Graphs linked to this page show pressure head, the height
to which water would rise above the sensor in a well or borehole. Interpreting
graphs of ground-water pressure head requires several kinds of information.
Knowledge of precipitation and artificial sources of ground-water recharge
are essential to interpreting changes in pressure head. Knowledge of an
instrument's installation, depth, sensitivity, accuracy, and range also
enter the interpretation. Other measurements or direct observations in open
boreholes may be needed to verify the pressure measurements. Final interpretation
depends on analysis of the data using numerical models. The annotated graphs
below describe some of the features of pressure-head data to aid the user
in understanding the graphs linked to this page.
NOISE AND SHORT-TERM PRESSURE CHANGES:
Two kinds of noise-- Data from the sensor at 75 ft in Boring
2 (above) at the Woodway landslide, show two kinds
of noise. The long downward spikes occur frequently in data from this sensor
and occasionally in data from other vibrating-wire sensors at this site.
In final data processing, these values will be removed. A second type of
noise that occurs in data from all the pressure transducers is visible as
tiny wiggles in the line traced by the data points.
Short-term pressure changes--After the noise is removed,
the trend of the data follows a smooth, gently undulating curve. The average
pressure head is slightly negative, indicating that the sensor is in an
unsaturated zone (above the water table). Detailed analysis of the data
is needed to identify the cause(s) of such short-term changes.
ANNUAL CYCLES:
Seasonal changes related to annual cycles--Data from several years
of observations at the Woodway landslide reveal
a pattern of seasonal water-level fluctuations. Water levels measured 46 ft
below the surface in B-1 rise from about December until April and fall thereafter
until about December of the following year when they begin rising again. Annual
precipitation appears to influence the amount of rise and fall.
Small wiggles in the curve reflect short-term changes in pressure head.
Some of the larger ones might follow significant precipitation events.
SHORT TERM RESPONSE TO PRECIPITATION:
Response of shallow ground water to rainfall--Rainfall
can trigger surprisingly rapid rises in shallow ground water. This data
from a landslide in Honolulu, Hawaii, shows that pressure head was gradually
declining before a rain storm on December 8 and 9, 1990 (in this graph,
the date is shown at the end of each day). After about 12 hours of rainfall,
the pressure head began to rise, and it continued rising until the end of
the storm. Pressure head declined rapidly for about a day after the storm
ended and thereafter it declined at a gradual, steady rate.
So far, similar short-term changes in pressure head have not been confirmed
at Woodway. Several observations indicate that pressure head might respond
more slowly to precipitation at Woodway than at Honolulu. Sensors at Woodway
are 45 to 90 feet deep below partially saturated soils compared with 11
feet at Honolulu. Below-average precipitation was recorded at stations near
Woodway during the 1997-1998 rainy season. The Woodway landslide occurred
about two weeks after a rainfall and rapid snow melt event, indicating that
pressure head changes at the depth where ground failure occurred may lag
several days or more behind precipitation.
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