Ground-Water Hydrology of the Upper Klamath Basin, Oregon and ...

22 Ground-Water Hydrology of the Upper Klamath Basin, Oregon and California
Ground-Water Discharge to Streams
Water flows to streams through a variety of mechanisms.
For convenience, streamflow is often broken into three
components: surface runoff, interflow, and baseflow. The
surface runoff component reaches the stream through
overland flow or flow in the top of the soil profile. Such
flow is typically rapid, and is responsible for the rapid rise in
streamflow during and immediately after storms or snowmelt.
Interflow, also termed subsurface runoff, reaches the stream
through flow in unsaturated or temporarily saturated zones
in the upper soil layers. Baseflow generally is considered to
be fed by ground-water discharge. Baseflow can originate
from a variety of scales of ground-water flow, ranging from
short, local flow paths to long and deep regional flow paths.
Baseflow generally is the source of water in streams in the late
summer and fall, when little or no precipitation or remaining
snow are available to provide surface runoff or interflow.
The flow in some streams in the upper Klamath Basin
consists entirely of ground-water discharge. Such streams
are characterized by consistent year-round flow with little
seasonal variability. An example of a typical ground-water-fed
stream is the Wood River at Fort Klamath (USGS streamgaging
station number 11504000), which during the period of
record from 1913 to 1936 had a mean annual flow of 215 ft 3 /s
and a standard deviation of daily mean flows of only 58 ft 3 /s.
The mean September flow of the Wood River was 199 ft 3 /s, or
about 93 percent of the mean annual flow. Other streams, in
contrast, have a relatively small component of ground-water
discharge and consist predominantly of surface runoff. Such
streams have large seasonal variability, with high flows during
and immediately after rainfall or snowmelt followed by low
or no flow during the dry periods of the year. Stream gaging
data from the Sycan River below Snake Creek near Beatty
(station 11499100) provide an example of a stream with a
large component of surface runoff. During the period of record
from 1973 to 2003 the Sycan River at this location had a mean
annual flow of 152 ft 3 /s and a standard deviation of daily mean
flows of 282 ft 3 /s. The mean September flow of the Sycan
River here is only 23 ft 3 /s, or about 15 percent of the mean
annual flow. Most streams in the Klamath Basin exhibit all
three components of discharge throughout the year. Identifying
the amount of streamflow supplied by ground-water discharge
is problematic during times of the year when there is
substantial contribution from overland flow and interflow.
However, in the late summer and fall, when there is scant
precipitation and snow has melted, streamflow is composed
largely of ground-water discharge. Exceptions to this
generalization include streams receiving substantial irrigation
return flow or water from reservoir releases. Where these
exceptions do not occur, or can be accounted for, streamflow
during the fall months (September–November) when
precipitation, runoff, and interflow are nearly absent provides
a good estimate of baseflow or ground-water discharge.
The location and quantity of ground-water discharge
entering the stream network was estimated at numerous
locations throughout the upper Klamath Basin (table 6, at
back of report). Estimates are, for the most part, based on
measurements of actual spring discharge or streamflow during
late summer and fall. For some spring-dominated streams
(Spring Creek, for example), streamflow over the entire year
could have been used to determine ground-water discharge.
However, to maintain consistency in the analysis, data from
the fall months were used exclusively where possible. Some
of the ground-water discharge estimates were based on the
OWRD natural streamflow analysis for the Klamath Basin
(Cooper, 2004). These estimates represent the median or
typical flow for a particular month over a 30-year base period,
from 1958 to 1997. For this study, the work was supplemented
by additional analysis, streamflow measurements, and
fieldwork performed from 1997 to 2005. When possible, the
estimate represents the typical ground-water discharge for the
fall over a base period from 1958 to 1987. Selection of this
base period is detailed in Cooper (2002). However, sometimes
the available data at a location was insufficient to generate an
estimate that represented the base period. In those cases, the
estimate may not reflect the long-term average conditions. The
data sources and techniques used to estimate ground-water
discharge are listed for each reach in table 6.
Estimates of ground-water discharge have inherent
uncertainty. One source of uncertainty is the streamflow
measurements on which they are based. For example, records
from stream gages are rated “excellent” when 95 percent of
the daily discharge values are within 5 percent of the true
value, “good” when 95 percent of the daily discharge values
are within 10 percent of the true value, and “fair” when 95
percent of the daily discharge values are within 15 percent of
the true value. Some of the estimates in table 6 are based on
regression models where estimates are derived by comparing
streamgage data that span periods that are short or outside of
the base period, or miscellaneous measurements with longterm
flow data from streams determined to be hydrologically
representative of the stream in question. Regression models
are another source of uncertainty. A detailed description of
the regression analyses is available in Cooper (2004). Some
ground-water discharge estimates in table 6 are based on
single measurements or averages of multiple measurements
and are not shifted to the base period. Consequently, there is
uncertainty as to the degree to which they represent long-term
average conditions. An assessment of the level of certainty
of each estimate is included in table 6, and most figures are
rounded to two significant figures. Rates of discharge at the
large spring complexes responsible for most of the groundwater
discharge in the basin are generally well known and
have the least associated uncertainty. Although there may be
uncertainty in the estimates of long-term average ground water
discharging at certain locations, the presence of discharge
at the listed locations is well established, and the general
distribution and magnitude of ground-water discharge in the
upper Klamath Basin is well understood.