Severity-area-duration analysis of 20th century drought in the conterminous United States Elizabeth A. Clark1, Konstantinos M. Andreadis1, Andrew A. Wood1, and Dennis P. Lettenmaier1 1. Department of Civil and Environmental Engineering, Box 352700, University of Washington, Seattle, WA 98195 NOAA 29th Annual Climate Diagnostics and Prediction Workshop Madison, Wisconsin 3 Drought Definitions 4 Severity-Area-Duration Analysis ABSTRACT Drought characterization typically involves severity, frequency and duration; however, the often neglected spatial extent of droughts also The overall impact of a drought event depends on several factors, influences their impact on water resources management. In the past, station data have been used to calculate drought severity for individual including severity, frequency, area, and duration (Dracup et al., Depth-area-duration analysis, widely used in characterization of precipitation extremes for specification of so-called design storms, climate divisions across the United States, but a basis for the spatial characterization of drought on a nationwide-scale has been lacking. 1980). Several drought indices have been defined for the provides a basis for evaluation of drought severity when storm depth is replaced by an appropriate measure of drought severity. For 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- characterization of drought severity over time. These indices the purposes of this paper, drought severity (S) is defined as S=(1-ΣP/t), where ΣP is the percentile of soil moisture or runoff summed duration analysis, widely used to characterize precipitation extremes for specification of so-called design storms, provides a basis for evaluation typically describe one of four drought types: agricultural, over duration t (in months). We adapted the computational method of WMO (1960) to calculate the average severity corresponding of drought severity when storm depth is replaced by an appropriate measure of drought severity. Precipitation and temperature data over the hydrologic, meteorological, and socioeconomic. Generally, to each standard area for each drought identified. In this study, we examine durations of 3, 6, 9, 12, 24, 36, 48, 72, and 96 months and continental U.S. for the entire period of observational record are now available in electronic form from the National Climatic Data Center, and agricultural drought relates to soil moisture, hydrologic to runoff areas from 10 grid cells, or approximately 25,000 km2, to the maximum drought extent, in increments of 20 grid cells, or these recently available digital data greatly facilitate the ability to reconstruct a U.S. drought history. We used these data, starting with 1916 and streamflow, meteorologic to precipitation and temperature, and approximately 50,000 km2. (prior to which station density was too sparse to allow production of meaningful simulations) as input to the physically-based Variable 2) Figure 5. Process of calculating area Infiltration Capacity (VIC) macroscale hydrologic model, which we ran to simulate soil moisture and runoff over the conterminous United socioeconomic to the disparity between supply and demand for 1) water-related goods (Wilhite and Glantz, 1985). Construction of SAD Curves (Figure 5): and area-averaged severities for States at 1/2o 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- Severity 1. Rank cells by severity & identify potential drought centers SAD analysis. (1) Start with step. We considered an area to be experiencing agricultural (or hydrologic) drought when soil moisture (or runoff) fell below the 20th percentile, We use simulated monthly soil moisture anomalies as a measure of 2. Search 3x3 neighborhood of drought center maximum severity grid cell, or 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 agricultural drought (Figure 4) and simulated monthly runoff 3. Average severities & add areas “drought center”. (2) Add area and for all drought events in the conterminous United States from 1916 to 2003. From these events, we constructed an envelope curve of the worst average severity with most severe drought events in the conterminous United States during the 20th century. These included the 1930s, 1950s, late 1980s, and 2003, all of which anomalies as a measure of hydrologic drought. Because the 4. Output severity and area at specified area intervals 3) absolute magnitude of these departures varies with climate region, 5. Compare the severity at ~25,000 km2 for each potential grid cell that is contiguous with have been previously cited as extreme drought events. we measure the departure from normal in terms of percentiles drought center and select center with maximum severity drought center. (3) Continue adding (Weibull estimate) based on the 1925 to 2003 climatology. next most severe grid cells until prescribed areal extent is reached. 1 Introduction Area Finally, we generalize an enveloping relationship of the most extreme drought events in the United States between 1925 and 2003, Drought is among the most costly natural disasters in the United States. In 1995, the Federal Emergency Management The algorithm used to identify areal extent of droughts begins with Agency (FEMA, 1995) estimated that the annual cost of U.S. droughts was in the range of $6-8B. To assess the potential spatial smoothing using a 3-by-3 median filter to minimize composed of the maximum severity drought events at each area interval and duration (Figure 6). impacts of current and future drought, water managers often compare current or potential drought severity with historic discontinuities in the original data. Based on the threshold level, VIC model Threshold Final drought SAD Envelope drought severities. This process, however, often overlooks the effects of areal extent on drought intensity. The purpose of pixels that are experiencing drought at a given monthly time step Initial output Spatial Temporal and Severity-Area- curves Highest curve for 20th percentile drought this study is to create a context for comparison of extreme drought events based not only on an event’s severity, but also on are identified and are classified using the modified clustering Weibull contiguity classification contiguity subdrought Duration for each severities each Total column percentiles soil moisture classification event duration its areal extent and duration. algorithm. The first pixel "under drought" is assigned to the first soil moisture 30th percentile class. Then the 3-by-3 neighborhood of this pixel is searched for Runoff runoff NOAA's National Climate Data Center (NCDC) recently released digitized Cooperative Observer Network (Co-op) station Figure 6. Overview of methodology from initial input to envelope curves. pixels "under drought" which are classified in the same drought precipitation and temperature data for much of the 20th century. This data set makes the extension of model simulations of cluster. This procedure is repeated until no pixels in the hydrologic conditions over most of the 20th Century possible. Such simulations provide a spatially and temporally neighboring pixels of the current cluster are "under drought" and a continuous data set. They also allow us to investigate historical droughts in new ways. new cluster is created for the next pixel below the drought 5 Envelope Curves threshold. After the initial partitioning step, a final classification Soil Moisture Runoff step excludes all clusters with areas below 10 half-degree pixels Dec 1976 to Sep 1976 to 2 Hydrologic Simulations and Validation from subsequent calculations. At the end of this step, the remaining Feb 1977 Nov 1976 The Variable Infiltration Capacity (VIC) macroscale hydrologic model (Liang clusters are defined as separate drought events for the current time et al., 1994) is applied to a half degree resolution grid mesh across the step. conterminous United States for the 1925 to 2003 period. Soil properties for a 3- Feb 2002 to 3-month Feb 2001 to Duration Feb 2003 Feb 2002 layer soil column are taken from Maurer et al. (2002) and aggregated from 1/8o 6-month Drought events are assigned a temporal extent based on the to 1/2o resolution. Vegetation fractions are based on the University of 3-month continuation of drought conditions (20th percentile or for lower soil 1 year Maryland's 1-km global land cover classification (Hansen et al., 2000), and LAI moisture, 30th percentile or lower for runoff) in at least one cell 6-month is derived from the gridded 1/4o monthly global LAI database of Myeni et al. 2 years 2 years between monthly time steps. If multiple clusters merge to form a 1 year (1997). The meteorological forcings used include observed precipitation and Feb 1930 to Jan 1934 to larger drought in later time steps, or if a single drought splits into 4 years 4 years Feb 1937 8 years Mar 1934 8 years temperature from NCDC Cooperative station data and wind from NCEP/NCAR multiple smaller droughts, the smaller droughts are considered part reanalysis (Kalnay et al. 1996). Vapor pressure, downward longwave and of the larger drought. In these special cases, the spatial extents and Sep 1953 to shortwave radiation, and air pressure are derived from the observed Oct 1951 to severities are calculated separately based on a contiguity constraint. Oct 1956 Sep 1956 precipitation and temperature. The VIC model accounts for sub-grid scale variability in soil, vegetation, From the envelope curve, we can see that there were five major Figure 4. Drought severity agricultural drought events between 1925 and 2003. These This envelope curve represents five major hydrologic drought precipitation, and topography and treats subgrid hydrologic variability based on soil moisture for occurred from 1928 to 1938, 1938 to 1942, 1947 to 1957, 1972 to events between 1925 and 2003. These include 1928 to 1938, statistically over coarse resolution grid cells (Figure 1). The vertical dimension January 1934 before 1978, and 1998 to 2003 (current). The 1970s drought, which 1948 to1957, 1962 to 1969, 1975 to 1978, and 1998 to 2003 of each cell is divided into three soil layers, in which the soil moisture storage processing. occurred in the western U.S., as well as the Great Lakes region, (current). As with agricultural drought, the 1970s drought was capacity is treated as a spatial probability distribution. Baseflow from deepest was most severe over 3-month durations for areas under 1 million most severe over the 3-month duration at small areas. The soil layer is produced according to the Arno model nonlinear baseflow sq. km. Similarly, the current drought dominates at smaller areas current drought dominates over a greater range of areas for formulation (Francini and Pacciani, 1996). Evapotranspiration, surface runoff for durations from 3 months to 4 years. As expected, the 1930s durations from 3-months to 4-years when defined by runoff and baseflow are calculated for each vegetation class, and the weighted sum of Figure 1. Schematic representation of drought was most severe at large spatial extents for all durations. shortages. The 1930s drought was most severe for the 3-month each, based on vegetation fraction, is assigned to the grid cell. Once generated, VIC hydrologic model. At the end of the 1930s (1938 to 1942), severe drought covered duration over areas from 2.5 million sq. km to 5 million sq. km. surface runoff and baseflow are released from the grid cell, though they can be areas larger than 5 million sq. km for durations from 3-months to The 1950s drought was most severe for larger areas and longer routed to simulate streamflow using the model developed by Lohmann et al. Figure 2. Six monthly hydrographs, 1-year. The 1950s drought experienced the highest severities for durations; however, if the current drought, which has a (1998). aggregated from daily flows. durations from 6- months to 8-years over mid-range areal extents, maximum duration of 5 years in this analysis, continues, it Validation Numbers correspond to location of from 1 million sq. km to 4 million sq. km. At the 8-year duration, could surpass the 1950s drought in severity at the 8-year The VIC model has been tested at one-eighth degree over the hydrograph flow. Observed flows the 1950s and 1930s droughts had similar severities across most duration. Obs. continental U.S. for the period from 1950 to 2000 (Maurer et al., 2002). from USGS gaging stations. areas, such that their appearance on the envelope curve alternates Model Runoff was routed through a stream network to simulate streamflow for Simulated flows from VIC model. Model between small and large areas. Obs. continental scale river basins with reasonable results, showing good Results from Maurer et al (2002) agreement of seasonal cycle, low flows, and peak flows between simulated streamflow and USGS observed streamflow (Figure 2). CONCLUDING REMARKS Figure 3. Comparison of soil Maurer et al. (2002) also compare VIC simulated soil moisture with moisture at 19 observing stations in Illinois observed soil moisture (Figure 3). The seasonal variability of •The most severe historical US droughts occurred during the 1930s and 1950s. Illinois and soil moisture from the soil moisture flux (soil moisture storage change), and its coefficient of •The current drought ranks among the most severe droughts, especially when measured by runoff and averaged over smaller areas. 17 1/8º modeled grid cells that variability and temporal persistence, were all reasonably well- •Further investigation of differences between runoff- and soil moisture-defined droughts is needed. contain the observation points. simulated. Authors would like to thank Alan Hamlet and HyoSeok Park, both at University of Washington, for their help on this project.
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