Elizabeth Clark*, Kostas Andreadis*, and Dennis Lettenmaier
Tuesday, October 19, 2004
3:30 to 5:00
Severity-area-duration analysis of 20th century drought in the conterminous United States
Drought characterization typically involves severity, frequency and duration;
however, the often neglected spatial extent of a drought also influences
its impact on water resources management. In the past, station data have
been used to calculate drought severity for individual climate divisions
across the United States, but a basis for the spatial characterization of
drought on a nationwide-scale has been lacking.
Spatially distributed hydrologic models provide a means for simulating both agricultural drought (related to soil moisture) and hydrological drought (related to runoff) over a grid mesh. The output of such models can be used to identify the spatial extent of drought. Depth-area-duration analysis, widely used in characterization of precipitation extremes for specification of so-called design storms, provides a basis for evaluation of drought severity when storm depth is replaced by an appropriate measure of drought severity. Precipitation and temperature data over the continental U.S. for the entire period of observational record are now available in electronic form from the National Climatic Data Center, and these recently available extended data greatly facilitate the ability to reconstruct a U.S. drought history.
We used these data, starting with 1916 (prior to which station density was too sparse to allow production of meaningful simulations) as input to the physically-based Variable Infiltration Capacity (VIC) macroscale hydrologic model, which we ran to simulate soil moisture and runoff over the conterminous United States at 1/2 degree spatial resolution. To standardize soil moisture and runoff anomalies, we computed soil moisture and runoff percentiles on a monthly basis for each grid cell. We then used cluster analysis to identify individual drought events and their spatial extent at a monthly time-step. We considered an area to be experiencing agricultural (or hydrologic) drought when soil moisture (or runoff) falls below the 20th percentile, based on the 1916-2003 climatology. To relate the area of each drought to its severity, we constructed a series of severity-area-duration curves for all drought events in the conterminous United States from 1916 to 2003. From these events, we constructed an envelope curve of the worst drought events in the conterminous United States during the 20th century. These included the 1930s, 1950s, late 1980s, and 2003, all of which have been previously cited as extreme drought events.