FROM THE PREFACE: This book attempts to combine two separate themes: a description of one of the links in the chain of the water cycle inside the earth's crust i.e., the subsurface flow; and the quantification of the various types of this flow, obtained by applying the principles of fluid mechanics in porous media. The first part is the more descriptive, and geological of the two. It deals with the concept of water resources, which then leads us on to other links in the cycle: rainfall, infiltration, evaporation. runoff, and surface water resources. The second part is necessary in order to quantify ground water resources. It points the way to other applications, such as solutions to civil engineering problems including drainage and compaction; and transport problems in porous media, including aquifer pollution by miscible fluids, multiphase flow of immiscible fluids, and heat transfer in porous media, i.e., geothermal problems. However, the qualitative and the quantitative aspects are not treated separately but combined and blended together, just as geology and hydrology are woven together in hydrogeology.