A summary of the hydrological conditions during the 1999 and 2000 water years is shown in Figure 11.4. This figure illustrates important features of the hydroclimatic regime of the Pacific Northwest region of the United States. In this region, there is a progressive decline of soil moisture, streamflow, and groundwater levels through the dry summer months, followed by replenishment of water during wet winters.
Precipitation during the 1999 and 2000 water years was slightly greater (+5 and +0%, respectively) than the long-term (1931-1977) average annual precipitation of 2470 mm measured at the WRRS. Mean monthly temperatures during the winter periods ranged from 0.5 to 3.0°C above average. During warmer than average conditions in 1999, precipitation occurred mainly as rain, and snowcover at the site consisted of isolated snow patches in canopy gaps. Snow conditions were more typical in 2000, when a shallow continuous snowcover developed at the site lasting approximately 10 weeks. The spring seasons in both years were characterized by wetter than average conditions, followed by drier than average summers (Link 2001).
During the 1999 period when throughfall was measured, 451mm of rainfall occurred and 103 mm (23%) was evaporated from the canopy. The proportion of interception loss was similar during the 2000 measurement period, with 619 mm of rainfall measured and 155 mm (25%) lost to evaporation. The canopy saturation storage capacity derived from the throughfall measurements increased from an average of 3.0 mm in the spring and fall, to 4.1 mm in the summer, probably as a result of seasonal leaf area changes. The storage volume per unit leaf area was about 0.4 mm m-2, much higher than values estimated in plantation forests. The high value for the WRCCRF canopy is apparently due to the additional storage capacity of stems, branches, and the large canopy epiphyte community consisting of lichens and bryophytes, that can absorb from 2 to 12
times their dry weight of water (Nash and Wirth 1988; Proctor 1982). Details of the throughfall measurements and derivation of canopy interception parameters are provided by Link et al. (2004b).
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