Table Of Content1
Future Opportunities and Challenges in Remote Sensing of Drought
Brian D. Wardlow1, Martha C. Anderson2, Justin Sheffield3, Brad Doorn4, Xiwu Zhan5, Matt
Rodell6, and Others
1National Drought Mitigation Center, University of Nebraska-Lincoln
2U.S. Department of Agriculture, Agricultural Research Service
3Department of Civil and Environmental Engineering, Princeton University
4National Aeronautics and Space Administration, Earth Sciences Directorate
5National Oceanic and Atmospheric Administration, Center for Satellite Applications and
Research
6 National Aeronautics and Space Administration, Goddard Space Flight Center
CONTENTS
16.1 Introduction
16.2 Future Opportunities
16.2.1 Soil Moisture Active/Passive (SMAP)
16.2.2 Surface Water and Ocean Topography (SWOT)
16.2.3 GRACE-II
16.2.4 Visible/Infrared Imager Radiometer Suite (VIIRS)
16.2.5 Landsat Data Continuity Mission (LDCM)
16.2.6 Next Generation Geostationary Satellites
16.2.7 Other Proposed Sensors
16.2.8 Enhancement of Land Data Assimilation Systems with Remotely Sensed Data
16.3 Challenges for the Remote Sensing Community
16.3.1 Engagement of the User Community
16.3.2 Accuracy Assessment
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16.3.2.1 Extended and Near Real-Time Assessment Campaigns
16.3.2.2 Consideration of Drought Impacts
16.3.4 Long-Term Data Continuity
16.4 Final Thoughts
References
16.1 INTRODUCTION
The value of satellite remote sensing for drought monitoring was first realized more than two
decades ago with the application of Normalized Difference Index (NDVI) data from the
Advanced Very High Resolution Radiometer (AVHRR) for assessing the effect of drought on
vegetation, as summarized by Anayamba and Tucker (2012; Chapter 2). Other indices such as
the Vegetation Health Index (VHI). (Kogan, 1995) were also developed during this time period,
and applied to AVHRR NDVI and brightness temperature data for routine global monitoring of
drought conditions. These early efforts demonstrated the unique perspective that global imagers
such as AVHRR could provide for operational drought monitoring through their near-daily,
global observations of Earth’s land surface. However, the advancement of satellite remote
sensing of drought was limited by the relatively few spectral bands of operational global sensors
such as AVHRR, along with a relatively short period of observational record.
Since the late 1990s, spectral coverage was expanded with the launch of many new satellite-
based, remote sensing instruments such as the Moderate Resolution Imaging Spectroradiometer
(MODIS), Medium Resolution Imaging Spectrometer (MERIS),Gravity Recovery and Climate
Experiment (GRACE), Advanced Microwave Scanning Radiometer - Earth Observation System
(AMSR-E), Tropical Rainfall Measuring Mission (TRMM), Advanced Microwave Sounding
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Unit (AMSU), [mr1]which have provided an array of new Earth observations and measurements to
map, monitor, and estimate a wide range of environmental parameters that are well suited for
drought monitoring. As illustrated by the various remote sensing methods presented in this
book, many components of the hydrological cycle can now be estimated and mapped from
satellite, providing the drought community with a more complete and comprehensive view of
drought conditions. Precipitation inputs (and deficits) in the form of both rainfall (Story, 2012;
Chapter 12 and AghaKouchak et al., 2012; Chapter 13) and snow (Kongoli et al., 2012; Chapter
15) can now be monitored over large areas through a combination of ground-based radar and
satellite-based optical and microwave observations. Natural water consumption on the land
surface can also be assessed with new innovative techniques applied to thermal data to estimate
evapotranspiration (ET) as demonstrated by Senay et al. (2012; Chapter 6), Anderson et al.
(2012; Chapter 7), and Marshall et al. (2012; Chapter 8). In addition, new perspectives on
vegetation conditions are provided by new hybrid vegetation indicators (Wardlow et al., 2012;
Chapter 3 and Tadesse et al., 2012; Chapter 4) and estimates of biophysical parameters such as
the fraction of absorbed photosynethetic active radiation (fAPAR) (Rossi and Niemeyer, 2012;
Chapter 5). Lastly, unprecedented insights into sub-surface moisture conditions (i.e, soil
moisture and groundwater) are possible through the analysis of microwave (Nghiem et al., 2012;
Chapter 9 ) and gravity anomaly (Rodell, 2012; Chapter 11) data, as well as land data
assimilation systems incorporating remotely sensed moisture signals (Sheffield et al., 2012;
Chapter 10).
Such remote sensing advancements are of paramount importance given the increasing demand
for tools that can provide accurate, timely, and integrated information on drought conditions to
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facilitate proactive decision making (NIDIS, 2007). Satellite-based approaches are key to
addressing significant gaps in the spatial and temporal coverage of current surface station
instrument networks providing key moisture observations (e.g., rainfall, snow, soil moisture,
ground water, and ET) over the United States and globally (NIDIS, 2007). Improved monitoring
capabilities will be particularly important given increases in spatial extent, intensity, and duration
of drought events observed in some regions of the world, as reported in the International Panel
on Climate Change (IPCC) report (IPCC, 2007). The risk of drought is anticipated to further
increase in some regions in response to climatic changes in the hydrologic cycle related to
evaporation, precipitation, air temperature, and snow cover (Burke et al., 2006; IPCC, 2007;
USGCRP, 2009). Numerous national, regional, and global efforts such as the Famine and Early
Warning System (FEWS), National Integrated Drought Information System (NIDIS), and Group
on Earth Observations (GEO), as well as the establishment of regional drought centers (e.g.,
European Drought Observatory) and geospatial visualization and monitoring systems (e.g,
NASA SERVIR) have been undertaken to improve drought monitoring and early warning
systems throughout the world. The suite of innovative remote sensing tools that have recently
emerged will be looked upon to fill important data and knowledge gaps (NIDIS, 2007; NRC,
2007) to address a wide range of drought-related issues including food security, water scarcity,
and human health.
16.2 FUTURE OPPORTUNITIES
Over the next decade, further progress in the application of satellite remote sensing for drought
monitoring is expected with the launch of several new instruments, many recommended by the
2007 Decadal Survey on Earth Science and Applications from Space (NRC, 2007). These
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instruments will enhance current capabilities in monitoring specific variables (e.g., soil moisture
and terrestrial water storage) and will enable estimation of new parameters such as surface water
elevation and storage. In addition, integration of new satellite observations into land data
assimilation systems (LDAS) is anticipated to improve various modeling outputs (e.g., soil
moisture and streamflow) that can be utilized for drought monitoring. In this section, several of
these new satellite-based sensors will be discussed, as well as the increased role of remotely
sensed data in LDAS models, to highlight opportunities that exist to further advance the
contributions of remote sensing for operational drought monitoring and early warning.
16.2.1 Soil Moisture Active/Passive (SMAP)
The Soil Moisture Active/Passive (SMAP) mission, which is currently scheduled to be launched
in the 2014-2015[mr2] time frame (NASA, 2010), is intended to provide global measurements of
soil moisture and freeze/thaw state, with drought monitoring and prediction as a targeted
application (Entekhabi et al., 2010). SMAP includes both L-band radar and radiometer
instruments, which will allow global mapping of soil moisture at a 10-km spatial resolution with
a 2-3 day revisit time. Combining the coincident radiometer and radar measurements from
SMAP will provide much higher spatial resolution soil moisture mapping capabilities than
previous instruments such as AMSR-E (Nghiem et al., 2012) and the European Soil Moisture
and Ocean Salinity (SMOS; Kerr et al., 2010) mission, which generate products at 25- and 50-
km resolutions, respectively. The SMAP 10-km soil moisture product will be achieved by
combining higher accuracy but coarser spatial resolution (40 km) radiometer-based soil moisture
retrievals, with higher accuracy but coarser spatial resolution (40 km), and higher spatial
resolution radar data (1 to 3 km) that has lower retrieval accuracy. In addition, the integration of
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these two types of data enables soil moisture to be estimated under a wider range of vegetation
conditions. Radar backscatter is highly influenced by land surface conditionscover and only
performs adequately in low vegetation conditions (Dubois et al., 1995), whereas the L-band
radiometers have improved sensitivity to soil moisture under moderate vegetation cover. SMAP
measurements will provide ‘direct’ sensing of surface soil moisture in the top 5 cm of the soil
profile (Entekhabi et al., 2010). Because many applications such as drought monitoring require
information about soil moisture in the root zone, the SMAP mission will also provide estimates
of soil moisture representative of a 1-m soil depth by merging SMAP observations with land
surface model estimates in a soil moisture data assimilation system (Reichle, 2008).
16.2.2 Surface Water and Ocean Topography (SWOT)
The Surface Water and Ocean Topography (SWOT) mission is designed to provide water surface
elevation (WSE) measurements for ocean and inland waters including lakes, reservoirs, rivers,
and wetlands. The primary SWOT instrument will be an interferometric altimeter that utilizes a
Ka-band synthetic aperture radar (SAR) interferometer with two antennas to measure WSE
(Durand et al., 2010a). Over land, SWOT will directly measure the area and elevation of water
inundation with a spatial resolution on the order of tens of meters. Specific observational goals
include a maximum 10-day temporal resolution and the capability to spatially resolve rivers with
widths greater than 50- to 100-m and water bodies with areas greater than 250-m2. The vertical
precision requirement of the WSE measurements is set at 10 cm over 1-km2 area and river slope
measurements within 1 cm over a 1-km distance (Biancamaria et al., 2010). Initial results in
applying simulated SWOT data for characterizing surface water changes of inland water bodies
have been promising (Durand et al., 2008; Lee et al., 2010). SWOT is anticipated to provide
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new information about the spatio-temporal dynamics of surface water (i.e., areal extent, storage,
and discharge) globally, which would have tremendous benefit for many applications including
water resource management and hydrologic drought monitoring and prediction. The SWOT
mission is currently targeted to be launched in 2020 (NASA, 2010), but several recent efforts
have explored the expected accuracy of SWOT products for measuring surface water storage of
lakes (Lee et al., 2010) and water surface profiles (Schumann et al., 2010), as well as the depth
and discharge (Durand et al., 2010b) of rivers.
16.2.3 GRACE Follow-On and GRACE-II
GRACE mission, which hasThe Gravity Recovery and Climate Experiment (GRACE) has
provided new insights into terrestrial water storage for drought monitoring as summarized by
Rodell (2012, in Chapter 11). GRACE has demonstrated the abilityis able to monitor
groundwater changes, as well as variations in shallow and root-zone soil moisture content
(Famiglietti et al., 2011; Rodell et al., 2009; Tiwari et al., 2009; Rodell et al., 2007; Yeh et al.,
2006). Outputs from GRACE related to surface and root-zone soil moisture and groundwater
storage are now being applied in operational drought monitoring activities (Rodell, 2012 in
Chapter 11; Houborg et al., 2010). The National Research Council Decadal Survey (NRC, 2007)
recommended that NASA launch an advanced technology gravimetry mission (GRACE II)
towards the end of the decade. A Gravity Recovery and Climate Experiment-II (GRACE-II)
mission has been recommended by the National Research Council Decadal Survey (NRC, 2007)
to extend the time series gravity field observations from the original GRACE mission, which has
provided new insights into terrestrial water storage for drought monitoring as summarized by
Rodell (2012, in Chapter 11). GRACE has demonstrated the ability to monitor groundwater
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changes, as well as variations in shallow and root-zone soil moisture content (Famiglietti et al.,
2011; Rodell et al., 2009; Tiwari et al., 2009; Rodell et al., 2007; Yeh et al., 2006). Outputs from
GRACE related to surface and root-zone soil moisture and groundwater storage are now being
applied in operational drought monitoring activities (Rodell, 2012 in Chapter 11; Houborg et al.,
2010). The proposed GRACE-II willmission would replace the microwave interferometer with a
laser and also fly at a lower altitude with a drag free system, enabling perhaps an order of
magnitude improvement in spatial resolution, making GRACE II have improved spatial
resolution and will provide information about variations in groundwater storage and other sub-
surface moisture at a spatial scale more directly applicable to wider range of water resource
characterization and management activities globally (NRC, 2007). However, taking into account
the value of GRACE for many applications beyond drought monitoring, including measurement
of ice sheet and glacier mass losses, groundwater depletion, and ocean bottom pressures, NASA
has given preliminary approval to a GRACE Follow-On (GRACE FO) mission targeted for
launch in 2016. GRACE FO would reduce an expected data gap after the terminus of GRACE
and provide time for the technology developments required for GRACE II. The configuration of
GRACE FO would be similar to GRACE, with incremental technological improvements that
should afford some level of error reduction / increased spatial resolution. GRACE-II has been
recommended for launch in the 2016-2020 time frame (NRC, 2007), which could result in a data
gap considering GRACE was launched in 2002 and had an expected 5-year lifetime. In
response, a GRACE Follow-On (GRACE FO) mission has been proposed in the interim to
continue the time series of gravitational field measurements until the GRACE-II mission can be
developed (NASA, 2010).
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16.2.4 Visible/Infrared Imager Radiometer Suite (VIIRS)
The Visible/Infrared Imager/Radiometer Suite (VIIRS) is the next-generation, moderate
resolution imaging radiometer that will be on-board the National Polar-orbiting Operational
Environmental Satellite System (NPOESS) Preparatory Project (NPP), scheduled for a 2011
launch, and the subsequent operational NPOESS series of satellites (first satellite scheduled for
launch in 2014) (Lee et al., 2010). The VIIRS instrument will be the operational successor to
AVHRR and MODIS (Townshend and Justice, 2002), collecting data in 22 spectral bands
spanning the visible, as well as the near, middle, and thermal infrared wavelength regions (Lee,
2006). The spectral bands of VIIRS were chosen primarily from two legacy instruments,
AVHHR and MODIS, which have both greatly contributed to several remote sensing tools for
drought monitoring discussed in earlier chapters. VIIRS is designed to provide daily global
coverage at spatial resolutions of 370-m (5 bands) and 740-m (17 bands) (Lee et al., 2006). In
addition, several data products, termed environmental data records, will be produced from the
VIIRS observations (Lee, 2006) including vegetation indices (NDVI and EVI, Enhanced
Vegetation Index) (Justice et al., 2010), land surface temperature (LST), and snow cover/depth
data, each of which can be used for operational drought monitoring. The generation of VI and
LST products from VIIRS is critical for extending the historical time series previously
established with AVHRR and MODIS. The placement of a VIIRS instrument on the NPP
platform was intended to provide continuity for observational datastreams from instruments on
NASA’s Terra and Aqua platforms (Townshend and Justice, 2002). This will be followed by
VIIRS instruments on a series of NPOESS platforms that are planned to be operational into the
2023-2026 time period (Lee et al., 2010). Future application of VIIRS VI and LST data for
drought monitoring will support tools such as the Vegetation Drought Response Index (VegDRI,
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Brown et al., 2008 and Wardlow et al., 2012 in Chapter 3) and estimation of ET (Anderson et al.,
2007) and ET-derived indices (Anderson et al., 2011 and Anderson et al., 2012 in Chapter 7).
16.2.5 Landsat Data Continuity Mission (LDCM)
Along with prescribing recommendations for new missions, the NRC Decadal Survey also
stressed the need for maintaining critical long-term Earth surveillance programs such as Landsat.
The Landsat satellite series have been collecting global earth observations in the visible and near
infrared bands since 1972 (15-60 m spatial resolution), and the thermal band since 1982 (60-120
m spatial resolution). The resulting dataset is the only long-term civilian archive of satellite
imagery at scales of human influence, resolving individual farm fields, deforestation patterns,
urban expansion, and other types of land cover change. Both Landsats 5 and 7 (currently active)
have functioned well past their expected lifetimes. The Landsat Data Continuity Mission
(LDCM) is scheduled for launch in December 2012, and will carry the Operational Land Imager
(OLI) that will continue six heritage shortwave bands as well as two new coastal and cirrus
bands with a30-m spatial resolution. The Thermal InfraRed Sensor (TIRS) will collect thermal
infrared data in two split-window channels to facilitate atmospheric correction. TIRS will enable
global mapping of ET and vegetation stress at sub-field scales.
While the revisit period of individual Landsat satellites (16 days or more, depending on cloud
cover) is not particularly well-suited for operational monitoring rapid changes in vegetation and
moisture conditions associated with drought events, multiple staggered systems could provide 8-
day (2 systems) or 5-day (3 systems) coverage. Increased temporal frequency could also be
achieved by increasing the swath width of data collection. The potential also exists to fuse high-
