Elizabeth A by HC120302183910


									                                                                                                            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
    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
                                                                                                                                                                                                                                                                                                                            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
                                                                                                                                                                  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.
                                                                             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
                                                                             Runoff was routed through a stream network to simulate streamflow for                Simulated flows from VIC model.                     Model            between small and large areas.
                                                                             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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