The spatial variability of soil aggregate stability and its relationship to runoff and soil erosion were examined in a catena of soils and vegetation in a semiarid environment at the Rambla Honda field site (Tabernas, Almeria, SE Spain) to evaluate the validity of structural stability as a soil erosion indicator in sandy loam range soils. The influence of soil properties and topography on the variability of aggregate stability was also examined. Methods include: 1) aggregate stability assessment at 12 sites (3 repetitions per site) on the hillslope by two methods: a) aggregate size distribution by dry sieving b) water drop test; 2) soil organic carbon content; 3) particle size distribution determination; 4) terrain attributes derived from a digital elevation model (1-m resolution); 5) monitoring runoff and erosion for nearly 3 years in eight (10 × 2 m) plots distributed over the hillslope. Results: 41% of the average soil mass is formed by > 2-mm aggregates. However, wet aggregate stability is poor, with a mean (of a total of 1440 aggregates) of only 26 drop impacts necessary to break up a wet aggregate (pF = 1). Significant relationships were found in the number of water drops required for aggregate breakdown and runoff and erosion rates. However, no significant relationships between the mean weight diameter of aggregates under dry conditions and runoff or erosion rates were observed. The relationships of aggregates with other soil properties, hillslope position and proximity to plants are also analysed. The most significant correlation found was between the number of drop impacts and soil organic matter content. The stability of topsoil aggregates seems to be a valuable indicator of field-assessed runoff and inter-rill erosion of sandy loam range soils under semiarid conditions.

Aggregate stability in range sandy loam soils Relationships with runoff and erosion

The assessment of soil erodibility to water erosion in the field is often expensive and time-consuming. This study was designed to reveal the effects of aggregate breakdown mechanisms on interrill erosion dynamics and develop an improved model for assessing interrill soil loss, which incorporated the soil aggregate stability tests as a substitute for the interrill erodibility parameter, from both disturbed and undisturbed samples for red soils in subtropical China. Six cultivated areas of sloping land with red soils were selected, and topsoil aggregate stability was analyzed using the Le Bissonnais method to determine the different disaggregation forces. Laboratory rainfall simulations were designed to distinguish the effects of slaking (at different wetting rates) and mechanical breakdown (with and without screening) on soil erosion characteristics. Field rainstorm simulations with medium and high rainfall intensities were conducted on runoff plots (2 m 1 m) with slope gradients varying from 10% to 20% for each soil type. A new instability index, Ka , which considers aggregate breakdown mechanisms in interrill erosion processes, was proposed based on the disturbed sample results. Ka showed a close relationship with erosion rates in both disturbed and undisturbed samples. Following from the results of undisturbed sample experiments, Ka was used as a substitute for the erodibility factor, and introduced into the WEPP model, establishing a new erosion predication formula for red soils which had a good correlation coefficient ( R 2  = 0.89**). This research made a good attempt at estimating the interrill erosion rate on the basis of aggregate stability from simple laboratory determinations. These results extend the validity of soil aggregation characterization as an appropriate indicator of soil susceptibility to interrill erosion in red soils from subtropical China. The formula based on the instability index, Ka , has the potential to improve the methodology used for assessing interrill erosion rates.

Interrill erosion from disturbed and undisturbed samples in relation to topsoil aggregate stability in red soils from subtropical China.

Rainfall splash erosion is a temporal dynamic process that is determined by the interaction of soil surface and rainfall conditions. The properties of aggregate, such as aggregate stability and size distribution, are recognized as factors that influence the variation in soil surface structure and sediment selectivity during splash erosion event. This study based on aggregate stability to investigate the effect of aggregates breakdown dynamics on splash erosion and sediment size selectivity processes of Ultisols in subtropical China during rainfall events. Laboratory experiments of rainfall simulation involving rainstorm simulations at 58.1 ± 2.8 mm h− 1 were conducted on four soils derived from Quarternary red clay and Shale. We compared the size distributions of aggregates produced by raindrop impact with the sediment size distributions resulted from transportation. Fractal dimension (D) and enrichment ratio (ER) were used to characterize aggregates size distribution and sediment size selectivity. Aggregates were prone to breakdown at the beginning of rainfall and a multiple regression equation (D = 1.02RMI + 1.77 T0.06 (R2 = 0.95, P < 0.01, n = 60)) was obtained to describe aggregate breakdown dynamics considering a relative aggregate stability index (RMI) and rainfall time (T). Total masses of erosion sediment decreased as a power function with an increasing size fraction. The behaviors of sediment size fractions (< 0.25 mm, 0.5–0.25 mm, 1–0.5 mm, 2–1 mm, 5–2 mm) showed that the transport selectivity was decided by the interaction of aggregates size distributions and the characteristics of soil surface structure. Sediment size selectivity led to the enrichment of 1–0.25 mm fraction compared with the breakdown products. This research contributes to knowledge about the dynamics of the splash mechanism, and the results could be valuable for the development of rainfall erosion models.

The dynamic response of splash erosion to aggregate mechanical breakdown through rainfall simulation events in Ultisols (subtropical China)

Sensory quality, bioactive constituents and microbiological quality of green and red fresh-cut lettuces (Lactuca sativa L.) are influenced by soil and soilless agricultural production systems

Cover crops to mitigate soil degradation and enhance soil functionality in irrigated land

The transient hydrodynamics and thermal behaviors of fluid flow in an open-ended vertical parallel-plate microchannel are investigated analytically under the effect of the hyperbolic heat conduction model. The model that combines both the continuum approach and the possibility of slip at the boundary is adopted in this study. The effects of Knudsen number Kn and thermal relaxation time τ on the microchannel hydrodynamics and thermal behaviors are investigated using the hyperbolic and the parabolic heat conduction models. It is found that as Kn increases, the slip in the hydrodynamic and thermal boundary condition increases. Also, this slip increases as τ decreases.

Transient Free Convection Fluid Flow in a Vertical Microchannel as Described by the Hyperbolic Heat Conduction Model

Summary

Detachment of soil particles by the processes of splash from rainfall and shallow flow from surface runoff is influenced by soil cohesion, soil aggregate properties, and characteristics of this flow. We have evaluated relationships between rates of detachment, aggregate size, and tensile strength of the soil. Soil and water losses were determined in the laboratory from sieved air-dry samples on three aggregate size ranges of two clay loam soils differing in particle-size distribution and organic matter. Tensile strength was measured for each aggregate size range. The results showed that as clod size increased, detachment rates increased and interaggregate tensile strength decreased. Wash erosion increased as initial clod size increased despite a decrease in runoff. Final rates of loss by splash were greater from the largest clods than from the smaller clods. Finally, splashed material was larger in size than material washed off. The fact that the size of the splashed material was larger than washed-off material shows that material in the wash suffered more impact by raindrops and thus was more likely to be fragmented.

Effect of soil aggregate size on infiltration and erosion characteristics

Basic viscoelastic fluid flow problems using the Jeffreys model

A fixed-bed dryer was designed and constructed and drying experiments with fixed beds of wheat were carried out under various conditions of drying air with wheat of several initial moisture contents. The air temperature and moisture content of wheat at various levels within the beds were measured periodically. A computer program based on energy and mass balances was developed to simulate the deep bed drying process. Experimental data from the dryer were compared with the results from this program. The results showed that there was good agreement between the simulated drying rates between the layers and those experimentally observed. In addition, there was a good agreement with respect to the shapes of the drying air temperature profiles.

An Experimental Study and Mathematical Simulation of Wheat Drying

An experimental investigation was carried out to study the possibility of decreasing the consumed energy in buildings in Jordan by decreasing the thermal conductivity of building bricks. Insulation materials with different weight percentages are mixed with the traditional materials of the building bricks used in Jordan. The used materials were polystyrene, polycarbonate, polyethylene of (2.5, 5, 10 & 15wt %), wood dust and natural cork of (1, 2, 3, & 4 wt %), glass wool and rock wool layer of ( 24, 36, 48 and 60 gm). The water absorption, compressive strength and thermal conductivity, of the prepared bricks were determined and the obtained results are discussed and compared. The results showed that addition of the insulating materials resulted in reduction of water absorption in the entire materials examined (a minimum of -4.14% for glass wool addition) except natural cork and wood dust (a maximum of +207.62% of natural cork addition).The results also indicated a decrease in the compressive strength as much as (90.71 %) for both glass wool and natural cork additions. A pronounced decrease in thermal conductivity was also noticed of about 70 % of its value for the case of glass wool and natural cork additions, at addition rate of 4% weight.

Experimental Investigations of the Effect of Some Insulating Materials on the Compressive Strength, Water Absorption and Thermal Conductivity of Building Bricks.

Ali M. Othman

Papers

Transient Free Convection Fluid Flow in a Vertical Microchannel as Described by the Hyperbolic Heat Conduction Model

Effect of soil aggregate size on infiltration and erosion characteristics

Basic viscoelastic fluid flow problems using the Jeffreys model

An Experimental Study and Mathematical Simulation of Wheat Drying

Experimental Investigations of the Effect of Some Insulating Materials on the Compressive Strength, Water Absorption and Thermal Conductivity of Building Bricks.