Milena Cislerova

On the reliability of unsaturated hydraulic conductivity calculated from the moisture retention curve

In comparison with direct measurements of unsaturated hydraulic conductivity, the methods of calculations from the moisture retention curve are attractive for their fast and simple use and low cost. These are the main reasons for their increasing use, mainly in spatial variability studies. On the other hand, it is known that their applicability is limited. The possibility of the use of the retention curve to indirectly determine hydraulic conductivities is analyzed as follows. The theoretical derivation of the relationK(h) − θ(h) is briefly discussed with regards to potential sources of inaccuracy. The sensitivity of the algorithm forK(h) calculation is studied as a response to possible inaccuracies in the retention curve determination. Conclusions about the usability of calculated hydraulic conductivities are drawn.

MR properties of water in saturated soils and resulting loss of MRI signal in water content detection at 2 tesla

This paper reports a systematic MRI study at 2 tesla of 23 soils, each separately saturated with a known amount of water. The percentage of that water which could be detected using various MR methods was determined by comparison with a liquid reference sample. A pulse-acquire sequence gave a bulk detection of between 47 and 94% of the known water content of saturated soil. Also for bulk measurements, the inversion-recovery sequence used for determining T1 values detected a range of 0.7–75% of the existing soil water. The CPMG sequence with an echo time (TE) of 1 ms used for determining the bulk T2 values gave lower values, in the range of 0.4–66% overall. A spin-echo MRI sequence with a TE of 2.9 ms gave an even lower bulk detection, ranging from 0.2 to 57%. These low values for the detectable water content of bulk saturated soil water reflect the loss of water magnetization which occurs even during short echo time MR sequence at 2 tesla field strength. The source of the above findings was investigated by measurements of the longitudinal (T1) and transverse (T2) relaxation times and spectral linewidths of the soil-water protons, and by conventional analysis of soil properties. The MR parameters of critical importance to water quantification are T2 and T2∗, shorter values of which lead to a progressively greater loss of signal intensity for all MR protocols. Those parameters are affected by the following soil chemical and physical features: soil magnetic susceptibility, and the content of free iron oxides, clay, sand, exchangeable cations (K, Na and Ca), and organic matter. The implication of this work is that the only soil water which can be detected quantitatively at 2 tesla using a conventional spin-echo MRI protocol with echo times of 3 ms or longer is that located in the relatively large soil pores. Using the protocols investigated in this work, water in smaller pores will only be detected accurately for soils which have relatively low paramagnetic-metal impurities and/or have low clay content. Future MR studies of soil water should consider the use of other MRI protocols (e.g. for solid state), and measurement at low magnetic fields.

CT derived porosity distribution and flow domains

Computed X-ray tomography images of dry undisturbed soil samples of the coarse sandy loam of Korkusova Hut (KH) and the fine sand of Hupselse Beek (HB) are analysed concerning the image resolution, the direction of scanning and the evaluation of boundaries between the fast and slow flow domains. The image resolution appears to be a less important factor than the direction of scanning. Additional data sets are recommended for partitioning the fast and slow flow domains. Comparison is made with the tracer breakthrough pictures, magnetic resonance (MR) images and outflow volumes obtained from infiltration outflow experiments. The two soils under study appeared to have essentially different flow dynamics. In the KH sandy loam soil, the fast flow occurs in regions of high local porosity. In case of the HB fine sand, the sample appeared to consist of a dense part, where the flow process took place, and a more porous part, which stayed dry. Flow instability was detected on MR images for the KH soil.

Changes of steady-state infiltration rates in recurrent ponding infiltration experiments

Abstract Ponding infiltration experiments on coarse acid brown soils (Cambisol) are described to show that the steady-state infiltration rate depends on the initial moisture content, which is in contrast with the theory. The effect is observed on three different scales: (1) in the field at randomly chosen places near grid points of a network for two recurrent days under various initial moisture content; (2) in the field, in cylinders installed at fixed places for several measurements under different initial moisture content; and (3) on undisturbed samples taken to the laboratory, where also the outflow of the sample was observed. The effect is most apparent in repetitive ponding infiltration experiments and is ascribed to trapped air which alters the volume available for the gravity dominated flow. Instead of piston-like flow, downward flow through macropores takes place. As a consequence, the ponding infiltration experiment does not supply theoretically assumed constants.

Transforming Hydrographs in the Hillslope Subsurface

Transforming Hydrographs in the Hillslope Subsurface To reveal and evaluate the mechanism of transforming rainfall into runoff in the region, where the subsurface flow plays a dominant role in the runoff formation, a continuous hydrological and climatic data monitoring has been set-up in the experimental catchment Uhlířská (the Jizera Mountains, CR). The soil profile (Dystric Cambisol), formed on the weathered granite bedrock, is shallow and highly heterogeneous. Beside a standard catchment data observation a hillslope transect was instrumented to control the flow dynamics in the soil profile. From three soil horizons, the subsurface outflow is recorded in the subsurface trench. Adjacent to the trench the soil water suction is scanned by triplets of automatic tensiometers. Within the soil profile the unsaturated regime prevails, nevertheless the soil keeps almost saturated. Nearly simultaneous reaction of suction on a rainfall in all soil horizons implies a rapid vertical flow. Local preferential flow paths are conducting infiltrating water at significantly variable rates when saturation is reached. Groundwater table, soil moisture and subsurface runoff measured at the hillslope transect and the total outflow from the catchment, are correlated. The outflow from the catchment is dominantly controlled by soil moisture however the mechanism of its generation is not yet fully understood. Transformace hydrogramu podpovrchového odtoku na svahu V oblasti s dominantním podpovrchovým odtokem bylo započato s kontinuálním hydrologickým a klimatickým monitoringem s cílem popsat a vyhodnotit transformaci srážky na odtok. Experimentální povodí Uhlířská se nachází v severní části České republiky v Jizerských horách. Půdní profil, klasifikovaný jako dystrická kambizem na zvětralém žulovém podloží, je mělký a velmi heterogenní. Svahový transekt byl vystrojen pro sledování dynamiky podpovrchového odtoku. Ve třech půdních horizontech je monitorován odtok a půdní sací tlak. V půdním profilu převládá nenasycený stav, ačkoliv je půdní vlhkost dlouhodobě blízko nasycení. Rychlé vertikální proudění je indikováno téměř současnou odezvou půdního sacího tlaku na srážku ve všech půdních horizontech. Po dosažení nasycení infiltrující voda protéká preferenčními cestami s výrazně odlišnými lokálními rychlostmi. Závislost hladiny podzemní vody, půdní vlhkosti, podpovrchového odtoku ve svahovém transektu na odtoku z povodí je významná. Odtok vody z povodí, které leží na zvětralém žulovém podloží, je dominantně určován půdní vlhkostí. Přes tato zjištění není mechanismus tvorby odtoku zatím jednoznačně popsatelný.

The relationships between MR parameters and the content of water in packed samples of two soils

Abstract The relationship between the known water content and that determined using Magnetic Resonance (MR) methods was studied for samples of two soils packed to various dry bulk densities; four basic MR protocols were employed (single 90° pulse, inversion-recovery, Carr–Purcell–Meiboom–Gill (CPMG), and spin-echo (SE) pulse sequences). The soil types were chosen from a previously studied range of soils to represent both an “easy” (fine sand) and a “difficult” (coarse sandy loam) system for MR application. Whereas for the “easy” soil, the MR-detected water relates linearly to the soil water content as expected, for “difficult” soil the observed relationships are non-linear and depend on MR properties of soil water and MR protocols used.

Ponded infiltration into soil with biopores — field experiment and modeling

Preferential movement of water in macropores plays an important role when the process of ponded infiltration in natural porous systems is studied. For example, the detailed knowledge of water flow through macropores is of a major importance when predicting runoff responses to rainfall events. The main objectives of this study are to detect preferential movement of water in Chernozem soil and to employ numerical modeling to describe the variably saturated flow during a field ponded infiltration experiment. The infiltration experiment was performed at the Macov experimental station (Calcari-Haplic Chernozem in Danubian Lowland, Slovakia). The experiment involved single ring ponded infiltration. At the quasi steady state phase of the experiment dye tracer was added to the infiltrating water. Then the soil profile was excavated and the penetration pattern of the applied tracer was recorded. The abundance of biopores as a product of fauna and flora was found. To quantify the preferential flow effects during the infiltration experiment, three-dimensional axisymmetric simulations were carried out by a two-dimensional dual-continuum numerical model. The water flow simulations based on measured hydraulic characteristics without consideration of preferential flow effects failed to describe the infiltration experiment adequately. The 3D axisymmetric simulation based on dual-permeability approach provided relatively realistic space-time distribution of soil water pressure below the infiltration ring.

A scaling-based interpretation of a field infiltration experiment

A simple scaling-based inverse-problem solution was employed to obtain values characteristics of field-scale saturated hydraulic conductivity and moisture capacity. For this purpose the variability of soil hydraulic properties was expressed in terms of invariant laboratory-determined reference characteristics and space-variable scaling factors, determined from a set of 36 double-ring ponded infiltration field experiments. The three-dimensional axial-symmetric numerical simulation of the infiltration experiments was used in a fictitious reference profile, together with the special scaling procedure, to evaluate the effective parameters and their space distribution over the area of interest.

The Infiltration-Outflow Experiment Used to Detect Flow Deviations

Simple recurrent infiltration-outflow experiments, together with dye tracing, show deviations from the theoretical assumptions on which classical soil-water flow modelling is based. The experimental set-up represents a well-defined system which enables the study of the dynamic character of soil properties expressed in the Richard’s-equation approach by soil hydraulic functions. The method may offer the possibility to develop an efficient classification of soils according to their dynamic behaviour. Results from four soils are presented.

Simulation of soil water dynamics in structured heavy soils with respect to root water uptake

In agricultural lands has the soil moisture uptake from the root system a significant effect on the water regime of the soil profile. In texturally heavy soils, where preferential pathways are present, infiltrated precipitation and irrigation water with diluted fertilizers quickly penetrate to a significant depth and often reach an under-root zone or even the ground-water level. Such a scenario is likely to happen during long summer periods without rain followed by heavy precipitation events, when a part of the water may flow through desiccated cracks.Since 2001 the effects of drip irrigation and nitrogen fertilization of potatoes (Solanum tuberosum L., cultivar Agria) have been monitored within the frame of a research project at the experimental site Valecov (Czech Republic). Based upon the measured data an attempt has been made to simulate the water regime of the soil profile at a selected experimental plot, considering the impact of preferential flow and root water uptake. The dual-permeability simulation model S_1D_Dual (VOGEL et al., 2000) was used for the simulation. The soil hydraulic parameters were inversely determined using Levenberg-Marquardt method. Measured and simulated pressure heads were utilized in the optimization criterion. The scaling approach was applied to simplify the description of the spatial variability of the soil profile.The results of simulations demonstrate that during particular rainfall events the water reaches significant depths of the soil profile via preferential pathways. The effect of the root zone is dominant during dry periods, when capillary water uptake from the layers below roots becomes important. This should be taken in account into the optimization of the drip irrigation and nitrogen fertilization schedule.