Review of Water Flow in Soils, 2nd Ed., by Tsuyoshi Miyazaki: Taylor & Francis Group, LLC, Boca Raton, Fla.; 2006; 418 pp.

This book is composed of nine chapters. The first six chapters and the last chapter are written by Miyazaki; Chapter 7 is co-authored with K. Seki, and Chapter 8 with S. Hasegawa and T. Kasubuchi. For an authored book this seems an unusual arrangement.

Introducing the system of units employed in the book, Chapter 1 discusses the structure of soils and the retentivity of water therein. The structure includes both solid phase and soil pores, as well as liquid phase and gas phase in soil pores. Matric potential, soil moisture characteristic curves, osmotic effect on retentitivity, and hysteresis encompass the material on the retentivity of water in soils.

Chapter 2, containing five parts, discusses physical laws of water flow in soils. Introducing Darcy’s law for water flux in one, two, and three dimensions, the first part goes on to present basic equations governing the flow of water in soils, unsaturated hydraulic conductivity and empirical relations thereof, and soil water diffusivity without any mathematical relations. The second part discusses the gas-phase configuration in unsaturated flow, including free and entrapped air in soils, forced-closed system, water flow in open systems, and significance of gas-phase configuration in fields. The discussion is comprehensive and has a lot of significance in irrigated agriculture and hydrologic modeling, as well as solute transport. The third part is on infiltration and discusses basic aspects of infiltration and infiltration into dry soils and wet soils. Mathematical equations of infiltration included are Horton, Holtan, Green–Ampt, and Philip. There are other useful empirical and conceptual equations, such as Kostyakov, SCS-CN, Singh-Yu, Smith, Parlange, and others, that have been found to be useful in irrigation engineering and watershed modeling, but have not been included. Steady flows, both upward and downward, are discussed in Part IV. The last part deals with transient flow in soils, including evaporation, redistribution, and water flow in deep soils. The chapter is quite good but would have been stronger if it had a discussion on stochasticity and reliability of different formulations. A discussion of errors and limitations of different formulations is helpful to the reader.

Refraction of water flow in soils is the subject matter of Chapter 3. It encompasses refraction of flux, theory of refraction, jump condition, visualization of refraction in layered soils, and verification of refraction theory experimentally as well as numerically. Refraction and anisotropy coefficient in saturated layered and unsaturated layered soils, and refractive anisotropy coefficient in two-layered and multilayered models, are also treated in the chapter. This provides a comprehensive treatment of refraction theory.

Chapter 4 deals with preferential flow. Beginning with a classification of flow, it goes on to discuss general features; vertical and lateral bypassing flow; criteria and hydrodynamics of fingering flow; and nature, experimental observation, qualitative features, and modeling of funneled flow. Both saturated and unsaturated funneled flow models as well as their further extensions are included. It would have been good to discuss how infiltration models discussed earlier are impacted by preferential flow. This would provide a connectivity to Chapter 2.

Water flow in slopes is treated in Chapter 5. Introducing the water balance on slopes, it deals with moisture conditions on slopes, causes of surface runoff, and subsurface flow in slopes. The discussion is quite useful from the standpoint of hydrologic modeling and irrigated agriculture. It would have been helpful if this chapter could also be tied to the discussion given earlier in Chapter 2, as well as Chapters 3 and 4.

Water flow under the effects of temperature gradients constitutes the subject matter of Chapter 6. From a hydrologic point of view it is quite informative. In the first part it deals with water vapor under a temperature gradient and presents a simple model considering isothermal diffusion of water vapor in soils, enhancement factor by one-way diffusion, nonisothermal diffusion in soils, and the limitation of the simple model. It also includes a discussion of the modified model considering mechanistic enhancement and condensation and evaporation in the water island model. The flow of liquid water under a temperature gradient is discussed next. Observations of water flow in soils under temperature gradients conclude this chapter.

Chapter 7 deals with effects of microbiological factors on water flow in soils. It discusses typical changes in permeability, microbiological effects on permeability, microorganisms in soils, microbiological effects on permeability of natural soils, effects of microbiological factors on water flow in clay liners of landfill sites, mircrobial clogging of soils, and microbial growth and substrate transport modeling.

Water regimes in fields with vegetation is treated in Chapter 8. Water flow in unsaturated subsoil in a paddy field and in an upland field, water balance in fields with vegetation, evaporation, soil water uptake by plant roots, and effects of vegetation on soil water regime are included in the chapter. This kind of discussion is highly useful for watershed hydrologic modeling.

The concluding Chapter 9 deals with heterogeneity of soils in fields. It discusses the concept of a representative elementary volume, similar media concept and scaling, nonsimilar media concept and scaling, spatial variability and geostatistics, and macropores.

On the whole the book is well written, easy to follow, and comprehensive. Much of the literature cited in the text is older than $10\text{years}$ . It would have been desirable if it had presented the inherent difficulties in dealing with flow in unsaturated soils, especially from a mathematical point of view, uncertainties associated with mathematical formulations being employed these days and the resulting errors, limitations of different formulations, and a guidance on choosing a particular formulation. The book would have been well served if it also had a discussion on the connectivity to such areas as hydrology, hydraulics, irrigation engineering, drainage engineering, watershed science, environmental science, geology, and other areas. To be fair, there is some discussion here and there to this effect in the text. The book will serve as a good text for a course on unsaturated flow either at the senior undergraduate level or the beginning graduate level. It would also be useful to have this book on one’s bookshelf.