Field Water Content Research Articles

Soil Shrinkage Relationships of Texas Vertisols: I. Small Cores

AbstractSoil shrinkage‐water content relationships are needed to describe pore size distribution, bulk density function, and potential vertical movement of Vertisols.Ten‐centimeter long, 10‐cm diam cores were collected from the 10‐ to 20‐, 40‐ to 50‐, 70‐ to 80‐, 100‐ to 110‐, and 130‐ to 140‐cm depths of eight central Texas Vertisols. Four sites were adjacent mound and depression samples from two gilgai complexes. Profile description and analytical characterization were obtained for each site. Soil shrinkage was measured vertically and horizontally during a drying cycle from field water content to oven‐dry. Some field matric potential profiles were obtained at sampling.The shrinkage curves are described in terms of structural, shrinkage, and residual water loss phases. All cores had normal, equidimensional shrinkage. Only sites sampled at matric potentials > −300 cm of water had structural water loss. The intersection between the structural and shrinkage phases (the swelling limit) was generally sharply defined when present and occurred at about −0.3 bar. Structural water loss increased with depth due to increasing matric potential, and the soil air content at the swelling limit was about 0.05 cm3/cm3. The water content at the swelling limit was related to the cation exchange capacity (CEC) and the −0.3 bar water content. The shrinkage limit was relatively constant (0.09–0.11 g/g) for seven of the sites, and all residual water loss occurred at < −15 bars. The total vertical shrinkage and the normal water loss were related to the CEC and to the coefficient of linear extensibility (COLE).

Winter water relations of native and introduced evergreens in interior Alaska

In view of wintertime browning of needles of pines introduced in Alaska, winter desiccation damage was suspected. However, examination of field xylem water potentials and relative water contents indicated little water stress in pine needles but severe stress in sunny native spruce needles and slightly less stress in shaded needles. Spruce were typical of drought tolerant plants, exhibiting field xylem water potentials of –10 to –40 bars and field relative water contents of 88 to 99%, closing stomates at water contents of 71 to 92%, and maintaining cuticular conductances from 0.013 to 0.088 cm s−1. In contrast, pines were drought avoiding, exhibiting field xylem water potentials of 0 bars and field water contents of 100%, keeping stomates closed all winter, and maintaining cuticular conductances from 0 to 0.033 cm s−1 and 0 to 0.074 cm s−1 in lodgepole pine and yellow pine, respectively. Needles of all species photosynthesized at temperatures from 0 to 10 °C with rates in spruce being 10 times those in yellow pine and 3 times those in lodgepole pine. While spruce needles had significantly more carbohydrate reserves at the end of the winter than did pine needles, the latter were apparently not severely carbohydrate depleted.

Fluctuations of the parameters of probe radiation traveling in a bleached zone created in an aerosol medium by radiation of different wavelength

The geometric-optics approximation is used in the derivation of expressions for the square of fluctuations of the intensity level and for the structure function of fluctuations of the phase of a beam of probe radiation traveling in a bleached disperse medium. Fluctuations of the parameters of the medium appear as a result of mixing of inhomogeneous temperature and water-content fields by a random-velocity wind. A comparison is made of the calculated fluctuations of the intensity with those measured in an artificial fog chamber.

Prediction of Field Soil Water Content

AbstractA 3‐year experiment was conducted to monitor and subsequently predict field water contents using the water transport model. Saturated hydraulic conductivities and soil water characteristic curve results were measured and averaged for each soil layer. The Millington‐Quirk (MQ) method was used to calculate average model parameters. Direct use of the MQ‐derived conductivity and diffusivity functions did not give an exact fit to the observed field results. However, when reasonable adjustments were made to the K(θ) and D(θ) functions the prediction was improved considerably. The methodology for “averaging” and utilizing field data to derive model parameters, and subsequent adjustment or optimization of the model parameters all require more research if we hope to use the unsaturated flow equation for field water content prediction.

Toposequences of Loess‐derived Soils in Southwestern Indiana

AbstractSoil properties are related to stratigraphy of parent materials and to hillslope position in southwestern Indiana where the landscape consists of gently rolling hillslopes and nearly level plains. Loess was uniform thickness downslope. Two distinct units were recognized. The upper (Peoria) loess contained about 15% less sand and had greater calcium carbonate equivalent, greater field water content, and lesser bulk density than the lower (sandy) loess. The Sangamon paleosol below the loess consisted of an upper slightly developed horizon and lower well developed horizons. The percentages of some heavy minerals were similar in the two loess units and similar in the two paleosol units, but differed between the loess and the paleosols. The lower loess showed slight soil formation. A fragipan was present only on summit and shoulder positions where the paleosol formed in weathered sandstone and shale, but not where it formed in outwash. Apparently there was slight soil formation in the lower loess before its surface was covered by the upper loess. Our data support the hypothesis that fragipans form in loess underlain by less permeable materials at certain depths and that silica (or silicate) acts as a bonding agent in fragipans. We believe that the fragipan forms by processes associated with the modern landscape, but its formation is influenced by pre‐Peoria soils or weathering zones.

A Laboratory Technique for Appraising in Situ Salinity of Soil

AbstractA laboratory technique has been devised for removal of solution from soils at field water content. A soil sample can be placed directly in a closable centrifuge tube, thereby minimizing changes which could be induced by altering gaseous equilibria. Water need not be added to the soil sample, so changes in electrical conductivity (EC) and in concentration of species present are avoided. The soil solution is obtained from the sample by immiscible displacement with CCl4 using centrifugation. The technique is simple and relatively rapid. Values for EC in successively displaced solution samples from a given soil sample were found to be essentially constant. The values for EC obtained for an assortment of soils were significantly higher than those obtained using 2:1 or saturation extracts but were well correlated with them.

Soil‐Water Regimes in Brookston and Crosby Soils

AbstractHydraulic conductivity and seasonal patterns of water content were measured and related to soil formation in associated very poorly drained Brookston soils (Typic Argiaquolls) and somewhat poorly drained Crosby soils (Aeric Ochraqualfs) in central Indiana. In the Brookston soil the water table was near the surface for only a short time during periods of heavy rainfall in the winter and spring. The water table then stabilized near 125 cm deep, where it was controlled by tile drains. In Crosby, also tile‐drained, the water table stayed near the surface longer in the spring, then became deeper than in Brookston. The hydraulic conductivity is relatively high in the Brookston B and C and Crosby B horizons but very low in the Crosby C horizon, compact glacial till. Since downward movement is restricted in the Crosby C horizon, water tends to move laterally through the B horizon into the Brookston soil where the tile are in more permeable horizons than they are in Crosby.Available water capacities of soils can be estimated by measuring the field water contents in a profile at the beginning of the growing season and during dry periods for several years using a soil under perennial vegetation. In these two soils, however, it appears that water movement from Crosby to Brookston and the profile storage capacity are both important in determining the water‐supplying capacity of the soils.

Ice Lensing, Thermal Diffusion and Water Migration in Freezing Soil

When a surface layer of the soil freezes, both heat and water diffuse from the unfrozen soil beneath to the frozen region. Often the soil does not freeze homogeneously but distinct ice lenses form. An analysis of the diffusion and ice nucleation processes suggests conditions under which ice lensing can be expected; in particular, it is shown that multiple ice lenses cannot form unless the soil thermal diffusivity is greater than the water diffusion coefficient. Analysis of a simple one-dimensional case (a semi-infinite mass of homogeneous soil whose surface temperature is suddenly lowered) gives the temperature and water-content fields as functions of time; these agree with those observed in an experimental study of freezing clay.

Ice Lensing, Thermal Diffusion and Water Migration in Freezing Soil

When a surface layer of the soil freezes, both heat and water diffuse from the unfrozen soil beneath to the frozen region. Often the soil does not freeze homogeneously but distinct ice lenses form. An analysis of the diffusion and ice nucleation processes suggests conditions under which ice lensing can be expected; in particular, it is shown that multiple ice lenses cannot form unless the soil thermal diffusivity is greater than the water diffusion coefficient. Analysis of a simple one-dimensional case (a semi-infinite mass of homogeneous soil whose surface temperature is suddenly lowered) gives the temperature and water-content fields as functions of time; these agree with those observed in an experimental study of freezing clay.