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Title:   Author  item
                                            AGHAKOUCHAK, A. - Collaboratoritem
                                            FARAHMAND, A. - Collaboratoritem
                                            TEIXEIRA, J. - National Aeronautics And Space Administration (NASA)item
                                            WARDLOW, B. - University Of Nebraskaitem
                                            MELTON, F. - National Aeronautics And Space Administration (NASA)item
                                            Anderson, Marthaitem
                                            HAIN  - University Of Maryland
  Submitted to: Reviews of Geophysics
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 5/18/2015
                                    Publication  Date: 5/23/2015
                                    Citation: Aghakouchak, A., Farahmand, A., Teixeira, J., Wardlow, B., Melton, F., Anderson, M.C., Hain 2015. Remote sensing of drought:  progress, challenges and opportunities. Reviews of Geophysics. doi: 10.1002/2014RG000456.Interpretive Summary: Indices used for operational monitoring of drought in the United States have traditionally been computed from ground-based measurements, primarily of precipitation (e.g., the Palmer Drought Severity Index).  However, spatial sampling in U.S. ground-based precipitation observation networks is insufficient for providing the localized information required for regional drought mitigation planning.  Sampling limitations are even more severe in other countries, leading to challenges in addressing issues of global food insecurity.   This paper reviews the advent of drought index maps computed using satellite imagery, which provide significantly improved spatial coverage in comparison with ground-based networks – particularly in data-sparse regions of the world.  Satellites can be used to map anomalies in most components of the hydrologic budget, including precipitation, soil moisture, groundwater and soil moisture storage, evapotranspiration, and snow water equivalent.  Ecological impacts of drought can also be monitored, reflecting anomalies in green vegetation amount and canopy stress detected in the shortwave reflectance and thermal emission wavebands. The paper reviews state-of-the-art remote sensing approaches to mapping hydrologic and ecologic drought indicators, and discusses methods for combining multiple indicators within in an objective framework for monitoring drought impacts at different timescales and relevant to different sectors.  Major research gaps, challenges and opportunities are identified, including the need to improve assessment of drought impacts on the carbon and nitrogen cycles, and to develop robust drought early warning detection systems.
                                Technical Abstract: This review surveys current and emerging drought monitoring approaches using satellite remote sensing observations from climatological and ecosystem perspectives. We argue that satellite observations not currently used for operational drought monitoring, such as relative humidity data from the Atmospheric Infrared Sounder (AIRS) mission, provide opportunities to improve early drought warning. Current and future satellite missions offer opportunities to develop composite and multi-indicator drought models. This proposal faces some major challenges including data continuity, unquantified uncertainty, sensor changes, and community acceptability, which will require future research. One of the major limitations of the currently available satellite observations is their short length of record. A number of relevant satellite missions and sensors (e.g., GRACE) provide only a decade of data, which may not be sufficient to study droughts from a climate perspective. However, they still provide valuable information about relevant hydrologic and ecological processes linked to this natural hazard. Therefore, there is a need for models and algorithms that combine multiple data sets and/or assimilate satellite observations into model simulations to generate long-term climate data records. Finally, the study identifies a major gap in indicators for describing drought impacts on the carbon and nitrogen cycle, which are fundamental to assessing drought impacts on the ecosystem.


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