Part 2: Science for Drought Planning
USGS and its partners carry out research and assessments to help water stakeholders understand how, why, and when precipitation deficits affect different parts of the hydrologic system. How does the lack of precipitation affect streams, rivers, reservoirs, snowpack, soil moisture, and groundwater?
Reliable information from such a broad spectrum of sources is critical if we are to plan ahead as much as possible and try to minimize the devastating effects that drought has on communities, the economy, and ecosystems.
Predicting the weather of drought
Recognizing that drought is one of the most costly natural disasters affecting the U.S., Congress established the National Integrated Drought Information System (NIDIS) Program at the National Oceanic and Atmospheric Administration (NOAA) in 2006 to help begin to move our society from a reactive response to drought to a proactive stance. As a central feature of a national drought early warning system, the NIDIS U.S. Drought Portal provides timely drought updates on current conditions through the U.S. Drought Monitor as well as seasonal outlooks for drought, potential agricultural impacts, and local, state, and regional breakdowns of drought impacts.
Groundwater, the go-to resource during drought
Groundwater, in particular, plays a vital role during drought conditions when it is often the resource that communities turn to for irrigation and for public supply. USGS studies show that aquifers vary in their response to drought conditions; water table (more shallow) aquifers are more likely to experience reduced recharge during drought periods, resulting in reduced water levels, while confined (or deeper) aquifers are more likely to experience increased water withdrawals, which also result in reduced water levels.
In a study of drought conditions across much of North Carolina during 1998–2002, precipitation deficits during the four-year period were among the highest recorded for some locations in North Carolina. Streamflows responded quickly to increases in precipitation in 2002, while groundwater levels continued to decline with recovery to normal levels lagging by eight months.
USGS assessments and research also help stakeholders to understand the role, status and trends in the magnitude of storage– such as in snowpack, aquifers, and reservoirs—all of which are needed for adequate supply, particularly during drought.
Reservoirs often provide water during drought to satisfy water supplies, legal water needs, or to sustain ecosystem health. USGS hydrology studies bring knowledge of streams and rivers, the range of variations in flow, and the reliable yield of reservoirs and aquifers, all of which are affected by the nature, timing, and extent of drought and increased water use.
USGS conducts snowpack-studies across the country, particularly in the western regions where winter snowpack accounts for between 60 to 80 percent of the annual water supply to more than 70 million people. These studies have highlighted the variability in snowpack and place current conditions in a 1,000-year context. USGS scientists used a regional network of tree-ring chronologies to reconstruct variations in snowpack accumulation over the Rocky Mountains. Over the past millennium, (see graph) snowpack variations generally seesawed with regional drought from north (blue line) to south (red line) (Pederson et al. 2011).
After 1980, however, both the Northern and Southern Rockies experienced an unusual and rapid snowpack decline attributed to warmer springs and earlier and faster snowmelt. USGS findings suggest that continued warming could exacerbate hydrological drought in snow-dominated basins, with significant implications for water resource planning and management in the West.
Drought and ecosystems
USGS scientists conduct many studies on drought-induced effects on ecosystems, including on the resistance and resilience of western forests to drought and wildfire. A recent USGS study, for example, examined trees that survived recent prescribed burns in the West, and found that pre-fire water deficits increased post-fire tree mortality probabilities in western conifer forests as a function of recent drought stress and higher growing season temperatures. USGS scientists are applying new analytical methods and indices to evaluate the role of both precipitation deficits and higher temperatures to estimate and predict past and future tree mortality during warmer droughts.
Additional ecosystem studies by USGS and Los Alamos National Laboratory, University of Arizona, and other university partners show that the current drought-related stress in forests of the southwest may exceed that of the most severe droughts in the last thousand years.
USGS Science for Looking Ahead
To look ahead, it helps to look back for context and pattterns. Decadal USGS streamflow monitoring shows that drought conditions over the last few years have undoubtedly been significant but probably not the “drought of the century” over any large area, such as compared to the Dust Bowl in the mid-thirties (see drought maps: 1934, 2013).
The longest term records by USGS are derived from paleoclimate records (such as from tree-ring analysis) that help to assess the dimensions and ecological impacts of drought over the past few centuries, putting events during the last decades in context with the last millennium. Our findings show that the severity and duration of droughts in the last 50-100 years have been less severe and do not represent the full scope of events that we can expect (Woodhouse et al. 2006; Ault et al. 2013; related information).
USGS watershed-scale studies related to climate-change projections indicate a steady increase in temperature progressing through the 21st century, generally resulting in snowpack reductions, changes to the timing of snowmelt, altered streamflows, and reductions in soil moisture. Despite some widespread similarities in climate change trends, USGS findings suggest that climate change will affect specific water basins in the U.S. differently, based on the particular hydrologic and geologic conditions in that area. So far, the USGS has applied models of climate change scenarios to fourteen basins across the U.S., from Oregon to Maine:
Local-scale hydrologic models allow regional managers to plan for changes in water resources that are specific to their area. For example, reduced snowpack in headwaters of the Colorado River could affect the amount and timing of streamflow to the Colorado River. Portions of Maine may see higher streamflows which could affect populations of endangered Atlantic salmon. Areas of the already drought-stressed Flint River Basin, one of Atlanta’s primary drinking water supplies, are projected to become even drier.
Adapting to drought
Droughts are in large part a natural phenomenon, which makes their severity and duration difficult to predict. However, human activities that result in increased water use and storage may amplify the pattern of severity and duration of droughts. Society must learn to expect the unexpected when it comes to drought and to rapidly adapt to changing conducts. By thoroughly assessing present hydrologic conditions and human uses of water and by striving to understand the lessons of the past, USGS science plays a critical role in drought mitigation and adaptation by helping reduce the level of uncertainty about the future.