Conclusion and Future Directions
Over the past decade, the application of satellite remote sensing for operational drought monitoring and early warning has rapidly advanced with the development of a suite of new tools, such as those discussed in this chapter. This advancement can be attributed to several catalysts, including the availability of new types of earth observations acquired by various space-borne sensors that have been launched since 2000; the development of extended time series of these observations, with many now spanning 15+ years; and the development of advanced computing capabilities and analytical methods to analyze and integrate these remotely sensed data into new drought indicators. Collectively, these advancements have allowed different components of the hydrologic cycle and biophysical characteristics of the landscape relevant to drought to be estimated and monitored, enabling a more complete view of drought to be obtained via satellite remote sensing.
Currently, the remote sensing community has several ongoing or planned efforts to continue expanding the types of tools and information that can be obtained from satellite observations for drought applications.
One emerging area is solar-induced fluorescence (SIF) of vegetation, which represents emitted radiation from chlorophyll pigments in a plant that occurs as part of photosynthesis. SIF can be an early-stage indicator of stress. The premise is that as a plant becomes stressed by drought, it will reduce its photosynthetic capacity (i.e., productivity), and the proportion of absorbed solar radiation emitted through fluorescence decreases as demonstrated by Sun et al. (2015). SIF has been shown to be a direct proxy of changes in photosynthetic capacity (Flexas et al. 2002; Damm et al. 2015) in response to the earliest stages of plant stress from events such as drought, which was demonstrated in the work of Sun et al. (2015) for the 2011 and 2012 droughts over the south-central United States and the US Corn Belt, respectively. Work has focused on the retrieval of SIF information from remotely sensed data acquired by the European Global Ozone Monitoring Instrument 2 (GOME-2) and the Japanese Greenhouse gases Observing Satellite (GOSAT; Frankenberg et al. 2011). Although both sensors were designed to measure atmospheric conditions, they have spectral bands placed in the visible red regions where SIF affects the absorption features over very narrow wavelength ranges called Fraunhofer lines. The satellite SIF data record for GOME-2 data ranges from 2007 to present, GOSAT data from 2009 to 2015, and the recent NASA orbiting carbon observatory-2 data (OCO-2; Frankenberg et al. 2014) from 2014 to present. In 2018, a follow- on OCO-3 sensor will be deployed on the international space station (ISS), providing data over a range of diurnal sampling times. Planned codeployment of the ecosystem space-borne thermal radiometer on the space station (ECOSTRESS) during a similar timeframe as OCO-3 will allow exciting opportunities for investigating synergies in SIF-TIR signals of plant stress.
Another emerging area of satellite remote sensing for drought monitoring is developing an indicator reflective of the vapor pressure deficit (VPD), which represents the difference between the amount of moisture in the air and the amount of moisture the air can hold when saturated. As the atmosphere dries, the VPD increases (i.e., decreasing humidity levels) and can be a precursor to drought onset or the intensification of existing drought conditions. Recent work by Behrangi et al. (2015) demonstrated that VPD can be calculated over large areas using remotely sensed near-surface temperature and relative humidity observations from the NASA atmospheric infrared sounder (AIRS) in combination with in situ dew point temperature and relative humidity data. Behrangi et al. (2016) found that the co-occurrence of high temperatures and low atmospheric humidity, which was expressed by high VPD, was an important factor in the development and evolution of the 2011 and 2012 droughts in the south-central and Corn Belt regions of the United States. For both events, the VPD showed marked increases during the formation and rapid intensification in drought conditions, demonstrating that remotely sensed VPD holds considerable potential to offer new atmospheric insights for drought early warning and assessment that complements the terrestrial information provided by the other tools presented in this chapter.
The application of satellite-based remote sensing for operational drought monitoring and early warning has significantly expanded and matured since the early 2000s, resulting in a suite of tools that characterize several hydrologic dimensions of drought. Given that several remote-sensed drought indicators now have relatively long-term (20-30 years) historical records, additional research on evaluating their spatial and temporal accuracy in characterizing drought patterns and conditions is a critical next step in effectively integrating this new information into drought decision-making activities. Evaluation will require comparing drought indicators' response to relevant observed impacts (e.g., crop yield reductions, reservoir levels, soil moisture depletion, economic losses, and reduction of ecosystem services) from historical drought events that have occurred within the satellite observational record. Drought impact/ remote sensing indicator comparisons will be challenging given that drought impact documentation is limited for many sectors; the impacts may be either direct or indirect, and often the reported impacts are collected at suboptimal spatial (e.g., a real county or district report vs. an individual pixel of satellite product) and temporal (e.g., annual impact report vs. weekly update of satellite product) scales. Some preliminary efforts have been undertaken to compare remotely sensed drought indicators with observed impacts (Otkin et al. 2015; Tadesse et al. 2015; Otkin et al. 2016), as well as more formal efforts to systematically collect drought impacts, as demonstrated by the drought impact reporter (droughtreporter.unl.edu/) for the United States. Work is needed in this area to establish triggers based on these indicators (e.g., three consecutive weeks of extreme drought detected by an indicator) that can be used by decision makers to implement a specific drought mitigation action (e.g., eligibility for assistance, demand reduction measures). Another key area of work is the long-term maintenance of these remotely sensed indicators into the future. As remote sensing tools and products such as those highlighted in this chapter become formally integrated into operational drought monitoring and early warning systems, sustaining the required satellite observations to maintain them will be key, as decision-making activities will be reliant on this information. This poses a challenge to the remote sensing community because a series of satellite sensors will be needed over time to replace aging instruments that degrade. This will require dedicated efforts to intercalibrate remote sensing observations between sensors to ensure comparable data inputs are used in the calculation of these indicators, resulting in consistent long-term data records.
Given the multifaceted nature of drought, it is clear that a single index is unlikely to tell the complete story of drought evolution, and so the question of interindex synergy is also raised. Ideally we would deploy a suite of diagnostic remote sensing tools that allow us to watch agricultural drought as it moves through its various phases—from atmospheric demand to enhanced evaporative loss to soil moisture depletion to canopy stress and degradation, and finally to yield loss and associated impacts. Such a multi-index screening, much like those used in the medical fields, may help us to catch signs of developing drought early and trace the progression to more and more serious consequences, allowing more effective and proactive adaptation to the evolving conditions.