Geoscience Reference
In-Depth Information
Volumetric soil water content plays many essential roles in our knowledge of
hydrological balancing of soil water regimes, of computing the consumption of
water by plants, of estimating wind and water erosion of soils, etc. If the volumetric
soil water content measured before a brief rain was 35 %, and immediately after the
rain a distinctly wetted 10-cm layer had a water content of 47 %, we learn that
12 mm of the rain infi ltrated the topsoil. If the rain manifested a measured height of
12 mm, we also learn that all of the rainwater infi ltrated the soil surface. However,
had the rain been 15 mm, we would know that 3 mm of rainwater was lost by sur-
face runoff or accumulated in puddles made on the soil surface. Various types of
observations and deductions obtained from knowledge about volumetric soil water
content shall be discussed in subsequent chapters.
Although the above sampling procedure to quantify the amount of water in a soil
is very simple and easily understood, it has the serious disadvantage of requiring
that holes be dug into the soil surface or the steps of excavated soil using any one of
the many types of soil augers. In other words, whenever the soil water content is
measured, a hole must be dug. To measure a change in soil water content during a
specifi c period of time, a minimum of two holes are required. The observation of
periodic changes of soil water content requires periodic digging of more and more
holes. Hence, a long-lasting series of periodic observations destroys the properties
of a naturally occurring soil profi le and renders it into a material similar to Swiss
cheese. The holes after digging and drilling enable preferential deeper fl ow of rain-
water into the soil profi le, fi lling the holes and all excavations with any soil or soil
materials does not adequately substitute for the original soil, and the data obtained
after almost any period of such observations were not at all realistic and usable. As
a result, new methods were developed to measure soil water content without destroy-
ing the original nature of soils.
8.1
Methods of Measuring Soil Water Content
Considering the most broadly used present-day procedures, the fi rst method for
measuring soil water content without destroying the continuity of a natural fi eld soil
was the neutron probe. Later on, dielectric techniques and dual needle heat pulse
methods were developed.
Neutron probes were developed about 50 years ago at the time when many radio-
isotopes were started to be used in the study of various material properties. The
great advantage of the method when compared with the up-to-that-time gravimetric
soil sampling was the permanent stable position of the access tube placed into a soil
profi le to any depth with minimal disturbance of local soil properties. A source of
fast neutrons was inserted into the access tube made of either steel or aluminum.
Today, the source consists of a mixture of radioactive americium ( 241 Am) and beryl-
lium (Be). Actually, 241 Am emits alpha particles that are absorbed by the Be nucleus
and emits fast neutrons. Fast neutrons emitted from the beryllium interact primarily
with hydrogen atoms of water within the soil. This interaction slows down the fast
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