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

Ground-Water Hydrology 17
100
90
80
Qs, n = 41
Ts, n = 48
Tv, n = 173
70
PERCENTILE
60
50
40
30
20
10
0
0.1 1 10 100 1,000 10,000 100,000 1,000,000
TRANSMISSIVITY, IN FEET SQUARED PER DAY
Figure 5. Cumulative distribution of transmissivity values estimated from specific-capacity tests for wells producing
from Quaternary sediment, Tertiary sedimentary rock, and Tertiary volcanic rock in the upper Klamath Basin, Oregon
and California.
Aquifer tests with observation wells are not affected by this
phenomenon. Second, the large number of specific-capacity
tests (173) represent a more or less random sampling of
wells (and varying characteristics) in the unit. Aquifer tests,
in contrast, are not random but tend to be conducted most
commonly on high yielding wells for specific purposes.
Regardless, transmissivity values calculated from both aquifer
tests and specific-capacity tests are useful for understanding
the hydraulic characteristics of hydrogeologic units and the
differences between units.
Ground-Water Hydrology
Ground water moves from areas where it enters the
ground, known as “recharge areas,” to areas where it leaves
the ground, known as “discharge areas.” On a regional scale,
recharge areas are typically high-elevation regions with large
amounts of precipitation compared to surrounding areas.
Ground water moves from recharge areas toward low-elevation
areas in response to gravity. In low-elevation areas, ground
water typically discharges to streams, lakes, or wetlands and
then is returned to the atmosphere through evapotranspiration
or leaves the basin as streamflow. Ground water can be
removed anywhere along its flow path by wells.
Ground water moves in response to differences in
hydraulic head, from areas of high head to areas of low
head. In unconfined aquifers—those without overlying lowpermeability
strata—hydraulic head can be thought of as the
elevation of the water table. In confined aquifers, hydraulic
head can be thought of as the elevation of the aquifer plus the
pressure of the confined ground water. Maps of hydraulic head
are useful for identifying recharge and discharge areas, and
for indicating the direction of ground-water flow. Although
the regional scale movement of ground water largely follows
topography, the actual flow paths that the ground water
follows and the rate of ground-water movement is controlled
by the permeability of the geologic materials through which
it flows. The rate of ground-water movement is proportional
to the hydraulic head gradient and the permeability of the
geologic materials.
Ground-water systems are dynamic, with rates of
recharge and discharge and hydraulic head varying in
response to external stresses. The largest external influence on
ground-water systems is climate. Drought cycles cause large
fluctuations in recharge, ground-water levels, and discharge to
springs and streams. Human-caused stresses, such as pumping
and artifical recharge from canal leakage and deep percolation
of irrigation water, also affect the ground-water system.
OR19-0048_fig05