This research investigates the historical streamflow record of the Snoqualmie River watershed, Washington and several of its smaller tributaries for evidence of changes in flood magnitudes over the period of record. The Snoqualmie River basin is of particular interest because of its proximity to the Seattle metropolitan area, and because of flood damage that has occurred in the floods of 1986, 1990, and 1995 (three of the five largest floods in the 67 year period of record). In addition, trend analysis of the climatic forcings, as represented by simulated records of the water available for runoff (WAR, also sometimes termed rain-plus-melt) were conducted.
We examined both the historical Annual Maximum Series
(AMS) and the Peaks over Threshold (POT) series for USGS gauges on
the main stem of the
Snoqualmie River Watershed
and four of its
subcatchments, the North Fork, Middle Fork, South Fork Snoqualmie
Rivers and the North Fork Tolt River.
(See Figure 1.) This work focuses on
the longest concurrent period of record for all five gauges, which
extends from 1961 to 1993. Figures 2a-d show the harvest history
for each of the subcatchments (
N. Fork Snoq.,
Mid. Fork Snoq.,
S. Fork Snoq.,
N. Fork Tolt.
)
expressed as the percent of each of the
subcatchments with trees less than 5 and 20 years old. Harvest
histories were reconstructed from data provided by Weyerhauser
Co., the Washington Department of Natural Resources, and the U.S.
Forest Service. The character of the harvest pattern is somewhat
similar for the four subcatchments: the transient snow zone (300-
900 m) tended to be harvested earlier than the higher elevations.
The timing of the harvesting is somewhat similar for the North Fork
Tolt and North Fork Snoqualmie. The peak harvest occurred earlier
for the Middle Fork, and later for the South Fork. The total fraction
of area harvested is significantly higher in the North Fork Tolt than
in the other three subcatchments. For the main stem (not shown),
the fraction of young trees tends to be smaller, and with an earlier
peak, than for the headwaters catchments.
To simulate WAR we used the Distributed Hydrology Soil Vegetation Model (DHSVM) of Wigmosta et al., (1994) as extended for use in forested mountain catchments by Storck, et al (1995). DHSVM requires observations of temperature, dewpoint temperature, windspeed, incoming solar radiation, incoming longwave radiation, cloud cover and precipitation. Of these data, all but the radiation data were available from the Stampede Pass weather station which is located at the crest of the Cascade Range just outside the Snoqualmie Watershed. Incoming solar radiation was estimated from calculated extraterrestrial radiation and adjusted for cloud cover. Incoming long wave radiation was calculated assuming an emissivity of one with a cloud temperature equal to the dewpoint temperature and a clear sky temperature equal to local air temperature. Since DHSVM was only used to estimate the precipitation and snowmelt series, the model was run for 16 points which represented the area-elevation curve. The snowmelt for these points was then integrated to produce time series of basin-total snowmelt. Precipitation at each elevation was adjusted from the Stampede Pass observations according to precipitation-elevation curves constructed for each basin, which were constrained to reproduce the mean annual water balance.
For these simulations, the vegetation cover for each point was set to old-growth Douglas Fir for the entire simulation. DHSVM was run at a three hour time step and the total precipitation, snow water equivalent and snowmelt during each time step was recorded. The model output was then used to construct a ROS index for each event in the POT record by calculating the three day (two days prior to and one day during each event) catchment-total snowmelt divided by the total precipitation for the same period. We segregated each POT series into three categories using the ROS index: events for which snowmelt was less than 5% of precipitation, 5-10%, and over 10%, respectively. Each of the WAR series were tested for trends using the Mann-Kendall test, as were each of the AMS and POT flood series. For all analyses, a significance level of 0.05 was used.
o For all of the headwaters streams, there are statistically significant increasing trends in the AMS, but only for the North Fork Tolt, the most heavily harvested subcatchment, were there statistically significant increases in POT series;
o There were no statistically significant increases in the POT series partitioned into large or small events for any of the subcatchments or the main stem;
o Only for the North Fork Tolt and main stem Snoqualmie, were there statistically significant (increasing) trends for partitions of the POT series by ROS index; for these two gauges, statistically significant increases occurred for those events with the smallest snowmelt component of total storm WAR.
o No statistically significant trends were found for a series of POT differences for the two subcatchments (North and Middle Fork) with the greatest contrast in harvest histories. The result was the same for three normalization schemes, and for partitions of the series by event magnitude and ROS index.
o These results are consistent in some respects, and contrast in others, with a related study of the Snoqualmie River main stem by Storck et al. (1995) which examined residuals of DHSVM simulations of AFS and POT series from observed peaks. Unlike this study, Storck et al found statistically significant increases in normalized residuals of the POT series for the smallest events, but like this study, they found those increases only in events with small ROS indices.
Coffin, B. A., and R. D. Harr, Effects of forest cover on volume of water delivery to soil during rain-on-snow, Final Report, Project SH-1, Sediment, Hydrology, and Mass Wasting Steering Committee, Timber/Fish/Wildlife program, Olympia, Washington, 1992.
Jones, J. A. and G. E. Grant, Peak flow responses to clearcutting and roads, Western Cascades, Oregon, in press, Water Resources Research, 1995
Storck, P. A., D. P. Lettenmaier, B. A. Connelly, T. W. Cundy, Implications of forest practices on downstream flooding: Phase II Final Report, Washington Forest Protection Association, 1995.
Wigmosta, M. S., D. P. Lettenmaier, and L. W. Vail, A distributed hydrology-vegetation model for complex terrain, Water Resources Research, 30(6), 1665-1679, 1994.