Moisture Content Gradient Research Articles

Microwave Bistatic Reflectivity Dependence on the Moisture Content And Matric Potential of Bare Soil

Results are presented of an experimental program to determine the functional dependence of the microwave reflectivity of nonvegetated soil surfaces upon volumetric soil moisture and matric potential. A combination evaporation-drainage field experiment was conducted on a bare Captina silt loam with reflectivity, soil moisture content, and matric potential monitored for extended time periods. Results show that for a restricted pressure range (approximately -0.05 to -0.75 bar) there is excellent linear correlation between the log of bistatic reflectivity and both volumetric moisture content and matric potential. Layering effects due to steep moisture content (and matric potential) gradients in the profile are demonstrated to have two distinct and significant effects on the reflectivity response. At near saturation of rough surfaces a very thin dry surface layer appears to modify the effective roughness. This leads to a saturation of reflectivity at high moisture contents. As the surface proceeds to dry further, deeper layers produce coherent interference patterns in the reflectivity response, particularly at the higher frequencies.

Observational Analysis of Tropical Cyclone Formation. Part II: Comparison of Non-Developing versus Developing Systems

Abstract The thermodynamic and dynamic fields surrounding the composite tropical weather systems described in Part I (McBride, 1981a) are examined for differences between non-developing and developing systems. The main findings are as follows: (i) Both non-developing and developing systems are warm core in the upper levels. The temperature (and height) gradients are more pronounced in the developing system, but the magnitudes are so small that the differences would be difficult to measure for individual systems. (ii) The developing or pre-typhoon cloud cluster exists in a warmer atmosphere over a large horizontal scale, for example, out to 8° latitude radius in all directions. (iii) There is no obvious difference in vertical stability for moist convection between the systems. (iv) There is no obvious difference in moisture content or moisture gradient. (v) Pre-typhoon and pre-hurricane systems are located in large areas of high values of low-level relative vorticity. The low-level vorticity in the vicinit...

Heat and mass transfer in hygroscopic capillary extruded products

AbstractThis work presents an analysis of heat and mass transfer of extruded corn meal. Isotherm, thermal conductivity, drying and center temperature data were experimentally obtained for the cylindrically extruded product.Modeling of the drying process was made by incorporating the mechanistic and irreversible thermodynamics approaches for determining heat and mass transfer in capillary‐porous media. The proposed model shows that both liquid and vapor fluxes can be expressed in terms of the same driving‐forces, namely, temperature and equilibrium moisture content gradients. A search technique allowed for the evaluation of both liquid and vapor conductivities.

Thermal diffusion of bound water in wood

There are few references in the wood science literature to nonisothermal moisture movement. Some experiments by Voight, Babbitt, and Choong indicate that thermal diffusion in wood may be very significant. Three equations are presented to represent nonisothermal moisture movement through wood in the transverse direction. The first, described in detail in a previous paper, is based upon two driving forces: Soret potential which results in thermal diffusion and chemical potential which results from a gradient of equilibrium relative humidity. All three equations include the same term for Soret potential and, in the second two equations, the thermal-diffusion term was derived by the application of activation theory. The isothermal term in these two equations utilizes moisture content gradient as the driving force and therefore a knowledge of the sorption isotherm is not necessary. The third equation contains an additional term for moisture-content activation to account for the increase in flux with an increased moisture content. All three equations give approximately the same result with the same input data at low moisture contents. The results diverge at high moisture contents and experimental data are required to determine which equation is most representative of the physical phenomenon of nonisothermal moisture movement.

A Mathematical Treatment of the Initial Stages of Drying of a Soil Column

AbstractA nonlinear diffusion equation governing the initial stages of drying of a soil column is numerically integrated using a method designed to adjust the time step to compensate for steep moisture content gradients. A balance of mass check is used to determine when the simulation becomes invalid. Resulting tables of relative moisture depletion are given for various soils and drying rates. In view of the improved technique of numerical integration and the attention to the balance of mass check, it is felt that the numerical results obtained are more reliable than previously available results.

The mean accuracy and the precision of using stem disc moisture contents to estimate stem dry weight of Pinus resinosa ait

The moisture content gradient of dominant-codominant red pine stems was examined. The moisture contents of discs removed from five positions on the stem were used to estimate the stem dry weight. Each disc when used alone, does not represent the moisture content of the stem accurately. All discs used together improve the precision of dry weight estimates but not to the degree expected. It is suggested that more than one disc be removed from the stem to adequately estimate the stem moisture content, and stem dry weight.

Surface microcracking induced by weathering of polycarbonate sheet

Polycarbonate sheet subjected to outdoor weathering for relatively short periods develops a network of surface microcracks on the side exposed to solar radiation. Artificial weathering and SEM microscopy were used to illustrate the process of microcrack formation. Microcracking occurs under the influence of light radiation in conjunction with cycling of either temperature and moisture or temperature alone. The use of radiation by itself, or even relatively severe cycling of temperature and humidity without radiation, does not induce microcracking. According to the proposed mechanism, the resin of the exposed surface undergoes, with weathering, a gradual reduction in strength owing to a lowering of its molecular weight as a result of photochemical degradation. Cyclic variation of temperature and humidity in natural and artificial weathering imposes on the surface material a type of stress fatigue. Such stress fatigue is caused by the non-uniform dimensional changes that result from thermal and moisture content gradients between the surface and the bulk of the resin and from inhomogeneities and defects. When the fatigue limit of the surface material at a microsite becomes lower than the physically induced stresses, the resin cracks.

The effect of atmospheric conditions on the length of visible cooling tower plumes

The governing equations for the initial phase of cooling tower plume behaviour are solved numerically to illustrate the quantitative effects of atmospheric temperature, moisture content, stability and moisture gradient on visible plume lengths. Some discussion of the atmospheric turbulence dominated phase of plume behaviour is given. Decreasing relative humidity with height and/or stable conditions in the atmosphere are found to lead to decreased plume lengths. The effects are most pronounced at low temperatures and high atmospheric relative humidities. The influence of the entrainment parameter on visible plume length is examined and found to be relatively minor when compared to the influence of atmospheric conditions. The numerical results are compared with a simple graphical solution and the results show considerable disparity.

Measurement of the Transport Coefficients for Coupled Flow of Heat and Moisture in a Medium Sand

AbstractA two‐column experiment was designed to measure the primary and cross‐coupling transport coefficients for the simultaneous flow of heat and moisture through a medium sand (Plainfield sand) in the liquid‐dominated regime of moisture flow. Values for the moisture diffusivity and thermal conductivity were obtained in the range from 20% to 90% of saturation. Thermal conductivity values fell within ± 7% of a curve calculated by the method of de Vries. In addition, the coefficient for transport of heat under gradients of moisture content was found to be zero within the accuracy of the experiment. The experiment was not sufficiently accurate to yield quantitative estimates of the coefficient for transport of moisture by temperature gradients, so a different experiment was performed to measure this coefficient indirectly. In this latter experiment the dependence of soil water potential upon temperature was found to be as much as five times larger than predicted by the surface tension model used by Philip and de Vries. These large values suggest that thermal effects should be considered for long‐term movement of soil moisture, and perhaps in certain circumstances for shorter term simulations in moist soil.

Reactions of Ammonium Ortho‐ and Polyphosphate Fertilizers in Soil: I. Mobility of Phosphorus

AbstractThe mobility of P applied as diammonium orthophosphate (DAP), triammonium pyrophosphate (TPP), or ammonium polyphosphate (APP) was studied in columns of Hartsells fine sandy loam. The fertilizer dissolved in soil moisture that moved towards the application site. This water movement was sometimes against a gradient in soil moisture content, but it was along a gradient in the total potential of soil water.The extent of P movement from all three sources was similar, but the distribution patterns were different. The extent of P movement was influenced more by the initial soil moisture content than by the source of P.A higher fraction of the added P was precipitated when the source was TPP or APP than when it was DAP. The ability of the polyphosphates to sequester soil Fe and Al did not prevent the precipitation of these phosphates nor did it make them more mobile than the orthophosphates. It only delayed the precipitation reaction to a degree that depended on the polyphosphate content of the fertilizer material.

Slope length of claypan soil affects runoff

Measurements of runoff after prolonged irrigation of grass plots on a claypan soil (Mexico silt loam) showed that recession yields per unit area increased with slope length. Yields from slopes of lengths varying from 76 to 323 feet indicated return flow (‘interflow’) during runoff recession of at least 0.1 inch from the longer plots. During a simulated wet season using irrigation to supplement rainfall in the summer of 1965, per unit area yields were greatest for long plots, except for small events or events of long duration. The seasonal yield was 1.69 inches, or about 19% more from the long than from the short plots. Hydraulic conductivity measurements indicate that the predominant path of interflow is in the upper inch or two of the soil surface. However, soil moisture content and hydraulic pressure gradient changes during recession runoff indicated that there may be an interflow contribution from the loessal soil layer between weathered till and claypan. (Key words: Runoff; soil moisture; infiltration)

Moisture distribution in granular material during drying in a fluidized bed

This paper gives the results of a determination of the moisture content gradient and an investigation of the moisture distribution in a granular material during drying in a fluidized bed.

Soil Moisture Transport Due to Thermal Gradients: Practical Aspects

AbstractThe present status of knowledge concerning thermally induced transport of moisture in soil is reviewed. This includes the various theories on mechanisms of transport and experimental data showing the magnitude of moisture flow in various porous materials. Wherever possible these data were chosen to show the relative importance of thermal versus head‐type flow and some general trends are noted.Simple equations are developed to describe the thermally induced moisture flow near the soil surface which arises from transient thermal gradients produced by the diurnal temperature cycle. Calculations of the moisture flux over 2 ten‐hour time intervals for a typical field situation indicate that thermal water transport should be considered whenever moisture, salt, or heat fluxes are being studied in the soil's surface layers.The upward flow of soil water against a moisture content gradient in the winter is also considered. A sample calculation of the amount of thermally driven moisture was made using data available in the literature. The result suggested that the thermal moisture flow was too small to account for the net movement of soil moisture into the frost zone.

Linear Equations for the Simultaneous Flow of Matter and Energy in a Continuous Soil System

AbstractSome transfer processes in soil are considered with respect to recent developments in the theory of thermodynamics of irreversible processes. The theory provides a general and systematic guide for setting up interrelated equations describing the transport of one or more components through soil. Darcy's law and the “diffusion equation” are shown to be components of this general development. Particular attention is given to the non‐isothermal flow of moisture and energy through soil. A general relation between the fluxes induced by the combined gradients of moisture content, temperature, and solutes is also suggested.

Simultaneous transfer of heat and moisture in porous media

Simultaneous differential equations for the transfer of heat and moisture in porous media under the combined influence of gravity and gradients of temperature and moisture content are developed. These equations are a generalization of those derived by Philip and de Vries [1957]. In particular a consistent distinction is made between changes of moisture content in the liquid and the vapor phase.The interaction between heat and moisture transfer in steady state heat conduction is discussed in detail using numerical values for two soils, a clay loam and a medium sand. The behavior is found to be dependent on the boundary conditions for moisture transfer, on the direction of the temperature gradient and on the ratio of two moisture diffusivities enterbag in the analysis.

The permeability of porous materials

The permeability of a porous material to water is a function of the geometry of the boundary between the solid component and the pore space. Expressions of the Kozeny type purporting to represent this function are based upon the particle size or specific surface of the solids, and whilst, for engineering practice, they have given satisfaction for saturated sands, they may fail badly in other cases. By developing a Kozeny type of expression for the particular structure of a bundle of capillary tubes of assorted radii, we demonstrate the cause of the failure. Such failure may be avoided by relating permeability to pore-size distribution, which is the factor of prime concern and which may be measured directly by even simpler means than are used to determine particle-size distribution. The pore-size distribution is arrived at by an interpretation of the moisture characteristic of the material, i.e. of the curve of moisture content plotted against pressure deficiency. A simple statistical theory, based upon the calculation of the probability of occurrence of sequences of pairs of pores of all the possible sizes, and of the contribution to the permeability made by each such pair, leads to an expression of the permeability as the sum of a series of terms. By stopping the summation at a selected upper limit of pore size one may calculate the permeability at any chosen moisture content and plot it as a function of that content. An example is presented, using a coarse graded sand specified by its moisture characteristic. To check these calculations, experimental determinations of the permeabilities of unsaturated materials are presented, using two different grades of sand and a sample of slate dust, the results being compared with computed values. The agreement seems good, and is certainly better than that provided by the Kozeny formula as developed, with difficulty, for the purpose. The limitations and possible improvements of our concept are very briefly discussed, and finally it is shown how a combined use of the moisture characteristic and the permeability (which is itself derivable from the moisture characteristic) leads to an expression for the coefficient of diffusion of water in the material as a function of moisture content. From this it should be possible, in principle, to calculate in suitable cases the course of water movement down a gradient of moisture content. Such a calculation awaits a satisfactory solution of the problem of non-linear diffusion.

Laboratory experiments on evaporation from fallow soil

Experiments on evaporation from freely drained soils are described. Under isothermal conditions characteristic winter field behaviour is obtained, even when the air drying power is greater than its normal English midsummer value. Characteristic summer field behaviour is obtained when the rapid drying of a thin surface layer is achieved, either by using an extremely high air temperature under ‘isothermal’ conditions, or by raising the surface temperature by means of radiation—the normal method in nature. The effect of a high salt concentration in the soil water is shown to lead to greater evaporation losses and to a tendency for the salt to concentrate in the more salty patches.It is suggested that mulching will only be beneficial during the isothermal part of the year, i.e. when soil surface and air temperature are approximately equal, and that it will have little effect on water conservation where the soil will be self mulched by the action of summer sunshine. The cause of this self-mulching action is briefly considered in the light of our limited knowledge of soil water dynamics; it appears to depend on the existence of a dual mechanism of water movement in soils—as liquid and as vapour—the rates of movement being very different functions of moisture content and moisture gradient.

OBSERVATIONS ON THE PERMEABILITY OF HYGROSCOPIC MATERIALS TO WATER VAPOUR: I. OBSERVATIONS AT RELATIVE HUMIDITIES LESS THAN 75%

It is pointed out that experimental work has shown that for the transpiration of moisture through hygroscopic materials two distinct regions of relative humidity can be distinguished: (i) A region below some value between 70 and 80% where the moisture movement is proportional to the vapour pressure difference, and (ii) a region of high relative humidity where the moisture movement is not directly proportional to the vapour pressure. A theory is advanced that these two regions correspond to the two conditions in which water may be present in a hygroscopic material: (i) the water may be molecularly adsorbed, that is to say, the water is bound by the affinity of the molecules of water for those of the solid on which it is adsorbed; (ii) the water may be held in small fissures in the sub-microscopic structure of the sorbing material by capillary forces. The mechanism by which the moisture will move through the material would be different in the two cases and would result in the distinction between regions of high and of low relative humidity.A short discussion is given of Fick's law showing the form in which it might be expected to apply in the two cases. Measurements are given for fibreboards showing that below 75% relative humidity the resistance of the board to moisture transpiration is proportional to the thickness. The moisture content gradients through fibreboard samples have been determined. The diffusance through a board in which the moisture content gradient is opposed to the vapour pressure gradient shows that the latter is the important factor, and the determination of the moisture content gradient indicates that at these low humidities Fick's law is applicable.