Base of drainage

Discharge areas away from the terminal base of drainage warrant identification in relation to drainage operations, water exploration, and water quality preservation. 

Fig. 2.8 Components of a karstic system sinkholes and fissures acting as inlets for rapid intake of runoff water dissolution channels water discharging in springs at local bases of drainage, for example, river beds caves exposed at former dissolution channels formed at former (higher) drainage bases.

Fig. 2.10 Rock beds in a subsidence basin. The part above the terminal base of drainage, for example, the sea, functions as a through-flow system (arrows). The deeper rock beds are fossil through-flow systems that host stagnant groundwater as they are (1) covered by impermeable rocks, (2) bisected by plastic impermeable rocks that have been squeezed into stretch joints in the competent rock beds and in between bedding plane thrusts, and (3) placed in a zone of zero hydraulic potential.

Lateral overflow, as in water reaching a filled tub. The water that fills the tub blocks the downflow direction of the added water (two-dimensional degrees of hydraulic freedom), and the latter overflows, that is, it flows laterally toward the bases of drainage (Fig. 2.11b). The observation that the water flows laterally indicates its surface (water table) is slightly tilted by the critical angle of flow. The latter is defined by the water viscosity. The water present in the deeper part of the tub does not flow—it is stagnant, bounded in a dead volume situated under... 

Let us expose an empty aquarium to rain. The vessel will fill up and water will overflow laterally toward the bases of drainage. The observation that the water is flowing indicates that the water surface, or water table, is slightly inclined (Fig. 2.12) by the critical angle of flow. This angle is determined by properties of the water, mainly viscosity, which in turn is determined by salinity and temperature. If the vessel is exposed long enough... 

Fig. 2.12 A basic experiment in water flow an aquarium exposed to rain. The vessel fills up and the additional rainwater overflows. At a steady state, all the arriving rainwater flows laterally toward the bases of drainage, and the water at the bottom of the vessel is stagnant (dead volume).

Terminal base of drainage (zero hydraulic potential)... 

Groundwater regimes are best understood by discussing entire groundwater drainage basins, that is, from the principal water divide to the terminal base of drainage, which in most cases is the sea (Fig. 2.14). 

Fig. 2.14 An entire groundwater system, from the water divide to the terminal base of drainage, built of permeable rocks. The following patterns of water motion are recognizable (1) a through-flow zone with vertical flow paths that join a lateral flow path toward the terminal base of drainage (2) a transition (mixing) zone and (3) a zone of stagnation occurring beneath the level of the terminal base of drainage (zero hydraulic potential).

Zone of through-flow. The term through-flow pertains to the section of the water cycle at which water is recharged, then flows through voids in the rocks, and eventually is discharged at a terminal base of drainage. This definition leaves out other modes of groundwater motion, which are not... 

The term terminal base of drainage is applied to emphasize that the level of final drainage is meant, not an intermediate base of drainage such as a lake, a valley, or a river. The terminal base of drainage is in general the sea. 

A closer look at the zone of lateral base flow (overflow). Overflowing water flows laterally by the critical angle. This angle is determined by the water viscosity, which in turn is dependent on the temperature and concentration of dissolved ions. Groundwater flows laterally toward the terminal base of drainage at a critical angle that is determined by the hydraulic conductivity, or permeability, of the rocks (k) the water viscosity, which depends on the temperature (7), and the concentration of dissolved ions (i) ... 

The zone of lateral base flow accumulates recharge water introduced over the surface. The flux, or amount of water flowing at each section, increases from the principal water divide in the downflow direction and reaches a maximal value at the drainage front. Thus the zone of lateral base flow has a certain thickness that is expected to grow toward the base of drainage. 

Fig. 2.18 A cross-section of a much-quoted model (following Freeze and Cherry, 1979, who cited Hubbert, 1940). The surface is described as undulating in a mode that can be expressed by a simple mathematical equation, and the water table is assumed to follow topography in a fixed mode. The stippled section describes a water system from a low-order divide to a nearby low-order valley the thick lines mark there impermeable planes that are an intrinsic part of the U-shape flow paths model, enlarged in Fig. 2.19. The cross-section emphasizes topographic undulations and disregards the location of the terminal base of drainage and the location of the main water divide.

Fig. 2.20 The modeled box of Figs. 2.18 and 2.19, plotted on a regional cross-section from a major watershed to the terminal base of drainage. The assumption of three impermeable planes is groundless, as discussed in the text.

Fig. 3.7 Stagnant confined systems are former (fossil) through-flow systems (1) that got buried beneath the level of the terminal base of drainage and (2) were covered by new rock systems that eventually got buried. System (3) is presently active as a through-flow system.

Similarly, hydrological models, based on precipitation distribution, topographic relief, location of the terminal base of drainage, hydraulic head data, and geology are also of a fragmented nature and seldom lead to a unique model solution. The situation may be significantly improved by the application of hydrochemical and isotopic checks, as discussed below. 

Flow velocities of water in an aquifer, calculated by gradients and transmissivities, provide the maximum possible values. These are subject to limitations imposed by stagnation conditions. In extreme cases, confined systems may be rich in fossil karstic conduits, but with no through-flow due to complete confinement and/or burial beneath the level of the terminal base of drainage. 

Runoff travels down the landscape to the base of drainage— the sea surface... 

Answer 1.1 It seems easy to grasp that along the seacoast all rivers flow into the sea. The observation that all rivers run into the sea describes runoff. Water flows down along the topographic relief, and the sea is the terminal base of drainage. 

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