Severity-area-duration analysis of 20 th century drought in the conterminous United States Elizabeth A. Clark 1, Konstantinos M. Andreadis 1, Andrew A. Wood 1, and Dennis P. Lettenmaier 1 1. Department of Civil and Environmental Engineering, Box , University of Washington, Seattle, WA NOAA 29th Annual Climate Diagnostics and Prediction Workshop Madison, Wisconsin ABSTRACT Drought characterization typically involves severity, frequency and duration; however, the often neglected spatial extent of droughts also influences their 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 to characterize 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 digital 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 o 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) fell below the 20 th percentile, based on the 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 From these events, we constructed an envelope curve of the worst drought events in the conterminous United States during the 20 th century. These included the 1930s, 1950s, late 1980s, and 2003, all of which have been previously cited as extreme drought events. Introduction 1 Severity-Area-Duration Analysis 35 Drought Definitions 4 Envelope Curves CONCLUDING REMARKS The most severe historical US droughts occurred during the 1930s and 1950s. The current drought ranks among the most severe droughts, especially when measured by runoff and averaged over smaller areas. Further investigation of differences between runoff- and soil moisture-defined droughts is needed. Authors would like to thank Alan Hamlet and HyoSeok Park, both at University of Washington, for their help on this project. Drought is among the most costly natural disasters in the United States. In 1995, the Federal Emergency Management Agency (FEMA, 1995) estimated that the annual cost of U.S. droughts was in the range of $6-8B. To assess the potential impacts of current and future drought, water managers often compare current or potential drought severity with historic drought severities. This process, however, often overlooks the effects of areal extent on drought intensity. The purpose of this study is to create a context for comparison of extreme drought events based not only on an event’s severity, but also on its areal extent and duration. NOAA's National Climate Data Center (NCDC) recently released digitized Cooperative Observer Network (Co-op) station precipitation and temperature data for much of the 20 th century. This data set makes the extension of model simulations of hydrologic conditions over most of the 20 th Century possible. Such simulations provide a spatially and temporally continuous data set. They also allow us to investigate historical droughts in new ways. The Variable Infiltration Capacity (VIC) macroscale hydrologic model (Liang et al., 1994) is applied to a half degree resolution grid mesh across the conterminous United States for the 1925 to 2003 period. Soil properties for a 3- layer soil column are taken from Maurer et al. (2002) and aggregated from 1/8 o to 1/2 o resolution. Vegetation fractions are based on the University of Maryland's 1-km global land cover classification (Hansen et al., 2000), and LAI is derived from the gridded 1/4 o monthly global LAI database of Myeni et al. (1997). The meteorological forcings used include observed precipitation and temperature from NCDC Cooperative station data and wind from NCEP/NCAR reanalysis (Kalnay et al. 1996). Vapor pressure, downward longwave and shortwave radiation, and air pressure are derived from the observed precipitation and temperature. The VIC model accounts for sub-grid scale variability in soil, vegetation, precipitation, and topography and treats subgrid hydrologic variability statistically over coarse resolution grid cells (Figure 1). The vertical dimension of each cell is divided into three soil layers, in which the soil moisture storage capacity is treated as a spatial probability distribution. Baseflow from deepest soil layer is produced according to the Arno model nonlinear baseflow formulation (Francini and Pacciani, 1996). Evapotranspiration, surface runoff and baseflow are calculated for each vegetation class, and the weighted sum of each, based on vegetation fraction, is assigned to the grid cell. Once generated, surface runoff and baseflow are released from the grid cell, though they can be routed to simulate streamflow using the model developed by Lohmann et al. (1998). Model Obs. 2 Hydrologic Simulations and Validation 1) 2) 3) 3-month 6-month 1 year 2 years 4 years 8 years Dec 1976 to Feb 1977 Feb 2002 to Feb 2003 Feb 1930 to Feb 1937 Oct 1951 to Oct month 6-month 1 year 2 years 4 years 8 years Feb 2001 to Feb 2002 Sep 1976 to Nov 1976 Jan 1934 to Mar 1934 Sep 1953 to Sep 1956 Soil MoistureRunoff From the envelope curve, we can see that there were five major agricultural drought events between 1925 and These occurred from 1928 to 1938, 1938 to 1942, 1947 to 1957, 1972 to 1978, and 1998 to 2003 (current). The 1970s drought, which occurred in the western U.S., as well as the Great Lakes region, was most severe over 3-month durations for areas under 1 million sq. km. Similarly, the current drought dominates at smaller areas for durations from 3 months to 4 years. As expected, the 1930s drought was most severe at large spatial extents for all durations. At the end of the 1930s (1938 to 1942), severe drought covered areas larger than 5 million sq. km for durations from 3-months to 1-year. The 1950s drought experienced the highest severities for durations from 6- months to 8-years over mid-range areal extents, from 1 million sq. km to 4 million sq. km. At the 8-year duration, the 1950s and 1930s droughts had similar severities across most areas, such that their appearance on the envelope curve alternates between small and large areas. This envelope curve represents five major hydrologic drought events between 1925 and These include 1928 to 1938, 1948 to1957, 1962 to 1969, 1975 to 1978, and 1998 to 2003 (current). As with agricultural drought, the 1970s drought was most severe over the 3-month duration at small areas. The current drought dominates over a greater range of areas for durations from 3-months to 4-years when defined by runoff shortages. The 1930s drought was most severe for the 3-month duration over areas from 2.5 million sq. km to 5 million sq. km. The 1950s drought was most severe for larger areas and longer durations; however, if the current drought, which has a maximum duration of 5 years in this analysis, continues, it could surpass the 1950s drought in severity at the 8-year duration. The overall impact of a drought event depends on several factors, including severity, frequency, area, and duration (Dracup et al., 1980). Several drought indices have been defined for the characterization of drought severity over time. These indices typically describe one of four drought types: agricultural, hydrologic, meteorological, and socioeconomic. Generally, agricultural drought relates to soil moisture, hydrologic to runoff and streamflow, meteorologic to precipitation and temperature, and socioeconomic to the disparity between supply and demand for water-related goods (Wilhite and Glantz, 1985). Severity We use simulated monthly soil moisture anomalies as a measure of agricultural drought (Figure 4) and simulated monthly runoff anomalies as a measure of hydrologic drought. Because the absolute magnitude of these departures varies with climate region, we measure the departure from normal in terms of percentiles (Weibull estimate) based on the 1925 to 2003 climatology. Duration Drought events are assigned a temporal extent based on the continuation of drought conditions (20th percentile or for lower soil moisture, 30th percentile or lower for runoff) in at least one cell between monthly time steps. If multiple clusters merge to form a larger drought in later time steps, or if a single drought splits into multiple smaller droughts, the smaller droughts are considered part of the larger drought. In these special cases, the spatial extents and severities are calculated separately based on a contiguity constraint. Area The algorithm used to identify areal extent of droughts begins with spatial smoothing using a 3-by-3 median filter to minimize discontinuities in the original data. Based on the threshold level, pixels that are experiencing drought at a given monthly time step are identified and are classified using the modified clustering algorithm. The first pixel "under drought" is assigned to the first class. Then the 3-by-3 neighborhood of this pixel is searched for pixels "under drought" which are classified in the same drought cluster. This procedure is repeated until no pixels in the neighboring pixels of the current cluster are "under drought" and a new cluster is created for the next pixel below the drought threshold. After the initial partitioning step, a final classification step excludes all clusters with areas below 10 half-degree pixels from subsequent calculations. At the end of this step, the remaining clusters are defined as separate drought events for the current time step. VIC model output Total column soil moisture Runoff Weibull percentiles Threshold 20 th percentile soil moisture 30 th percentile runoff Spatial contiguity Initial drought classification Temporal contiguity Final drought and subdrought classification Severity-Area- Duration SAD curves for each event Highest severities Envelope curve for each duration The VIC model has been tested at one-eighth degree over the continental U.S. for the period from 1950 to 2000 (Maurer et al., 2002). Runoff was routed through a stream network to simulate streamflow for continental scale river basins with reasonable results, showing good agreement of seasonal cycle, low flows, and peak flows between simulated streamflow and USGS observed streamflow (Figure 2). Maurer et al. (2002) also compare VIC simulated soil moisture with Illinois observed soil moisture (Figure 3). The seasonal variability of soil moisture flux (soil moisture storage change), and its coefficient of variability and temporal persistence, were all reasonably well- simulated. Obs. Model Validation Figure 3. Comparison of soil moisture at 19 observing stations in Illinois and soil moisture from the 17 1/8º modeled grid cells that contain the observation points. Figure 2. Six monthly hydrographs, aggregated from daily flows. Numbers correspond to location of hydrograph flow. Observed flows from USGS gaging stations. Simulated flows from VIC model. Results from Maurer et al (2002) 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. For 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 over duration t (in months). We adapted the computational method of WMO (1960) to calculate the average severity corresponding 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 areas from 10 grid cells, or approximately 25,000 km 2, to the maximum drought extent, in increments of 20 grid cells, or approximately 50,000 km2. Finally, we generalize an enveloping relationship of the most extreme drought events in the United States between 1925 and 2003, composed of the maximum severity drought events at each area interval and duration (Figure 6). Construction of SAD Curves (Figure 5): 1. Rank cells by severity & identify potential drought centers 2. Search 3x3 neighborhood of drought center 3. Average severities & add areas 4. Output severity and area at specified area intervals 5. Compare the severity at ~25,000 km2 for each potential drought center and select center with maximum severity Figure 5. Process of calculating area and area-averaged severities for SAD analysis. (1) Start with maximum severity grid cell, or “drought center”. (2) Add area and average severity with most severe grid cell that is contiguous with drought center. (3) Continue adding next most severe grid cells until prescribed areal extent is reached. Figure 6. Overview of methodology from initial input to envelope curves. Figure 4. Drought severity based on soil moisture for January 1934 before processing. Figure 1. Schematic representation of VIC hydrologic model.