Sensing of Natural Soil Properties - Precision in Crop Farming - page 78

After having sensed the real relative permittivity or dielectric constant, the soil
water content excluding bound water can be estimated using Topp's equation
(Topp et al. 1980 ):

2 3

Volumetric soil watercontent =− 0 053 0 029 . + . ε − 0 00055 . ε + 0 00000 . 43 ε

ε is the real relative permittivity or dielectric constant.
This equation provides for reliable results (Stoffregen et al. 2002 ) under the
premise that ε was sensed precisely for the respective soil.
Sensing of permittivity is possible either on the basis of velocities or of reflec-
tions of waves. For wave velocities, time signals are recorded. Thus the sensing
occurs in the time domain. A well established method of this kind is the time
domain reflectometry (TDR) . Electromagnetic waves are guided along transmis-
sion lines or cables within the soil. The time that is needed depends on the permit-
tivity of the soil and thus indicates its water content. This method provides for rather
reliable results and therefore often is used as a reference. Topp's equation was
obtained using this method.
But unfortunately, methods that sense on a time domain up to now are not yet
suited for on-the-go site-specific operations. These methods therefore are left out
here. Instead, methods that rely on signals from reflected radiation will be dealt
with. These methods operate either from satellites or on-the-go from terrestrial
vehicles. Yet it will be shown that using reflected radiation can make it difficult to
obtain accurate dielectric constants of soils.

5.2.3.2 Water Sensing from Satellites by Permittivity

Water sensing from satellites is used extensively for observing the earth's atmo-
sphere with the objective of weather forecasting within large areas. But contrary
to this, sensing of soil moisture from satellites within single fields and thus for
site-speciic farming still is not state of the art. In the past, neither the spatial- nor
the temporal resolutions did correspond to the needs.
Yet both drawbacks are slowly disappearing. The spatial resolution for data from
some modern radar satellites that operate in an active mode now even is going
down to 1-100 m 2 , which is sufficient. In addition, providing for information on a
daily basis may become feasible because of more satellites. And the capability of
micro- or radar waves to penetrate the space between the satellites and the earth
never has been a problem. These waves have almost all weather capabilities -
contrary to those from the visible- and infrared range (Sect. 3.3 ) . An exception from
this all weather capabilitiy may hold solely at times of heavy rainfall. Hence some
general perspectives for signals about soil moisture from satellites by means of
micro- or radar waves are encouraging.
But what about the ability of micro- or radar waves to provide sufficiently
accurate signals about soil water? The relations between soil moisture and real