Hannes Flühler

SUSCEPTIBILITY OF SOILS TO PREFERENTIAL FLOW OF WATER : A FIELD STUDY

Flow pathways of water and solutes in soils form distinct patterns, which are not a priori predictable. Macropore structure is a prime cause, but other factors, such as differing initial or boundary conditions, may also predispose a soil to produce bypassing of infiltrating water. This study was conducted to assess the flow pathways of water in different soils and to investigate the effect of initial water content on the flow pattern. Dye-tracing experiments were carried out at 14 different field sites. The sites represent a good portion of soils used for agricultural crop production in Switzerland. Each site consisted of two 1.4 by 1.4 m plots, one of which had been covered with a plastic roof for two months before the experiment to achieve different initial water contents. Forty millimeters of water containing the dye Brilliant Blue FCF (C.I. Food Blue 2) were applied within 8 hours onto the plots with a sprinkling apparatus. One day after irrigation the plots were excavated, and the stained pattern was examined on a vertical 1 by l m soil profile. The spatial structure of flow patterns showed remarkable differences. In most soils, water bypassed the soil matrix. In some soils, dye penetrated beyond l m depth, whereas in others it remained in the top 50 cm. Structured soils were more prone to produce bypass flow, deep dye penetration, and pulse splitting than nonstructured soils. The initial water content had a less pronounced effect in some soils and no effect in others.

Transport of chemicals through soil: mechanisms, models, and field applications

Publisher Summary The chapter presents and evaluates current experimental information and theoretical approaches used to represent chemical transport and transformations in unsaturated soil. It focuses on the field regime and discusses the current approaches used for modeling chemical transport in natural media. The field evidence regarding preferential flow is quite consistent in one respect. Preferential flow can occur under a variety of circumstances and is not restricted to clay-rich soils with significant structural voids. Fluid transport through well-defined structural voids is not predictable unless the distributions of voids, aperture sizes and shapes, depths of penetration, and interconnectivity are known. Progress is being made slowly in characterizing transport through rock fractures but there the geometry is much more stable in time than it is in the soil regime. Laboratory studies have demonstrated clearly that soil structure is almost certain to introduce mass transfer limitations to equilibrium between the dissolved and sorbed phases in soil. The chapter concludes that there are fundamental differences between the transport characteristics of the laboratory and field environments.

Export of dissolved organic carbon and nitrogen from Gleysol dominated catchments – the significance of water flow paths

In this study, we estimated whether changes in hydrological pathwaysduring storms could explain the large temporal variations of dissolvedorganic carbon (DOC) and nitrogen (DON) in the runoff of threecatchments: a forest and a grassland sub-catchment of 1600m2 delineated by trenches, and a headwater catchment of 0.7km2.The average annual DOC export from the sub-catchments was 185 kg DOCha−1 y−1 for the forest, 108 kg DOCha−1 y−1 for the grassland and 84 kgDOC ha−1 y−1 for the headwatercatchment. DON was the major form of the dissolved N in soil and streamwater. DON export from all catchments was approximately 6 kg Nha−1 y−1, which corresponded to 60% ofthe total N export and to 50% of the ambient wet N deposition. DOC andDON concentrations in weekly samples of stream water were positivelycorrelated with discharge. During individual storms, concentrations andproperties of DOC and DON changed drastically. In all catchments, DOCconcentrations increased by 6 to 7 mg DOC l−1 comparedto base flow, with the largest relative increment in the headwatercatchment (+350%). Concentrations of DON, hydrolysable amino acids, andphenolics showed comparable increases, whereas the proportion ofcarbohydrates in DOC decreased at peak flow. Prediction of DOC and DONconcentrations by an end-member mixing analysis (EMMA) on the base ofinorganic water chemistry showed that changes in water flow pathslargely explained these temporal variability. According to the EMMA, thecontribution of throughfall to the runoff peaked in the initial phase ofthe storm, while water from the subsoil dominated during base flow only.EMMA indicated that the contribution of the DOC and DON-rich topsoil washighest in the later stages of the storm, which explained the highestDOC and DON concentrations as the hydrograph receded. Discrepanciesbetween observed and predicted concentrations were largest for thereactive DOC compounds such as carbohydrates and phenolics. Theyoccurred at base flow and in the initial phase of storms. This suggeststhat other mechanisms such as in-stream processes or a time-variantrelease of DOC also played an important role.

Transport of Chloride Through an Unsaturated Field Soil

A chloride tracer was applied to the surface of a vegetable field and then leached downward by rainfall and irrigation. Tracer concentrations in a vertical two-dimensional region down to a depth of 2.4 m were monitored with suction cups that, were installed horizontally from a tunnel. The uniformly applied tracer pulse split into a slowly moving main pulse and a series of fast pulses. The first of the fast pulses reached a depth of 2.2 m after an infiltration of just 31 mm of natural rainfall, whereas the peak of the main pulse was still at a depth of 0.84 m by the end of the experiment after an infiltration of 0.853 m. The movement of the main pulse can be described by a convection-dispersion process in a homogeneous medium, provided that time is replaced by cumulative infiltration. However, the values of the parameters that produce a maximum agreement between the model and the observed main pulse have no physical basis, and consequently prediction of solute movement, based on measurements of soil properties, is not possible.

Lateral solute mixing processes — A key for understanding field-scale transport of water and solutes

Abstract Lateral mass exchange mechanisms affect the spreading of solutes in the main direction of flow. Modeling vertical solute spreading requires therefore an understanding of the lateral transport across regions of varying velocities. Experimental observations show that the variable extent and rates of lateral mixing cause dramatically different transport regimes, which can neither be predicted nor explained mechanistically in terms of known state variables. In this paper, we show that the various solute flow regimes are sensitive to the relative magnitudes of the vertical and lateral solute particle velocities. Different concepts of lateral mass exchange are discussed, and we postulate that physically meaningful parameters describing lateral mixing would be a clue for deciding a priori which solute transport regime is appropriate for a particular soil or soil horizon.

Transport of Anions and Herbicides in a Loamy and a Sandy Field Soil

A field experiment was conducted to investigate preferential transport of herbicides and to explore processes that cause rapid movement. Bromide, chloride, and three herbicides (triasulfuron, atrazine, and terbuthylazine) with different mobility characteristics were applied to six 1.4 × 1.4 m field plots on a loamy and a sandy soil. At both sites, three of the plots were covered with a plastic roof 1 month before the beginning of the experiment to achieve different initial water contents between the plots. Two days before the beginning of the tracer experiment, crops were removed, and the soil surface was homogenized with a spade to a depth of 15–20 cm. One day after application of the chemicals the plots were irrigated with a sprinkling apparatus. The cumulative amounts of infiltration until the time of sampling were 30, 60, and 90 mm within 1, 2, and 3 days, respectively. A trench was excavated, and soil cores were taken horizontally from a 1 × 1 m profile in a regular 0.1 × 0.1 m grid. The loamy and the sandy soil showed completely different transport patterns. In the loamy soil the bulk mass of herbicides remained in the top layer; however, considerable amounts of herbicides were transported below the root zone. A few percent for triasulfuron and atrazine and <1% for terbuthylazine were detected below 0.5 m depth after 90-mm cumulative infiltration. Traces of all herbicides were found down to 0.9 m. The depth distributions for anions and all herbicides were similar. These results show that the herbicides were only partly adsorbed by the soil matrix. A fraction of these chemicals was transported with or without minor adsorption along cracks or fissures. In the sandy soil, chemical movement was confined to the top 0.4 m, and the penetration depth of the herbicides was consistent with their mobility characteristics: triasulfuron showed greatest mobility, atrazine was moderately mobile, and terbuthylazine was the least mobile of all three.

Longitudinal and lateral dispersion in an unsaturated field soil

Lateral and longitudinal dispersion was quantified in a field soil under water-unsaturated conditions. The relatively mobile dye tracer Brilliant Blue FCF was applied as a line source and leached into the soil at two different rates of infiltration, 4 and 24 mm d−1, respectively. The resulting tracer plume was photographically recorded at vertical soil profiles excavated perpendicularly to the line source after ∼50, 100, and 200 mm of cumulative infiltration. An image analysis technique was used to determine two-dimensional concentration distributions from the photographs. Average horizontal and vertical concentration distributions were analyzed using the two-dimensional advection-dispersipn equation. Model parameters were fitted to optimize the agreement between measured and modeled averaged concentration profiles in both horizontal as well as vertical directions. Dispersivities showed a dependency on flow rates and amount of cumulative infiltration, but this dependency appeared to be related to the degree of irregularities of observed flow patterns. Large dispersivities were associated with higher degree of irregularities in the flow patterns and vice versa. Layer boundaries played a significant role for redirecting flow when flow rates were high and cumulative infiltration was large. This study demonstrates (1) that more than just the vertical concentration profiles are needed to define the transport regime under unsaturated conditions, and (2) that even subtle layer boundaries affect the lateral mixing regime and exert a marked influence on the transport in the main flow direction.

Multiple tracing of fast solute transport in a drained grassland soil

Fast transport of fertilizers and other agrochemicals into subsurface drainage systems has been recognized as a serious threat to surface waters. We report on a tracer experiment carried out on a 7.3×20 m2 plot on a loamy grassland soil to determine the flow paths to a tile drain at 1 m depth. The experiment consisted of a series of consecutive tracer applications including seven solutes and liquid manure that were applied either on the entire plot or on limited bands. Based on the discharge behavior under natural conditions, we estimated the effective hydraulic conductivity of the subsoil to be in the order of 8–29 cm day−1. Under experimental conditions, the soil transmitted 120 mm day−1 into the subsurface drain and two vertical profiles without producing surface runoff. Only part of the soil water, corresponding to 6–27 mm of the soil depth, contributed to the fast hydrological response. The transport of the tracers was very fast. Within 7–16 h after application of the conservative Br−, Cl− and HDO and the slightly sorbing substances brilliant blue (BB) and amino-G-acid (AG), these tracers reached relative concentrations in the outflow between 19% and 35% of the input concentrations. From the mass balance for water and solutes, it follows that the tracers were quickly transported over lateral distances of several meters. The manure constituents dissolved reactive P (DRP), NH4+ and Cl−, applied as liquid manure on the surface on a 1 m wide band above the tile drain, reached the drain within 5 min after application. After the early peak of DRP and NH4+, their concentration in the drain decreased quickly to background levels, whereas Cl− exhibited a second peak. Despite the fast transport and the small soil volume conducting water and solutes, the interaction between irrigation water and soil matrix was intimate enough to retain the two sorbing tracers. From the stained flow paths, the hydrologic behavior of the field under natural conditions and the hydrometric data during the experiment, it follows that the fast lateral tracer transport occurred mainly close to soil surface and not through the subsoil. Only in the immediate vicinity of the tile drain and of two lateral pits at the edge of the experimental plot water was redirected downwards and discharged from the tile drain and the bottom parts of the profiles, respectively. Hence, effluent from tile drains may not be representative for water reaching the subsoil or shallow ground water in undisturbed soils.

FOSMEX: Forest Soil Moisture Experiments With Microwave Radiometry

The microwave Forest Soil Moisture Experiment (FOSMEX) was performed at a deciduous forest site at the Research Centre Julich (Germany). An L- and an X-band radiometer were mounted 100 m above ground and directed to the canopy. The measurements consist of dual- and single-polarized L- and X-band data and simultaneously recorded ground moisture, temperature, and meteorological data. The canopy L-band transmissivity was estimated from a subset of the FOSMEX data, where the ground was masked with a metalized foil. For the foliage-free canopy, the reflecting foil diminished the L-band brightness by ap24 K, whereas brightness increased by ap14 K when the foil was removed from below the foliated canopy. Depending on the assumption made on the scattering albedo of the canopy, the transmissivities were between 0.2 and 0.51. Furthermore, the contribution of the foliage was quantified. Although, the evaluation revealed the semitransparency of the canopy for L-band frequencies, the brightness sensitivity with respect to ground moisture was substantially reduced for all foliation states. The effect of ground surface moisture was explored in an irrigation experiment. The L-band measurements were only affected for a few hours until the water drained through the litter layer. This emphasizes the significance of the presence of litter for soil moisture retrieval from remotely sensed L-band brightness data. The FOSMEX database serves for further testing and improving radiative transfer models used for interpreting microwave data received from future spaceborne L-band radiometers flying over areas comprising a considerable fraction of deciduous forests.

Microwave L-band emission of freezing soil

We report on field-measured microwave emission in a period of frost penetration into a grassland soil. The measurements were recorded with a high temporal resolution using an L-band radiometer mounted on a 7-m high tower. The observation period (December 2002 to March 2003) included two cycles of soil freezing and thawing with maximum frost depth of 25 cm. In situ soil temperature and liquid water content were measured at five depths down to 45 cm. Soil moisture profiles were calculated using the COUP numerical soil water and heat model in combination with measured soil properties and meteorological data monitored at the site. The L-band radiation data clearly showed the penetration and thawing of seasonal soil frost. We calculated soil reflectivities based on in situ measured and modeled soil moisture profiles by applying a coherent radiative transfer model. The calculated reflectivities were compared with the radiometrically determined soil reflectivities. It was demonstrated that the quantitative consistency between these reflectivities was significantly improved by applying an impedance matching approach accounting for surface effects. In this particular case, the dielectric structure of the uppermost soil horizon was largely influenced by soil roughness, vegetation, and snow cover. The radiometrically measured soil reflectivities were fitted using a radiative transfer model in combination with a roughness model assuming a soil surface roughness of 25 mm. The analysis during a period of frost penetration shows coherent behavior of the soil reflectivity. Temporal oscillation of the measured L-band radiation appears to be a coherent effect. This effect has the potential to be used for estimating the frost penetration velocity.