Horizontal watershed

The horizontal or hydraulic watershed indicates the depth in the ground at which the hydraulic potential is: ${\displaystyle \psi _{h}=0}$

• above the horizontal watershed the hydraulic potential is negative ( ), there is an upward movement of the water ${\displaystyle \psi _{h}<0}$
• below the horizontal watershed the hydraulic potential is positive ( ), there is a downward movement of the water. ${\displaystyle \psi _{h}>0}$

The hydraulic potential is calculated as the sum of the gravitational and matrix potential .

${\displaystyle \psi _{\mathrm {h} }=\psi _{\mathrm {m} }+\psi _{\mathrm {z} }}$

Since the matrix potential is negative by definition:

${\displaystyle \psi _{\mathrm {m} }<0,}$

If the matrix potential is greater than the gravitational potential, a negative hydraulic potential arises :

${\displaystyle |\psi _{m}|>\psi _{\text{z}}\quad \Rightarrow \quad \psi _{\text{h}}<0}$

With strong evaporation or water uptake by plants, the soil dries out from the surface, the matrix potential decreases (the amount increases), the remaining water in the soil is more strongly bound. This leads to an upward movement of water in the upper soil area above the horizontal watershed. In the course of the summer, with an evapotranspiration that exceeds the precipitation , the drying soil area expands further downwards, i.e. H. the horizontal watershed shifts downwards. In autumn and winter, on the other hand, when there is little or no evapotranspiration, the soil water supply is replenished , the matrix potential rises (up to 0), and the horizontal watershed moves upwards in the soil until it disappears completely.

Substances dissolved in the soil water such as nitrate , which are shifted downwards, are available to the plants for uptake as long as the soil water is above the horizontal watershed. Dissolved ingredients, but in the leachate are below the horizontal watershed (or below a depth that the horizontal divide during the growing season can not reach), have a washed out be considered, although depending on the thickness and permeability of the unsaturated zone of the Transport to entry into the groundwater can take months or years.

See also

• Seepage line

Web links

• Hydroscript , hydrology lecture by M. Schöniger & J. Dietrich