198
E.T. Engman
stage and in some cases some type of soil moisture description usually in the form
of an antecedent precipitation index. In many cases poor forecasts are attributed to
lack of information about the initial conditions, i.e., soil moisture. For example,
hydrologists at the NOAA Kansas City River Forecast Center believe that soil
moisture has been their most troublesome parameter affecting forecasts (Wiesnet,
1976) and Georgakakos et al. (1996) have identified soil moisture as the most
sensitive variable controlling runoff in the development of an operational flash
flood prediction system.
As important as this seems to our understanding of hydrology, soil moisture has
not had widespread application in the modeling of these processes. The main reason for this is that it is a very difficult variable to measure, not at a point in time,
but at a consistent and spatially comprehensive basis. The large spatial and temporal variability that soil moisture exhibits in the natural environment is precisely the
characteristic that makes it difficult to measure and use in Earth science applications. For the most part our understanding of the role of soil moisture in hydrology
has been developed from point studies where the emphasis has been on the variability of soil moisture with depth. Much of our failure to translate this point understanding to natural landscapes can be traced to a realization that soil moisture
varies greatly in space but with no obvious means to measure the spatial variability. As a parallel consequence, most models have been designed around the available point data and do not reflect he spatial variability that is known to exist.
9.2 General Approach
It has been shown that the soil moisture can be measured to some extent by a variety of techniques using all parts of the electromagnetic spectrum. Successful measurement of soil moisture by remote sensing depends upon the type of reflected or
emitted electromagnetic radiation. Table 9.1 summarizes the advantages and disadvantages of each approach. However, it will be seen that only the microwave
region of the spectrum can provide a quantitative approach to estimate soil moisture under a variety of topographic and vegetation cover conditions.
Gamma radiation techniques. Airborne soil moisture measurement by gamma
radiation is based on detecting the difference between the natural terrestrial gamma
radiation flux for wet and dry soils. The presence of water in the upper soil layers
increases the attenuation of the gamma radiation from below; thus the flux is less
for wet soils then for dry soils. Quantitative estimates of soil moisture require
calibration flight lines to determine the background soil moisture value, Mo, and
the background gamma count rate, Co. The current soil moisture, M can be estimated according to
M = C/Co(lOO+1.11Mo)-lOO
1.11
(9.1)
where C is the measured gamma count rate. A more complete description can be
found in Carroll (1981). Because the atmosphere also attenuates the gamma radia-
E.T. Engman
stage and in some cases some type of soil moisture description usually in the form
of an antecedent precipitation index. In many cases poor forecasts are attributed to
lack of information about the initial conditions, i.e., soil moisture. For example,
hydrologists at the NOAA Kansas City River Forecast Center believe that soil
moisture has been their most troublesome parameter affecting forecasts (Wiesnet,
1976) and Georgakakos et al. (1996) have identified soil moisture as the most
sensitive variable controlling runoff in the development of an operational flash
flood prediction system.
As important as this seems to our understanding of hydrology, soil moisture has
not had widespread application in the modeling of these processes. The main reason for this is that it is a very difficult variable to measure, not at a point in time,
but at a consistent and spatially comprehensive basis. The large spatial and temporal variability that soil moisture exhibits in the natural environment is precisely the
characteristic that makes it difficult to measure and use in Earth science applications. For the most part our understanding of the role of soil moisture in hydrology
has been developed from point studies where the emphasis has been on the variability of soil moisture with depth. Much of our failure to translate this point understanding to natural landscapes can be traced to a realization that soil moisture
varies greatly in space but with no obvious means to measure the spatial variability. As a parallel consequence, most models have been designed around the available point data and do not reflect he spatial variability that is known to exist.
9.2 General Approach
It has been shown that the soil moisture can be measured to some extent by a variety of techniques using all parts of the electromagnetic spectrum. Successful measurement of soil moisture by remote sensing depends upon the type of reflected or
emitted electromagnetic radiation. Table 9.1 summarizes the advantages and disadvantages of each approach. However, it will be seen that only the microwave
region of the spectrum can provide a quantitative approach to estimate soil moisture under a variety of topographic and vegetation cover conditions.
Gamma radiation techniques. Airborne soil moisture measurement by gamma
radiation is based on detecting the difference between the natural terrestrial gamma
radiation flux for wet and dry soils. The presence of water in the upper soil layers
increases the attenuation of the gamma radiation from below; thus the flux is less
for wet soils then for dry soils. Quantitative estimates of soil moisture require
calibration flight lines to determine the background soil moisture value, Mo, and
the background gamma count rate, Co. The current soil moisture, M can be estimated according to
M = C/Co(lOO+1.11Mo)-lOO
1.11
(9.1)
where C is the measured gamma count rate. A more complete description can be
found in Carroll (1981). Because the atmosphere also attenuates the gamma radia-