Advancements in Satellite Remote Sensing for Drought Monitoring
Brian D. Wardlow, Martha C. Anderson, Christopher Hain,
Drought is a complex climatic phenomenon of global importance with major, wide-ranging impacts to many sectors of society, including agriculture, the economy, energy, health and water, and other natural resources. In many regions of the world, drought is a common, recurring natural event that can have significant, detrimental economic, social, and environmental impacts. For example, the annual impact of drought in the United States is estimated at $6-8 billion (NCDC 2014), with even farther-reaching effects in developing regions that can result in famine, malnutrition, loss of life, and social and political conflict. Changes in climate and the projected increase in climatic extremes such as drought (Dai 2012) coupled with increasing demands on finite water supplies and food production capabilities have raised the significance of developing effective drought early warning and mitigation strategies.
Drought monitoring is a key component for effective drought preparedness strategies, providing critical information on current conditions that can be used to trigger mitigation actions to lessen the impact of this natural hazard. However, drought can be both complex and challenging to monitor because it lacks a single universal definition, which makes the detection and assessment of key drought characteristics such as severity, geographic extent, and duration difficult (Mishra and Singh 2010). Three operational, physically based definitions were developed by Wilhite and Glantz (1985) to differentiate and describe different types of drought: meteorological, agricultural, and hydrologic. The temporal length of dryness needed to initiate and recover from a drought event and the specific environmental factors affected (e.g., rainfall deficits vs. plant health vs. reservoir water levels) are primary factors distinguishing among these different types of drought. In general, the time period associated with the manifestation or cessation of drought increases as we progress from meteorological through to hydrologic drought. As a result, a period of dryness may result in the emergence of one type of drought (e.g., meteorological) but not the others, while in the case of more prolonged or more severe dry events, several types of drought may be occurring at the same time. As a result, a number of drought indicators related to precipitation, soil moisture, vegetation health, and surface and groundwater have been developed to characterize specific types of drought and have been analyzed collectively in efforts such as the US Drought Monitor (USDM) (Svoboda et al. 2002) to more fully describe drought conditions (see also Chapters 7 and 9).