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Seminar Abstract

Andy Wood, Dennis Lettenmaier and Arun Kumar - Nov 13, 2001

Experimental Real-time Seasonal Hydrologic Forecasting

We are developing an "end-to-end" streamflow forecasting approach which uses NCEP Global Spectral Model (GSM) climate forecasts to drive the Variable Infiltration Capacity (VIC) macroscale hydrologic model. The method has been applied on an experimental basis in the East Coast U.S. (in summer of 2000) and over the Columbia River basin of the U.S. Pacific Northwest (PNW) during the summer 2001 PNW drought. The linkage of the climate model and hydrologic model combines the climate model forecast signal with the persistence of land surface states, with the intent of improving streamflow forecasts at lead times of six months.

Each month, precipitation and temperature ensemble forecasts produced by GSM are adjusted to remove climate model bias, and the bias-adjusted ensembles are then used to drive the hydrologic model, which in turn produces ensemble streamflow forecasts with six month lead times. Initial hydrologic conditions for the forecasts are estimated by driving the VIC model with observed meteorological data (produced from NCDC cooperator stations) and updated for the most recent three months using archived real-time Land Data Assimilation System (LDAS) gridded forcings.

In both regional applications, the climate model forecasts tended toward normal, and anomalous initial hydrologic states predominated in the hydrologic response over the forecast period. In the East Coast application, critically low soil moisture in winter and spring of 2000 soil held over into summer, despite a return to more normal atmospheric conditions. A retrospective analysis of a year with significant climate forecast anomalies for the region (during the 1997-98 extreme ENSO event), however, showed that streamflow prediction skill can derive from the climate forecast signal as well. In the PNW, the forecast simulations predicted, among other effects, the severe deficit in snowpack, runoff and soil moisture, hence summer streamflow, starting in March. Prediction skill in summer 2001 is judged to have derived primarily from hydrologic persistence associated with the initial states of snowpack during the spring and summer.

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