Figure 6.1 Measurement volume with a void space between the tine assembly.
Void spaces between the tine assembly can also occur from changing soil conditions. Factors
such as shrink/swell clays, tree roots or pebbles may introduce a void space. The following
sections describe some of these and other factors.
6.4.1 Evapotranspiration
Water in the soil will be pulled downward by gravity, however during dry periods or in arid
regions, the net movement of water is up toward the surface. Water will move upward in the soil
column by a phenomenon called Evapotranspiration (ET). ET is the direct evaporation out the
soil plus the amount of water being pulled out of the soil by plants. Factors such as wind,
temperature, humidity, solar radiation and soil type play a role in the rate of ET. If ET exceeds
precipitation, there will likely be a net upward movement of water in the soil. With the net
upward movement of soil water, ET forces dissolved salts out of solution and thus creating saline
soil conditions.
6.4.2 Hydrology and Soil Texture
Sandy soils drain better than soils that are clay rich. In general, the smaller the soil particle size
distribution, the slower it will drain. Sometimes silt may have the same particle size distribution,
as clay but clay will retain more water for longer periods of time than silt. This can be explained
by the shape of the soil particles. Clay particles are planar where as silt particles are spherical.
Water basically gets stuck between the planar plate shaped clay particles and thus slows the flow
of water.
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