Advances in seasonal hydrologic prediction. Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington GEOSS Workshop XXXIII: Using Earth Observations for Water Management San Francisco December 18, 2009. Background
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Dennis P. Lettenmaier
Department of Civil and Environmental Engineering
University of Washington
GEOSS Workshop XXXIII: Using Earth
Observations for Water Management
December 18, 2009
The University of Washington west-wide seasonal hydrologic forecast system
Current and recent research -- assimilation of satellite data
Is there hydrologically useful skill in climate forecasts?
Concluding thoughtsTalk Outline
Aug1. Background: The importance of Seasonal Hydrologic Forecasting
Snow water content on April 1
McLean, D.A., 1948 Western Snow Conf.
April to August runoff
Overview: ESP Hydrologic prediction strategy prediction in the western U.S.
ESP data flow
The ESP “spider web”
6-month ESP streamflow forecasts for western U.S. and Mexico effective 12/7/09
Forecast System Initial State information hydrologic forecast system
Simulated Initial Condition
Simulated Initial Condition
Flow location maps give access to monthly hydrograph plots, and also to raw forecast data.
Clicking the stream flow forecast map also accesses current basin-averaged conditionsStreamflow Forecast Details
MODIS updating of snow covered area and also to raw forecast data.
local scale weather inputs
Hydrologic model spin up
streamflow, soil moisture, snowpack, runoff
NCDC met. station obs. up to 2-4 months from current
LDAS/other real-time met. forcings for remaining spin-up
End of Month 6 - 12
1-2 years back
25th Day of Month 0
Change in Snowcover as a Result of MODIS Update for April 1, 2004 Forecast
Snowcover before MODIS update
Snowcover after MODIS update
Unadjusted vs adjusted forecast errors, 2001-2003, for reservoir inflow volumes (left plot) and reservoir storage (right)
General finding is that NCEP GSM climate forecasts do not add to skill of ESP forecasts, except…
April GSM forecast with respect to climatology (left) and to ESP (right)
Summary: During strong ENSO events, for some river basins (California, Pacific Northwest) runoff forecasts improved with strong-ENSO composite; but Colorado River, upper Rio Grande River basin RO forecasts worsened.
October GSM forecast w.r.t ESP: unconditional (left) and strong-ENSO (right)
Columbia R. Basin
fcst more impt
ICs more impt
Rio Grande R. Basin
VIC model long-term (1960-99) simulations at ½ degree spatial resolution assumed to be truth
DEMETER reforecasts with ECMWF seasonal forecast model for 6 month lead, forecasts made on Feb 1, May 1, Aug 1, Nov 1 1960-99
9 forecast ensembles on each date
Forecast forcings (precipitation and temperature) downscaled and bias corrected using Wood et al approach (also incorporated in UW West-wide system)
On each forecast date, 9 ensemble members also resampled at random from 1960-99 to form ESP ensemble
Forecast skill evaluated using Cp for unrouted runoffDEMETER forecast evaluation
Test sites spatial resolution assumed to be truth
Missouri River at Fort Benton spatial resolution assumed to be truth
Snake River at Milne spatial resolution assumed to be truth
Hydrologic prediction skill at S/I lead spatial resolution assumed to be truthtimes comes mostly from initial conditions.
Hence more focus on data assimilation, and its implications for hydrologic forecast skill, needs more attention.
The role of model error in hydrologic predictions needs more focus – how do we best weight land models in multimodel ensemble?
Do hydrologists (and the land data assimilation community) need to expend more effort on hydrologic forecasting?Concluding thoughts
Streamflow forecast skill, observed streamflow simulated (left panel) and forecasted (right two) using model soil moisture and SWE; MAMJ streamflow conditioned on January 1 model conditions
The challenge: Different land schemes have different soil moisture dynamics
soil moisture at cell
NE moisture dynamics
SEAreas for spatially averaged soil moisture percentiles
Box sizes are 5 x 5 degrees
NW moisture dynamics
SE moisture dynamics
Soil Moisture Percentiles w.r.t. 1920-2003 moisture dynamics
US Drought Monitor