 Installation

Schematic of a weighable gravitation lysimeter.

A 0.25 m filter layer (sand, coarse sand, gravel) is placed at the bottom of each lysimeter to avoid disturbances to natural flux. Three special digital load cells, which are situated on top of three aluminium pedestals. TDR (Time Domain Reflectrometry) probes, suction cups, tensiometers and thermometers recording hourly values are installed at depths of 0.30 m, 0.90 m and 1.50 m beneath the surface. The amount of seepage water is measured continuously with a tipping bucket and collected in a storage container for taking water samples for chemical analysis. All frequently measured parameters are stored in a data-logger.

Arrangement 

 The soil and crop conditions in the lysimeters should be close to the natural conditions.  Water added, water retained by the soil and water lost through evaporation, transpiration and deep percolation measurements involve weighing which may be made with scales or by floating the lysimeters in water on a suitable heavy liquid, in which case the change in liquid displacement is computed against water loss from the tank.

The tanks must be permanently buried in the ground and surrounded by a large area of crop of the same height if the readings made are to bear relation to losses from the crop in the field. The water table is maintained at a specific depth in the tank by connecting it to a supply tank provided with a float mechanism which has an arrangement for receiving excess water that tends to build up in the tank.

 Water is applied in measured amounts to the lysimeter, as irrigation is applied to the surrounding cropped area.  The overflow and deep percolation are measured.  The water received either from the reservoir or precipitation excluding the overflow, constitutes the water used by the crop.  To ensure proper drainage, the bottom of an isolated soil column will often require the artificial application of a moisture suction equivalent to that present at the same depth in the natural soil.

The actual evapotranspiration can easily be derived using the following equation: ETa = P - D ± ΔS (1) ETa actual evapotranspiration (mm) P precipitation (mm) D amount of seepage (drainage) water (mm) Δ S change in the amount of water stored (mm), based on measuring the weight change of the soil column over time;

Limitations  Temperature  Water table  Soil texture  Density, etc…..

References ALSG (2004). Austrian Lysimeter Study Group – ATV-DVWK (2002). ´Evapotranspiration in dependence from land use, plant cover and soil´. (ATV-DVWK Merkblatt 504: Hennef).