture dynamics and contribute to a comprehensive theory of coupled energy and moisture transport. As the dynamics of physical systems are dictated by conservation equations, it is common practice to estimate quantities that cannot be directly measured (e.g. vapor fluxes) from the residuals of balance equations. In laboratory as well as in field experiments, residuals are calculated from measurements that are sparse in space and discrete in time. We have theoretically and numerically shown how the use of discrete data may lead to large residuals even without measurements or model errors. We have demonstrated that residuals are very sensitive to the distance between the measurements and that they increase in presence of nonlinear processes, such as moisture transport. A careful error analysis is required to assess the reliability of residuals computed from discrete data and avoid misinterpretations of the underlying physical processes. Another crucial element dictating soil moisture dynamics and evaporation into the atmosphere is soil water-retention. Popular parametric models of the retention curve present a matric potential that becomes infinite at residual saturation, which is the water content below which liquid flow stops. Few extensions of the water-retention curves have been proposed, which remain rarely used in practice. We have studied the effects of different models on liquid and vapor transport. We have shown that parameterizations that allow vapor flux below residual saturation yield larger vapor fluxes that result in a drier soil surface, whereas deep soil remains relatively wet. When diurnal forcing is considered, potential evaporation is reduced in the energy-limited regime and enhanced in the moisture-limited regime. We have established that in this case extended water-retention models predict larger heat fluxes that might critically impact models at whole scales (possibly including global circulation models). Discrimination between different theoretical descriptions and parametric models requires experiments that monitor moisture and energy dynamics. This demands the use of accurate sensors and measurement techniques that are able to simultaneously monitor several quantities (e.g. soil moisture, temperature, soil thermal properties, solute concentration in soil water). As in general different probes are used, problems arise due to different footprints and sampled soil volumes. Multi functional heat pulse probes (MFHPPs) have been conceived to overcome these issues by allowing simultaneous measurements of all required variables. We have assessed MFHPPs ability to measure ground heat flux, which has to be accurately known to avoid significant imbalances in the surface energy budget. We have employed an experimental setup able to generate a non-uniform heat-flux field and we have demonstrated that MFHPPs can reliably capture the magnitudes and directions of the fluxes. This allows more complete information on ground heat flux than that obtained by the classic probes (e.g. ground heat flux plates). To monitor soil state in large-scale field applications we have investigated the possibility to infer distributed thermal conductivity and soil moisture profiles with an optical fiber. Information is obtained by monitoring soil response to an active heat pulse generated from the metal shield armoring the optic cable, and analyzing the temperature differences of the cooling phase, monitored each meter by a distributed temperature sensing system (DTS). We have shown that soil moisture measurements inferred from soil thermal response (recorded by the DTS) are in good agreement with independent measurements in wet and intermediately wet soils; however in dry soils moisture content is underestimated and further investigations are required to improve this technique for dry conditions. In this study we have significantly improved the knowledge of coupled energy and moisture transport on both theoretical-numerical and experimental sides. We have proven that reliable measurements can be obtained at different spatiotemporal scales, and provided both prognostic and diagnostic methods to optimize their use. Finally, we have suggested crucial improvements to numerical models which might have implications beyond field-soils dynamics.

EPFL2013

Etude des processus hydrologiques par l'application du traçage environnemental

Abstract The main objective of this research is to improve the comprehension of the hydrological behaviour of natural catchments. This study is based essentially on the observation and the measurement of hydrological processes on the Haute-Mentue experimental research basin. The main originality of this work is to associate different types of measurements in order to obtain a better vision of hydrological processes responsible for streamflow generation. First the hydrological behaviour is studied at the catchment scale by the application of environmental tracing. Then hillslope measurements are conducted so as to identify the form of water flows through the catchment. Before the application of the environmental tracing technique on the Haute-Mentue, an uncertainty analysis of hydrograph separation models is proposed. In order to improve their interpretation in terms of hydrological processes, it is essential to estimate systematically their uncertainty. Two types of uncertainty are distinguished: the model uncertainty, which is affected by model assumptions, and the statistical uncertainty, which is due to the temporal variability of the chemical tracer concentrations of the components. The statistical uncertainty is studied using a Monte Carlo procedure. The model uncertainty is investigated by the comparison of alternative hypotheses concerning the chemical concentration of mixing model components and their spatio-temporal variability. This analysis validates in a way the use of hydrograph separation for the study of catchment hydrological behaviour. In fact it indicates that despite the uncertainty, the flow sources which generate the stream flow are clearly identified. However, the precision and the coherence of hydrograph separations can be improved by taking account of the temporal and spatial variability of component chemical concentrations. After this analysis, hydrographs recorded at the outlets of four Haute-Mentue sub-basins are separated by the application of a three-component mixing model based on the silica and calcium concentrations of water. This model allows the distinction of the following three components: direct precipitation, soil water and groundwater. The analysis of hydrograph separations of these four sub-basins allows the study of the spatial and temporal variability of hydrological responses. Concerning the spatial variability, despite the uncertainty of hydrograph separation, it appears that the hydrological behaviour of each sub-basin is different. The spatial variability of hydrological responses, which is observed essentially at the soil water contribution level, seems mainly due to the catchment morphology and geology. In general, soil contribution decreases with the steepness of the soil and the permeability of geological formation. As for the temporal variability of hydrological responses, it seems essentially to be controlled by the catchment humidity. Soil contribution increases with increasing basin humidity. A statistical analysis based on the development of multiple linear regressions allows the confirmation of the influence of these physico-climatic factors on the spatio-temporal variability of hydrological responses. In order to discover the processes responsible for the important soil water contributions highlighted by the application of environmental tracing, two experiments, one with a TDR system and the other with a rainfall simulator, were conducted within the catchment. On the whole, these two experiments indicate that the water flow processes are spatially quite heterogeneous and depend on very local properties. In fact, a geostatistical analysis of TDR measurements shows that the spatial variability of soil water content is very important at a local scale (1-20 meters). The soil infiltration capacity, determined by the simulation of artificial rainfall, is also spatially quite variable. This variability depends on the physical properties of the soil and geology but also on very local properties as macropores. After these different experiments, all of the observations done at the local scale as well as at the catchment scale are combined in order to get the maximum information about hydrological processes which are at the origin of streamflow. First, the TDR measurements partially confirm the environmental tracing results indicating that the soil water is an important potential source of subsurface flows. In view of the important spatial variability of soil water content and more particularly of the drainage dynamics, it seems that the stream flow generation is essentially controlled by processes at the local scale. The experiment with the rainfall simulator confirms this observation showing that the capacity of soil to transmit a water flow varies strongly in space. According to this experiment it seems, in the particular case of the Haute-Mentue, that soil water flows contributing to the stormflow generation moves essentially through the macropore system. In order to identify the form of subsurface flows and indirectly to confirm the role of macropores, an artificial tracing experiment was conducted. The transit speeds highlighted during this experiment are quick enough to conclude that the water contained in hillslopes contributes to the stromflow generation. Considering the physical and hydrodynamic properties of soils, only preferential flows can explain these transit speeds. In fact, from this experiment, it is not possible to know the nature of these preferential flows, but according to previous results it seems to be macropore flows. Finally, according to the information collected in this study, a conceptual model is proposed in order to explain the hydrological behaviour of the Haute-Mentue. This model, which is based essentially on the extension of contributing areas and on the role of preferential flows, allows to explain in terms of mechanisms the hydrological and chemical responses highlighted by hydrograph decompositions while respecting observations done at the local scale.

EPFL2000

Moisture and energy dynamics in fields soils

Francesco Ciocca

EPFL2013

Etude des processus hydrologiques par l'application du traçage environnemental

EPFL2000