Customized Water Level Trends |
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Customized Water Level Trends |
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AquiferTest provides the option to create a user-defined correction factor, and apply this to the observed drawdown data.
In confined and leaky aquifers, rhythmic fluctuations of the hydraulic head may be due to the influence of tides or river-level fluctuations, or to rhythmic variations in the atmospheric pressure. In unconfined aquifers whose water tables are close to the surface, diurnal fluctuations of the water table can be significant because of the great difference between day and night evapotranspiration. The water table drops during the day because of the consumptive use by the vegetation, and recovers during the night when the plant stomata are closed (Kruseman and de Ridder, 1990).
To access the User Defined Data Corrections, go to the Water Levels tab, click on the Add Data Correction button (not the arrow beside it) and the following dialog will appear:
In the Data Correction dialog, enter a name for the correction, then select a formula type. There are four formula types to choose from (see individual sections below). All four corrections have the following general behavior:
where:
is the corrected drawdown [L]
is the measured drawdown [L]
is the specified drawdown correction [L]
Depending upon selected type of correction, there will be input fields for the different coefficients (A, B, C, and D) as described below. Determining the values of the coefficients is a complex process, which depends on the type of data correction and the cause of the displacement.
NOTE: It is not possible to apply a data correction only to a certain period of time, it always applies to all data. It is only possible to limit data correction to a particular well.
The Simple Delta S adjusts the water levels up or down by a constant value. The formula used is:
where:
is the drawdown correction [L]
is a specified constant offset [L]
This is a simple correction. An example use of this type of correction may be to correct an offset of a logger measurement (e.g. the cable used to hang the data logger from the well is lengthened or shortened) or to correct for a change in the reference elevation of a well (e.g. if the well construction changes from a well with a stick-up to a flush-mounted well).
The Linear Time Dependent correction adjusts the water level linearly over time. The formula used is:
where:
is the drawdown correction [L]
is a specified rate constant [L/T]
is the elapsed time since the start of the test [T]
This is a general trend correction for cases where the overall changes of water levels in the aquifer can be approximated by a linear function over the duration of the pumping test. An Example would be seasonal drainage.
The Linear Time Dependent correction adjusts the water level logarithmically over time. The formula used is:
where:
is the drawdown correction [L]
is a specified offset [L]
is a specified constant [-]
is a specified rate constant, usually negative [-/T]
is the elapsed time since the start of the test [T]
This is a decay-like trend correction for cases where the overall changes of water levels in the aquifer can be approximated by an exponential decay function over the duration of the pumping test. An example would be drainage of an aquifer following a precipitation event.
The Periodic Time Dependent correction adjusts the water level by a sinusoidal offset over time. The formula used is:
where:
is the drawdown correction [L]
is the amplitude of the wave [L]
is a specified phase shift [-]
is the period of the wave [-/T]
is the elapsed time since the start of the test [T]
D is an exponent that can be used to adjust the amplitude wave over time [-/T]
This is a decay-like trend correction for cases where the overall changes of water levels in the aquifer can be approximated by an exponential decay function over the duration of the pumping test. An example would be drainage of an aquifer following a precipitation event. A common example of a period time dependent correction is tidal correction. For tidal corrections, the coefficients can be estimated as follows:
A (amplitude): half amount of the tidal change during one period (high - low tide)
B (phase shift): calculated as follows; For example, if low tide occurs 2 hours after after the start of the test, B = (π/2) + [ (2h/ 6.2h) * π ]. Please note that B is dimensionless, so it must be given in radians
C (period) = ( π/12 h 25 min)
D (exponent): = 1
The range of application indicates whether the correction applies only to the current well data set, or to all wells. For example, a local trend usually affects all wells, while a periodic correction of the Tidal influences depends on the distance to the sea, and therefore must be unique for each observation well.
When defining the coefficients, be aware of the sign (positive or negative). The result of the calculation is added to the drawdown values; i.e. if the value is positive, the drawdown increases; for negative values, the drawdown decreases.
For example, if you have a local trend where the water table decreases 1cm/d, the value must then be defined as negative, so that the appropriate amount is subtracted from the observed drawdown. Alternatively, if the trend shows the water table elevation rising 1cm/day, the value must then be defined as positive, so that the appropriate amount is added to the observed drawdown data.
Upon clicking OK, the data correction will be applied to the measured drawdown data, and an additional column will appear in the data table. This column will contain the corrected drawdown using this data correction; the corrected drawdown will be used in the analysis to calculate the aquifer parameters.
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