This volume certainly is a Conference Proceedings, the Proceedings of the NATO Advanced Research Workshop (ARW) on "Unsaturated Flow in Hydrologic Modeling" held at "Les Villages du Soleil" near ArIes, France from June 13 to 17, 1988. Let me therefore acknowledge properly, at the very beginning, the gratitude of all the participants to the NATO Science Committee for its generous support and worthwhile goal of bringing together scientists of many countries to communicate and share their experiences. Particular thanks are extended to the director of the program, Dr. Luis Vega da Cunha for his interest and understanding. On the other hand this volume is also, and probably more so, a Textbook that fills a gap in the field of unsaturated flow. Many treatises on the subject present the theory in its different aspects. Hardly any explain in details how the different pieces can be put together to address realistic problems at the basin scale. The various invited contributions to the ARW were structured in a subject progression much as chapters are organized in a book. The intent of the ARW was to assess the current state of knowledge in "Unsaturated Flow" and its use in "Hydrologic Modeling Practice". In a sense the interest in fundamentals of unsaturated flow in this ARW was not just for the sake of knowledge but also and primarily for the sake of action. Can such fundamental knowledge be utilized for better management of the water resource? was the basic question.
Table of Contents1. Perspective and Fundamentals of Unsaturated Flow.- Background Concepts and Principles.- Multiphase Flow in Porous Media: Description at the Pore and Macroscopic Scale.- Flow of Water and Air in Soils: Theoretical and Experimental Aspects.- 2. Components in the Description of Unsaturated Flow.- Infiltration and Ponding Time.- Liquid Moisture Redistribution: Hydrologie Simulation and Spatial Variability.- Evaluation of the Hydrological Response of a Drainage Network.- Interflow.- Unsaturated Flow Induced by Evaporation and Transpiration.- Prediction of Water Table Mound Development and Aquifer Recharge from an Infiltrating Area.- 3. Parameters Estimation and Numerical Techniques.- Estimation of Soil Water Retention and Hydraulic Properties.- Remote Sensing of Watershed Characteristics and Rainfall Input.- Numerical Simulation of Richards Equation: Current Approaches and an Alternate Perspective.- 4. Treatment of Unsaturated Zone in Hydrologic Models.- Loss Rate Representation in the HEC-1 Watershed Model.- Comparative Evaluation of Unsaturated Flow Methods in Selected USDA Simulation Models.- The Unsaturated Components of SWATCH: A Multiprocess Watershed Model for Runoff Generation and Routing.- Spatially Distributed Modeling: Conceptual Approach, Coupling Surface Water and Groundwater.- Influence of Spatial Variability of Saturated Hydraulic Conductivity on the Infiltration Process.- 5. Miscellaneous.- Analytic Steady-State Solution to One-Dimensional Unsaturated Water Flow in Layered Soils.- The Constant Head Percolation Test-Improved Equipment and Possibilities of Assessing the Saturated Hydraulic Conductivity.- Mobility of Residual Phase Organics in the Vadose Zone.- The Movement of Water in Swelling Clays.- General List of Symbols.