Virgil L. Quisenberry

The effect of preferential flow on the short and long-term spatial distribution of surface applied solutes in a structured soil

Abstract We measured the lateral distribution of Br in a well-structured soil (clayey, kaolinitic, thermic Typic Kanhapludults) over a period of 40 days following a leaching event with Br-tagged water (Event-1) and the subsequent vertical transport of solute that occurred following a second leaching event (Event-2). The objectives were to determine the time required for solutes to become uniformly distributed laterally after Event-1, and the effect that degree of spatial distribution had on the subsequent vertical movement induced by Event-2. Six field plots, 1.2 by 1.2 m, were exposed to Event-1. Three of the plots were sampled 1, 5, or 40 days later. The three remaining plots were exposed to Event-2 either 1, 5, or 40 days following Event-1 and they were sampled the day following Event-2. A total of 319 samples were taken from a 38.5 by 38.5 cm area in the center of each plot at each of five depths ranging from 5 to 60 cm. Bromide was practically uniformly distributed in the surface layer immediately following Event-1, as indicated by a coefficient of variation (CV) of 25% among the 319 samples, but became progressively more non-uniformly distributed as depth increased. At 60 cm, the CV had increased to 240%. The longer we delayed sampling following Event-1, the more uniform was the lateral distribution of Br. During the 40 days following Event-1, Br became practically uniformly distributed across all layers, and the center of mass moved from 8 to 16 cm below the surface. Event-2 caused Br to move deeper within the soil profile when it occurred the day following Event-1 than when it occurred later, but because drainage occurred during the 40-day delay, the center of Br mass was located 26–27 cm below the surface after Event-2 regardless of the delay time.

One‐dimensional infiltration with moving finite elements and improved soil water diffusivity

A problem of significant interest to environmental scientists is the flow of water and solutes through the vadose zone. The partial differential equations which govern this flow are typically time-dependent and nonlinear. Valid solutions to these equations require (1) accurate relationships between various coefficients and variables on which they depend (e.g., coefficient of diffusivity and water content) and (2) sophisticated numerical methods which can handle complexities such as sharp moving fronts. In cases where coefficients are not known explicitly, curve-fitting techniques are needed to smooth out scattered experimental data. Nonlinear coefficients can then be calculated. A constrained least squares spline fit is compared to empirical function fits which have appeared recently. Then, a state-of-the-art numerical technique is used to accurately model transient flow through unsaturated homogeneous soils. The moving finite element method of Miller and colleagues is an adaptive approach in the sense that the grid moves so that nodes are concentrated where they are most needed. As a result, better accuracy is achieved with fewer nodes than are required for standard fixed-grid methods. Petzolds robust Gear-type solver DASSL is used for time-integration. Numerical results are compared to experimental data. Mass balance errors are neglible, and accurate solutions are obtained at all time steps. Though only one-dimensional problems are considered here, the numerical approach generalizes to heterogeneous media and problems in higher dimensions.

Relating model parameters to basic soil properties

Relating model parameters to basic soil characteristics can help to differentiate and classify soils based on their flow and transport characteristics and ultimately helps to develop a sound management tool to protect groundwater from industrial and agricultural contaminants. In this study, the model parameters (effective diffusion path-length or aggregate half-width, boundary soil water pressure, boundary hydraulic conductivity, saturated hydraulic conductivity, tortuosity in macropores, dispersivity, mixing depth) obtained from simulation of water flow and solute transport for 3 soils (Maury, Cecil, Lakeland) with contrasting properties were related to see whether these derived parameters can be related to variation in fundamental soil properties such as texture and structure and thus the flow and transport characteristics of the soils. The boundary is a division point in which the soil porosity is divided into macropores and micropores. The ANOVA test showed that the parameter values of effective diffusion path-length and tortuosity in macropores for 3 soils were not different from each other, but the parameter values of saturated and boundary hydraulic conductivities including the texture (clay content) were statistically different. Moreover, the means of boundary soil water pressure, dispersivity, and mixing depth for 3 soils were significantly different. These results suggest that relating model parameters to basic soil properties in order to differentiate and classify soils based on their flow and transport characteristics is promising and needs further study.

Soil Physical and Moisture Properties

Any science has key basic concepts that should be mastered before pursuing additional ones. This chapter describes in simplified terms, the key components involved with soil physical and moisture properties. Soil physical properties covered in this chapter include soil particle analysis, soil particle and bulk density, soil porosity, and the relationship between soil volume and soil mass. When exploring soil moisture properties, topics covered include describing soil moisture, plant-available water, soil water potential and its components, infiltration and percolation rates, water movement in soil, perched water table, saturated hydraulic conductivity, and soil moisture retention curves. The chapter then integrates these basic components into applied useful principles and practices which subsequent chapters will be built upon. The chapter especially covers and explains the usefulness of constructing and interpreting soil moisture retention curves and how this information is critical to the successful evaluation of a rootzone soil mixture. Numerous examples are covered in the chapter with applied questions at the end it. The chapter also includes figures and illustrations to help explain and understand many of the key concepts introduced.

Water Management and Conservation

The final chapter covers one of the most important biological issues facing all outdoor activities, namely proper water management and conservation. The latest scientific-based concepts and methods pertaining to this topic are discussed in the chapter. Included are water use, quantifying soil water present and determining levels needed, ET rates and modeling means, water-use rates, irrigation strategies, irrigation system calibration, water budgeting, water quality issues, salts, ions, alkalinity, suspended solids, soil salinity, and managing poor quality water. In addition, the latest means of conserving water are listed and covered in detail as well as hydrophobic soils and their management with various soil wetting agents and adjuvants. Numerous examples are provided to support the concepts and topics covered. Since many areas are facing mandatory justification of water allocation, examples are provided on how to scientifically determine water needs for successful plant survival including how to calculate and document this. Appropriate illustrations and photographs are included to help demonstrate and support the various concepts discussed.

An adaptive-grid least squares finite element solution for flow in layered soils

Groundwater flow in unsaturated soil is governed by Richards equation, a nonlinear convection-diffusion equation. The process is normally convection-dominated, and steep fronts are common in solution profiles. The problem is further complicated if the medium is heterogeneous, for example when there are two or more different soil layers. In this paper, the least squares finite element method is used to solve for flow through 5 layers with differing hydraulic properties. Solution-dependent coefficients are constructed from smooth fits of experimental data. The least squares finite element approach is developed, along with the method for building an optimized, nonuniform grid. Numerical results are presented for the 1D problem. Generalization to higher dimensions is also discussed.

Solute and Bacterial Transport through Partially-Saturated Intact Soil Blocks

Steady-state transport of water, chloride and bacteria was measured through intact blocks of Maury and Cecil soils, under partially saturated conditions. Major objectives were to determine if transport occurs uniformly or via preferential flow paths, and if soil physical properties could be used to predict breakthrough. The blocks were instrumented with TDR probes and mounted on a vacuum chamber containing 100 cells that collected eflluent. After each experiment the blocks were sampled for soil physical properties. The fluxes showed no spatial autocorrelation and the eflluent variance was not statistically different between soils. Less than 3% of the influent bacteria appeared in the effluent. Maximum bacterial breakthrough occurred after 0.25 water-filled pore volumes had been leached, and was greater for Cecil soil than for Maury soil. The chloride breakthrough curves were fitted to the convection dispersion equation. The best predictor of dispersivity was volumetric water content (R 2 = 0.28, P<0.01 ), with dispersivity increasing with decreasing water content. Lower water contents lead to more tortuous flow paths and thus, a broadening of the velocity distribution. Soil structural controls on solute dispersion under partially saturated conditions are likely to be indirect, and related to differences in water content at given flux produced by differences in pore-size distribution. Focus Categories: ST, NPP, AG