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

A one-dimensional dual-porosity model has been developed for the purpose of studying variably saturated water flow and solute transport in structured soils or fractured rocks. The model involves two overlaying continua at the macroscopic level: a macropore or fracture pore system and a less permeable matrix pore system. Water in both pore systems is assumed to be mobile. Variably saturated water flow in the matrix as well as in the fracture pore system is described with the Richards' equation, and solute transport is described with the convection-dispersion equation. Transfer of water and solutes between the two pore regions is simulated by means of first-order rate equations. The mass transfer term for solute transport includes both convective and diffusive components. The formulation leads to two coupled systems of nonlinear partial differential equations which were solved numerically using the Galerkin finite element method. Simulation results demonstrate the complicated nature of solute leaching in structured, unsaturated porous media during transient water flow. Sensitivity studies show the importance of having accurate estimates of the hydraulic conductivity near the surface of soil aggregates or rock matrix blocks. The proposed model is capable of simulating preferential flow situations using parameters which can be related to physical and chemical properties of the medium.

A dual-porosity model for simulating the preferential movement of water and solutes in structured porous media

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

While some of the world's most productive agriculture is on artificially drained soils, drainage is increasingly perceived as a major contributor to detrimental off‐site environmental impacts. Howe...

Hydrologic and water quality impacts of agricultural drainage

Phosphorus (P) loss from agricultural fields and watersheds has been an important water quality issue for decades because of the critical role P plays in eutrophication. Historically, most research has focused on P losses by surface runoff and erosion because subsurface P losses were often deemed to be negligible. Perceptions of subsurface P transport, however, have evolved, and considerable work has been conducted to better understand the magnitude and importance of subsurface P transport and to identify practices and treatments that decrease subsurface P loads to surface waters. The objectives of this paper were (i) to critically review research on P transport in subsurface drainage, (ii) to determine factors that control P losses, and (iii) to identify gaps in the current scientific understanding of the role of subsurface drainage in P transport. Factors that affect subsurface P transport are discussed within the framework of intensively drained agricultural settings. These factors include soil characteristics (e.g., preferential flow, P sorption capacity, and redox conditions), drainage design (e.g., tile spacing, tile depth, and the installation of surface inlets), prevailing conditions and management (e.g., soil-test P levels, tillage, cropping system, and the source, rate, placement, and timing of P application), and hydrologic and climatic variables (e.g., baseflow, event flow, and seasonal differences). Structural, treatment, and management approaches to mitigate subsurface P transport-such as practices that disconnect flow pathways between surface soils and tile drains, drainage water management, in-stream or end-of-tile treatments, and ditch design and management-are also discussed. The review concludes by identifying gaps in the current understanding of P transport in subsurface drains and suggesting areas where future research is needed.

Phosphorus transport in agricultural subsurface drainage: a review.

Carbofuran, alachlor, atrazine, bromide (a tracer), and nitrate are applied to two tile‐drained plots of com, one of which is conventionally tilled and another that is no‐till. Soil water samples are collected from the plots with suction lysimeters and ground‐water samples are collected from wells and tile drains. The no‐till plot first showed low concentrations of atrazine and alachlor ranging from 0.1 to 0.4 μg/L below the root zone one month after application, but the conventionally tilled plot showed no pesticides below the root zone until late fall, when atrazine was detected at 0.4 μg/L. Dye studies indicated that, in the no‐till plots, some of the pesticide bypasses the root zone through surface‐connected ma‐cropores, whereas in the conventionally tilled plot, most of the pesticide is adsorbed within the root zone as a result of greater contact with the soil. Concentrations of bromide (which is not adsorbed) in samples collected from wells and suction lysimeters do not differ between the two plots....

Preferential movement of pesticides and tracers in agricultural soils

A numerical model for preferential solute movement in structured soils.

To examine the impact of small reservoir irrigation development in Africa, the performance and productivity of two small reservoirs and irrigation schemes in the Upper East Region of Ghana were investigated in this study. Hydrologic data measured included daily irrigation volumes and daily evaporation. Farmer cost inputs, excluding labor, and harvest data were also recorded. There was a strong contrast in water availability between the two systems, the Tanga system having a higher amount of available water than did the Weega system. The concept of relative water supply was used to confirm this disparity; Tanga was an inefficient system with a relative water supply of 5.7, compared to a value of 2.4 for the efficient Weega system. It was also concluded that the dissimilar water availabilities resulted in the evolution of very different irrigation methods and coincided with different management structures. Where there was more water available per unit land (Tanga), management was relaxed and the irrigation inefficient. Where there was less water available per unit land (Weega), management was well structured and irrigation efficient. The productivity of water (US$ m−3) of the Tanga system was half that of the Weega system, when analyzed at a high market price for crops grown. In terms of productivity of cultivated land (US$ ha−1), however, the Tanga system was 49% more productive than the Weega system. The difference in the productivity of land is primarily a result of increased farmer cash inputs in the Tanga system as compared to the Weega system. The difference in the productivity of water can be attributed to the varying irrigation methods and management structures, and ultimately to the contrasting water availability. Copyright © 2008 John Wiley & Sons, Ltd.



L'impact du developpement de petits reservoirs d'irrigation en Afriques est etudie en analysant la performance et le benefice economique de deux petits reservoirs avec de differents schemes d'utilisation dans une region au nord-est du Ghana. Les donnees hydrologiques utilisees dans cette etude comprennent les volumes journaliers d'irrigation, ainsi que des mesures journaliers d'evaporation. En outre les couts des investissements des agriculteurs ainsi que des donnees de recoltes ont ete enregistres. La quantite d'eau disponible etait considerablement differente dans les deux systemes: les ressources en eau du systeme de Tanga etaient nettement superieures par rapport a celles du systeme de Weega. Le concept de l'approvisionnement relatif en eau a ete utilise pour demontrer cette disparite: Tanga avait un systeme inefficace avec un taux d'approvisionnement relatif en eau de 5.7 compare a un taux de 2.4 du systeme efficace de Weega. Il a ete conclu que les differences au niveau de la disponibilite d'eau sont a la base d'une evolution de methodes et s'accordent avec de concepts de gestion d'irrigation tres contrastes. Quand il y avait plus d'eau disponible par unite de surface (Tanga), la gestion etait peu organisee et l'irrigation moins efficace. Par contre, quand les ressources en eau etaient limitees (Weega) la gestion etait bien structuree et l'irrigation tres efficace. En termes d'eau les agriculteurs de Tanga recevaient seulement la moitie du benefice economique des agriculteurs de Weega quand le prix du marche etait eleve pour les produits recoltes. Par contre, en termes de surface cultivee, les agriculteurs de Tanga faisaient 49% plus de profit par rapport aux agriculteurs de Weega. La difference du benefice economique de la terre cultive est principalement une consequence des investissements eleves des agriculteurs dans le systeme de Tanga compare au systeme de Weega. La difference du benefice economique de l'eau peut etre attribuee aux variations des methodes d'irrigation et des structures de gestion et donc finalement aux disponibilites en eau tres contrastees. Copyright © 2008 John Wiley & Sons, Ltd.

/pdf/water-use-and-productivity-of-two-small-reservoir-irrigation-3qtjf7tnqd.pdf

Water use and productivity of two small reservoir irrigation schemes in Ghana's upper east region

In Kitissou, M.; Ndulo, M.; Nagel, M.; Grieco, M. (Eds.). The hydropolitics of Africa: a contemporary challenge. Newcastle, UK: Cambridge Scholars

http://www.smallreservoirs.org/full/publications/reports/Q12-TUD-SRP-Hydropolitics-07.pdf

The small reservoirs project: research to improve water availability and economic development in rural semi-arid areas

The flow of pesticides through preferential paths in soils.

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