Showing papers on "Hydraulic conductivity published in 1976"

A new model for predicting the hydraulic conductivity of unsaturated porous media

Abstract: A simple analytic model is proposed which predicts the unsaturated hydraulic conductivity curves by using the moisture content-capillary head curve and the measured value of the hydraulic conductivity at saturation. It is similar to the Childs and Collis-George (1950) model but uses a modified assumption concerning the hydraulic conductivity of the pore sequence in order to take into account the effect of the larger pore section. A computational method is derived for the determination of the residual water content and for the extrapolation of the water content-capillary head curve as measured in a limited range. The proposed model is compared with the existing practical models of Averjanov (1950), Wyllie and Gardner (1958), and Millington and Quirk (1961) on the basis of the measured data of 45 soils. It seems that the new model is in better agreement with observations.

A slug test for determining hydraulic conductivity of unconfined aquifers with completely or partially penetrating wells

Abstract: ~. help ed. steady Proc. With the slug test the hydraulic conductivity or transmissibility of an aquifer is determined from the rate of rise of the water level in a well after a certain volume or 'slug' of water is suddenly removed from the welL. The slug test is simpler and quicker than the Theis pumping test because observation wells and pumping the well are not needed. With the slug test the portion of the aquifer 'sampled' for hydraulic conductivity is smaller than that for the pumping test even though with the latter, most of the head loss also occurs within a relatively small distance of the pumped well and the resulting transmissibility primarily reflects the aquifer conditions near the pumped welL. Essentially instantaneous lowering of the water level in a well can be achieved by quickly removing water with a bailer or by partially or completely submerging an object in the water, letting the water level reach equilibrium, and then quickly removing the object. If the aquifer is very permeable, the water level in the well may rise very rapidly. Such rapid rises can be measured with sensitive pressure transducers and fast-response strip chart recorders or x-y plotters. Also it may be possible to isolate portions of the perforated or screened section of the well with special packers for the slug test. This not only reduces the inflow and hence the rate of rise of the water level in the well, but it also makes it possible to determine the vertical distribution of the hydraulic conductivity. Special packer techniques may have to be developed to obtain a good seal, especially for rough casings or perforations. Effective sealing may be achieved with relatively long sections of inflatable stoppers or tubing. The use of long sections of these materials would also reduce leakage flow from the rest of the well to the isolated section between packers. This flow can occur through gravel envelopes or other permeable zones surrounding the casing. Sections of inflatable tubing may have to be long enough to block off the entire part of the well not used for the slug test. High inflation pressures should be used to minimize volume changes in the tubing due to changing water pressures in the isolated section when the head is lowered. So far, solutions for the slug test have been developed only for completely penetrating wells in confined aquifers. Cooper et at. (1967) derived an equation for the rise or fall of the water level in a well after sudden lowering or raising, respectively. Their equation was based on nonsteady flow to a pumped,

Hydraulic conductivity in frozen soils

Abstract: An apparatus has been developed for investigation of hydraulic conductivity of frozen soils. The test procedure is isothermal and involves the passage of water from one reservoir into the frozen sample and out of the frozen sample into a second reservoir. The water in the reservoirs remains unfrozen because it contains dissolved lactose. The concentration of lactose is such that, initially, the water in the reservoirs is in thermodynamic equilibrium with the water in the soil. On application of pressure to one reservoir a known hydraulic gradient is established and flow takes place. Flow is shown to vary linearly with hydraulic gradient. The hydraulic conductivity coefficient depends on soil type and temperature and is related to the unfrozen water content. At temperatures within a few tenths of 0°C the coefficient apparently ranges from 10−5 to 10−9 cm sec−1, and decreases only slowly below about −0·5°C. Soils known to be susceptible to frost heave are shown to have significant hydraulic conductivities well below 0°C.

A New Model for Predicting the Hydraulic Conductivity

Abstract: A simple analytic model is proposed which predicts the unsaturated hydraulic conductivity curves by using the moisture content-capillary head curve and the measured value of the hydraulic conductivity at saturation. It is similar to the Childs and Coilis-George (1950) model but uses a modified assumption concerning the hydraulic conductivity of the pore sequence in order to take into'account the effect of the larger pore' section. A computational method is derived for the determination of the residual water content and for the extrapolation of the water content-capillary head curve as measured in a limited range. The proposed model is compared with the existing practical models of Averjanov (1950), Wyllie and Gardner (1958), and Millington and Quirk (1961) on the basis of the measured data of 45 soils. It seems that the new model is in better agreement with observations.

One‐dimensional simulation of aquifer system compaction near Pixley, California: 2. Stress‐Dependent Parameters

Abstract: A major problem facing hydrologists is how to predict land subsidence. A key to this problem is the development of a reliable method for evaluating aquitard parameters. For assumed values of hydraulic conductivity and storage, vertical compaction and expansion of idealized aquitards can be computed (predicted) by an appropriate diffusion equation from known (projected) water level changes in adjacent aquifers. If water levels within the aquifers are observed and the resulting field compaction and expansion are measured, the parameters themselves can be evaluated. Such field measurements are available at a site near Pixley, California, for the composite behavior of a series of 21 doubly draining aquitards. By means of a linear partial differential equation with constant coefficients within one digital model, average hydraulic conductivity for idealized aquitards was evaluated from the field data to be 3 × 10−3 ft yr−1 (2.9 × 10−9 cm s−1), and average nonrecoverable specific storage was evaluated to be 2.3 × 10−4 ft−1 (7.5 × 10−4m−1). A second model allows parameters of nonrecoverable compaction to be stress dependent by assuming for any single material that the product of hydraulic conductivity and an incremental effective stress is a constant and that the product of nonrecoverable specific storage and past maximum effective stress is a constant. The latter assumption is a standard simplification of laboratory consolidation data; the former is introduced in the present paper. This second model improved simulation of observed compaction and expansion severalfold with virtually no increase in computer time. By using a double transformation of applied stress within the second model to linearize the nonlinear partial differential equation, hydraulic conductivity was evaluated to decrease by more than an order of magnitude during 12 years of record from 3.4 × 10−3 ft yr−1 (3.3 × 10−9 cm s−1) near the midplane of an idealized aquitard to 3.0 × 10−4 ft yr−1 (2.9 × 10−10 cm s−1) near the drainage faces of the idealized aquitard. Average nonrecoverable specific storage was evaluated to decrease from 2.3 × 10−4 ft−1 to 1.9 × 10−4 ft−1 (7.6 × 10−4 m−1 to 6.2 × 10−4 −1). In both models the average recoverable specific storage was evaluated to be 4.6 × 10−6 ft−1 (1.5 × 10−5 m−1).

Relative Importance of Soil Resistance and Plant Resistance in Root Water Absorption

Abstract: Soil water potentials, leaf water potentials, and transpiration rates of sweet corn (Zea mays L.), growing in a greenhouse, and grain sorghum (Sorghum bicolor L.), growing in a field, were determined to evaluate the magnitude of the combined soil and plant resistances to water flow in the plant system. Using a theoretical analysis of water absorption by roots, soil resistance was estimated. Plant resistance was inferred by the difference between the measured combined resistances and the estimated soil resistance. A wide range of soil hydraulic conductivity values for the plant rooting media which included nutrient solutions, peat-vermiculite-sand mixture, and a sand and clay soil maintained at various water potentials provided variations in calculated soil resistances. Our results showed that when root density was not unusually low, plant resistance to water transport was much larger than soil resistance, until the threshold soil hydraulic conductivity reached about 10-6 to 10-7 cm/day. This conductivity usually occurred at about -1 and -8 bars for the sandy and clay soils, respectively. These findings emphasize the need to consider plant resistance in water-uptake calculations when using equations that evaluate water potential gradients along the water flow path.

Infiltration and water movement in an in situ swelling soil during prolonged ponding

Abstract: Infiltration, swelling, and water movement were studied during ponding on a swelling clay soil. The soil was uniform in texture and clay mineralogy to 2 m depth. Most structural heterogeneity, caused by gilgai and shrinkage cracks, had been removed by grading, cultivation, and pre-ponding irrigations. Measurements were made of infiltration, moisture content, soil water potential, hydraulic conductivity, bulk density, vertical soil swelling, and the effect of overburden on tensiometer readings. Infiltration was rapid and water penetrated deeply during the first ponding day. Thereafter, qualitative agreement was found between measured infiltration and that expected from theory from 1 to 45 days after ponding. From 45 to 120 days after ponding, the development of a time-variable flow restriction near the soil surface prevented the attainment of a final, steady infiltration rate. During ponding a transient water table developed, moisture profiles were distinctly hydric, and seepage to a deep water table or aquifer was not negligible. Core sample values of hydraulic conductivity agreed with those obtained from mean flux and potential gradients, although conductivity and infiltration rate varied greatly from place to place. Measured swelling compared favourably with that calculated from bulk density changes. The maximum measured soil swelling, in the rather narrow range of moisture contents involved, was 25 mm. This is consistent with reported data on similar soils. Mean values of a = U/P near saturation at 0.2 and 0.4 m depth were between 0.20 and 0.25, indicating that the effect of overburden potential on flow was not large.

Variability of Hydraulic Conductivity in Two Subsurface Horizons of Two Silt Loam Soils

Abstract: Hydraulic conductivity was measured in the B22t and B31t horizons of 12 pedons of two soils developed in loess deposits overlying glacial till. Conductivity measurements were made with the crust test technique for unsaturated conditions and with a new related in situ method for saturated conditions. Nonlinear regression yielded simple well-fitting curves of the form log K = log(bψ⁻ᶜ). Variability within and between major horizons in these soil series was found to be relatively low. The four horizons in these two silt loam soils had statistically identical hydraulic conductivity characteristics, even though morphological soil structure and soil genesis differed significantly.

Spatial variability of in situ unsaturated hydraulic conductivity of maddock sandy loam

Abstract: Unsaturated soil hydraulic conductivity (K) versus depth was measured in situ in five infiltration plots within a 0.01 hectare area on a soil developed from lacustrine materials in a glacial lake bed. Sufficient water was allowed to infiltrate each plot to wet the soil profile to 152 cm. The soil surface was covered to prevent evaporation and during the ensuing drainage period, soil water pressure was monitored with triplicate tensiometers at each depth of 15, 30, 45, 61, 91, 122, and 152 cm. Soil water characteristic data determined on triplicate cores taken from the same depth as the tensiometer cups, were used in conjunction with the soil water pressure head data to compute the hydraulic conductivity. Significant spatial variability of hydraulic conductivity at the 1 percent level was found. In addition, K significantly varied with depth, at the 1 percent level, generally increasing, due to the heterogeneous nature of soil in the vertical direction. Hydraulic conductivity as a function of porosity was computed for each site by the modified Green and Corey method. Agreement of these theoretical K values with those measured in situ depended on the soil water content at which the matching factor was selected. Agreement between methods was best when the matching factor was selected at the lowest water content at which K was measured in the field for that particular soil depth.

Infiltration of Water on a Cattle Feedlot

Abstract: j NFILTRATION into a feed-x lot surface could not be mea-sured on site because of extremely low infiltration rates. Four undisturbed soil cores from an active cattle feed-lot, encased in heat-shrink plas-tic, were used in the laboratory to measure infiltration. Six undisturbed feedlot cores containing the manure surface, interface layer (2 cm manure and top of soil) and soil below were cut into sections 10 cm long. Air permeability, hydraulic conduc-tivity, air-water permeability ratio, and bulk density were measured on each section. The interface layer of manure and soil develops as a result of hoof action and manure cover. It is the most restrictive layer to water and air movement in the feedlot soil profile. The hydraulic conductivity increased 28-fold from the interface layer to the next layer and was about the same as the comparable depth in adjacent cropland.

Determining the Hydraulic Conductivity of Soil Cores by Centrifugation

Abstract: Two centrifugal techniques are proposed for determining the hydraulic conductivity of cores of natural soil. Experimental results are presented for one technique in which the change in weight of one end of the sample, previously centrifuged, is measured with a balance. The mathematical equations describing this redistribution process were developed and fitted to the data to ascertain the soil water diffusivity D. The value of the hydraulic conductivity K was obtained from K = bD, where b is also calculated. Calculated values of K agreed with previously published values. The second technique for which a theory is presented but no experimental values are provided depends upon the measurement of the volumetric outflow of water from a soil core when the speed of centrifugation is suddenly increased.

On the stochastic foundations of the theory of water flow through unsaturated soil

Abstract: The parabolic differential equation that describes the isothermal isohaline transport of water through an unsaturated soil is shown to be the mathematically rigorous result of a fundamental stochastic hypothesis: that the trajectory of a water molecule is a nonhomogeneous Markov process characterized by space- and time-dependent coefficients of drift and diffusion. The demonstration is valid in general for heterogeneous anisotropic soils and provides for three principal results in the theory of water flow through unsaturated media: (1) a derivation of the Buckingham-Darcy flux law that does not rely directly on experiment, (2) a new theoretical interpretation of the soil water diffusivity and the hydraulic conductivity in molecular terms, and (3) a proof that the soil water diffusivity for anisotropic soil is a symmetric tensor of the second rank. A dynamic argument at the molecular level is developed to show that the fundamental Markovian hypothesis is physically reasonable in the case of water movement through an unsaturated soil.

A model describing soil-plant-water relations for potatoes

Abstract: A simple steady state model is derived which describes the diurnal water potential fluctuations in leaves and tubers of potatoes. The magnitude of these fluctuations is shown to depend on transpiration rate, hydraulic properties of the soil, rooting depth and density, resistance to flow of water within the plant, and the leaf water potential at which stomatal closure occurs. Model predictions agree quite well with measurements made in the field and in the growth chamber. The model is used to predict the lower limit of readily available moisture for potatoes and shows the important environmental and plant factors.

Determining both Water Characteristics and Hydraulic Conductivity of a Soil Core at High Water Contents from a Transient Flow Experiment

Abstract: A method of calculating both the water characteristics and hydraulic conductivity of a short soil core at high water contents from a single transient flow experiment is described and evaluated. The method is based on measurements of cumulative inflow or outflow of the water entering or leavi

Pumping of Aquifer with Viscoelastic Properties

Abstract: The land subsidence and the piezometric drawdown resulting from fluid withdrawal from a confined aquifer system are analyzed. The aquifer material is assumed to behave as a viscoelastic Merchant body, representing both primary and secondary consolidation. The resulting ground-water flow equation is solved for the problem of unsteady radial flow in an extensive confined aquifer of uniform thickness to a single fully penetrating well, pumped at a steady rate of flow. Comparison of the result for the special case of an elastic aquifer with the Theis solution indicates that for most practical situations the obtained solution should yield good results. The parameters of the model which are the hydraulic conductivity, the compressibilities of primary and secondary consolidation, respectively, and the viscosity of the secondary consolidation, can be derived from records of piezometric drawdown and surface compaction.

Hydrogeology of the South Fork of Long Island, New York

Abstract: The South Fork of Long Island, New York, is underlain by unconsolidated Pleistocene and Cretaceous sediments resting on crystalline bedrock. A two-layered aquifer system contains fresh ground water with saline ground water in the deeper strata. The average horizontal hydraulic conductivity of the upper aquifer is 49 m/day and of the lower aquifer is 25 m/day. The average annual precipitation of 1.14 m is the only natural source of fresh water. After consumptive losses the precipitation provides about 1.85 × 10 8 m 3 /yr to recharge the water table. Discharge of fresh ground water occurs primarily as undersea outflow to the ocean at the perimeter of the area. The safe yield of the area is estimated to be 9.15 × 10 4 m 3 /day.

Effect of River Sand on the Permeability of a Sodic Soil

Abstract: The effect of river sand on the permeability of a highly sodic soil has been studied under laboratory and field conditions. A sharp decrease in hydraulic conductivity occurred with time where sand and soil were mixed in the ratio of 175: 25. With further increase in the amount of soil, there appeared not much difference in hydraulic conductivity and was nearly constant at 0.1 × 10−2 cm/hr, which was comparable to the treatment containing soil alone. Covering the soil with 1, 2 and 3 cm thick sand layer increased the saturated hydraulic conductivity by 48, 63 and 97 times, respectively, whereas mixing of sand in different amounts in the upper 7.5 cm soil layer did not alter the permeability characteristics compared to the control. At 168 hours the cumulative intake in the treatments where sand was mixed at the rates of 0, 9 and 18 kg/m2 was 10.86, 11.48 and 10.72 cm, respectively. When different depths of sand layers were surface applied, the cumulative intake in 191 hours was 9.35, 12.12, 12.06 and 12.58 cm under control, 2, 4 and 6 cm sand layers, respectively. It is discussed that the clay displacement and subsequent clogging of the pores created by sand additions possibly resulted in reduced water flux, whereas, the surface application of sand prevented the formation of crust just below the sand layer resulting in higher water intake by the soil.

Variable thickness transient groundwater flow model theory and numerical implementation. [Radionuclide migration in ground water at Hanford]

Abstract: Modeling of radionuclide movement in the groundwater system beneath the Hanford Reservation requires mathematical simulation of the two-dimensional flow in the unconfined aquifer. This was accomplished using the nonlinear, transient Boussinesq equation with appropriate initial and boundary conditions, including measured Columbia River stages and rates of wastewater disposal to the ground. The heterogeneous permeability (hydraulic conductivity) distribution was derived by solution of the Boussinesq equation along instantaneous streamtubes of flow employing a measured water table surface and a limited number of field-measured hydraulic conductivity values. Use of a successive line over-relaxation technique with unequal time steps resulted in a more rapid convergence of the numerical solution than with previous techniques. The model was used to simulate the water table changes for the period 1968 through 1973 using known inputs and boundary conditions. A comparison of calculated and measured water table elevations was made at specific well locations and the quality of the verification simulation was evaluated using a data retrieval and display system. Agreement between the model results and measured data was good over two-thirds of the Hanford Reservation. The capability of the model to simulate flow with time-varying boundary conditions, complex boundary shapes, and a heterogeneous distribution of aquifermore » properties was demonstrated.« less

A Markovian Stochastic Basis for the Transport of Water Through Unsaturated Soil

Abstract: The differential equation that describes the isothermal, isohaline transport of water through a homogeneous, isotropic, unsaturated soil is shown to result from a fundamental stochastic hypothesis: that the trajectory of a water molecule is a nonhomogeneous Markov process characterized by space- and time-dependent coefficients of drift and diffusion. The demonstration makes possible a new theoretical interpretation of the water diffusivity and the hydraulic conductivity at the molecular level and results in a derivation of the Buckingham-Darcy flux law that does not rely directly on experiment.

Movement of moisture in the unsaturated zone in a loess-mantled area, southwestern Kansas

Abstract: A study of moisture movement associated with four ponding tests in a loess-mantled area near Garden City, Kans., provides significant information on the potential of using the area for artificial recharge by water spreading. Infiltration during the four ponding tests stabilized at rates ranging from 0.7 to 2.2 feet (0.2 to 0.7 meter) per day. The large differences in infiltration rates reflect changes in the hydraulic conductivity of the soil horizons developed in loess materials. The underlying loess has an appreciably greater hydraulic conductivity than the soil. Removing the soil zone should increase infiltration rates, provided that the underlying loess is not severely compacted during excavation or during subsequent recharge operations. When the wetting front of infiltrated water is in the loess, the moisture-buildup pattern shows the characteristic wetting and transmission zones observed for infiltration in homogeneous materials. When the wetting front penetrates the underlying alluvium, the wetting front becomes indistinct. The loess has the capacity to take large quantities of water into temporary storage. If adequate time is allowed between water applications for the loess to drain, the amount may be as much as 1 cubic foot (0.03 cubic meter) of water for each 6 cubic feet (0.17 cubic meter) of the material. At the ponding site, several fine-grained strata are in the unsaturated alluvium underlying the loess. Because these strata have relatively high hydraulic conductivities, they did not act as effective perching beds during the ponding tests. Strata of this type probably would not cause sufficient mounding to impede infiltration significantly or cause water-logged conditions during water spreading. The ground-water mound that developed after application of 21 feet (6 meters) of water has a maximum thickness of 2 feet (0.6 meter) at the edge of the pond. The boundary of the mound moved laterally a distance of 50 feet (15 meters) from the edge of the pond in 2 days. The mound, which dissipated very slowly because of additional drainage from the unsaturated zone, was discernible 3 months after ponding. Although the mound on the saturated zone spread rapidly as a result of pressure transmission, the recharged water actually spread slowly by lateral displacement. Because accumulated salts are leached from the unsaturated zone, the specific conductance of water arriving at the water table is higher than that of the applied water. The amount of increase is dependent on the extent of leaching from previous applications of water and on dilution by infiltrated water. A general appraisal of the ponding-test data indicates that the study area has an excellent potential for artificial recharge by water spreading. Infiltration rates in the loess-mantled area would be favorable for water spreading, and fine-grained strata in the unsaturated zone would not significantly impede downward percolation. Water that is put into underground storage by utilizing the dewatered part of the reservoir would help to sustain the productivity of the aquifer system. INTRODUCTION The extensive use of ground water for irrigation has caused appreciable declines in the water table in several loess-mantled areas in southwestern Kansas because the rate of withdrawal greatly exceeds the rate of replenishment. The life of the alluvial aquifer underlying the loess could be extended by more efficient irrigation practices and by artificial recharge to supplement natural replenishment. Ponding tests in a loess-mantled area have yielded significant information on the potential of these deposits for artificial recharge by water spreading as well as information basic to the development of more efficient irrigation practices. This report describes infiltration rates under various test conditions, buildup of moisture during ponding and the subsequent redistribution after ponding, mounding on perching beds, and effects of ponding on the water table in terms of mounding and water-quality changes. Owing to the homogeneity of the loess materials, the data provide a classic field demonstration of water movement in the unsaturated zone according to theoretical projections. Green, Dabiri, Weinaug, and Prill (1970) have used a computer to simulate moisture changes with depth, as monitored by neutron moisture logs from this study. Excellent agreement between the calculated and the observed data was obtained. Figure 1 shows the study area, 9 mi (14.5 km) northwest of Garden City in Finney County, south­ western Kansas, and the distribution of loess deposits that mantle about 60 percent of the 11-county area that forms southwestern Kansas. Much of the irriga­ tion in southwestern Kansas takes place in the areas mantled by loess. Because soil surveys show the loess deposits of the 11 counties to be similar in physical and chemical properties, the results of this study have application throughout this area. Details of the ponding site and pertinent facilities of the study area are shown in figure 2. The test pond is MOVEMENT OF MOISTURE IN A LOESS-MANTLED AREA, SOUTHWESTERN KANSAS

Zoning Aquifers for Tertiary Treatment of Wastewatera

Abstract: Soils and aquifers can function as effective and economical filter systems for advanced treatment of conventionally treated sewage and other wastewater. The wastewater is applied to the land with low-rate or high-rate infiltration systems. Physical, chemical, and biological processes in the soil improve the quality of the wastewater as it percolates through the vadose zone and into the aquifer to become renovated water. The quality of the renovated water, however, often is not as good as that of the native ground water. To utilize the land for treatment of wastewater, without trading a problem of surface-water pollution for one of ground-water contamination, the spread of renovated water in the aquifer must be restricted. This can be accomplished by locating the system so that the renovated water drains naturally into a stream or other surface water, or by artificially removing renovated water from the aquifer with wells or drains at some distance from the application area. Examples are given of various systems that utilize these principles, and general design criteria are presented. Proper design involves analysis of underground-flow systems for various system geometries. Methods for measuring hydraulic conductivity, particularly in the vadose zone, are briefly reviewed.