Hydraulic head

Groundwater

In general, the total mechanical energy of ground water shall consist of:

  • Pressure energy
  • Able energy, also called potential energy, relative to a reference level (eg, height above sea level ) and
  • Kinetic energy, also called kinetic energy,

That is, it is important Bernoulli's energy equation:

With

  • Water pressure p ( in groundwater pore water pressure ) ( force per unit area [ F/L2 ], often Pa or N · m-2)
  • Volume V
  • Mass m of the water element
  • Acceleration due to gravity g ( often in m · s -2)
  • Able potential z, that is, the height of the measuring point on the reference line
  • Flow velocity v.

Since the flow velocities are very small in groundwater in general, one can neglect the fraction of the kinetic energy to a good approximation:

In addition, the specific energy per equivalent weight of a water element from the dimension analysis of the part of a geometric height:

Thus, the hydraulic potential h in the groundwater hydraulics defined as follows:

The pressure altitude is here expressed as:

With

  • Specific gravity of the fluid ( force per unit volume [ F/L3 ], often N · m -3),   Density of the fluid ( mass per unit volume [ M/L3 ], often kg · m -3).

The hydraulic potential can be relatively easily measured. For this purpose, a tube ( standpipe ) introduced at a Verfilterung in the aquifer ( by drilling, pile drivers, etc.), in low permeable soils the pore water pressure is measured by a load cell ( piezometers ). The hydraulic potential at the measurement point is then represented by

  • The height / depth of the water level in the pipe relative to a reference point ()
  • Measured by the load cell pressure level, combined with the position of the load cell relative to a reference level ().

Differences in energy level at two points of the aquifer lead to groundwater movement between these two points. Groundwater flows always from higher to lower hydraulic potential. The decrease of the hydraulic potential ( piezometric head ) corresponds to a " loss of energy ", a part of the energy is namely by the internal friction between the water and the solid rock ( grains, fractures, etc.) into heat energy. Depending on the ratio between the pressure and potential energy groundwater can even against gravity flow ( Artesian well ).

Unsaturated area

In order to model the water movements in the ground, one would have to know in principle the total potential, which is defined as the sum of all forces acting on the water in the bottom part potential. Since the total potential is, however, difficult to determine in practice, is used for the description of water movement instead the hydraulic potential as an approximation. This is the sum of simple determinable potentials matric potential and gravitational potential, the gas potential is usually not taken into account:

Where the hydraulic potential vanishes (), is the horizontal watershed.

To capture the availability of water for a plant, a different combination of partial potentials is used, the water potential.

Hydroelectricity Dimensional analysis Specific weight
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