In the saturated zone, groundwater fills the pore spaces completely and porosity is therefore a direct measure of storage volume. Part of this water(specific retention) cannot be removed by pumping or drainage because of molecular and surface tension forces. Specific retention is the ratio of volume of water retained against gravity drainage to gross volume of the soil Ground surface Soil water zone 伞 Well Interme (Vadose water Pellicular and gravitational water Phreatic Groundwater Capillary fringe.Groundwater Figure 1.3 Subsurface moisture zones(from Bear and Verruijt, 1987) 1.4 Groundwater flow systems Groundwater flows are usually three-dimensional. Unfortunately, the solution of such problem by analytic methods is complex unless the system is symmetric. In other cases, space of the coordinate directions may be so slight that assumption of two-dimensional flow is satisfactory. Many problems of practical importance fall into this class. Sometime one-dimensional flow can be assumed, thus further simplifying the solution Fluid properties such as velocity, pressure, temperature, density, and viscosity often vary in time and space. When time dependency occurs, the issue is termed an unsteady flow problem and solutions are usually difficult. On the other hand, situations where space dependency alone exists are steady flow problems. Only homogeneous (single-phase) fluids are considered here Boundaries to groundwater flow systems may be fixed geologic structures or free water surface that are dependent for their position on the state of the flow. a hydrologist must be able to define these boundaries mathematically if the groundwater flow problems are to be solve Porous media through which groundwater flow may be classified as isotropic, anisotropIc heterogeneous, homogeneous, or several possible combinations of these. An isotropic medium has uniform properties in all directions from a given point. Anisotropic media have one or more properties that depend on a given direction. For example, permeability of the 6In the saturated zone, groundwater fills the pore spaces completely and porosity is therefore a direct measure of storage volume. Part of this water (specific retention) cannot be removed by pumping or drainage because of molecular and surface tension forces. Specific retention is the ratio of volume of water retained against gravity drainage to gross volume of the soil. Figure 1.3 Subsurface moisture zones (from Bear and Verruijt, 1987) 1.4 Groundwater flow systems Groundwater flows are usually three-dimensional. Unfortunately, the solution of such problem by analytic methods is complex unless the system is symmetric. In other cases, space dependency in one of the coordinate directions may be so slight that assumption of two-dimensional flow is satisfactory. Many problems of practical importance fall into this class. Sometime one-dimensional flow can be assumed, thus further simplifying the solution. Fluid properties such as velocity, pressure, temperature, density, and viscosity often vary in time and space. When time dependency occurs, the issue is termed an unsteady flow problem and solutions are usually difficult. On the other hand, situations where space dependency alone exists are steady flow problems. Only homogeneous (single-phase) fluids are considered here. Boundaries to groundwater flow systems may be fixed geologic structures or free water surface that are dependent for their position on the state of the flow. A hydrologist must be able to define these boundaries mathematically if the groundwater flow problems are to be solved. Porous media through which groundwater flow may be classified as isotropic, anisotropic, heterogeneous, homogeneous, or several possible combinations of these. An isotropic medium has uniform properties in all directions from a given point. Anisotropic media have one or more properties that depend on a given direction. For example, permeability of the 6