differently depending on the dominant vegetation type/structure [20] as well as the
biomass and condition of vegetation [126, 127]. In areas of open water without
vegetation, specular reflection occurs and a dark signal (weak or no return) is
observed [128]. Specular reflectance also occurs in wetlands dominated by lower
biomass herbaceous vegetation when a layer of water is present [129]. Conversely,
the radar signal is often increased in forested wetlands when standing water, such as
flooding, is present due to the double-bounce effect, i.e., the radar pulse is reflected
strongly by the water surface away from the sensor (specular reflectance) but is then
redirected back towards the sensor by a second reflection from a nearby tree trunk
[128, 130].
Wetland InSAR is a unique application of the interferometric synthetic aperture
radar (InSAR) technique that detects elevation changes of aquatic surfaces. Most
other InSAR applications detect displacements of solid surfaces [131]. The technique works because the radar pulse is backscattered twice (double bounce) from
the water surface and vegetation. Wetland InSAR provides high spatial resolution
hydrologic observations of wetlands and floodplains that cannot be obtained by any
terrestrial-based methods. InSAR observations of wetlands have been acquired over
various wetland environments using L-, C-, and X-bands and different polarizations
[132, 133]. L-band data are most suitable for the wetland InSAR applications.
However, the X- and C-band radar signals, which primarily interact only with the
upper sections of the vegetation, were also found to be useful. In general, promising
applications of InSAR for wetland observations include high spatial resolution
water level monitoring, detection of flow patterns and flow discontinuities, and
constraining high-resolution flow models [131].
7 Drought Monitoring and Prediction
Accurate mapping and monitoring of drought severity is important for water
management and drought mitigation efforts. A system for drought monitoring and
prediction can be a vital tool to facilitate drought response while saving money,
time, and lives [134]. Drought indicators can be based on one variable or a
combination of variables. Different indicators describe various aspects of droughts;
however, holistic, comprehensive drought assessment requires multiple indicators.
Drought indices integrate large amounts of data, such as precipitation, vegetation
condition, snowpack, streamflow, and other water presence/supply indicators, to
monitor drought severity in a comprehensive framework and to measure how much
the climate in a given period has deviated from historically established normal
conditions [135, 136]. These indicators can be obtained from different sources,
including satellite observations, model simulations, and reanalysis of past data.
One example of a system for monitoring and predicting drought is the Global
Integrated Drought Monitoring and Prediction System (GIDMaPS) developed in
2012 by researchers at the University of California, Irvine [83]. The system gathers
and synthesizes land-atmosphere model simulations and remote sensing
Using Remote Sensing to Map and Monitor Water Resources in Arid and Semiarid. . .
51
biomass and condition of vegetation [126, 127]. In areas of open water without
vegetation, specular reflection occurs and a dark signal (weak or no return) is
observed [128]. Specular reflectance also occurs in wetlands dominated by lower
biomass herbaceous vegetation when a layer of water is present [129]. Conversely,
the radar signal is often increased in forested wetlands when standing water, such as
flooding, is present due to the double-bounce effect, i.e., the radar pulse is reflected
strongly by the water surface away from the sensor (specular reflectance) but is then
redirected back towards the sensor by a second reflection from a nearby tree trunk
[128, 130].
Wetland InSAR is a unique application of the interferometric synthetic aperture
radar (InSAR) technique that detects elevation changes of aquatic surfaces. Most
other InSAR applications detect displacements of solid surfaces [131]. The technique works because the radar pulse is backscattered twice (double bounce) from
the water surface and vegetation. Wetland InSAR provides high spatial resolution
hydrologic observations of wetlands and floodplains that cannot be obtained by any
terrestrial-based methods. InSAR observations of wetlands have been acquired over
various wetland environments using L-, C-, and X-bands and different polarizations
[132, 133]. L-band data are most suitable for the wetland InSAR applications.
However, the X- and C-band radar signals, which primarily interact only with the
upper sections of the vegetation, were also found to be useful. In general, promising
applications of InSAR for wetland observations include high spatial resolution
water level monitoring, detection of flow patterns and flow discontinuities, and
constraining high-resolution flow models [131].
7 Drought Monitoring and Prediction
Accurate mapping and monitoring of drought severity is important for water
management and drought mitigation efforts. A system for drought monitoring and
prediction can be a vital tool to facilitate drought response while saving money,
time, and lives [134]. Drought indicators can be based on one variable or a
combination of variables. Different indicators describe various aspects of droughts;
however, holistic, comprehensive drought assessment requires multiple indicators.
Drought indices integrate large amounts of data, such as precipitation, vegetation
condition, snowpack, streamflow, and other water presence/supply indicators, to
monitor drought severity in a comprehensive framework and to measure how much
the climate in a given period has deviated from historically established normal
conditions [135, 136]. These indicators can be obtained from different sources,
including satellite observations, model simulations, and reanalysis of past data.
One example of a system for monitoring and predicting drought is the Global
Integrated Drought Monitoring and Prediction System (GIDMaPS) developed in
2012 by researchers at the University of California, Irvine [83]. The system gathers
and synthesizes land-atmosphere model simulations and remote sensing
Using Remote Sensing to Map and Monitor Water Resources in Arid and Semiarid. . .
51