Watershed Management Plan - Mason County

Chapter
WRIA 16 – State of the Watershed 2
Figure 2. The Hydrologic Cycle
(Golder Associates, 2003)
In WRIA 16, the majority of precipitation falls during the winter months, as
moisture-laden clouds coming off the Pacific Ocean drop up to 250 inches
of precipitation per year on the Olympic Mountains. By the time these
clouds travel east to reach the lower portions of WRIA 16 and Hood Canal,
most of their precipitation has already fallen. Consequently, precipitation
along Hood Canal is about 60 inches per year. The highest, peak stream
flows in many streams closely follow precipitation patterns; peak flows
occur in winter when most rain falls. Streams that are fed significantly by
snowpack, however, may not have their maximum flows in winter. Such
streams often experience peak flows in late fall (before precipitation turns
to snow) and again in late spring (when warming air temperatures melt the
snow) (Golder Associates, 2003).
The precipitation that falls in WRIA 16 feeds both the surface water and
groundwater of the basin.
In WRIA 16, snowmelt
from the Olympic mountain
is critical to spring and
summer streamflow in the
Dosewallips, Duckabush,
and Hamma Hamma
Rivers, as well as in the
North Fork of the
Skokomish River
SURFACE AND GROUNDWATER QUANTITIES
The Level 1 Technical Assessment estimated that the total annual volume of
precipitation in WRIA 16 averages approximately 3.4 million acre-feet per
year (AF/yr). An acre-foot is a volume of water equivalent to one foot of
precipitation over an acre of land and is a common unit used by
Final Plan for County Adoption – May 11, 2006 page 11