Extended Droughts Affecting Pacific Northwest Water Resources Systems

Kristyn Gray, Alan Hamlet

12/28/98

Introduction

Water resources systems in the Pacific Northwest are in many instances quite robust to "normal" climate variability. Long term planning for these systems is frequently based on providing acceptable performance during the most extreme hydrologic events encountered in the historic record, and as a result there is a considerable margin of safety during more normal periods. The Columbia River Basin reservoir system, for example, until recently based certain crucial parts of its operating system on a critical period covering water years 1929-1931. With the change in operating system, this critical period has now changed to a shorter period in 1936-1937. These prolonged low-flow sequences have been used to define firm energy production limits, for example.

For water resources systems of this kind, forecasting of conditions in the "normal" range may have some value for certain uses, but the forecasting of extreme events such as multi-year droughts or extreme floods, probably provides the greatest value to management decision processes.

Definition of Extended Drought

1. For each calendar month the virgin flow at The Dalles must be 0.9 std deviations or more below the long-term mean, and not exceed that threshold for longer than 3 sequential months at any time during the sequence.

2. The sequence of low-flow fitting the definition above must be more than 12 months long

Six primary droughts fitting this definition are recorded in the observed flow record for the Columbia from 1900 to the present. The drought periods are:

Feb 1905--Jun 1906, Dec 1928--Feb1932, Oct1935--Aug1937, Jan1944--Aug1945, Jun1987--Sep1988, May1992--Oct1995

Droughts of this kind are characterized by low-flow conditions in the first year that deplete storage and prevent reservoir refill, followed by a second or even third year of low-flow that exacerbates impacts to system uses that rely on system storage (e.g., ag, recreation, non-firm energy, fish flows).

The following figure shows all El Nino Water years compared to Non-El Nino years with the extended droughts listed above identified. Note the preponderance of PDO positive and El Nino conditions.

Management Implications

The statistics for these extended droughts are:

5 out of 6 events are in PDO positive (Warm Phase)

4 out of 6 contain more than one El Nino event

3 out of 6 contain back-to-back El Nino events

These then are the primary climactic conditions associated with extended droughts: PDO positive/El-Nino conditions, and El Nino conditions in sequential years. Note, however, that these conditions are not always in evidence, the notable exceptions being 1936-1937 (ENSO Neutral) and 1944-1945 (La Nina). Other severe short-term droughts such as those encountered in water years 1926, 1977, and 1992 (All PDO Positive/El Nino) may also have significant effects to water resources objectives in the PNW.