"Subsurface flow, in hydrology, is the flow of water beneath earth's surface as part of the water cycle. Water beneath the land surface occurs in two principal zones, the unsaturated zone and the saturated zone. In the unsaturated zone, the spaces between particle grains and the cracks in rocks contain both air and water. The voids in the saturated zone are completely filled with water. Although a considerable amount of water can be present in the unsaturated zone, this water cannot be pumped by wells because capillary forces hold it too tightly. Subsurface flow to maintain base flow and its contribution to high flow is of high significance. The high contribution of subsurface flow to stream flow has usually been determined based on the application of tracers. However, there are some studies that challenge tracer test applications. These studies have shown that tracer test applications lead to a high percentage of subsurface flow contribution because advection and dispersion effects are not individually considered in the mass balance equation. This volume is a practical guide to the practices, procedures, and rules for dealing with comprehensive examination of water movement as well as the movement of various pollutants in the earth's subsurface. The multidisciplinary approach integrates earth science, fluid mechanics, mathematics, statistics, and chemistry. It focuses on the contribution of subsurface flow to high flows, although a brief description of their role in low flows is included. Different suggested mechanisms, considering their applicability, strengths and inadequacies are also presented. In addition, the application of tracer experiments is elaborated. The challenges of modeling surface/subsurface flow interactions are addressed. In addition, Watershed-Scale Hydrological Modeling Methods; Surface Water Interactions in a Mountain-to-Coast Watershed: and a universal approach scaling relationship between fracture stiffness and fluid flow are also explained. This book is intended for the practicing professionals and students, mathematical modelers, hydrogeologists and water resources specialists."