A system for real-time prediction of hydrological and agricultural drought over the continental U.S. Dennis P. Lettenmaier Andrew W. Wood, and Kostas Andreadis Department of Civil and Environmental Engineering American Geophysical Union Fall Meeting San Francisco December 13, 2006

www.hydro.washington.edu/forecast/monitor

SW Monitor Background directly related to published retrospective drought reconstruction work: Andreadis et al. (2005, JHM) Severity-Area-Duration analysis Andreadis and Lettenmaier (2006, GRL) CONUS drought trends enabled by recent NCDC extension of digital data archives back to 1915 Currently implemented in near real-time (daily updates) for nowcast only

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 Depth-Area-Duration analysis is commonly used by engineers to characterize extreme precipitation events for the purposes of defining a design storm. Drought is not very different from precipitation; it has an area, a duration, and rather than precipitation depth, we can use a measure of severity in our analyses. For predefined drought durations (3-month to 8-year), plots of average severity versus areal extent are created for each event. The maximum severity for different area values, at each drought duration, are taken from these curves to produce an envelope curve.

Droughts change over time! In each of these figures, the upper panel shows percent severity based on soil moisture. Red is most severe (100%), yellow is less severe (80%). The lower panel shows the classification of distinct drought events, with each color denoting a separate event. The question remains: How can we define a distinct drought event if droughts are continuous in one month, but discontinuous in the next month? Our algorithm identifies droughts at each time step, checks for common pixels between time steps, and groups the cluster with overlapping pixels at the current timestep with that in the last time step. If a drought seperates, as shown here, or merges, each subdrought (distinct) cluster is grouped as a continuation of the single larger drought event. However, in the subsequent steps, each subdrought is treated separately to ensure that only contiguous areas are compared.

Soil Moisture SAD index – entire record Apr 1934-June 1934 July 2002-July 2002 3 month 6 month 1 year 2 years Feb 1955-Feb 1956 4 years 8 years

SW Monitor Information Flow Index Station Method Gridded Forcing Creation SW Monitor Information Flow NOAA ACIS Prcp Tmax Tmin Coop Stations 1930s 1955+ VIC Retrospective Simulation Daily, 1915 to Near Current Hydrologic State VIC Real-time Simulation (~1 month long) Hydrologic State (-1 Day) Hydrologic values, anom’s, %-iles w.r.t. retrospective PDF climatology (PDF) of hydrologic values w.r.t. defined period vals, anoms %-iles w.r.t. PDF

Historic archive: UW SW Monitor

Drought recovery – the concept Real-time applications! Drought recovery probability described by soil moisture percentiles: (a) Current drought area (based on August 1933); and for different lead times, maps showing the probability (in each grid cell experiencing drought) that soil moisture percentiles will recover. (b) The grid cell-specific recovery probabilities are derived from real-time soil moisture simulations up to the current date, after which simulations are driven by ensemble climate forecasts based on a variety of sources -- e.g., ESP, climate index-conditioned ESP, and the CPC seasonal climate outlooks

Initial soil moisture percentiles 2/2006 1 month lead, forecast for 3/2006 3 month lead, forecast for 5/2006 6 month lead, forecast for 8/2006)

California-Arizona drought Feb Mar Apr May Jun Jul Aug

Texas drought Feb Mar Apr May Jun Jul Aug

Initial Condition

One month lead -- observed

Three month lead -- observed

Six month lead -- observed

To be done Replace current ESP approach with ensemble version of CPC “official” forecast, perhaps others included in westwide system Resolve issues associated with higher frequency (than monthly) update Resolve issue of incorporating ancillary (precipitation and other) data in approach that is inherently based on long station records Implement in real time

Research questions How much drought prediction skill is there in initial hydrologic conditions (e.g., soil moisture) vs climate prediction, and under what conditions, locations, and lead times? What level of complexity is required of land surface schemes to predict other drought-affected variables (especially streamflow, and effects of groundwater)? What effect have changes in drought characteristics over time (especially in the western U.S.) had on ability to represent drought probabilities?