Severity-area-duration analysis of 20th century drought in the conterminous United States Climate Impacts Group Weekly Seminar Oct. 18, 2004 Elizabeth Clark, Konstantinos Andreadis, Dennis Lettenmaier

Outline Background Motivation VIC Model Drought Definitions Technique of drought classification Preliminary results for continental U.S. 1930s, 1950s, current drought Comparison of most severe agricultural and hydrologic droughts Implications for water managers Future research

Motivation Recent availability of precipitation and temperature data make model simulation of hydrologic conditions for long periods possible. Such simulations provide a spatially and temporally continuous data set. They also allow us to investigate historical droughts in new ways.

VIC Model Soil Parameterization 3 soil layers Variable infiltration curve in upper layer partitions subsurface and quick storm response Gravity-driven vertical soil drainage Non-linear baseflow drainage from lowest layer

6 Sample Hydrographs Good agreement of Seasonal cycle Low Flows Peak Flows Model Obs. Maurer et al., 2002

Comparison with Illinois Soil Moisture 19 observing stations are compared to the 17 1/8º modeled grid cells that contain the observation points. Persistence Moisture Level Moisture Flux Variability Obs. Model Maurer et al., 2002

Drought Definitions Meteorological Drought Precipitation and Temperature Agricultural Drought Soil Moisture Hydrologic Drought Streamflow/Runoff Socioeconomic Drought Disparity between supply and demand

Palmer Drought Severity Index PDSI : measures meteorological drought using a method that accounts for precipitation, evaporation, and soil moisture conditions. Dai & Trenberth (2004) find correlation between annual PDSI and streamflow and correlation between PDSI and soil moisture during warm season. Snow interferes with soil moisture calculations. Despite standardization, dependence on termination criteria results in questionable distribution of severe droughts.

PDSI-based studies Cook et al. (1999) used tree-ring chronologies to reconstruct US droughts from EOFs used for regionalization purposes Examined PDSI signal over those regions “Dust Bowl” dominated the entire period Other notable droughts: 1950, 1965, 1977.

PDSI-based studies Dai et al. (1998) used a monthly PDSI dataset from o x 2.5 o grid over the globe Major droughts identified: 1930s, 1950s and 1988 Correlation between PDSI and ENSO signals Increase in percentage areas of severe drought during the last 2-3 decades, over many ENSO-sensitive regions

Drought spatial analysis from other studies Most other studies have used station data Pre-defined climate regions Statistical methods such as Correlation analysis, (Oladipo 1986) Empirical Orthogonal Functions (EOF), (Cook et al. 1999, Hisdal and Tallaksen 2003) Simulation provides continuous spatial and temporal mapping of hydrologic variables

Hydrologic Simulations Based on physical processes Not dependent on scattered or temporally disjoint station data Allow for direct analysis of parameters of interest, i.e. runoff and soil moisture Use of percentile values standardizes over heterogeneous regions and is independent of initialization and termination criteria

How can we use information from long term hydrologic model simulations to synthesize the following drought characteristics: severity, intensity, extent, and duration?

Severity-Area-Duration Analysis Based on the Depth-Area-Duration technique from probable maximum precipitation analysis Replace depth with measure of drought severity S=(1-ΣP/t) S=severity, ΣP = total percentile (soil moisture or runoff), t = duration

How do we define drought extent? Severity

Droughts change over time!

SAD Construction 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 km 2 for each potential drought center and select center with maximum severity Modified from WMO (1960) computational method of DAD analysis 1) 2) 3)

Methodology VIC model output Total column soil moisture Runoff Weibull percentiles Threshold 20 th percentile and lower soil moisture 30 th percentile and lower 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

1930s Drought Soil Moisture 80 Percent severity 100 Soil moisture-defined drought

1930s Drought Runoff 80 Percent severity 100 Runoff-defined drought

1950s Drought Soil Moisture 80 Percent severity 100 Soil moisture-defined drought

1950s Drought Runoff 80 Percent severity 100 Runoff-defined drought

Current Drought Soil Moisture Soil moisture-defined drought 80 Percent severity 100

Current Drought Runoff 80 Percent severity 100 Runoff-defined drought

Soil Moisture 3 month 6 month 1 year 2 years 4 years 8 years Apr June 1934 Feb Feb 1956 July July 2002

Runoff 3 month 6 month 1 year 2 years 4 years 8 years Jun Jun 2003 Dec Dec 1939 Nov Nov 1956 Feb Feb 1977

Rising Temperatures and Declining Streamflow in West US Higher temperatures are resulting in earlier snow melts, up to days earlier (Pagano et al., 2004). Upper Colorado River basin reported to be experiencing worst streamflow deficit in 80 years & 7 th worse in past 500 years (Piechota et al., 2004).

Implications for Water Management Similar to Depth-Area-Duration analysis, Severity-Area-Duration analysis provides a basis for a sort of “design drought” estimation. This estimates an upper bound for anticipated drought severities.

Future Research Real-time applications! Figure from Andy Wood.

Conclusions The most severe historical US droughts occurred during the 1930s and 1950s. The current drought ranks among the most severe droughts, especially when averaged over smaller areas. Future research promises to provide water managers with new tools for real-time drought forecasting.

Acknowledgements Thanks to Alan Hamlet, Andy Wood, and HyoSeok Park.