1. Hydrologic Forecasting
Alan F. Hamlet
Dennis P. Lettenmaier
JISAO/CSES Climate Impacts Group
Dept. of Civil and Environmental Engineering
University of Washington

2. Winter Climate of the Western U.S.
DJF Temp (°C)
NDJFM Precip (mm)

3. Runoff Timing in the PNW is Determined Primarily by
Winter Temperature Regimes

4. Typical Effects of Urbanization on a Small Watershed
Des Moines Creek
(developed)
Source: Booth D.B., 2000, Forest Cover, Impervious-Surface Area, and the Mitigation of Urbanization Impacts in King County, WA

5. Effects of the Pacific Decadal Oscillation (PDO) and El Niño Southern Oscillation (ENSO) on Columbia River Summer Streamflows
Red = Warm ENSO, Blue = Cool ENSO, Green = ENSO neutral
Cool PDO
Cool PDO
Warm PDO
Warm PDO

6. Global Surface Temperatures are Increasing Rapidly

7. Flood Control and Hydropower Management
Weather Forecasts
Flood Control and Hydropower Management
Flood Forecasts
Seasonal to Interannual Climate Forecasts
Seasonal Streamflow Volumes
Water Resources Management
Climate Change Scenarios
Long-Range Streamflow Forecasts
Water Resources Planning
1hr - 1 week
1– 24 months
years
Forecast Lead Time

8. Schematic Diagram of a Hydrologic Forecasting System
Initial Hydrologic State
Soil Moisture
Snowpack
Hydrologic
Forecast:
Streamflow
Soil Moisture
Snowpack
Evaporation
Hydrologic Model
Future Temperature and
Precipitation
Forecast

9. Simulated Water Balance for the Pacific Northwest

10. Relative Roles of Future Precipitation and Initial Hydrologic State at Different Forecast Dates
In October
future
precipitation
dominates the inputs to the water balance.
In April inputs to the water balance from future
precipitation and storage are comparable.
99%
46%
Simulated Long-Term Water Balance for the Pacific Northwest

11. Examples of Hydrologic Forecasting Systems

12. Example of a Short Time Scale Flood Forecasting System
MM5 mesoscale atmospheric model
DHSVM distributed hydrologic model
Estimated
Hydrologic
State
Streamflow Forecast
River Stage Forecast

13. Example of a Seasonal Forecasting System Based on Regression Models
NRCS SNOTEL Network
NRCS/NWRFC Water Supply Forecasts
Hydrologic
Index
Regression
Equation
Streamflow
Volume

14. UW West-Wide Seasonal Hydrologic
Example of a Seasonal Forecasting System Using a Physically-Based Hydrologic Model
Temperature and
Precipitation
Forecast
Estimated
Hydrologic
State
VIC Hydrologic Model
UW West-Wide Seasonal Hydrologic
Forecast System
Hydrologic
Forecast

15. Background: Forecast System Schematic
NCDC met. station obs. up to 2-4 months from current
local scale (1/8 degree) weather inputs
soil moisture
snowpack
Hydrologic model spin up
SNOTEL Update
streamflow, soil moisture,
snow water equivalent, runoff
25th Day, Month 0
1-2 years back
LDAS/other real-time met. forcings for spin-up gap
Hydrologic forecast simulation
Month
INITIAL STATE
SNOTEL
Update
ensemble forecasts
ESP traces (40)
CPC-based outlook (13)
NCEP GSM ensemble (20)
NSIPP-1 ensemble (9)

16. Natural Streamflow (cfs)
Climate forecasts based on ENSO predictions can provide useful information about future streamflows with lead times up to 12 months.
Natural Streamflow Columbia River at The Dalles, OR
Retrospective tests in the Columbia River basin have shown that during cool or warm events, ENSO-based
streamflow forecasts are superior to assumptions of “normal” conditions about 65 % of the time on Oct 1
Red = Unconditional mean
Blue = Ensemble mean
Black = 2005 Observed
Natural Streamflow (cfs)

17. Conclusions
Useful hydrologic forecasts based on weather or climate forecasts are available with lead times ranging from a few hours (flood forecasts) to 50 years or more (climate change scenarios).
Many operational hydrologic forecasting systems are currently based on statistical models, however dynamic, physically-based tools are increasingly being used in both academic and operational forecasting systems.
Dynamic forecasting systems based on weather or climate models directly linked to physically-based hydrologic models have important advantages in a rapidly evolving climate system. Short-term forecasts based on weather models have already reached a useful state of development, but many challenges remain at seasonal or longer time scales.