Die Bestimmung der Wasserspannungs- /Wasserleitfähigkeits-Beziehung von Bodenaggregaten

Determination of the unsaturated hydraulic conductivity of soil aggregates by use of microtensiometers The hydraulic properties of single aggregates were measured with the use of microtensiometers. They are small enough (tip diameter 1 mm, length 1–2 mm) that two of them can be installed inside an aggregate within a distance of 1–3 mm. The changes of water suction are measured with pressure transducers and recorded by a micro-computer. Results obtained for different aggregates show, that at the same water suction, the hydraulic conductivity of single aggregates is up to 2 orders of magnitude smaller than that of the bulk soil. The cross-over-suction value for aggregates can also be derived.     

Ein autoregressives Verfahren zur Bestimmung der gesättigten und ungesättigten hydraulischen Leitfähigkeit

An autoregressive procedure to predict the hydraulic conductivity — Comparison of measured and predicted results An instantaneous profile method was used to measure the unsaturated hydraulic conductivity. Relatively new techniques involving undisturbed soil samples instrumented with minitensio-meters and Time-Domain-Reflectometry (TDR) mini-probes were used for the experiments. The laboratory method allows a high spatial and temporal resolution. Laboratory measurements were carried out for 40 soil horizons with a wide spectrum of texture and bulk density. In addition, retention curves were measured using the standard pressure plate apparatus. Using this homogeneous set of data, an autoregressive model was developed which allows a stepwise calculation of the hydraulic conductivity for a water potential range of —30 up to —600 hPa. This model was developed for loamy sands, sandy, silty and clayey soils in conjunction with data from the retention curves. The calculation procedure starts with the determination of an initial unsaturated conductivity (k) close to field capacity, i.e., for water potential from —60 hPa up to —100 hPa. This first value is then used to predict other conductivity values using appropriate changes in soil water content corresponding to a defined range of the soil water potential. Subsequently, the hydraulic conductivities for higher and lower potentials were estimated considering the k value of the previous step in combination with the data of the retention curve of the next water potential range. The advantage of this empirical model is the indirect consideration of soil structure, in contrast to the closed-form van Genuchten-Mualem (vGM) model. To demonstrate these effects on different fitting procedures, the vGM model was also used to describe soil hydraulic functions. The accuracy of both, the vGM model and the autoregressive one, were compared for various fitting procedures and soils.     

Soil physical characteristics of peat soils

Drainage and intensive use of fens lead to alterations in the physical characteristics of peat soils. This was demonstrated using parameters of water balance (available water capacity) and the evaluated unsaturated hydraulic conductivity. Deriving the distribution of the pore size from the water retention curve was flawed because of shrinkage due to drainage, especially at high soil water potentials. These errors became greater as the peat was less influenced by soil‐genetic processes. The water retention curves (desorption) evaluated in the field and the laboratory satisfactorily corresponded. However, the wetting‐ and drainage‐curves obtained in the field differed up to 30 vol.‐% water content at same soil water potentials. These differences were largely due to a wetting inhibition.     

Soil Structural Stability and Water Retention Characteristics Under Different Land uses of Degraded Lower Himalayas of North‐West India

The lower Himalayan regions of north‐west India experienced a severe land‐use change in the recent past. A study was thus conducted to assess the effect of grassland, forest, agricultural and eroded land uses on soil aggregation, bulk density, pore size distribution and water retention and transmission characteristics. The soil samples were analysed for aggregate stability by shaking under water and water drop stability by using single simulated raindrop technique. The water‐stable aggregates (WSA) >2 mm were highest (17·3 per cent) in the surface layers of grassland, whereas the micro‐aggregates (WSA < 0·25 mm) were highest in eroded soils. The water drop stability followed the similar trend. It decreased with the increase in aggregate size. Being lowest in eroded soils, the soil organic carbon also showed an adverse effect of past land‐use change. The bulk density was highest in eroded lands, being significantly higher for the individual aggregates than that of the bulk soils. The macroporosity (>150 µm) of eroded soils was significantly (p < 0·05) lower than that of grassland and forest soils. The grassland soils retained the highest amount of water. Significant (p < 0·05) effects of land use, soil depth and their interaction were observed in water retention at different soil water suctions. Eroded soils had significantly (p < 0·05) lower water retention than grassland and forest soils. The saturated hydraulic conductivity and maximum water‐holding capacity of eroded soils were sufficiently lower than those of forest and grassland soils. These indicated a degradation of soil physical attributes due to the conversion of natural ecosystems to farming system and increased erosion hazards in the lower Himalayan region of north‐west India. Copyright © 2013 John Wiley & Sons, Ltd.     

Aggregate characterization as compared to soil bulk properties

The aim of this paper is to clarify the effect of soil aggregation on the physical and chemical properties of structured soils and as compared with the homogenized material. Aggregation and aggregate strength do not only depend on biological activity and organic exudates, but also on the intensity, number and time of swelling, and drying events. Such aggregates are not only more dense than the structured bulk soil, the intra-aggregate pore distribution consists not only of finer pores, but they are also more tortuous. Thus, water and ion fluxes by mass flow as well as ion transportation by diffusion are delayed, whereby the length of the flow path in such tortuous finer pores further retards chemical exchange processes. Futhermore, the chemical composition of the percolating soil solution differs more from that of the corresponding homogenized material the stronger and denser the aggregates are. From the mechanical point of view, the strength of single aggregates, determined as the angle of internal friction and cohesion, depends on the number of contact points or the forces, which can be transmitted at each single contact point. However, internal soil parameters, like grain size distribution or chemical composition, further affect the strength. The more structured the soils are, the higher is the proportion of the effective stress on total stress, but even in single aggregates neutral stresses can be revealed. This is true because of the relationship to the smaller value of the hydraulic conductivity and higher tortuosity. Finally, some dynamic effects on aggregation and aggregate deterioration are discussed.     

The effect of soil texture and organic amendment on the hydrological behaviour of coarse-textured soils

To gain more insight into the hydrological behaviour of coarse-textured soils, the physical properties of artificially created soil mixtures with different texture were determined. The mixtures were prepared according to the specifications of the United States Golf Association (USGA) for constructing putting greens. In addition, the effect of 10 vol.% organic matter addition was studied. The soil moisture retention and hydraulic conductivity relationships of the different mixtures were determined and their hydrological behaviour was studied using the numerical model SoWaM. Both texture and organic matter addition substantially affected the hydraulic properties. Hydraulic conductivity significantly increased with increasing coarseness while moisture retention decreased. On the other hand, organic matter addition reduced saturated hydraulic conductivity by a factor of 10 to 100 and distinctly increased moisture retention capacity. The amounts of total available water were increased by the addition of organic matter between 144% (slightly coarse texture) and 434% (very coarse texture). Results indicate that the mixtures can contain only 2–16% plant available water and therefore need frequent irrigation to maintain plant growth. Addition of organic matter seems a good solution to reduce the irrigation water requirements but it increases the risk of ponding or runoff because of large reductions in the saturated hydraulic conductivity sometimes to below the rate of 3.6 m/day recommended by the USGA.     

The effect of dairy sewage sludge amendment on repellency and hydraulic conductivity of soil aggregates from two depths of Eutric Cambisol

Appropriate management of sewage sludge is an important worldwide issue due to the still growing amount of wastewaters. In the study we examined to what extent the addition of dairy sewage sludge compared with mineral fertilization affects porosity, repellency index, and hydraulic conductivity of variously sized aggregates from two soil depths of Eutric Cambisol derived from loess: 5–15 cm and 25–35 cm. The repellency index was calculated as a ratio of ethanol and water sorptivity. Data on water and ethanol sorptivities of initially air‐dry soil aggregate fractions were obtained from steady state flow measurements using an infiltration device. Hydraulic conductivity was determined by measuring water infiltration at five pressure heads: –8, –6, –4, –2, and 0 cm of water column with the same device as for sorptivity determination. Addition of sewage sludge to the soil decreased the soil repellency index by an average of 27% in topsoil and 32% in subsoil for both aggregate sizes, respectively, and increased hydraulic conductivity about four times in both layers. Smaller aggregates (15–20 mm diameter) from soil amended with sewage sludge, in comparison with larger ones (30–35 mm diameter), had a higher repellency index by 36 and 24% in topsoil and subsoil, respectively. As for aggregates from soil with mineral fertilization, those differences were smaller and equal to 15% in subsoil, in topsoil smaller aggregates even had slightly lower repellency index (by 5%). Aggregates taken from the upper soil layer were more water repellent and had smaller hydraulic conductivity than those taken from subsoil, regardless of soil treatment and aggregate size.     

Change of soil hydrological properties of fens as a result of soil development

This study deals with the change and evaluation of hydrological properties of peat soils (Histosols) in the course of soil development. Ash content, volumetric water content, and dry bulk density, unsaturated hydraulic conductivity, water retention function, and wetting properties were measured for 84 fen sites in 19 fen regions of North‐Eastern Germany. Soil development resulted in porosity decrease. On the contrary, the macropore space and the capillary rise increased. With the start of consolidation processes and the development of segregation structure, a'noticeable reduction of the macropores and unsaturated hydraulic conductivity were observed. In course of soil development and decreasing of aggregate size, these processes reversed. Both parameters increased from segregation structure horizon to earthyfied fen and weak moorshyfied fen horizon, until they partly exceeded the starting values of pedogenetic almost unchanged fen in strongly moorshyfied stadium. Differences in wetting properties of peat could not be explained by the changes of peat properties in the course of soil development.     

Comparison of seven water retention functions used for modelling soil hydraulic conductivity due to film flow

The unsaturated soil hydraulic conductivity accounting for film flow is important for understanding soil hydrological and biological processes, especially in arid and semi‐arid regions. Recently, a theoretically based hydraulic conductivity model was developed to describe the hydraulic conductivity as a function of water content. We have used this model to compare seven soil water retention functions commonly used for predicting soil hydraulic conductivity due to film flow. A total of 30 soils, varying in basic properties, were selected from the Unsaturated Soil Hydraulic Database to evaluate the seven functions. The Webb method was applied to identify the critical soil matric potential (h _c) below which thin film flow controls water movement. Soil hydraulic conductivity measurements at matric potential below h _c were then used for curve fitting according to the seven functions. Slight differences were observed among the functions in predicting soil hydraulic conductivity due to film flow. Six of the seven functions in combination with the hydraulic conductivity model described the hydraulic conductivity due to film flow well, according to the terms of the coefficient of efficiency. The relatively poor performance of the one exception was due to the fact that the linear shape of the function made it less flexible at low matric potentials. In addition, the effect of textural class on its performance was substantial, showing a poorer fit for the sand soil compared with the loam and clay soils. These findings have important applications related to soil and water resources conservation especially in arid and semi‐arid regions.     

Changes in shrinkage of restored soil caused by compaction beneath heavy agricultural machinery

Compaction is a major cause of soil degradation. It affects not only the porosity of the soil, but also the soil's hydrostructural stability. Soil that is restored after temporary removal is particularly sensitive to compaction. We investigated the effects of trafficking with a heavy combine harvester on the shrinkage behaviour of a restored soil that had been gently cultivated for several years. We tested the hypothesis that compaction decreases the hydrostructural stability of restored soil by analysing simultaneously measured shrinkage and water retention curves of undisturbed soil samples. Shrinkage strongly depended on clay and organic carbon content. Taking account of this influence and normalizing the shrinkage parameters with respect to these soil properties, we found pronounced effects of trafficking on shrinkage. Ten passes with the combine harvester decreased the structural porosity by about 40% at maximum swelling and by about 30% at the shrinkage limit and increased the bulk density by 8% at maximum swelling and by 10% at the shrinkage limit, but did not significantly affect the porosity of the soil plasma. Moreover, trafficking modified shrinkage, increasing the slopes of the shrinkage curve in the basic and structural shrinkage domains by about 30% and more than 150% after 10 passes, respectively. Evidently the aggregate structure was strongly destabilized. The results indicate that the hydrostructural stability of the soil was still very sensitive to compaction by trafficking even 5 years after restoration. The analysis of shrinkage seemed well suited for the assessment of compaction effects on soil structure.     

Free‐Form estimation of soil hydraulic properties using Wind’s method

Transient evaporation experiments offer the potential to determine simultaneously the soil hydraulic properties necessary to simulate water flow in unsaturated soils. We present a new algorithm for determining the retention and conductivity curve from evaporation experiments which uses Wind’s method with a free‐form soil water retention function. Our algorithm estimates nodal values of volumetric water content and derives a smooth and monotone retention curve by cubic Hermite interpolation. A multilevel routine increases the number of nodes and their adequate number is identified by a performance criterion which balances goodness of fit, the cross correlation between the estimated water contents and the number of degrees of freedom. We calculate point values of unsaturated hydraulic conductivity by the instantaneous profile method and discard unreliable conductivity estimates by a statistical filter criterion. Results for three synthetic data sets including an uncertainty analysis of the estimated retention curves show that the algorithm is suitable to identify, both correctly and precisely, the soil hydraulic properties. An application to a real data set confirms these results. In order to enable the free‐form functions to be used in numerical flow simulations, we extrapolate the retention function to the dry range and compute a coupled conductivity function based on the Mualem model. Major advantages of the proposed method are the enormous flexibility provided by the free‐form functions, the low level of parameter cross‐correlation in comparison with classic parametric functions, and the possibility of assessing the uncertainty of the retention curve individually in different ranges of pressure head.     

A relatively simple scaling method for describing the unsaturated hydraulic functions of stony soils

Few if any methods exist to estimate the effects of stone content (stoniness) on the unsaturated soil hydraulic properties. A relatively simple scaling method is presented to estimate the hydraulic conductivity of unsaturated stony soils having different stone contents. A key assumption of the method is that van Genuchten's water retention parameters α and n of the fine soil fraction are the same as those of the stony soil. The method further assumes a linearly decreasing relationship between the saturated hydraulic conductivity and the stone content, based on previous numerical simulations. Using the proposed method, it is possible to calculate the hydraulic conductivity of unsaturated stony soils, knowing the saturated hydraulic conductivity of the fine soil fraction, the retention curve of the fine soil fraction, and the particular stoniness of the soil.     

A new equation for the soil water retention curve

The soil water retention curve is a fundamental characteristic of unsaturated zone flow and transport properties. Recent studies show that an air‐entry value is needed in a soil water retention equation in order to provide a better prediction of relative hydraulic conductivity. A new equation considering the air‐entry value is proposed to describe the soil water retention curve. The performance of the proposed equation is contrasted with a well‐supported equation by comparing measured and calculated data for 14 soils, representing various soil textures, which range from sandstone to clay. Results show that the proposed equation provides adequate characterization of the soil water retention curves. The equation for predicting relative hydraulic conductivity is derived from the proposed soil water retention equation. An empirical equation for relative hydraulic conductivity is also used. Our results show that the agreement between the predicted and measured relative hydraulic conductivities is improved by the combinations of the proposed equation and the relative hydraulic conductivity equations. The proposed equation is mathematically simple and it can easily be implemented in unsaturated flow and multiphase flow numerical models.     

室内基于土壤水分再分布过程推求紫色土导水参数

选择三峡库区3种不同质地的紫色土,室内通过土壤水分再分布试验,探讨基于土壤水分再分布过程推求导水参数对于紫色土的适用性.结果显示,结合土壤水分垂直和水平再分布过程推求的紫色土水分扩散率与实测值具有很好的一致性,但推求的非饱和导水率偏差较大.然而,选用单一的土壤水分再分布过程结合实测水分特征曲线推求的紫色土非饱和导水率与实测值具有良好的一致性.湿润锋湿度与湿润剖面平均湿度不同函数关系对推求非饱和导水率和水分扩散率差异不明显.此外,基于土壤水分再分布过程推求导水参数方法比较适合低湿土壤的非饱和导水参数推求.     

Soil physical properties related to soil structure

The aim of this paper is to clarify the effect of soil aggregation on soil physical and chemical properties of structured soils both on a bulk soil scale, for single aggregates, as well as for homogenized material. Aggregate formation and aggregate strength depend on swelling and shrinkage processes and on biological activity and kinds of organic exudates as well as on the intensity, number and time of swelling and drying events. Such aggregates are, most of all, more dense than the aggregated bulk soil. The intra-aggregate pore distribution consists not only of finer pores but these are also more tortuous. Thus, water fluxes in aggregated soils are mostly multidimensional and the corresponding water fluxes in the intra-aggregate pore system are much smaller. Furthermore, ion transport by mass flow as well as by diffusion are delayed, whereby the length of the flow path in such tortuous finer pores further retards chemical exchange processes. The chemical composition of the percolating soil solution differs even more from that of the corresponding homogenized material the stronger and denser the aggregates are.

The rearrangement of particles by aggregate formation also induces an increased apparent thermal diffusivity as compared with the homogenized material. The aggregate formation also affects the aeration and the gaseous composition of the intra-aggregate pore space. Depending on the kind and intensity of aggregation, the intra-aggregate pores can be completely anoxic, while the inter-aggregate pores are already completely aerated. The higher the amount of dissolved organic carbon in the percolating soil solution, the more pronounced is the difference between the gaseous composition in the inter- and in the intra-aggregate pore system.

From the mechanical point of view, the strength single aggregates, determined as the angle of internal friction and cohesion, depends on the number of contact points or the forces, which can be transmitted at each single contact point. The more structured soils are, the higher the proportion of the effective stress on the total stress is, but even in single aggregates positive pore water pressure values can be revealed. Dynamic forces e.g. due to wheeling and/or slip processes can affect the pore system as well as the composition of the soil by: (1) a rearrangement of single aggregates in the existing inter-aggregate pore system resulting in an increased bulk density and a less aerated and less rootable soil volume, (2) a complete homogenization, i.e. aggregate deterioration due to shearing. Thus, the smaller texture dependent soil strength coincides with a more intensive soil compaction due to loading. (3) Aggregate deterioration due to shearing results in a complete homogenization, if excess soil water is available owing to kneading as soon as the octahedral shear stresses and the mean normal stresses exceed the stress state defined by the Mohr-Coulomb failure line. Consequently, normal shrinkage processes start again.

Thus, the rearrangement of particles and the formation of well defined single aggregates even at the same bulk density of the bulk soil both affect, to a great extent, various ecological parameters. Environmental aspects can also be correlated, or at least explained with the processes in soils, as a major compartment of terrestial ecosystems, if the physical and chemical properties of the structure elements and their composition in the bulk soil are understood.     

Soil Water Relations and Aeration Status of Single Soil Aggregates,Taken from a Gleyic Vertisol

Clayey soils have the potential to swell and to shrink depending on their hydraulic and hydrological status. Thus bulk density values vary in a range of 1.0 to 2.0 g cm^?3 in the case of a gleyic Vertisol, by which also other soil physical properties e.g. the pore size distribution of the bulk soil as well as of the soil aggregates are affected. Intraaggregate airfilled porosities are reduced by shrinkage and are relatively low. Thus it appeared to be difficult to determine the airfilled porosity of the aggregates below pF 1.5. For that reason and because of the influence of pore forms we were not able to get a clear relation of diffusion constant K with airfilled porosity. Regarding soil aeration status, the existence of anoxic microsites in the interior of unsaturated soil aggregates has been proved by microelectrode measurements of oxygen partial pressure and redox potential distribution in single soil aggregates. We verified restrained oxygen supply to the aggregate center as well as reduced redox potentials only for aggregates of the A horizon. There the microbial activity, measured as soil respiration as well as the source for C and N was by a factor 2 to 4 higher than in the subsurface horizons.     

Hydraulic energy indices reveal spatial dependence in a subtropical soil under maize crop in Southern Brazil

Soil physical quality (SPQ) assessment is an important part in the evaluation of soil use, management, and conservation. It can be assessed using several physical properties, hydraulic indices, and functions. Soils from tropical and temperate regions represent different physical behaviors, and the quantification of their physical properties is important to support soil evaluation and modelling. The objective of this study was to evaluate the SPQ in a subtropical field under maize crop cultivation according to its physical properties, hydraulic indices, and functions in an attempt to infer the spatial variability and to determine the behavior of soil physical structure across a spatial domain. Commonly used soil key physical variables, such as texture, bulk density, total porosity, saturated hydraulic conductivity, and organic carbon content, were measured in a regular grid with a soil sampling density of 30 points per hectare, covering an area of 0.5 ha. Saturated hydraulic conductivity varied strongly between subsamples and in the field, suggesting the heterogeneity of the soil structure regarding water drainage. The physical variables were combined with other indicators, which were based on the soil water retention curve and the pore size distribution (PSD) function. Correlation analysis was performed to verify the relationship between the measured and calculated variables, and some strong linear correlations were revealed, such as between aeration energy index and microporosity (r = 0.608) and water retention energy index with microporosity (r = 0.532) and with bulk density (r = 0.541). For most sampled locations, the shape and location parameters of PSD showed results outside of the optimum ranges, whereas the hydraulic energy indices and cumulative hydraulic energy functions presented values that were similar to those found for some tropical soils described in the literature. The spatial variability of these indices was described using semivariograms and kriged maps, indicating the variability of the SPQ in this field.     

Use of an inverse method and geostatistics to estimate soil hydraulic conductivity for spatial variability analysis

A field method for determining the soil hydraulic properties using a parameter estimation technique is presented. Input data for the inverse problem are soil-water potentials and soil-water contents measured at different soil depths and different times during a field transient drainage experiment. For the water retention function the parametric relation suggested by Van Genuchten was adopted. For the hydraulic conductivity function the relation proposed by Van Genuchten and the exponential relation were adopted.

With the proposed method soil hydraulic properties along a transect of a volcanic Vesuvian soil were determined using as boundary condition the unit gradient of total potential at the bottom of the soil profile. Geostatistics were used to describe the spatial variability of hydraulic conductivity characteristics of the soil here considered.

Finally, results obtained using this method were compared with those of the simplified method suggested by Sisson and Van Genuchten based on a unit gradient water flow model.