Methodology for 1-Year Lead Time Experimental Streamflow Forecasts for the Columbia River
Center for Science in the Earth System Climate Impacts Group
Department of Civil and Environmental Engineering
University of Washington
Alan F. Hamlet
Dennis P. Lettenmaier
Background and Description of Methodology
The methods used to produce these retrospective forecasts are described in more detail by Hamlet and Lettenmaier (1999a).
Investigation of interactions between recurrent patterns of climate
variability and Columbia River streamflow have revealed strong
correlations between Columbia River flow from April-September and two recurrent climate
phenomona: the El Nino/Southern Oscillation (ENSO) (see for example
Battisti and Sarachik 1995) and the Pacific Decadal Oscillation (PDO)
(Mantua et al 1997). These climate patterns, and particularly the
phase of ENSO, can be forecast with lead times of six months to one
year, and are used here in conjunction with a macro-scale hydrology model
driven by resampled historical meteorological data to create
streamflow forecast ensembles for the coming water year. The forecasts
for the water year (Oct-Sept) can typically be delivered in June of
the preceding year, when good climate forecasts are typically available (Hamlet and Lettenmaier 1999a)
Several web sites present more information on Pacific Northwest climate variability and ENSO forecasting in more detail:
The JISAO CSES Climate Impacts Group website
CIG Website
provides general information on PNW climate variability and recurrent climate patterns like ENSO and PDO.
The Pacific Northwest Marine Environmental Laboratory El Niño Theme Page
http://www.pmel.noaa.gov/toga\-tao/el-nino/nino-home.html
provides general information about ENSO and links to real-time monitoring and forecast information that
are used to select a categorical ENSO forecast upon which the streamflow forecasts are based.
The Climate Categories
Combinations of two climate indicators are used to create six climate categories for investigations of the relationship between streamflow and climate states and for retrospective forecasting.
The ENSO (El Nino/Southern Oscillation) climate categories are created using the NINO 3.4 index averaged from Dec-Feb. Annomalies more than 0.5 standard deviations above the mean are assigned as ENSO positive (El Nino), and annomalies more than 0.5 standard deviations below the mean are assigned to ENSO negative (La Nina). Other years are considered ENSO neutral. The phase of ENSO can be forecast with lead times of about 1 year using coupled ocean atmosphere models and/or statistical procedures.
PDO, on the other hand, is retrospectively assigned a bimodal warm or cool phase based on
decadal scale oscillations that have occurred in the historic
record. From 1948-1976 the PDO index was predominantly negative (cool
phase), and from 1977-? the PDO index has been predominantly positive
(warm phase) (Mantua et al, 1997).
Although a PDO transition from predominantly warm (positive index values) to cool (negative index
values) in 1997 remains uncertain, the extremely
high streamflow event in 1997 followed by predominanty negative PDO index values from 1999-2002 suggests that such a shift may have taken place.
(see Predicting PDO Transitions)
Most recently, Newman et al. (2003) have demonstrated the skill of simple regression techniques for predicting the PDO index on an interannual basis. We currently use these techniques to estimate the PDO index for the coming winter.
Six climate categories shown below in the scatter plots are created using combinations of the ENSO and PDO phase. Red triangles in the figures show average April-Sept hydrologic response of water years in the climate category. Blue circles show the hydrologic response of those water years not in the category. The red and blue horizontal lines show the average difference between the those years in the category and those outside the category. Climate Category 1, for example, shows a strong signal for low-flow response from the Columbia Basin.
Creating the Streamflow Forecast
The Variable Infiltration Capacity (VIC) macro-scale hydrology model (Matheusen et al., 2000; Nijssen et al, 1997; Liang et al, 1994) covering the Columbia Basin above The Dalles at 1/4 degree resolution is used with gridded meteorological data derived from station data to simulate streamflow associated with different water years. The model exhibits some bias under certain conditions, but successfully captures the variability in streamflow at The Dalles from 1948-1997 quite well (Hamlet and Lettenmaier, 1999b). Technical information on this hydrologic model is available from links to the UW Hydrology Home Page below:
VIC Hydrologic Model
Streamflow outcome during the forecast year is primarily a function
of initial soil moisture conditions and meteorological events
(precipitation and temperature) which will occur in the forecast
period (Oct-Sept for these forecasts). To incorporate these variables in the forecast
method, the hydrology model is initialized using a meteorological time
series from the historic record which is similar in character to the
year preceding the forecast year. Then meteorological data from
different years within the forecast climate category are used to drive
the initialized hydrology model to create the forecast ensemble
members. So, for example, water year 1998 was forecast as Climate
Category 4 (cool PDO/El Nino), so water years from Climate Category 4
were used to drive the hydrology model after initialization using
water year 1974 (an extremely wet year similar in character to
1997). The figure below shows a schematic for the forecasting
method.
The figure below shows a timeline for the streamflow forecasting process.
Retrospective Streamflow Forecast Archive
Retrospective forecasts from water year 1951 to 2000 have been produced and are available at:
Forecast Archive