Showing papers on "Hydraulic conductivity published in 1984"

Response of soils to sodic and saline conditions

Abstract: The interaction between soil particles and the soil solution depends on the types and amounts of soil clays. Montmorillonite is the most reactive clay. Ca-clay forms tactoids (quasi crystals) and has limited swelling and dispersion. Na-clay forms single platelets and disperses freely. In a mixed Na/Ca system, “demixing” of the cations occurs as the Na ions concentrate on the external and the Ca ions on the internal surfaces of the tactoids. The demixing explains why a small percentage of exchangeable Na sharply increases the zeta potential and dispersion of montmorillonite clay. The hydraulic conductivity of a soil depends on both Na and the total salt concentration of the percolating solution. High hydraulic conductivity may be maintained, even at high exchangeable sodium percentage (ESP) values, if the solution concentration is above a critical (threshold) level. When waters of very low salt content are used, decreases in hydraulic conductivity and clay dispersion occur even in soils with low ESP values (<10). The main mechanism for hydraulic conductivity reduction in waters of medium to high salinity (replacecodegt0.5 dS/m) is clay swelling. High content of expansible clays increases the susceptibility of soils to intermediate (10

Effect of pH on Saturated Hydraulic Conductivity and Soil Dispersion

Abstract: The adverse effects of exchangeable sodium on soil hydraulic conductivity (K) are well known, but at present only sodicity and total electrolyte concentration are used in evaluating irrigation water suitability. In arid areas, high sodicity is often associated with high dissolved carbonate and thus high pH, but in humid areas high sodicity may be associated with low pH. To evaluate the effect of pH (as an independent variable) on A", solutions with the same SAR and electrolyte level were prepared at pH 6, 7, 8, and 9. Saturated A' values were determined at constant flux in columns packed at a bulk density of 1.5 Mg m'. At pH 9, saturated K values were lower than at pH 6 for a montmorillonitic and a kaolinitic soil. For a vermiculitic soil with lower organic carbon and higher silt content, pH changes did not cause large K differences. Decreases in A" were 1 Contribution from the U.S. Salinity Laboratory, USDA-ARS, U.S. Salinity Laboratory, Riverside, CA 92501. Received 16 Dec. 1982. Approved 9 Sept. 1983. 2 Geochemist, Soil Scientists, and Plant Physiologist, respectively. The permanent address of R. Lavado is Institute de Geomorfologia y Suelos, La Plata, Argentina. not reversible on application of waters with higher electrolyte levels. The results from the K experiments were generally consistent with optical transmission measurements of dispersion. Although anion adsorption was at or below detection limits and cation exchange capacity (CEC) was only slightly dependent on pH, differences in pH effects on A" among soils are likely due to differences in quantities of variable-charge minerals and organic matter. Additional Index Words: sodium adsorption rate, electrolyte concentration, optical transmission. Suarez, D.L., J.D. Rhoades, R. Lavado, and CM. Grieve. 1984. Effect of pH on saturated hydraulic conductivity and soil dispersion. Soil Sci. Soc. Am. J. 48:50-55. T ADVERSE EFFECTS of high levels of exchangeable sodium on the hydraulic conductivity (K) of soils are well established. The onset of reduced K at a given soil exchangeable sodium percentage (ESP) SUAREZ ET AL.: EFFECT OF pH ON SATURATED HYDRAULIC CONDUCTIVITY AND SOIL DISPERSION 51 varies with total electrolyte concentration and soil properties (Quirk and Schofield, 1955; McNeal and Coleman, 1966; Frenkel et al., 1978; Shainberg et al., 198la). Despite these and numerous other studies, the exact levels of ESP and electrolyte concentration at which reductions in K will occur for a given soil still cannot be predicted accurately. In published laboratory column studies, neutral or slightly acidic chloride salt solutions (pH =; 6.0) were normally used with a carbon dioxide partial pressure (Pco2) near atmospheric (10"kPa). The pH values of water in the columns were likely =: 6.0 in the upper portion and 7.5 to 8.0 in the lower portions when CaCO3 was present. Arid and semiarid soils are usually calcareous in the subsurface. Sodic soils in this environment are generally associated with high pH and high dissolved carbonate and bicarbonate concentrations. With such soils, pH values can exceed 10 but are more likely in the range of 8 to 9.5 near the surface. Sodic soils may occur also under acid conditions (pH 1.3 L of each solution was passed through the column. The pH 6 solutions were maintained at pH 6 by bubbling CO2 gas into the 5-L Pyrex solution reservoir. An exit tube at the bottom of the reservoir was placed at the same height as the peristaltic pump to eliminate CO2 degassing during the pumping operation. The pH of the leachate was measured in the U tubes before degassing could occur. Influent and effluent solutions were analyzed for Ca, Mg, Na, and K by atomic absorption, alkalinity by acid titration, and Cl by AgCl titration (Rhoades and Clark, 1978). At the conclusion of the experiment, the columns were separated into 1-cm sections and analyzed for exchangeable cations. For purposes of comparison, the K data were scaled to the initial K(K,) values determined with the 100 mmolc L", SAR 20 solution, or 500 mmolc L~, SAR 40 solution. Based on six replications on the Bonsall soil, SAR 40, pH 9 treatment, log K/Kt standard deviations of 0.08, 0.18, 0.19, and 0.11 were determined for solutions of 250, 100, 50, and 25 mmol,; L", respectively.

Predicting Hydraulic Conductivity of Clay Liners

Abstract: Data are presented from four projects in which rates of leakage from ponds lined with clay significantly exceeded the rates that would have been predicted on the basis of laboratory permeability tests. The actual hydraulic conductivities of the clay liners were generally found to be 10 to 1,000 times larger than values obtained from laboratory tests on either undisturbed or recompacted samples of the clay liner. The source of difficulty with laboratory permeability tests is the problem of obtaining a representative sample of soil for testing. Neither recompacted samples nor small, undisturbed samples are likely to contain a representative distribution of desiccation cracks, fissures, slickensides, or other hydraulic defects that may be present in the liner. Field permeability tests were performed for three of the four case histories and yielded results that compared well with field performance. Field permeability tests seem to produce much better results than laboratory tests. The four projects had certai...

Infiltration into a class of vertically non-uniform soils

Abstract: An infiltration model based on the Green-Ampt assumptions is developed for a class of non-uniform soils in which saturated hydraulic conductivity decreases as an exponential function of depth, and the storage-suction factor C = ΔθΔΨ is a constant. An analysis of measured hydraulic conductivity and porosity data suggests that the model may be useful in some soils where the changes of hydraulic characteristics with depth are of this form. A method for using the model with time variable rainfall rates is given. The model is applied to simulate the experimental results of Childs & Bybordi (1969) who measured infiltration into layered sand profiles. Agreement was satisfactory even for this case.

An Analysis of Dispersion in a Stratified Aquifer

Abstract: The dispersion of a conservative solute produced as a result of vertical variations of hydraulic conductivity in a horizontal stratified aquifer of finite thickness is analyzed by applying the moment method of Aris to solve the governing advection-dispersion equation describing mass transport. In the analysis it is assumed that the aquifer is of constant thickness and of infinite lateral extent, the hydraulic conductivity is a known function of the vertical coordinate only, and the flow is unidirectional, parallel to the stratification. The applicable Aris moment equations are developed in a suitable nondimensional form. Analytical solutions are obtained for the zeroth and first moments and for the time derivative of the second moment of the longitudinal concentration distribution for the case of an instantaneous plane source for several idealized hydraulic conductivity profiles (parabolic, linear, step function, and cosine profiles and their even periodic extensions). The analysis gives the time-dependent variation of the longitudinal macrodispersivity for these idealized cases throughout the transient development of the dispersion process. The results of the analysis are applied to a field-measured hydraulic conductivity profile, and predicted values of the longitudinal macrodispersivity are compared with field results. An important conclusion from the analyses is that nonuniformities in the hydraulic conductivity profile which persist over long distances may produce rather large values of longitudinal macrodispersivity which are comparable to those observed in some aquifers and which are much larger than those predicted by some previous stochastic analyses. Implications of the analytical results for field dispersion problems are discussed.

Laboratory Studies of the Effects of the Capillary Fringe on Streamflow Generation

Abstract: This paper describes laboratory experiments that were conducted to investigate the groundwatersurface water interactions during the process of streamflow generation. In particular, the experiments were designed to examine the role of the capillary fringe in the runoff processes and its influence on the groundwater response to precipitation. The experimental results clearly show that if the zone of tension saturation extends to, or near, ground surface, the application of a small amount of water can cause an immediate rise in the water table, and the magnitude of the rise is much greater than would be expected on the basis of normal specific yield values for sandy materials. Because of the sloping surface of the laboratory model the rising water table caused the rapid generation of hydraulic gradients directed towards the toe of the slope (the stream) resulting in the immediate generation of groundwater discharge to the stream. Two experiments using chloride as a tracer and having rainfall rates of 4.3 and 1.9 cm h−1 showed that the discharge of preevent water to the stream preceded event water and that at early times, preevent water was the predominant component of streamflow. In addition, the results of the two tracer experiments show that the significance of preevent water increases with decreasing rainfall rate and that for the 1.9 cm h−1 rainfall rate preevent water was the main component of the stream hydrograph. The proportions of preevent and event water in the streamflow after steady conditions were reached were determined largely by the extent of the seepage face. This in turn would be determined by the hydraulic conductivity of the medium, the surface slope, and the rainfall intensity.

Influence of tillage systems on soil physical parameters and infiltration after planting

Abstract: Simulated rainfall trials were conducted immediately after planting during the third crop year in a continuous-corn rotation of an established tillage experiment designed to evaluate the effects of residue harvest on soil physical properties. Tillage systems were (a) fall moldboard plow, spring disk; (b) fall chisel plow, spring disk; and (c) modified no-till. Each tillage plot was split, and corn stover was harvested each year from the same half of the plots. Commercial harvesting equipment was used to remove about 70% of the above-ground plant material. Soil physical parameters measured at the time of simulated rainfall were (a) random roughness, (b) bulk density, (c) saturated hydraulic conductivity, (d) penetrometer resistance, and (e) pore size distribution. Residue harvest in the first two crop years had no measurable effect on the soil physical properties. The plots differed in bulk density, penetrometer resistance, saturated hydraulic conductivity, and pore size distribution in the Ap horizon, but not in the lower horizons. Artificial rainfall was applied at 5.6 centimeters per hour (2.2 inches) under relatively low energy (55% of natural rainfall energy) for 1 hour and then again for 45 minutes about 24 hours later. Water runoff and soil erosion differences were observed among the tillage treatments. The no-till system produced the greatest runoff regardless of residue harvesting. Soil erosion was greatest with the no-till system when residue had been harvested. The soil physical parameters measured indicated that the no-till system forms an undesirable surface condition characterized by high bulk density, high penetrometer resistance, low saturated hydraulic conductivity, and low volume of macropores, all of which can promote rapid water runoff under normal rainfall conditions.

Impact of Harvesting and Site Preparation on Physical Properties of Piedmont Forest Soils

Abstract: The impact of intensive forest management practices on soil bulk density, aeration porosity, and saturated hydraulic conductivity was examined to a depth of 0.3 m before harvest, after harvest, and after site preparation. Harvesting caused significant changes in soil physical properties to an average depth of 0.17 m in whole tree harvest plots and 0.22 m in skid trail plots. Disking was effective in restoring soil physical properties to preharvest levels in the upper 0.07 to 0.12 m of soil. Soil compaction in chop/burn plots may result in reduced root growth because of mechanical impedance, reduced aeration, or both. 21 references.

Tilled soil subsidence during repeated wetting.

Abstract: A total of 15.2 cm of water was applied without raindrop impact energy to six replicates of four freshly tilled soils to determine its effect on subsidence, random roughness, bulk density, and hydraulic conductivity. Texture of the soils ranged from sandy loam to clay loam. Random roughness degraded to a relatively constant value about 0.5 cm below initial values for all soils except one which was degraded to 1.3 cm below the initial value. Bulk density increased about 0.1 g/cm3 for three soils while the fourth increased 0.2 g/cm3. Regression equations were developed to predict these changes. Coefficients of determination ranged from 0.84 to 0.97 for both random roughness and bulk density. Hydraulic conductivity also decreased with added water. Maximum decreases ranged from 3.0 cm/h to 8.9 cm/h...

Fluid Conductivity Testing of Fine‐Grained Soils

Abstract: Advective fluid flow, which depends directly on hydraulic conductivity, controls contaminant migration through soils in many cases. The state of practice in laboratory measurements of hydraulic conductivity of soils was reviewed and found to be extremely variable, with the result that measurements on a given soil material often give variations of several orders of magnitude. An experimental program was completed using compacted clay soils to study several facets of laboratory testing practice, and a suitable test procedure for consistent results was developed. Recommendations on several important aspects of procedure are presented. The procedure was also evaluated with a synthetic lead‐zinc tailings leachate, and alteration of the test soils' hydraulic conductivity was evaluated and correlated with probable chemical interactions of the test permeants and soils.

In situ estimation of hydraulic conductivity using simplified methods

Abstract: Five simplified methods of estimating the relationship between hydraulic conductivity K and water content θ were compared in this study. Redistribution of water following constant rate infiltration (steady state) was monitored for a 10-day period at 100 locations and seven depths at each location within a 5000-m2 fallow sandy loam field. All the methods assumed a unit hydraulic gradient during redistribution and an exponential relationship between k and θ of the form K(θ) = k0 exp [β(θ − θ0)]. The five methods were the θ, flux, and CGA methods (Libardi et al., 1980) and two methods based on a Lax solution of the Richards’ equation (Sisson et al., 1980). Water content data were used to calculate K0 and β by each method at each depth and location. Soil water flux was estimated for selected depths using appropriate mean and variance values of K0 and β for the field. Relative differences between the methods are briefly discussed.

Evaluation of Slug Tests in Wells Containing a Finite-Thickness Skin

Abstract: The effects of a finite-thickness skin (low-permeability zone surrounding the well bore face) on the response of slug tests is investigated by using a numerical model and a simple analytical solution. The results show that, for skins of finite thicknesses, estimates of hydraulic conductivity provided by slug tests can be more representative of the skin than of the surrounding formation. When a finite-thickness skin is present, the slug test response is shifted along the horizontal axis, making estimates of hydraulic conductivity unreliable. This result is different from that obtained by using an analytical solution (Ramey et al., 1975) for a skin of infinitesimal thickness.

A New System for Ground Water Monitoring

Abstract: This paper describes a new system for ground water monitoring, “the BAT System,” which includes the following functions: (a) sampling of ground water in most types of soils, (b) measurement of pore water pressure, and (c) in situ measurement of hydraulic conductivity. The system can also be used for tracer tests. The system utilizes a permanently installed filter tip attached to a steel or PVC pipe. Installation is normally performed by pushing the tip down to the desired depth. The filter tip can also be buried beneath a landfill. The primary feature of the new system is that the filter tip contains a self-sealing quick coupling unit, which makes it possible to temporarily connect the filter tip to adapters for various functions, e.g. water sampling and for measurement of pore pressure and hydraulic conductivity. The new technique makes sampling of both pressurized water and gas possible. Samples are obtained directly in hermetically sealed, pre-sterilized sample cylinders. Sampling of ground water and measurement of pore pressure can be repeated over a long period of time with undiminished accuracy. This technique is also well-adapted for taking water samples from different strata in a soil profile, in both the saturated and unsaturated zones. Actual installations range from 0.5 to 60m depth.

Evaporation From a Bare Soil Evaluated Using a Soil Water Transfer Model and Remotely Sensed Surface Soil Moisture Data

Abstract: Daily surface soil moisture data, obtained with a C band scatterometer on a bare soil, provide the upper boundary condition for a numerical soil water transfer model. The model uses an implicit finite difference scheme to solve the nonlinear Richards equation. The soil is represented by two layers differing in hydraulic conductivity. Following calibration by comparison of calculated and measured soil moisture profiles, the model provides estimates of the water budget and the boundary fluxes. The estimates of evaporation computed from the model compare favorably to those derived from soil water content measurements by the neutron probe when only the radar data are used to define the surface boundary condition. When known precipitation rates are also used, evaporation is underestimated.

Tillage system and residue cover effects on infiltration in northwestern Corn Belt soils

Abstract: Simulated rainfall trials were conducted in wheel-tracked and nonwheel-tracked interrows for conventional, reduced, and no-till cropping systems immediately after planting during the first and tenth years of continuous corn. Results showed that (a) tractor wheel-tracked interrows were vulnerable to runoff regardless of tillage system and (b) infiltration declined after runoff started. Runoff occurred quickly on the no-till, non-wheel-tracked interrow, indicating the presence of some infiltration restricting factor(s). Bulk density, penetrometer resistance, saturated hydraulic conductivity, and pore-size distribution were measured during the third crop year of continuous corn to determine soil physical properties that developed with tillage systems and the effect these properties had on infiltration. The no-till treatment—modified in this trial by cultivation during the previous growing season to form ridges—was characterized by a higher bulk density, greater penetrometer resistance, reduced saturated hydraulic conductivity, and a lower volume of macropores. The no-till treatment exhibited reduced infiltration. Surface crop residue effectively increased the rainfall energy required to start runoff under the reduced tillage system, but residue had no effect when soil surface conditions (no-till and wheel-tracked interrows) had developed that restricted water movement into the soil.

The reliability of packer tests for estimating the hydraulic conductivity of aquifers

Abstract: Summary This paper investigates the suitability of the double packer testing technique when it is used to estimate how the hydraulic conductivity of an aquifer varies with depth. A mathematical model of the radial and vertical flow from a borehole in a typical aquifer is devised and used to investigate the nature of flow in the aquifer due to the injection of water between two packers. It is shown that the existing formulae usually provide adequate estimates for zones of both high and low hydraulic conductivity. Other features that are considered include the effect of the length of fissures, the presence of boundaries, the possibility of water flowing around the packers, the length and spacing of the packers and the influence of the free water surface.

The relationship between aggregation and other soil properties in cracking clay soils

Abstract: This paper explores the processes responsible for clay dispersion, and the formation of large dry aggregates, in cracking clay soils. It also isolates the soil factors causing variations in dry aggregate size using regression analysis. Twelve cracking clay soil samples were selected on visual differences in dry aggregate size distribution following seedbed preparation, and a range of soil structural and chemical properties were measured. The per cent dry aggregates > 5 mm was found to increase with resistance to mechanical abrasion, stability to wet sieving after capillary wetting, and dispersion ratio, indicating that large dry aggregates are formed as a result of binding by dispersed clay. Both raindrop impact and puddling by cultivation may be involved in dispersion. There was a strong relationship between dry aggregate size in the 0-10 cm layer and salt content in the subsurface (60-90 cm) layer. Coarse surface aggregation is explained in terms of limited profile hydraulic conductivity. For the soils studied, the properties of the surface layer appear to be responsible, at least in part, for the limitation in profile hydraulic conductivity. Dry aggregate size in the 0-10 cm layer was not simply correlated with any of the chemical properties of that layer. However, equations containing two (ESP and CEC per gram of clay) or three (ESP, per cent clay and CEC) independent variables were derived to explain variations in dry aggregate size, both for the 12 soils studied and for a wider range of Queensland cracking clay soils.

Radial Variation in the Axial Conductivity of Populus and its Significance in Heat Pulse Velocity Measurement

Abstract: Conductivity (defined as permeability divided by viscosity) was estimated as a function of radial position in the sapwood at three height levels, for two species of Populus. Stem sections 210-260 mm long and comprising the entire cross section were de-aerated and then infused axially with toluidine blue dye solution until emergence just occurred from the opposite end. Measurements were made of: (i) maximum penetration within each ring, and (ii) axial penetration at 10 mm radial increments starting at the cambium. The corresponding conductivity values were calculated for each measurement, and the values were fitted to a quadratic equation for each stem section. The equations were found to be suitable for calculating stem flux from sap velocities derived from heat pulse velocities measured at different depths in the xylem.

Soil Physical Characteristics of Reduced Tillage in a Wheat-Fallow System

Abstract: SOIL physical characteristics of reduced tillage for fallow-wheat {Triticum aestivum L.) were compared for two soils in western Nebraska. The soil physical environment influences the amount of water that enters soil, the availability of water for plants, and the microenvironment important to soil biological processes. Fallow tillage (plow, subtill, no till) did not affect bulk density in Alliance silt loam (fme silty, mixed mesic, Aridic Argiustolls). For Duroc loam (fme silty, mixed mesic Pachic HaplustoUs), in which native sod was compared to the above three wheat-fallow tillage methods, numerous effects on soil physical properties were found. Reduced tillage reduced bulk density in the 0- to 76-mm layer. Hydraulic conductivity (H.C.) in the surface 300 mm was greater for native sod than for plowed and subtilled soil. Air permeability (ka) was lowest and water permeability (kW) was highest in the native sod. The ratio, ka/kW, an indicator of soil structural stability, showed native sod was most stable and the plowed soil was least stable. Infiltration was least for plowed soil, and, after 4 h, infiltration into subtilled, chemical, and native sod was 2.2, 2.4, and 4.7 times greater, respectivley. Based on precipitation records and infiltration characteristics, water would seldom run off the surface of these soils.

Using soil physical properties to estimate hydraulic conductivity

Abstract: We measured the hydraulic conductivity of a coarse-textured C horizon at 15 sites. The hydraulic conductivity over the soil water potential range of 0 to - 10 kPa was measured in situ with the crust test method, and the data were fit to a simple empirical equation. The resulting regression coefficients were found to be significantly correlated at the 1% confidence level with the percentage of sand, percentage of silt, bulk density, and porosity of the soil. An equation based only on the sand content of the soil was developed that described 83% of the hydraulic conductivity variation. The equation also successfully described the hydraulic conductivity of a soil with similar physical properties, but was inadequate when applied to a fine-textured soil. The technique shows potential of serving as an easy, reliable, and accurate means for estimating soil hydraulic conductivity.

A Geostatistical Approach to Solute Transport in Heterogeneous Fields and Its Applications to Salinity Management

Abstract: In heterogeneous fields in which the soil water properties and the initial salinity vary under a deter- ministic water application rate uniform throughout the field, the salinity during leaching might also differ from place to place. A geostatistical approach was used to investigate the spatial variability of three soil properties: the saturated hydraulic conductivity Ks, the soil characteristic α = d log (K)/d(ψ) (K being the soil hydraulic conductivity and ψ being the pressure potential), and the dispersivity λ, as well as the initial salinity EC0, by using actual measured field data. These properties were used as input parameters of a simplified water and salt flow model, which in turn was coupled with the conditional simulation method to analyze the salinity profile and its spatial distribution during leaching in a 187-ha plot of land. Analysis of the results showed that 107 hours of continuous leaching (6527 m3 ha−1) were required to obtain an average salinity of EC* = 5 dS/m for the field layer between the soil surface and the 40-cm depth. By considering the leaching of the different sites in the field, rather than that of the entire field, it was shown that, theoretically, the amount of water for leaching required to obtain EC* = 5 dS/m uniformly throughout the field can be reduced to 4038 m3 ha−1 (a reduction of 38%). Practically, since the field had to be subdivided into small subplots and because of engineering design requirements for the water supply system, the amount of water for leaching required to obtain the above-mentioned value of EC* for the entire field (5193 m3/ha) could be reduced by only 20%.

Early stage infiltration of water into horizontal and vertical soil columns

Abstract: Water content distributions during horizontal, vertical-up, and vertical-down infiltration into air-dry soil, were measured experimentally using gamma radiation attenuation equipment. A sufficient number of soil columns were analyzed to ascertain fiducial limits of both measured and calculated soil water properties and parameters. The first three coefficient functions of Philip’s solution of the Richards’ flow equation applied to infiltration as well as to the soil hydraulic conductivity and soil water diffusivity as functions of soil water content were ascertained from the measured water content distributions.