Book details how drought is detected from space

Policymakers and relief agencies can use data from satellites to deliver relief from some of drought’s worst effects, including financial setbacks in the United States and famine in developing countries. Farmers and ranchers can use the same information to incorporate big-picture information into their own decision-making. A new book, "Remote Sensing of Drought: Innovative Monitoring Approaches," details the rapidly evolving state of the art of detecting drought remotely from space.

The book, published this spring by CRC Press, is edited by Brian Wardlow, a geographer and remote sensing professor in the School of Natural Resources who is affiliated with the National Drought Mitigation Center and Center for Advanced Land Management Information Technologies; Martha Anderson, U.S. Department of Agriculture, Agricultural Research Service; and James P. Verdin, U.S. Geological Survey, Earth Resources Observation and Science Center.

The book is the first in a series, "Drought and Water Crises," edited by Donald A. Wilhite, director of UNL’s School of Natural Resources and founding director of the National Drought Mitigation Center.

“One of the hardest parts of managing drought, a slow-moving natural disaster, is recognizing it in time to take action,” Wilhite said. “Scientists and decision makers use a variety of data to describe drought, such as precipitation, soil moisture and stream flow. All of those depend on having instruments or observations at multiple points across a network, and inferring what’s in between. Satellite data literally provides a big picture of what’s going on, and we now have several ways to use satellites to detect drought. This book gives scientists and decision makers a timely catalog of their options and an understanding of the current state of the art, including new or emerging technologies that will advance the science of drought monitoring in the next decade.”

Scientists view satellite remote sensing as a good way to fill in information gaps between widely-spaced, ground-based measurements, particularly in countries that don’t have good observation and reporting networks.

“Drought monitoring using remote sensing may in fact contribute the most added value in data-poor regions of the globe — areas that lack extensive ground-based monitoring networks,” said Anderson. “Agricultural communities in these regions will benefit from the timely and spatially detailed drought early warning information provided by satellites, allowing them to better adapt to changing conditions on the ground.”

Bradley Doorn, program manager for Agriculture and Water in the Applied Science Program at NASA and co-author of the book’s final chapter, said, “Our nation and world are facing unprecedented fresh water resource challenges, drought being key among them. While remote sensing is not a panacea for any of our drought challenges, our drought challenges will not be resolved without a dedicated and integrated remote sensing capability.”

Even in the United States, where there is more weather and climate monitoring infrastructure, satellite-derived drought indicators and maps can provide continuous spatial coverage, filling in gaps between monitoring station locations. In the United States, remotely sensed data is one of the indicators incorporated into the U.S. Drought Monitor, which synthesizes many types of data into a single map for the convenience of policymakers and the media. Some drought relief programs are based on the Drought Monitor. Farmers and ranchers can also go online to view maps of drought based on remote sensing to see what conditions are like in other parts of the country or world. Internationally, satellite-derived data helps relief organizations such as the Famine Early Warning System Network (FEWS NET) anticipate food and water shortages in developing countries.

“There’s an emerging tool kit of satellite-based tools available for drought monitoring,” Wardlow said. “It’s like your Swiss Army knife. There are several key variables of the hydrologic cycle that influence drought conditions, which can be estimated or monitored from satellites. One tool is not going to fully characterize all aspects of drought.”

For example, the most commonly used tool in the kit, the Normalized Differentiated Vegetation Index, calculated by using the difference in reflected radiation in the red and near-infrared ranges of the spectrum, can be used to detect drought-related stress in plants, but not may not show early-stage drought, or can take longer over certain land cover types such as dense forests, Wardlow said. On the other hand, a measurement of evapotranspiration from satellite, which represents water evaporating and transpiring from the leaves of plants, can be more responsive.

The book includes peer-reviewed chapters written by leading remote sensing scientists in the fields of agriculture, climate, ecology, environmental modeling, natural resources, and water. They describe and illustrate current state-of-the-art tools available for operational drought monitoring and early warning, Wardlow said. Chapters focus on the NDVI, which has been applied for more than three decades; the Vegetation Drought Response Index and the Vegetation Outlook; estimation techniques of absorbed photosynthetically active radiation in use in Europe; various measures of ET; microwave-based estimates of soil moisture; hydrologic modeling approaches and products from the North American Land Data Assimilation System; gravity-based estimates of groundwater and soil moisture; and radar and other satellite-based approaches to estimate rainfall; and snow cover characterization methods.

Unlike high spatial resolution satellite imagery that people may have seen on the web, such as Google Earth, the operational remotely sensed drought-monitoring tools featured in the book provide broad-scale information, with pixel sizes of a kilometer or more.

“Remote sensing provides many tools to monitor drought,” Wardlow said, and “individually they’re not all going to work well all the time for every location, but collectively, they can give you a pretty good picture of what’s going and compliment traditional ground-based climatic and hydrologic observations to make a better, proactive decisions related to agriculture, food security, water, and natural resources.”

— Kelly Helm Smith, Natural Resources