ONFINED AOUIFER UNCONFINED AQUIFER A B LEAKY AQUIFER c D MULTI-LAYERED LEAKY AQUIFER SYSTEM Figure 1. 2 Different types of aquifers(from Kruseman and de ridder 1990) Intermediate zone This belt extends from the bottom of the soil-water zone to the top of the capillary fringe and may change from non-existence to several hundred meters in thickness. The zone is essentially a connecting link between a near-ground surface region and the near- water-table region through which infiltrating fluids must pass Capillary zone a capillary zone extends from the water table to a height determined by the capillary rise that can be generated in the soil. The capillary band thickness is a function of soil texture and may fluctuate not only from region to region but also within a local area Saturated zone
Figure 1.2 Different types of aquifers (from Kruseman and de Ridder, 1990) Intermediate zone This belt extends from the bottom of the soil-water zone to the top of the capillary fringe and may change from non-existence to several hundred meters in thickness. The zone is essentially a connecting link between a near-ground surface region and the near-water-table region through which infiltrating fluids must pass. Capillary zone A capillary zone extends from the water table to a height determined by the capillary rise that can be generated in the soil. The capillary band thickness is a function of soil texture and may fluctuate not only from region to region but also within a local area. Saturated zone 5
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 6
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. 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
medium might be greater along a horizontal plan than along a vertical one. Heterogeneous media have non-uniform properties of anisotropy or isotropy, while homogeneous media are uniform in their characteristics Local flow system Relief Direction of fl Local flow system Intermediate flow system Figure 1. 4 Definition sketches of groundwater systems(from Toth, 1963) 7
medium might be greater along a horizontal plan than along a vertical one. Heterogeneous media have non-uniform properties of anisotropy or isotropy, while homogeneous media are uniform in their characteristics. Figure 1.4 Definition sketches of groundwater systems (from Toth, 1963) 7
8
2. Fundamental Equations of Groundwater Flow 2.1 Darcy's Law Darcy's experiment In 1856, A French engineer Darcy did a laboratory experiment as sketched in Figure 2.1. A one-dimensional steady flow in the sand column is created by keeping water levels in the left and right reservoirs constant P,/Dg Area a P2/Dg Flow n Reference level Figure 2.1 Darcys experiment Darcy measured the total discharge through a sand column when changing the difference between water levels in two reservoirs. He found that the total discharge was proportional to the difference of water levels O=-KA ere Q: total discharge,LTI A: cross-sectional area, LL] K: coefficient of permeability, [LT ];
2. Fundamental Equations of Groundwater Flow 2.1 Darcy's Law Darcy's experiment In 1856, A French engineer Darcy did a laboratory experiment as sketched in Figure 2.1. A one-dimensional steady flow in the sand column is created by keeping water levels in the left and right reservoirs constant. Reference level Flow Area A L n2 n1 P1/Dg P2/Dg z2 z1 Q Figure 2.1 Darcy's experiment Darcy measured the total discharge through a sand column when changing the difference between water levels in two reservoirs. He found that the total discharge was proportional to the difference of water levels: L - Q = - K A ϕ2 ϕ1 (2.1) where: Q : total discharge, [L3 T-1]; A : cross-sectional area, [L2 ]; K : coefficient of permeability, [LT-1]; 9