Effects of El Nino on Inflow to Chester Morse Lake
1946-1993 Water Years
Alan Hamlet, University of Washington
hamleaf@u.washington.edu
10/15/97
Definition of El Nino Years
El Nino years are defined as those years for which the average Oct-March SOI (Southern Oscillation Index) is less than -0.75. Other years are defined for this study as non-El Nino.
Flow Data Source
Monthly flow data were extracted from Earth Info CDs for the Cedar and Rex River upstream of Chester Morse Lake (both near Cedar Falls, WA).
Adjustments to Flow Data to Estimate Inflow to Chester Morse Lake
Estimates of inflow to Chester Morse Lake were based on the sum of Cedar and Rex river flows multiplied by a fixed correction factor. The factor is the ratio of the total basin area at Chester Morse Lake (78.4 sq mi) divided by the sum of the Cedar and Rex drainage areas at the flow measurement points (40.7+13.4 sq mi). The correction factor is 1.45.
Data were missing in water year 1992 for the Rex gaging station and were filled in using data from the Cedar based on the ratio of the long-term monthly averages at the two gaging locations.
Monthly Summary
The following figure shows average monthly flows segregated as El Nino and Non-El Nino years. Water year 1983 is included as an example of a single year, since some forecasters believe that the coming year shows similar characteristics to 1983 (i.e. very strong El Nino effects).
Yearly Summary
The following figure shows the distribution of average annual inflow to Chester Morse Lake. Note the frequent lower than average inflow for El Nino years. 91% of El Nino flows are below the full period average of 527 cfs.
April-September Summary
The following figure shows the distribution of average April-September inflow to Chester Morse Lake. The results are similar in character to the annual average flows, however the spread is somewhat larger. 82% of the El Nino Flows are below the full period average of 481 cfs. Note especially the El Nino year 1992, which has extremely low inflow for this period. Analysis of snow pack showed that this year had zero snow pack on April 1 due to unusually warm winter temperatures (Figure 4).
April 1 Snow Pack Summary
The following figure shows the distribution of April 1 snow water equivalent at Olallie Meadows (near Snoqualmie Pass). Note the general similarity between the distribution of April-September average streamflow to Chester Morse Lake and the distribution of snow pack measurements.
