Geoscience Reference
In-Depth Information
in the cup. If the pressure in the cup falls below the air-entry value of the largest pores,
air will enter the tensiometer and all the water may be adsorbed by the soil. In practice
an air-entry value near -900 cm is chosen. Smaller pores increase the hydraulic resis-
tance of the ceramic cup and thus the reaction time of the tensiometer.
Question 4.21: What is the equivalent diameter of the largest pore in a tensiometer cup
that can be used to measure pressure heads of -900 cm? Hint: Use Eq. ( 4.7 ) for capillary
rise with surface water tension σ = 0.07 N m -1 and wetting angle φ = 0º.
Because air is compressible and has a large thermal expansion coeficient, isolated
air bubbles inside tensiometers make them very sluggish in following changes in
soil water pressure and make them sensitive to temperature changes. If air is abun-
dant in the system, it can make accurate measurements altogether impossible. The
air problems can be nearly eliminated by illing tensiometers with de-aerated water
and selecting ceramic cups with pores smaller than 2 μm. However, air may still dif-
fuse through the tubing connecting the cup with the pressure measuring device and
through the water in the ceramic cup pores. The former can be eliminated by using
impermeable tubing, such as copper, but the latter can be reduced only by using thick
ceramic cups of low porosity which increase the reaction time of the tensiometer.
Therefore, tensiometers must be lushed periodically with de-aerated water to drive
accumulated air out of the system (Koorevaar et al., 1983 ).
4.11 Measurement of Soil Water Content
4.11.1 Gravimetric and Volumetric Soil Water Content
The quantity of water in soil may be expressed as volumetric water content θ (cm 3 cm -3 )
or as gravimetric water content w (g g -1 ). Figure 4.29 deines the volumes and masses
of solids, water, air and pores in a soil. The volumetric water content is the volume of
liquid water per volume soil and is calculated as θ = V w / V total , where V w is the water vol-
ume and V total is the total soil volume. The gravimetric water content is the mass of water
per mass of dry soil and equals w = M w / M s , where M w is the water mass and M s the solid
mass (note that M s is used, and not M total ). As the density of the solid phase varies in nat-
ural soils, volumetric water contents are easier to use than gravimetric water contents.
For instance, if we know that a soil has a volumetric water content θ , we may directly
calculate the water storage in a soil layer with thickness Δ z as the product θ × Δ z cm.
Therefore volumetric water contents are commonly used in applied soil physics. The
two most applied methods to determine the soil water content are by oven drying and
by time domain relectrometry, which are discussed in the next sections.
Question 4.22: Sometimes gravimetric water contents should be converted to volu-
metric water contents. Derive from Figure 4.29 that these water contents are related by
θ ρ
= d
w
w
where ρ d is the soil dry bulk density (g cm -3 ) and ρ w is the density of water (= 1 g cm -3 ).
Search WWH ::




Custom Search