Soil water repellency: its causes, characteristics and hydro-geomorphological significance

This review discusses the causes and consequences of 'non-equilibrium' water flow and solute transport in large structural pores or macropores (root and earthworm channels, fissures and interaggregate voids). The experimental evidence suggests that pores larger than c. 0.3 mm in equivalent cylindrical diameter allow rapid non-equilibrium flow. Apart from their large size and continuity, this is also due to the presence of impermeable linings and coatings that restrict lateral mass exchange. Macropores also represent microsites in soil that are more biologically active, and often more chemically reactive than the bulk soil. However, sorption retardation during transport through such pores is weaker than in the bulk soil, due to their small surface areas and significant kinetic effects, especially in larger macropores. The potential for non-equilibrium water flow and solute transport at any site depends on the nature of the macropore network, which is determined by the factors of structure formation and degradation, including the abundance and activity of soil biota such as earthworms, soil properties (e.g. clay content), site factors (e.g. slope position, drying intensity, vegetation) and management (e.g. cropping, tillage, traffic). A conceptual model is proposed that summarizes these effects of site factors on the inherent potential for non-equilibrium water flow and solute transport in macropores. Initial and boundary conditions determine the extent to which this potential is realized. High rain intensities clearly increase the strength of non-equilibrium flow in macropores, but the effects of initial water content seem complex, due to the confounding effects of soil shrinkage and water repellency. The impacts of macropore flow on water quality are most significant for relatively immobile solutes that are foreign to the soil and whose effects on ecosystem and human health are pronounced even at small leached fractions (e.g. pesticides). The review concludes with a discussion of topics where process understanding is still lacking, and also suggests some potential applications of the considerable knowledge that has accumulated in recent decades.

A review of non-equilibrium water flow and solute transport in soil macropores: principles, controlling factors and consequences for water quality

Mathematical models have become indispensable tools for studying vadose zone flow and transport processes. We reviewed the history of development, the main processes involved, and selected applications of HYDRUS and related models and software packages developed collaboratively by several groups in the United States, the Czech Republic, Israel, Belgium, and the Netherlands. Our main focus was on modeling tools developed jointly by the U.S. Salinity Laboratory of the USDA, Agricultural Research Service, and the University of California, Riverside. This collaboration during the past three decades has resulted in the development of a large number of numerical [e.g., SWMS_2D, HYDRUS-1D, HYDRUS-2D, HYDRUS (2D/3D), and HP1] as well as analytical (e.g., CXTFIT and STANMOD) computer tools for analyzing water flow and solute transport processes in soils and groundwater. The research also produced additional programs and databases (e.g., RETC, Rosetta, and UNSODA) for quantifying unsaturated soil hydraulic properties. All of the modeling tools, with the exception of HYDRUS-2D and HYDRUS (2D/3D), are in the public domain and can be downloaded freely from several websites.

Development and Applications of the HYDRUS and STANMOD Software Packages and Related Codes

The original review of macropores and water flow in soils by Beven and Germann is now 30 years old and has become one of the most highly cited papers in hydrology. This paper attempts to review the progress in observations and theoretical reasoning about preferential soil water flows over the intervening period. It is suggested that the topic has still not received the attention that its importance deserves, in part because of the ready availability of software packages rooted firmly in the Richards domain, albeit that there is convincing evidence that this may be predicated on the wrong experimental method for natural conditions. There is still not an adequate physical theory linking all types of flow, and there are still not adequate observational techniques to support the scale dependent parameterizations that will be required at practical field and hillslope scales of application. Some thoughts on future needs to develop a more comprehensive representation of soil water flows are offered.

/pdf/macropores-and-water-flow-in-soils-revisited-1zhff8ac1e.pdf

Macropores and water flow in soils revisited

Soil structure as an indicator of soil functions: A review

http://www.pc-progress.com/Documents/Jirka/Simunek_et_al_MacroFl_2002.pdf

Review and comparison of models for describing non-equilibrium and preferential flow and transport in the vadose zone

A simple empirical model of root water uptake

A decision-support tool (MACRO—DB) for predicting pesticide fate and mobility in soils is described. MACRO—DB consists of soil, pesticide, climate and crop databases linked to parameter estimation routines and a simulation model (MACRO). The system currently allows access to three soils databases: SEISMIC ( c. 400 UK benchmark soils), MARKDATA (26 Swedish soils), and a database that the user can develop independently. Automatic estimation procedures (pedo-transfer functions) translate the soil information into model parameter values. Two pesticide databases are available. The PETE database contains information for over 600 common compounds. The user can also develop a parallel database for new compounds. Sorption and degradation constants are calculated automatically, combining soil and compound properties. A weather database contains long-term daily meteorological data and a weather generator enables synthetic daily weather data to be derived from long-term average climatic data. A separate database contains information on typical planting and harvest dates, root depths, etc. for some common agricultural crops. To account for parameter uncertainty, the user can simulate ‘worst-case’ or ‘average-case’ soil scenarios, utilizing available information on the mean and typical ranges of soil organic carbon content and pH. The user can view and analyse simulation results with flexible, in-built, graphical procedures.

MACRO—DB: a decision-support tool for assessing pesticide fate and mobility in soils

Soil compaction by vehicular traffic modifies the pore structure and soil hydraulic properties. These changes potentially influence the occurrence of preferential flow, which so far has been little studied. Our aim was to study the effect of compaction on soil hydraulic and transport properties in subsoil. A randomized block design trial at two sites on a well-structured clay soil in central Sweden was established. Plots with two levels of compaction were created at both sites, in the following referred to as trafficked and control. The trafficked treatment was created by 4 passes track-by-track with a three-axle dumper with a maximum wheel load of 5.8 Mg. After one year, undisturbed soil columns (20 cm height × 20 cm diameter) from both trafficked and control plots at a depth of 30–50 cm were sampled. The columns were analyzed using X-ray CT imaging, together with measures of the degree of preferential transport derived from non-reactive tracer breakthrough curves and measurements of saturated hydraulic conductivity ( K s ) and air permeability at the field moisture content ( K a ). Although the traffic treatment did not cause any compaction effects at one of the two sites, it did result in significant reductions in saturated hydraulic conductivity, air permeability and number of macropores at the second site. At this site, the traffic also significantly reduced the strength of preferential flow, presumably due to compaction-induced disruption of macropore continuity. In apparent contrast, some previous studies have shown increases in the strength of preferential flow as a result of compaction. We propose a conceptual model to explain these apparently contradictory results, which suggests that preferential flow should be strongest at some intermediate level of compaction.

Nick Jarvis

Papers

Review and comparison of models for describing non-equilibrium and preferential flow and transport in the vadose zone

A simple empirical model of root water uptake

MACRO—DB: a decision-support tool for assessing pesticide fate and mobility in soils

Effects of subsoil compaction on hydraulic properties and preferential flow in a Swedish clay soil

Persistent subsoil compaction and its effects on preferential flow patterns in a loamy till soil