Recent Remote Sensing Advancements
Although NDVI and VHI have both proved useful for drought monitoring, they provide only a partial view of drought conditions, focusing on vegetation health and agricultural drought. Saturation effects at high levels of NDVI, background contamination at low levels, and the empirical nature of the NDVI-TIR combination in the VHI further limit their capabilities over a broad range of surface and drought conditions (Karnieli et al. 2010). Given that drought is a complex natural hazard and several components of hydrologic cycle influence drought conditions, additional information regarding other hydrologic parameters such as evapotranspiration, soil moisture, groundwater, and precipitation is needed to provide a more comprehensive picture of drought conditions. Historically, the capability to estimate these types of hydrologic variables operationally from satellite remote sensing for drought monitoring has been limited because either the available satellite-based sensors did not acquire the necessary observations to retrieve such information or the historical record of appropriate satellite observations lacked sufficient length to calculate meaningful drought anomalies. However, since the early 2000s, a number of new satellite-based sensors have been launched, providing new types of earth observations acquired at a high temporal frequency (in some cases, with 1- to 2-day revisit time) and over a broader spectral extent, expanding the suite of remote sensing tools that can functionally monitor these various components of the hydrologic cycle.
The Moderate Resolution Imaging Spectroradiometer (MODIS), for example, is on board NASA's Terra and Aqua platforms and collects 1-km spectral observations globally on a near daily basis in the visible and NIR regions, extending the global time series of the NDVI data record that was established with the AVHRR. MODIS spectral observations also extend into the middle infrared (MIR) region, which can be used to assess plant water content, as well as the TIR region, which can be used to develop thermal- based tools for evapotranspiration estimation. Microwave sensors such as the Advanced Microwave Scanning Radiometer for Earth Observing System (EOS) (AMSR-E) and Quick Scatterometer (QuikSCAT) collect key observations that can be used to estimate soil moisture (Bolten et al. 2010). Gravity field observations from NASA's Gravity Recovery and Climate Experiment (GRACE) also provide new insights into water cycle variables including soil moisture and groundwater (Rodell and Famiglietti 2002). Collectively, the availability of this suite of new remote sensing observations, with time- series datasets extending for more than a decade, coupled with advancements in environmental models and algorithms and computing capabilities, has resulted in the rapid emergence of many new remote sensing tools that monitor different aspects of the hydrologic cycle that influence drought conditions.
This chapter will discuss several satellite-based remote sensing tools that have been developed for drought monitoring and early warning. Key examples will be presented that characterize different components of the hydrologic cycle related to drought that include vegetation status and health, ET, soil moisture, groundwater, and precipitation. The tools highlighted in this chapter are either currently operational or hold the potential to be operational in the near future. The chapter will include a brief introduction to remote sensing-related prediction tools that can provide drought early warning information. A short discussion of upcoming satellite missions that hold considerable potential to further advance drought monitoring, as well as directions of future research in this field, will also be presented.