T.H. Skaggs

Agricultural Research Service, Kerrville, Texas, United States

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Publications (48)50.62 Total impact

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    T. H. Skaggs, D. L. Suarez, D. L. Corwin
    Vadose Zone Journal 06/2014; · 2.20 Impact Factor
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    ABSTRACT: Subsurface porous clay pipe irrigation is widely considered to be a very promising method for small-scale irrigation in arid regions. Unfortunately, salt accumulation at and near the soil surface using this method may affect germination of direct-seeded crops. Predicting salt movement and accumulation with clay pipe irrigation will allow producers to anticipate the need for leaching to control salinity in the soil root zone. The HYDRUS-2D model was used to simulate the accumulation of salt from a subsurface clay pipe irrigation system, installed at 30 cm depth, during the growing season of okra (Abelmoschus esculentus) irrigated with water having a salinity of 1.1 dS m−1. The loamy soil profile had an initial salinity of 2.3 dS m−1. Predicted electrical conductivity (ECe) values at the end of the growing season correlated significantly (R2 = 0.952) with measured saturated paste ECe data obtained at the end of the field experiments. Salinity was found to be relatively low around the pipes, but increased with distance away from the pipes. Measured and predicted soil salinity levels were especially higher above the clay pipes. Our results indicate that proper management of salt accumulation is vital for sustainable crop production whenever subsurface irrigation systems are being implemented.
    Agricultural Water Management 10/2013; 121:73–80. · 2.20 Impact Factor
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    ABSTRACT: Analytical solutions of the advection-dispersion equation and related models are indispensable for predicting or analyzing contaminant transport processes in streams and rivers, as well as in other surface water bodies. Many useful analytical solutions originated in disciplines other than surface-water hydrology, are scattered across the literature, and not always well known. In this two-part series we provide a discussion of the advection-dispersion equation and related models for predicting concentration distributions as a function of time and distance, and compile in one place a large number of analytical solutions. In the current part 1 we present a series of one-and multi-dimensional solutions of the standard equilibrium advection-dispersion equation with and without terms accounting for zero-order production and first-order decay. The solutions may prove useful for simplified analyses of contaminant transport in surface water, and for mathematical verification of more comprehensive numerical transport models. Part 2 provides solutions for advec-tive-dispersive transport with mass exchange into dead zones, diffusion in hyporheic zones, and consecutive decay chain reactions.
    Journal of Hydrology and Hydromechanics 01/2013; 61:146-160. · 0.65 Impact Factor
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    ABSTRACT: Contaminant transport processes in streams, rivers, and other surface water bodies can be analyzed or predict-ed using the advection-dispersion equation and related transport models. In part 1 of this two-part series we presented a large number of one-and multi-dimensional analytical solutions of the standard equilibrium advection-dispersion equa-tion (ADE) with and without terms accounting for zero-order production and first-order decay. The solutions are extend-ed in the current part 2 to advective-dispersive transport with simultaneous first-order mass exchange between the stream or river and zones with dead water (transient storage models), and to problems involving longitudinal advective-dispersive transport with simultaneous diffusion in fluvial sediments or near-stream subsurface regions comprising a hyporheic zone. Part 2 also provides solutions for one-dimensional advective-dispersive transport of contaminants sub-ject to consecutive decay chain reactions.
    Journal of Hydrology and Hydromechanics 01/2013; 61(3):250-259. · 0.65 Impact Factor
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    ABSTRACT: The advection–dispersion transport equation with first-order decay was solved analytically for multi-layered media using the classic integral transform technique (CITT). The solution procedure used an associated non-self-adjoint advection–diffusion eigenvalue problem that had the same form and coefficients as the original problem. The generalized solution of the eigenvalue problem for any numbers of layers was developed using mathematical induction, establishing recurrence formulas and a transcendental equation for determining the eigenvalues. The orthogonality property of the eigenfunctions was found using an integrating factor that transformed the non-self-adjoint advection–diffusion eigenvalue problem into a purely diffusive, self-adjoint problem. The performance of the closed-form analytical solution was evaluated by solving the advection–dispersion transport equation for two- and five-layer media test cases which have been previously reported in the literature. Additionally, a solution featuring first-order decay was developed. The analytical solution reproduced results from the literature, and it was found that the rate of convergence for the current solution was superior to that of previously published solutions.
    International Journal of Heat and Mass Transfer 01/2013; 56(s 1–2):274–282. · 2.32 Impact Factor
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    ABSTRACT: â–º We measured Br transport in clayey soils packed in large lysimeters. â–º Irrigation water salinity strongly affected Br leaching. â–º Replicated trials offered unique view of transport variability. â–º Predictive, mechanistic modeling of transport in clayey soils remains challenging.
    01/2012;
  • Todd H. Skaggs
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    ABSTRACT: Critical path analysis (CPA) is a method for estimating macroscopic transport coefficients of heterogeneous materials that are highly disordered at the micro-scale. Developed originally to model conduction in semiconductors, numerous researchers have noted that CPA might also have relevance to flow and transport processes in porous media. However, the results of several numerical investigations of critical path analysis on pore network models raise questions about the applicability of CPA to porous media. Among other things, these studies found that (i) in well-connected 3D networks, CPA predictions were inaccurate and became worse when heterogeneity was increased; and (ii) CPA could not fully explain the transport properties of 2D networks. To better understand the applicability of CPA to porous media, we made numerical computations of permeability and electrical conductivity on 2D and 3D networks with differing pore-size distributions and geometries. A new CPA model for the relationship between the permeability and electrical conductivity was found to be in good agreement with numerical data, and to be a significant improvement over a classical CPA model. In sufficiently disordered 3D networks, the new CPA prediction was within ±20% of the true value, and was nearly optimal in terms of minimizing the squared prediction errors across differing network configurations. The agreement of CPA predictions with 2D network computations was similarly good, although 2D networks are in general not well-suited for evaluating CPA. Numerical transport coefficients derived for regular 3D networks of slit-shaped pores were found to be in better agreement with experimental data from rock samples than were coefficients derived for networks of cylindrical pores.
    Advances in Water Resources 10/2011; 34(10):1335–1342. · 2.41 Impact Factor
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    ABSTRACT: Most analytical solutions available for the equations governing the advective–dispersive transport of multiple solutes undergoing sequential first-order decay reactions have been developed for infinite or semi-infinite spatial domains and steady-state boundary conditions. In this study, we present an analytical solution for a finite domain and a time-varying boundary condition. The solution was found using the Classic Integral Transform Technique (CITT) in combination with a filter function having separable space and time dependencies, implementation of the superposition principle, and using an algebraic transformation that changes the advection–dispersion equation for each species into a diffusion equation. The analytical solution was evaluated using a test case from the literature involving a four radionuclide decay chain. Results show that convergence is slower for advection-dominated transport problems. In all cases, the converged results were identical to those obtained with the previous solution for a semi-infinite domain, except near the exit boundary where differences were expected. Among other applications, the new solution should be useful for benchmarking numerical solutions because of the adoption of a finite spatial domain. KeywordsMulti-species transport-Finite domain-Analytical solution-Integral transform
    Transport in Porous Media 04/2010; 85(1):171-188. · 1.55 Impact Factor
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    J S Pérez Guerrero, T H Skaggs
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    ABSTRACT: s u m m a r y Mathematical models describing contaminant transport in heterogeneous porous media are often formu-lated as an advection–dispersion transport equation with distance-dependent transport coefficients. In this work, a general analytical solution is presented for the linear, one-dimensional advection–dispersion equation with distance-dependent coefficients. An integrating factor is employed to obtain a transport equation that has a self-adjoint differential operator, and a solution is found using the generalized inte-gral transform technique (GITT). It is demonstrated that an analytical expression for the integrating factor exists for several transport equation formulations of practical importance in groundwater transport mod-eling. Unlike nearly all solutions available in the literature, the current solution is developed for a finite spatial domain. As an illustration, solutions for the particular case of a linearly increasing dispersivity are developed in detail and results are compared with solutions from the literature. Among other applica-tions, the current analytical solution will be particularly useful for testing or benchmarking numerical transport codes because of the incorporation of a finite spatial domain. Published by Elsevier B.V.
    Journal of Hydrology 01/2010; · 2.96 Impact Factor
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    Vadose Zone Journal 01/2010; 9(2). · 2.20 Impact Factor
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    ABSTRACT: In the irrigated western U.S. disposal of drainage water has become a significant economic and environmental liability. Development of irrigation water management practices that reduce drainage water volumes is essential. One strategy combines restricted drainage outflow (by plugging the drains) with deficit irrigation to maximize shallow groundwater consumption by crops, thus reducing drainage that needs disposal. This approach is not without potential pitfalls; upward movement of groundwater in response to crop water uptake may increase salt and sodium concentrations in the root zone. The purposes for this study were: to observe changes in the spatial and temporal distributions of SAR (sodium adsorption ratio) and salt in a field managed to minimize drainage discharge; to determine if in situ drainage reduction strategy affects SAR distribution in the soil profile; and to identify soil or management factors that can help explain field wide variability. We measured SAR, soil salinity (EC1:1) and soil texture over 3 years in a 60-ha irrigated field on the west side of the San Joaquin Valley, California. At the time we started our measurements, the field was beginning to be managed according to a shallow groundwater/drainage reduction strategy. Soil salinity and SAR were found to be highly correlated in the field. The observed spatial and temporal variability in SAR was largely a product of soil textural variations within the field and their associated variations in apparent leaching fraction. During the 3-year study period, the percentage of the field in which the lower profile (90-180cm) depth averaged SAR was above 10, increased from 20 to 40%. Since salinity was increasing concomitantly with SAR, and because the soil contained gypsum, sodium hazard was not expected to become a limiting factor for long term shallow groundwater management by drain control. It is anticipated that the technology will be viable for future seasons.
    Agricultural Water Management 01/2010; 97(5):673-680. · 2.20 Impact Factor
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    A.A. Siyal, T.H. Skaggs
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    ABSTRACT: Sub-surface irrigation with porous clay pipe can be an efficient, water saving method of irrigation for many less developed arid and semi-arid regions. Maximizing the efficiency of clay pipe irrigation requires guidelines and criteria for system design and operation. In this study, experimental and simulated (with HYDRUS (2D/3D)) soil wetting patterns were investigated for sub-surface pipe systems operating at different water pressures. Predictions of the soil water content made with HYDRUS were found to be in good agreement (R2 = 0.98) with the observed data. Additional simulations with HYDRUS were used to study the effects of various design parameters on soil wetting. Increasing the system pressure increased the size of the wetted zone. The installation depth affects the recommended lateral spacing as well as the amount of evaporative water loss. For a given water application, the potential rate of surface evaporation affected the shape of the wetted region only minimally. Soil texture, due to its connection to soil hydraulic conductivity and water retention, has a larger impact on the wetting geometry. In general, greater horizontal spreading occurs in fine texture soils, or in the case of layered soils, in the finer textured layers.
    Agricultural Water Management. 06/2009;
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    ABSTRACT: The Ejina Basin is an extremely arid subwatershed in Northwest China. The predominant natural tree species in the area, Populus euphratica, depends on groundwater for sustenance. In recent decades, groundwater overdraft and increased water diversions from the Heihe River caused water table elevations to decline, such that large areas of P. euphratica have withered, creating a highly visible symbol of ecological change and desertification in the Ejina Basin. Ecological restoration efforts aimed at saving existing woodlands and cultivating new stands of P. euphratica are underway. To provide a better scientific basis for ecological restoration plans, it is necessary to understand the effect of water table elevation on P. euphratica water uptake. In this work, we used the HYDRUS-1D software package to study groundwater movement into the root zone and the uptake of groundwater in a 10-year-old P. euphratica woodland. Additionally, we examined the changes in uptake that would occur for different water table elevations. The model calibration was confirmed by comparing predicted soil moisture contents during the P. euphratica growing season with field measured values. The results indicate that in 2000, with an average water table depth of 2·64 m, P. euphratica at the study site obtained about 53% of its water from groundwater during the middle part of the growing season (day of year 160–290). Simulations made with constant water table depths found that increasing the water table depth from 2 to 3 metres resulted in a 74% reduction in transpiration. Many factors can influence the optimal water table depth at a given site. An advantage of the modelling approach is that these factors can be systematically varied, creating a site-specific impact assessment of water management options that may alter water table depths, thus aiding ecological restoration efforts. Copyright © 2009 John Wiley & Sons, Ltd.
    Hydrological Processes 05/2009; 23(17):2460 - 2469. · 2.50 Impact Factor
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    ABSTRACT: Transport equations governing the movement of multiple solutes undergoing sequential first-order decay reactions have relevance in analyzing a variety of subsurface contaminant transport problems. In this study, a one-dimensional analytical solution for multi-species transport is obtained for finite porous media and constant boundary conditions. The solution permits different retardation factors for the various species. The solution procedure involves a classic algebraic substitution that transforms the advection-dispersion partial differential equation for each species into an equation that is purely diffusive. The new system of partial differential equations is solved analytically using the Classic Integral Transform Technique (CITT). Results for a classic test case involving a three-species nitrification chain are shown to agree with previously reported literature values. Because the new solution was obtained for a finite domain, it should be especially useful for testing numerical solution procedures.
    Transport in Porous Media 04/2009; 80(2):373-387. · 1.55 Impact Factor
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    ABSTRACT: This paper presents a formal exact solution of the linear advection–diffusion transport equation with constant coefficients for both transient and steady-state regimes. A classical mathematical substitution transforms the original advection–diffusion equation into an exclusively diffusive equation. The new diffusive problem is solved analytically using the classic version of Generalized Integral Transform Technique (GITT), resulting in an explicit formal solution. The new solution is shown to converge faster than a hybrid analytical–numerical solution previously obtained by applying the GITT directly to the advection–diffusion transport equation.
    International Journal of Heat and Mass Transfer 04/2009; 52:3297-3304. · 2.32 Impact Factor
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    ABSTRACT: In irrigated semi-arid and arid regions, accurate knowledge of groundwater recharge is important for the sustainable management of scarce water resources. The Campo de Cartagena area of southeast Spain is a semi-arid region where irrigation return flow accounts for a substantial portion of recharge. In this study we estimated irrigation return flow using a root zone modelling approach in which irrigation, evapotranspiration, and soil moisture dynamics for specific crops and irrigation regimes were simulated with the HYDRUS-1D software package. The model was calibrated using field data collected in an experimental plot. Good agreement was achieved between the HYDRUS-1D simulations and field measurements made under melon and lettuce crops. The simulations indicated that water use by the crops was below potential levels despite regular irrigation. The fraction of applied water (irrigation plus precipitation) going to recharge ranged from 22% for a summer melon crop to 68% for a fall lettuce crop. In total, we estimate that irrigation of annual fruits and vegetables produces 26 hm3 y−1 of groundwater recharge to the top unconfined aquifer. This estimate does not include important irrigated perennial crops in the region, such as artichoke and citrus. Overall, the results suggest a greater amount of irrigation return flow in the Campo de Cartagena region than was previously estimated.
    Journal of Hydrology 01/2009; 367:138-149. · 2.96 Impact Factor
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    ABSTRACT: Methods to predict soil hydraulic properties frequently require information on the particle size distribution (PSD). The objectives of this study were to investigate various protocols for rapidly measuring PSD using the laser diffraction technique, compare the obtained PSD with those determined using the traditional hydrometer-and-sieves method (HSM), and assess the accuracy of soil hydraulic properties predicted from the measured PSD. Ten soil samples encompassing a wide textural range were analyzed using the HSM and 3 different laser diffraction methods (LDM1, LDM2, and LDM3). In LDM1, the soil sample was thoroughly mixed before analysis. In LDM2, the sand fraction was sieved out and analyzed separately from the silt-clay fraction. LDM3 was similar to LDM2 except that the silt-clay fraction was diluted so that a large sample volume could be used while maintaining an acceptable level of obscuration. LDM2 and LDM3 improved the agreement between the PSD with the HSM in comparison to LDM1, without the need of altering the Mie theory parameters or the use of scaling factors. Moreover, a reasonable prediction of measured saturated hydraulic conductivity and water retention curve was achieved when using the PSD from LDM2 and LDM3, in conjunction with bulk density information.
    01/2009;
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    Haishen Lü, Yonghua Zhu, Todd H. Skaggs, Zhongbo Yu
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    ABSTRACT: In the hilly regions of China, developing sustainable agriculture requires implementing conservation management practices that prevent soil erosion and conserve soil and water resources. In the semiarid northwest Loess Plateau, the primary conservation management practice is terracing. Numerical simulation of soil water dynamics in terraces is potentially an efficient means of investigating the effects of terrace design on moisture retention, but little information is available on the accuracy of such simulations. In this work, we evaluated the accuracy of HYDRUS-2D simulations of water infiltration and redistribution in fallow, level, dryland terraces located in the Loess Plateau. The simulated soil water content distributions were in good agreement with experimental data. Modeling analyses showed that about one-third of the evaporative water losses occurred from the terrace riser surface. To prevent such losses, it is advisable to mulch the riser and minimize the riser surface area. The simulations also demonstrated that with other dimensions equal, wide terraces retain more water on a percentage basis than narrow ones due to a lower evaporating surface area are per unit volume of water storage. With other design considerations being equal, wide beds and minimal riser surface areas will likely enhance water capture and retention. Future analyses of terrace moisture dynamics may additionally include simulations of root water uptake, surface ponding, and runoff.
    Agricultural Water Management. 01/2009;
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    ABSTRACT: Pitcher irrigation is an ancient, but very efficient irrigation system used in many arid and semiarid regions. Small pitchers are often used because they are less expensive than large ones. However, questions exist about whether the patterns and extent of soil wetting obtained with small pitchers are comparable to those achieved with larger pitchers. This work addresses these questions through a combination of experimental and simulation studies involving three pitcher sizes, identified here as large (20 L), medium (15 L), and small (11 L). Saturated hydraulic conductivities of the pitcher materials were measured using a constant head method; the measured values ranged from 0.07 cm d-1 for the large pitcher to 0.14 cm d-1 for the smaller sizes. To determine the zone of wetting, the pitchers were buried down to their necks in a sandy loam soil and filled with water. Water content distributions were determined after 1 and 10 days at locations 20, 40, and 60 cm away from the pitcher center at soil depths of 0, 20, 40, and 60 cm. Moisture distributions predicted with the HYDRUS-2D simulation model were found to be in close agreement with the experimental results, showing root-mean-square-error values between 0.004 and 0.023. The close agreement suggests that HYDRUS-2D is a suitable tool for investigating and designing pitcher irrigation systems. Experimental and numerical results showed that a small pitcher half the size of a larger one, but with double the hydraulic conductivity, will produce approximately the same wetting front as the larger pitcher. Simulations for the large pitcher further showed, as expected, more horizontal spreading of water in a fine-texture soil as compared with a coarse-texture soil.
    01/2009;
  • T. H. Skaggs, P. J. Shouse
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    ABSTRACT: A variety of mechanistic models have been proposed to describe the effect of drought and salinity stresses on the uptake of water by plant roots. Examples from the literature demonstrate that, in practice, it has been difficult to discriminate among these models owing to the difficulty of measuring relevant soil and plant parameters under a range of stress conditions. A new greenhouse installation aimed at obtaining data relevant to modeling uptake under stressed conditions will be discussed and preliminary data presented.
    AGU Fall Meeting Abstracts. 12/2008;

Publication Stats

529 Citations
50.62 Total Impact Points

Institutions

  • 2009–2013
    • Agricultural Research Service
      Kerrville, Texas, United States
    • Sindh Agriculture University
      • Department of Land and Water Management
      Tando Jām, Sindh, Pakistan
  • 2000
    • University of California, Riverside
      Riverside, California, United States