Brian D. Wardlow, Michael J. Hayes, Mark D. Svoboda, Tsegaye Tadesse, and Kelly H. Smith
Droughts are complex, naturally recurring events that affect many sectors of society and are among the most costly of all natural hazards. Proactive, risk-based drought management approaches are increasingly being implemented to mitigate the wide-ranging impacts of drought. This type of approach requires accurate, timely, and accessible information regarding drought conditions and their specific impacts in order to be effective. The National Drought Mitigation Center (NDMC) at the University of Nebraska-Lincoln is developing a suite of drought monitoring and assessment tools to provide such information to diverse user communities in order to support drought planning and mitigation activities. In this paper, we review four tools being developed for the United States that provide information about current and historical drought conditions across the nation, future outlooks of vegetation conditions, and sector-specific impacts at local, regional, and national scales.
Droughts are identified as one of the main natural hazards in the United States, affecting more people than any other natural hazard (NSTC, 2005). At the end of 2008, the spatial extent of drought and abnormally dry conditions across the United States had diminished to below 40% of the country for the first time since October 2005 (Fuchs, 2009). Even with this reduction, major drought events are still occurring in parts of the Southeast, Texas, and California. These current droughts, as well as a series of recent droughts that have affected almost all parts of the country since 1995, emphasize that droughts are a normal part of climate in the United States. A report from the Geological Society of America (GSA) (2007) stated that these recent droughts had caused substantial economic, environmental, and social impacts in many regions of the country. The report also warned that the frequency, intensity, duration, and impacts of droughts across the United States are expected to increase in the future based on the combination of increasing demands for limited water supplies and the effects of climate change. Globally, the world’s vulnerability to drought is increasing not only from our changing climate, but also because of significant increases in population, especially in water-short areas, and other factors such as changes in land use and government policies, and environmental degradation. Collectively, these changes place an ever-increasing strain not only on the United States’ but also the world’s finite water resources. Understanding our vulnerability to and risk from this natural hazard has never been more important than it is today as we head deeper into this century.
[/caption]Droughts have several distinct characteristics that make the monitoring of droughts different from monitoring other natural hazards. Droughts tend to be slow-onset phenomena, which makes it difficult to recognize the beginning of specific events and for officials to respond appropriately as an event is developing. Droughts also lack a specific definition that can identify a drought’s beginning or end, or even the spatial extent of the areas experiencing (or not experiencing) drought. These complex characteristics pose challenges in monitoring drought. For example, the duration of a drought can range from a few months to many years and its geographic coverage can extend from a few hundred to thousands of square kilometers, across which conditions can vary from moderate to extreme dryness. In addition, the effects of drought differ among sectors. Crops may experience drought stress because of shorter-term dryness over weeks to months, whereas another sector such as water resources may not consider the conditions to be drought because it requires a longer-term period of dryness on the order of months to years to be negatively affected. Therefore, given this inherent complexity of droughts, the ability to effectively monitor droughts and assess their impacts is critical to support activities that mitigate the effects of this natural hazard.
Recently, a paradigm shift began in the United States to move toward more proactive, risk-based drought management strategies to better prepare for and mitigate the effects of this natural hazard (NIDIS, 2007). In 2006, the National Integrated Drought Information System Act was enacted with the goal of improving the United States’ capacity to manage drought-related risks (NIDIS, 2006). The development of new tools and data delivery mechanisms is critical to providing decision makers with accurate, timely, and objective information regarding current, future, and historical drought conditions, as well as their associated impacts. However, no single method or indicator can fully capture all facets of drought. The key to reducing societal vulnerability to drought will depend on our ability to improve monitoring and forecast skills and adopt a more risk-based management approach that emphasizes mitigation and preparedness. As a result, multiple tools and approaches are needed to better characterize drought, and their informational products should be up-to-date and provided in a variety of easily accessible formats (e.g., figures, maps, tabular statistics, and narrative text) to increase their value and relevancy to a diverse user community (WGA, 2004).
The National Drought Mitigation Center (NDMC) at the University of Nebraska-Lincoln is beginning to address this need by developing a suite of drought tools through a partnership with the United States Department of Agriculture’s (USDA) Risk Management Agency (RMA). The NDMC was founded in 1995 and has worked extensively nationally and internationally in a variety of areas including drought monitoring, planning and preparedness, impact assessment, and education/outreach. This experience has formed the basis for the four new drought tools being developed at the NDMC that will be highlighted in this paper. These tools are designed to provide decision makers with a wide range of information including current and historical drought conditions across the United States, future outlooks of vegetation conditions, and sector-specific impacts from local to regional scales. The development of these tools has relied on the interdisciplinary expertise of the NDMC faculty and staff in the fields of climatology, geography, human dimensions, hydrology, geographic information systems (GIS), remote sensing, and agricultural economics.
U.S. Drought Monitor – Decision Support System (USDM – DSS)
An integrated climate/drought monitoring and early warning system has been identified as a critical need by scientists, natural resource managers, and policy makers (Riebsame et al., 1991; U.S. Congress, OTA, 1993; Wilhite, 2000; Hayes et al., 2005). As a result, this has been an important goal of the NDMC since the program’s inception in 1995. In 1999, the NDMC initiated a new activity in cooperation with the USDA and the Climate Prediction Center (CPC)/National Oceanic and Atmospheric Administration (NOAA). These three agencies working cooperatively established the weekly U.S. Drought Monitor (USDM) (Svoboda et al., 2002) product, which provides a current assessment of drought conditions around the country. The current Drought Monitor Decision Support System (DMDSS) is a web-based tool (Figures 1 and 2) that allows users to view current and historic information related to drought (precipitation, streamflow, drought category) on a more localized area to be better able to plan for drought. The product’s website is hosted by the NDMC. This USDM website contains a suite of maps and products that depict the current climate/water situation for the United States, as well as forecast and outlook information. New products are continually being developed as part of this effort. The NDMC also hosts and supports a drought listserver of more than 260 drought specialists that provide input and ground-truthing into the USDM development each week.
The USDM maps and related products continue to receive much attention from a wide variety of users, including major national and local media outlets. This product is now the most widely used and recognized drought monitoring product in the United States, and the methodology for map development and dissemination is being considered for application in other countries. This map is used by the USDA for numerous policy decisions and is regularly reviewed at USDA’s interagency drought council meetings. The passage of the 2008 Farm Bill has increased the number of programs that rely on the USDM for eligibility. It is also used by the U.S. Congress, Internal Revenue Service (IRS), and drought task forces of various states around the country in a variety of ways to help assess their drought situations.
The NDMC is also involved in the production of the monthly North American Drought Monitor (NADM), which is a collaborative project with the USDM agencies and agencies in Canada and Mexico (Figure 3). Each year a Drought Monitor forum is held to discuss the experiences of both the USDM and the NADM products and to solicit input from the diverse users of these products.
The NDMC’s Monitoring Program Area is heavily involved in efforts to improve the USDM (Figure 4) so that it is easier to use and of greater value to policy and other decision makers, including agricultural producers and the general citizenry. Efforts are currently underway to revamp and improve the temporal and spatial resolutions of the analyses that go into making the USDM product, including the use and integration of more satellite-derived remote sensing technologies and near real-time incorporation of drought impacts from the NDMC’s Drought Impact Reporter (DIR), discussed later in this article. In 2006, a new project was initiated by the NDMC to enhance the current capacity of the USDM by developing a new drought monitoring web portal with enhanced flexibility to assess the severity and spatial extent of drought conditions at all scales. This tool’s robustness will be further enhanced for decision support through the provision of more local assessments of drought and water supply conditions, impacts, and historical context. Users will be able to define their area of interest and query the tool to receive more locally relevant assessments of drought severity, impacts, data, and forecasts. This effort will also support the goals of NOAA in the implementation of NIDIS and will be implemented into the NIDIS web portal upon completion.
Vegetation Drought Response Index (VegDRI)
The Vegetation Drought Response Index (VegDRI) is a new vegetation drought monitoring tool being developed for the continental United States by researchers at the NDMC and United States Geological Survey (USGS) Center for Earth Resources Observation and Science (EROS). VegDRI integrates satellite-based observations of vegetation conditions with climate-based drought indices, as well as other biophysical information such as land use/land cover (LULC) type, soil characteristics, elevation, and ecological setting, to produce 1-km spatial resolution maps that depict the level of drought stress of vegetation.
The development of VegDRI builds upon the traditional, remote sensing-based method of large-area vegetation monitoring, which has relied upon the analysis of normalized difference vegetation index (NDVI) (Rouse et al., 1974) data collected from satellite observations. Satellite-derived NDVI has proven to be a useful indicator of the general state and health of vegetation from local to global scales over the past two decades (Tucker et al., 1985; Townshend et al., 1987; Reed et al., 1994; Jakubauskas et al., 2002; DeBeurs and Henebry, 2004), but identifying the specific causes of stress expressed in the NDVI data is difficult using this indicator by itself. Many environmental factors such as fire, flooding, hail damage, plant disease, pest infestation, and LULC change can produce anomalies in the NDVI similar to drought. As a result, VegDRI incorporates data from two climate-based drought indices, the Standardized Precipitation Index (SPI) (McKee et al., 1995) and the Palmer Drought Severity Index (PDSI) (Palmer, 1965; Hayes et al., 1999), as indicators of dryness, which are analyzed in combination with NDVI information acquired from the NOAA satellite-based Advanced Very High Resolution Radiometer (AVHRR) instrument. Several biophysical characteristics of the environment that can influence climate-vegetation interactions and the level of drought stress experienced at a given location are also accounted for in VegDRI’s calculation. Readers are referred to Brown et al. (2008) for a more detailed technical description of the specific data inputs and methodology used to calculate VegDRI.
|Figure 6. VegDRI (July 28, 2008) and U.S.
Drought Monitor maps over North Dakota
for July 28 and July 29, 2008, respectively.
The operational production of VegDRI maps began in 2005 for a 7-state region of the northern Great Plains, and each year, the geographic coverage has gradually expanded. It will culminate in complete continental coverage of the United States by spring 2009. Currently, the maps are updated in near real-time once every two weeks, with plans to move to a weekly update cycle in the near future. Figure 5 shows a VegDRI map for July 28 over the 22-state operational production area in 2008. A modified version of the PDSI classification scheme is used for VegDRI, in which index values are classified into one of eight drought severity classes ranging from “extreme drought” to “extreme moist” conditions. The 1-km spatial resolution of the VegDRI maps enables more spatially detailed patterns of vegetation drought stress to be monitored than the USDM, which is considered the current state-of-the-art drought monitoring tool for the United States. Information provided at VegDRI’s spatial scale is more appropriate for local-scale decision making than most traditional drought index maps. The improved spatial detail of VegDRI is demonstrated in Figure 6 for North Dakota in late July 2008. In the example, most of North Dakota was experiencing drought conditions, with the most severe conditions in the west/northwest part of the state. VegDRI characterized considerably more sub-county spatial variability in drought conditions for western North Dakota as well as other parts of the state, than the broader-scale drought depictions in the USDM map.
VegDRI maps and other informational products are available to the general public on the Internet at two website locations. The first website is a VegDRI Webpage hosted by the NDMC that presents current and historical VegDRI maps (downloadable as pdf or image file), change maps, drought area summary statistics, technical information about the index, and evaluation and outreach summaries. The second website is the USGS’ Drought Monitoring Viewer, which provides a GIS environment to interactively display and customize views of the VegDRI maps. The Viewer allows users to overlay multiple geographic layers of information (political and administrative boundaries, roads, streams, LULC type, satellite-derived NDVI, and estimated precipitation maps) to view in combination with a time series of VegDRI maps. Work is currently underway to produce annual time-series animations of the VegDRI maps and a 20-year historical archive of VegDRI calculation dating back to 1989.
Vegetation Outlook (VegOut)
The Vegetation Outlook (VegOut) is another new vegetation drought monitoring tool being developed for the continental United States by researchers at the NDMC and the University of Nebraska at Kearney (UNK). The VegOut is an experimental tool that predicts general vegetation conditions based on the analysis of “historical patterns” of the satellite-based vegetation condition observations, climate-based drought indices, general biophysical information about the environment (e.g., elevation, LULC type, and soil available water capacity), and several oceanic indices (e.g., El Niño and Southern Oscillation [ENSO] indices).
The VegOut models are built upon a derivative of AVHRR-based NDVI observations called seasonal greenness (SG), which is the time integrated (or accumulated) NDVI above a baseline NDVI value (termed ‘latent’ NDVI that represents the non-vegetated background signal from soil and plant litter) from the start of the growing season to a specified time during the growing season (Brown et al., 2008). In the VegOut model, the SG is standardized to a measure called the standardized seasonal greenness (SSG) to make all values directly comparable across large areas and time. The VegOut model uses historical SSG observations along with several other variables discussed below to predict future SSG values, which are representative of future general vegetation conditions. Two commonly used traditional climate-based drought indices, SPI and PDSI (Tadesse et al., 2005), are also integrated to represent dryness in the VegOut models along with the biophysical variables, which can influence the effect of climate on vegetation conditions. Oceanic indices represent the final component of the VegOut models and are integrated into the predictions because of the indirect temporal and spatial relationships between ocean-atmosphere dynamics and climate-vegetation interactions (teleconnection patterns such as El Niño and La Niña) that have been observed (Barnston et al., 2005; Tadesse et al., 2005; Lyon, 2004; Los et al., 2001; Asner et al., 2000; Verdin et al., 1999). For more information about the VegOut modeling approach, readers are referred to Tadesse and Wardlow (2007).
Using the VegOut models, 1-km resolution maps are produced that predict the level of drought stress on vegetation at multiple time intervals into the future (e.g., 2-, 4-, and 6-week vegetation condition outlooks). Figure 7 (a to g) shows the outlooks of vegetation conditions (i.e., predictions for those three time intervals calculated on May 15, 2006, for a 15-state region of the central United States. The spatial patterns of predicted vegetation conditions in the series of VegOut maps presented in Figures 7 b to d had relatively strong agreement with the observed patterns from satellites on the dates that corresponded to the three vegetation outlooks. Currently, the three outlook maps ranging from 2- to 6-week predictions are being produced and tested. Outlooks longer than 6 weeks are possible, but the predictive accuracy of the VegOut models must be thoroughly tested before these longer vegetation outlooks can be routinely produced. Assessment of VegOut’s predictive accuracy is currently underway and initial results of cross-validation testing (using holding years) reveal that the seasonal 2- to 6-week VegOut models have relatively high prediction accuracy. For example, the correlation coefficient (R2) of observed and predicted values ranged from 0.94 to 0.98 for 2-week, 0.86 to 0.96 for 4-week, and 0.79 to 0.94 for 6-week predictions across the growing season.
The VegOut products are currently experimental, but semi-operational production of VegOut maps for the central United States (i.e., 15 states) similar to those shown in Figures 7 b to d is planned in 2009. The VegOut maps will be made available to the general public on the NDMC’s website.
Drought Impact Reporter (DIR)
Drought is a slow-moving natural hazard that affects millions of people worldwide each year. The meteorological phenomenon triggers a cascade of impacts across agricultural, hydrological, economic, environmental, and social systems. Understanding these impacts is crucial for drought planning, mitigation, and response. However, there has been no standard method for reporting, distributing, or archiving drought impact data in the United States, so the quality and quantity of data required for effective drought management is often not available.
In July 2005, the NDMC took the first step toward addressing this issue by developing the Drought Impact Reporter (DIR). It is an interactive, web-based mapping tool designed to compile and display impact information across the United States in near real-time from a variety of sources such as media, government agencies, and the public. This aggregated information will help policy makers and resource managers identify and quantify the occurrence and severity of impacts. It also provides an opportunity that would not otherwise exist for resource managers, agricultural producers, and others affected by drought to add their experiences and observations to the public record.
From January 1, 2006, through December 31, 2008, there were 7,517 impacts recorded in the DIR. Of those, 89 percent were culled from media reports, with the rest entered by members of the public or government agency staff. Impacts were categorized as affecting agriculture (19 percent of the total impacts), water/energy (30.5 percent), environment (7 percent), fire (16.4 percent), social (11 percent), and other sectors (13 percent).
Figure 8 shows the interface of the current DIR, which can be accessed via the web at http://droughtreporter.unl.edu/. In this example for the state of Texas, the total number of impacts is color-coded in the state map by county over a specified period of time (e.g., week or month). A specific county may be selected and the number of impacts for each of the five specific categories is summarized along with a link to the full text report for a specific impact. Users also have the option to select a specific impact and view a listing of all counties that are affected.
In early 2009 the NDMC will unveil version 2 of the DIR, incorporating several enhancements, including 1) more user-friendly design and enhanced search and mapping functionality, 2) distinctions between “reports” and “impacts”, which will track early awareness of drought before a quantifiable impact occurs, and 3) a systematic media sample, to enable quantitative analyses of the media coverage of drought.
Summary and Conclusions
The tools presented in this paper represent a major initiative by the NDMC and its collaborators to provide improved drought information to a wide range of decision makers in the United States. As new technologies (e.g., field measurements and satellite sensors), techniques, and data sets become available in the future, this suite of operational drought monitoring tools will be enhanced and new tools developed. In addition, the tools and concepts can be modified and transferred to other parts of the world to improve drought monitoring capabilities internationally. The NDMC has a long history of working with various countries and international organizations on drought planning strategies and monitoring techniques.
Outreach is also an important element of the NDMC that has been incorporated into the development and application of these tools. The NDMC conducts multiple workshops across the United States each year to introduce users to these tools, to collect feedback regarding their usefulness and the specific needs of users, and to work with decision makers on the application of the information for a specific interest. Updates on the tools and outreach activities, as well as other drought-related information of interest (e.g., annual review of drought conditions and forecasts), is provided to the general public in an on-line quarterly newsletter called DroughtScape.
This work should be viewed as another step in the evolution of drought monitoring. Given the complexity of drought and the impact of climate change in the future, considerable work is still needed to develop new drought monitoring tools and observational networks to improve our capacity to better mitigate the effects of drought locally, nationally, and internationally. Another important task will be the effective integration and analysis of information from multiple tools by decision makers to make well-informed and timely decisions.
The work for VegDRI was conducted under USDA RMA Partnership Agreement #05-IE-0831-0208. The development for the remainder of the tools presented in this paper was supported under USDA RMA Partnership Agreement #02-IE-0831-0228. Support for the DIR was also provided by NOAA grant #NA07OAR4310464. The authors acknowledge the High Plains Regional Climate Center (HPRCC), particularly Bill Sorensen and Jun Li, for providing the historical climate for many of the tools. From the NDMC, we thank Melissa Widhalm and Denise Gutzmer of the NDMC for providing the DIR statistics, Karin Callahan for data processing and operational production support for VegDRI and VegOut, and Deb Wood for her editorial comments.
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Brian Wardlow is an assistant professor and remote sensing specialist at the NDMC and also serves as a faculty member in the Geography Program and the School of Natural Resources (SNR) at the University Nebraska-Lincoln. He leads the GIScience and Analysis program area at the NDMC and oversees a variety of remote sensing and GIS-related research projects. Prior to joining the NDMC, he was a NASA Earth System Science (ESS) Graduate Research Fellow during his Geography Ph.D. program at the University of Kansas and also worked as a remote sensing scientist at the USGS Center for EROS. His specific research interests include drought monitoring, land use/land cover mapping, vegetation dynamics, and climate-vegetation interactions.
Michael Hayes is the director of the NDMC and an associate professor in the School of Natural Resources at the University of Nebraska-Lincoln. His responsibilities include conducting research on the economic, environmental, and social impacts of drought; developing new drought monitoring and impact assessment methodologies; assisting with the development and review of drought plans; and helping to organize and conduct drought workshops and conferences.
Mark Svoboda serves as the NDMC Monitoring Program Area Leader. A climatologist, his duties include overseeing the Center’s national drought monitoring activities. His responsibilities include providing expertise on climate and water management issues by working closely with states, federal agencies and international governments as well as the media and private sector. He also provides supervision for applied research projects at the center and helps develop data products and tools to meet users’ needs. Mark helped develop and establish the U.S. Drought Monitor (USDM) in 1999 and serves as one of the principal authors of both the weekly USDM and monthly North American Drought Monitor products. He is heavily involved with drought monitoring, assessment, and prediction committees at state, regional, and national levels. He currently sits on the NOAA’s NIDIS Program Implementation Team and was appointed a co-chair for the NIDIS’s Web Portal development team.
Tsegaye Tadesse is a climatologist at the NDMC. He has been involved in research projects on the development of new drought monitoring tools in support of other monitoring activities of the NDMC and other collaborating organizations using satellite-, climate-, and oceanic-based data employing data mining techniques. Before he joined the NDMC in 1998, he was an expert on short-, medium-, and long- range weather forecasting at the National Meteorological Services Agency (NMSA) of Ethiopia.
Kelly Helm Smith is a communications and drought resources specialist at the NDMC. Her education and experience include journalism and community and regional planning.