Method of minimum flux in saturation layer for calculating stable water infiltration through layered soil
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Based on the principle of soil water dynamics, a new method called the method of minimum flux in saturation layer (MFSL) for calculating the stable infiltration rate from multi-layer soil was proposed. The HYDRUS-1D was applied to simulate a series of infiltration processes with different surface water head, and under different hydraulic properties in the layered soil. The simulation results provide a standard to verify MFSL. The results show that MFSL tend to underestimate the stable infiltration rate, yet mostly less than 5%. Therefore MFSL is a method with clear physical background and less error. Compared with the modified Green-Ampt model for calculating infiltration through layered soil, MFSL does not need to consider the water entry suction in the lower soil layer, and can deal with the complexity of the multi-layer soil characteristics presented in reality. Therefore, MFSL is more suitable for the estimation of stable water infiltration rate in layered soil. It provides a convenient and useful tool for water resources evaluation, environment contamination evaluation, and the related projects design.Keywords:
Infiltration (HVAC)
Pressure head
Saturation (graph theory)
Hydraulic head
Soil horizon
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Preferential flow is common in clay or expansive clay soils, involving water bypassing a large portion of the soil matrix. Dye tracer experiment and numerical modeling are used to simulate the surface runoff and subsurface preferential flow patterns influenced by the soil fracture network of a relatively steep hillslope system (slope angle equals to 10 degrees). The result of the experiments indicates that part of the water is infiltrated through cracks, leading to the delay of the initial runoff-yielding time and reduction of the discharge of the surface runoff. The soil water flow is mainly in the matrix when the intensity of precipitation is low. With the increasing of precipitation, soil water movement may become in the form of preferential flow through cracks. In addition, the nonuniformity of soil water infiltration and the depth of the average water infiltration increase as the precipitation intensity increases. To this end, the complete coupling model was established by using the surface-matrix-crack (SMC) model to simulate water flow within discrete fracture as well as to simulate water flow in the soil matrix based on the concept of dual permeability using the traditional Richards’ equation. In this model, the “cubic law” of fluid motion in cracks within smooth parallel plates and the two-dimensional diffusion wave approximation to Saint-Venant equations with momentum term ignored (two-dimensional shallow water equations) were used. The model divides soil water infiltration into two forms and uses the overall method to calculate the exchange of water between the crack networks and matrix regions as well as the exchange water between surface runoff and infiltration water. Results indicate that the SMC model has better performance compared with the traditional equivalent continuum model when those models are used to simulate the surface runoff movement and the soil water movement in the presence of cracks.
Infiltration (HVAC)
Richards equation
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Citations (13)
Water movement in the unsaturated zone is an important hydrological process. Richard’s equation is windily used to describe both soil water infiltration and soil water absorption. Various methods have been developed to solve Richard’s equation. Wang et al. (2003) have developed an algebraic model for the description of soil water infiltration, based on Parlange’s solution of Richard’s equation and on soil retention curve and hydraulic conductivity equation given by Brooks and Corey. Their model utilizes experimental measurements of the cumulative infiltration volume and the wetting front distance as functions of time in order to describe soil water infiltration. The objective of this paper is to test the accuracy of the Wang et al. algebraic model for the one-dimensional (vertical) soil water infiltration. A vertical infiltration experiment was conducted on a sandy soil, for the measurement of the cumulative infiltration volume and the wetting front distance. Soil water content was determined at selected times and positions, using gamma ray absorption. Additionally the hydraulic conductivity K(θ) and the soil retention curve Ψ(θ) were determined. The algebraic model developed by Wang et al., was found simple to use since the required data are the cumulative infiltration (F), the wetting front distance (zf) and the initial and saturated soil water content (θi and θs respectively). The results show a fair agreement between calculated and measured values on soil water content profiles, hydraulic conductivity and on the water mass balance.
Infiltration (HVAC)
Water retention curve
Richards equation
Water balance
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Steady infiltration from surface point sources into two-layered cylindrically confined and unconfined soil regions with roots extracting water is analyzed with a linearized form of the Richards equation. In the upper layer, from which plant roots withdraw water with a uniform extraction coefficient (λ), the hydraulic conductivity at saturation decreases (−1 ≤ β < 0) or increases (β > 0) exponentially with depth. The lower soil layer is homogeneous, and the soil texture coefficient α is the same for both layers. The evaporation loss is taken to be proportional to the matric flux potential (MFP) at the soil surface, with a proportionality coefficient m. It is shown that the point-source solutions for the modeled systems are physically relevant, i.e., consistent with an exponential hydraulic conductivity function, not only for positive but also for negative β if λ ≥ 0.5α2(1 − β). Also, the fractions of water lost via evaporation at the soil surface, extraction by plant roots, and deep percolation are found to be identical for both laterally confined and unconfined regions. Numerical examples illustrate the behavior of the solutions for various combinations of the coefficients λ, β and m, the root-zone depth, and the lateral extent of the soil domain. The water uptake rate (qup) increases with increasing λ and root-zone depth and the wetted regions contract as the water uptake increases. Without evaporation, the water uptake rate decreases in the order qup (β < 0) > qup (β = 0) > qup (β > 0). Evaporation losses significantly reduce both the water uptake rate and the extent of wetting. For β < 0 the effect of evaporation on qup is larger than for β ≥ 0.
Infiltration (HVAC)
Richards equation
Saturation (graph theory)
Water retention curve
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One‐dimensional, vertical infiltration of water through a crust of constant nonzero hydraulic resistance is theoretically examined in two ways, a numerical solution and a similarity reduction of the problem for early to intermediate times. The soil‐water content at the crust‐soil interface increases with time to approach a predictable final steady water content asymptotically. A greater proportion of the flow takes place at intermediate to low water contents, and without an appreciable effect of gravity, as the crust resistance increases. For the water content at the crust‐soil interface, the cumulative infiltration, and the wet front progress, simple expressions arise from the similarity reduction analysis, which is based on specific functional forms of the soil‐water diffusivity and suction head, and a relatively small initial soil‐water content. For early to intermediate times of flow, the similarity analysis describes adequately the calculated numerical solution flow data for Yolo soil. For intermediate to large times, a Green and Ampt type solution is linked to the similarity expression for cumulative infiltration with fair success.
Infiltration (HVAC)
Hydraulic head
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Citations (23)
Richards equation
Pressure head
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Citations (6)
Infiltration (HVAC)
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Citations (9)
The problem of infiltration at constant flux at the soil surface has been solved approximately in an analytical closed form. The solutions may be valuable to the practicing engineer when dealing with sprinkler irrigation or infiltration of rain. Infiltration into a semi‐infinite soil column as well as infiltration into a soil column of finite length with a constant water table were considered. Analytical and numerical results were compared for a few cases. It was found that the analytical solutions provide quite a satisfactory prediction of the moisture content at the soil surface and the advance of the wetting front. The results are presented in a dimensionless form; the analytical results are the same for all soils, and apparently the numerical results are also the same for all soils.
Infiltration (HVAC)
Dimensionless quantity
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Citations (66)
In the northwest of China, soil structure with sand interlayer is a common soil configuration. For soil structure with sand interlayer, infiltration rate will turn to steady infiltration rate when wetting front passes through the upper interface of sand layer. Normally, in the traditional Green-Ampt infiltration model, the water entry suction is difficult to determine. Furthermore, due to the hydraulic conductivity of wetting pattern is assumed to the saturated hydraulic conductivity of upper soil in the model, the error exists when calculating the steady infiltration rate of soil with sand interlayer. In order to compute the steady infiltration rate of the soil with sand interlayer accurately and conveniently, hydraulic conductivity coefficient, which was less than 1, was introduced to quantify the extent of water conductivity for upper layer soil on the basis of Green-Ampt infiltration model. More importantly, under the influence of various factors, the improved Green-Ampt infiltration model for soil with sand interlayer was set up, which included the undetermined parameters to the hydraulic conductivity coefficient and water entry suction. With the HYDRUS-1D software, the procedure of steady infiltration under various factors including soil texture, soil initial water content, pressure head, sand depth and sand thickness was simulated. At the same time, the law of infiltration and its influencing factors were analyzed according to the simulation results. The results showed that steady infiltration rate was mostly influenced by soil texture, pressure head and sand depth, but the steady infiltration rate of soil with sand interlayer was not affected by the soil initial water content and the thickness of sand interlayer. In view of the above, the steady infiltration rate could be acquired through simulation by changing sand depth while remaining pressure head, initial water content and sand thickness constant. Then the values of hydraulic conductivity coefficient and water entry suction were determined by fitting with the improved Green-Ampt infiltration model. The determination coefficient of fitting curve was larger than 0.99. In addition, the value of hydraulic conductivity coefficient changed in the range of 0.91 to 0.99. The average value of 0.95 was adopted in order to simplify the calculation. After that, the determined hydraulic conductivity coefficient was put into the improved Green-Ampt infiltration model. The improved Green-Ampt infiltration model was used further to match, with the purpose to determine the water entry suction. It was found that water entry suction was inversely proportional to the reciprocal of air entry value. Therefore, according the reciprocal of air entry value, which was one of the soil physical characteristic parameters, the calculation formula of water entry suction by estimation was put forward. In order to verify the reliability and universality of the improved model in this research, the water infiltration experiments of the soil with sand interlayer in Qinwangchuan area were conducted. The model was verified both by the experimental results and the available literature data. The improved model was proved to be better than the conventional calculation method for steady infiltration rate because the undetermined parameters were less, the solution for parameters was simpler, and the practicability was better. Simultaneously, the error was basically within 5%. In this paper, the developed model relied on the saturated hydraulic conductivity of upper soil and the reciprocal of air entry value of van Genuchten-Mualem model of sand layer soil. And the two parameters were the basic parameters of soil hydraulic properties, which could be obtained from the simple laboratory tests. Therefore, this model can provide valuble information for agricultural water management and engineering seepage controlling technique.
Infiltration (HVAC)
Soil gradation
Pressure head
Soil texture
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Citations (7)
Simple recurrent infiltration-outflow experiments, together with dye tracing, show deviations from the theoretical assumptions on which classical soil-water flow modelling is based. The experimental set-up represents a well-defined system which enables the study of the dynamic character of soil properties expressed in the Richard's-equation approach by soil hydraulic functions. The method may offer the possibility to develop an efficient classification of soils according to their dynamic behaviour. Results from four soils are presented.
Outflow
Infiltration (HVAC)
Tracing
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Citations (11)
A simple model of ponded infiltration into homogenous soils bounded by, and at steady state equilibrium with, a water table is presented. The model parameterizes the wetting front profile as a tension‐saturated zone above a zone in which the moisture content decreases linearly with depth. Application of the model requires numerical evaluation of an integral relating the time since the onset of ponding to the infiltration rate or wetting front position. Wetting fronts predicted by the simplified model compare favorably with laboratory measurements of Vachaud and Thony (1971) and, for a wide range of soil types, with those predicted by finite‐element integration of the governing flow equations. The presence of a shallow water table causes the infiltration rate to decrease faster than it would under semi‐infinite conditions with uniform initial moisture content. Infiltration equations for soils bounded by water table may be used in distributed hydrologie models and other applications where the shallow position of the water table significantly affects the partitioning of precipitation forcing during storms.
Infiltration (HVAC)
Ponding
Richards equation
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Citations (26)