Geoscience Reference
In-Depth Information
Soil moisture content
Soil water potential
Initial soil
moisture
content
profile
A
Initial
profile
Water
movement
upwards
Final soil
moisture
content
profile
Average “zero flux” plane
B
Water
movement
downwards
Final
profile
A
Water lost to evaporation
B
Water lost to drainage
Figure 7.13
Measuring evaporative water loss using soil water depletion with regions in the profile with upward and
downward water loss defined by the zero flux plane obtained from measurements of soil water potential. (From
Shuttleworth, 1993, published with permission.)
surrounding area, an identical crop must be grown in the lysimeter to maintain
a similar soil moisture status.
Soil moisture depletion
Evaporation is sometimes measured by monitoring incoming precipitation and the
change in soil moisture beneath sample areas of vegetation and soil. The change in
soil moisture can be determined in different ways, including using neutron probes,
capacitance probes, and time-domain reflectometer sensors. Adequate spatial
sampling is required for an accurate estimate of evaporation, and drainage must
either be easily quantified or negligible. This method is particularly useful for
comparing evaporation from undisturbed plots of different crops. The method
becomes more accurate if there are co-located measurements of soil tension to
determine the average
zero flux plane
which separates regions in the soil profile in
which movement is primarily upward from those where it is downward, see Fig. 7.13.
Comparison of evaporation measuring methods
Attributes of the several evaporation measuring methods described above are
given in Table 7.1 along with their relative strengths and weaknesses, the scale at
which measurement is made, and likely errors in each method.
