Determination of Soil Water Retention Curve

The water retention function 6 (h) usually is determined in the laboratory on undisturbed soil cores by proceeding through a series of wetting and drainage events and taking measurements at equilibrium conditions, or in the field by measuring simultaneously water contents and pressure potentials during a transient flow experiment. Reviews of direct methods for determining the soil water-retention curve can be found in the literature for laboratory analyses on soil cores [14] and for field soils [15].

In the soil water-pressure range from 0 to about —2.5m, drying water-retention values often are measured in the laboratory by placing initially saturated undisturbed soil cores on a porous material (e.g., sand-kaolin bed, mixture of glass and diatomaceous powders), which then is subjected to varying soil water-pressure heads. The selected porous bed is held in a container, which usually is made of ceramics or Perspex and is provided with a cover to prevent evaporative losses. After reaching conditions of water equilibrium at a fixed pressure head, water content in each core is measured gravimetrically. Before removing a soil core from the porous bed for weighing, it is thus important to know whether equilibrium has been reached. This condition can be monitored conveniently by placing on the upper surface of the soil core a tensiometer, consisting, for example, of a sintered glass slab connected to a pressure transducer.

In-situ 0 (h) data points can be obtained readily at different soil depths from simultaneous measurements of 0 and h using field tensiometers for pressure potentials and a neutron probe, or TDR probes, for water content.

Indirect methods to determine the soil water-retention curve also have been proposed and are discussed in Section 5.2.8.

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