Todd H. Skaggs

Analytical Solutions for Solute Transport in Three‐Dimensional Semi‐infinite Porous Media

This paper presents several analytical solutions for three-dimensional solute transport in semi-infinite porous media with unidirectional flow using first-type (or concentration) and third-type (or flux) boundary conditions at the inlet location of the medium. The solutions may be used for predicting solute concentrations in homogeneous media, verification of more comprehensive numerical models, and laboratory or field determination of solute transport parameters. The transport equation incorporates terms accounting for advection, dispersion, zero-order production, and first-order decay. General solutions were derived for an arbitrary initial distribution and solute input with the help of Laplace, Fourier, and Hankel transforms. Specific solutions are presented for rectangular and circular solute inflow regions, as well as for solutes initially present in the form of parallelepipedal or cylindrical regions of the medium. The solutions were mathematically verified against simplified analytical solutions. Examples of concentration profiles are presented for several solute transport parameters using both first- and third-type boundary conditions. A mass balance constraint is defined based on a prescribed solute influx; the third-type condition is shown to conserve mass whereas the first-type condition was found to always overestimate resident solute concentrations in the medium.

The Use of Numerical Flow and Transport Models in Environmental Analyses

This chapter provides an overview of alternative approaches for modeling water flow and contaminant transport problems in soils and groundwater. Special focus is on flow and transport processes in the variably saturated vadose zone between the soil surface and the groundwater table. The governing flow and transport equations are discussed for both equilibrium and nonequilibrium flow conditions, followed by three examples. The first example shows how one-dimensional root-zone modeling can be used to estimate short- and long-term recharge rates, including contaminant transport through the vadose zone. A second example illustrates a two-dimensional application involving drip irrigation, while the third example deals with two-dimensional nonequilibrium transport of a pesticide in a tile-drained field soil. Also discussed are alternative pore-scale modeling approaches that may provide a better understanding of the basic physical and geochemical processes affecting fluid flow and contaminant transport in saturated and variably saturated media.

Moving Forward on Remote Sensing of Soil Salinity at Regional Scale

Soil salinity undermines global agriculture by reducing crop yield and impairing soil quality. Irrigation management can help control salinity levels within the soil root-zone. To best manage water and soil resources, accurate regional-scale inventories of soil salinity are needed. The past decade has seen several successful applications of soil salinity remote sensing. Two salinity remote sensing approaches exist: direct assessment based on analysis of surface soil reflectance (the most popular approach), and indirect assessment of root-zone (e.g., 0-1 m) soil salinity based on analysis of crop canopy reflectance. In this perspective paper, we call on researchers and funding agencies to pay greater attention to the indirect approach because it is better suited for surveying agriculturally important lands. A joint effort between agricultural producers, irrigation specialists, environmental scientists, and policy makers is needed to better manage saline agricultural soils, especially because of projected future water scarcity in arid and semi-arid irrigated areas. The remote sensing community should focus on providing the best tools for mapping and monitoring salinity in such areas, which are of vital relevance to global food production.