Barometric Trend Analysis and Correction |
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Barometric Trend Analysis and Correction |
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During the pumping test, changes in the barometric pressure can have an affect on the recorded drawdown data, and should be considered during the data analysis. AquiferTest includes the tools to correct drawdown data for barometric effects, using data pre-processor tools. Barometric pre-processing generally involves the following steps:
1.Collecting baseline data (barometric pressure vs. water level) prior to, or after, the test;
2.Use this data to calculate the barometric efficiency (BE) of the aquifer.
3.During the pumping test, collect time vs. water level data AND time vs. barometric pressure data.
4.Using the BE value, determine the equivalent water level measurement at the observed time. If the pressure is not recorded at the same time as the water levels, linear interpolation may be used to find and correct the next available water level measurement.
5.Apply the correction to the observed drawdown data.
6.Use the corrected water levels for determining the aquifer parameters.
NOTE: Barometric correction tools are only available in the AquiferTest Pro edition.
In wells or piezometers penetrating confined and leaky aquifers, the water levels are continuously changing as the atmospheric pressure changes. When the atmospheric pressure decreases, the water levels rise in compensation. When the atmospheric pressure increases, the water levels decrease in compensation. By comparing the atmospheric changes, expressed in terms of a column of water, with the actual changes in water levels observed during the pre-test period, it is possible to calculate the barometric efficiency of the aquifer. (Kruseman and de Ridder, 1990)
The barometric efficiency (BE) is a parameter of the aquifer, and specifies how it reacts to changes in atmospheric pressure. The BE value usually ranges between 0.2 and 0.75. The BE is defined as the ratio of change in water level in a well (Δh) to the corresponding change in atmospheric pressure (Δp).
where:
is the change in water level [L]
is the density of the fluid [M/L3]
is the acceleration due to gravity [L/T2]
is the change in atmospheric pressure [M/LT2]
The density of water () is a function of the temperature and solute concentration. The default value of in AquiferTest of 999.7 kg/m³ is based on an assumptions that the aquifer and its groundwater have a temperature of 10°C and very low solute concentrations. The density of water may depart significantly from the default value when temperatures vary the density of water is 999.7 kg/m³ and you should adjust the constants as applicable based on the conditions encountered in the aquifers you evaluate.
The acceleration of gravity on the surface of the earth () is approximately 9.81 m/s² or 32.2 ft/s2 for most places on the surface of the Earth, However, the value of can vary between a low as 9.78 m/s² near the equator or at extreme altitudes (> 8,000 m) and high as 9.83 m/s² near the North and South Poles. Latitude, altitude, and the variable density of the Earth affect the value of . In most cases, the variability of is sufficiently small; however, you may wish to adjust the default value in AquiferTest (9.81 m/s2).
The values for both and may be adjusted using the settings available in the Constants tab, which is accessed by selecting Tools > Options from the main menu.
To calculate the change of water level in an aquifer caused by the atmospheric pressure change alone, rearrange the formula for the BE, to get:
The Barometric Efficiency (BE) may be entered directly into AquiferTest (in the Pumping Test tab), or it may be calculated. To calculate the BE value, the user must provide pressure vs. water level data recorded from a well near the test site, before or after the test.
Once the BE is known, the measured drawdown can be corrected. To do so, the user must provide time vs. pressure data recordings over the duration of the pumping test. However, the atmospheric pressure measurements do not have to be recorded at the point(s) in time as the water level measurements. In cases where there is a time offset between water level and atmospheric pressure measurements, AquiferTest uses linear interpolation between the previous and next available pressure values to estimate the atmospheric pressure at the time(s) when the water levels were recorded. An example is illustrated below:
In the figure above, you can see how AquiferTest interpolates the atmospheric pressure p(a) for the time of water level measurement (i.e. the water level when the elapsed time is 2 days, where no measured value for p(a) is available.
AquiferTest will use the values of (1.5) and (2.5) for linear interpolation and to calculate a straight line function of the form y = mx + b.
Once the coefficients m and b are calculated, the value of t = 2 days will be inserted into the equation, y = mx + b, and the result is the value of used for
the calculation of .
From the changes in pressure observed during the test, and the known relationship between and , the water level changes as a result of changes in pressure alone () can be calculated for the test period for each well. Subsequently, the actual drawdown () during the test can be corrected for the water level changes due to atmospheric pressure :
For falling atmospheric pressures, the correction will be positive:
For rising atmospheric pressures, the correction will negative:
For more information, see Kruseman and de Ridder (1990).
The BE value can be defined in the Pumping Test tab, or it may be calculated based on observed data. To calculate the BE value, locate the Bar.Eff. (BE) field in the Aquifer Properties frame of the Pumping Test tab, and press the button beside the BE field.
A blank window for barometric data entry will appear.
In this window, enter Pressure vs. Water Level data. This data must be recorded before or after the test, at a location near the test well. The data values can be entered in the grid on the left hand side. Or to import data, click on the appropriate link above the table. Data may be imported in .TXT or .XLS formats.
When importing data, observe the following requirements:
1. the source file must be in the same units as the test
2. data file must be .TXT or .XLS, with two columns of data (pressure and water level)
Once the data is entered, the dialog will look similar to the following:
The dialog displays a graph with the data and fits a line – and calculates the BE value.
Click [OK] to accept the barometric efficiency value. This value will now appear in the BE field in the Pumping Test tab.
Once the BE value has been determined, it can be used for correcting the observed drawdown data. To do so, load the Water Levels tab, and ensure there is time drawdown data for an existing well. Then, select “Add Barometric Correction” and the following window will appear:
In this window, enter time vs. pressure data, that was recorded simultaneously as the time drawdown data. As mentioned earlier, if the time measurements were not recorded at exactly the same time intervals, AquiferTest will use interpolation to correct the next available water level measurement.
When importing data, observe the following requirements:
1. the source file must be in the same units as the test
2. data file must be .TXT or .XLS, with two columns of data (time vs. pressure)
The example below shows a sample data set of time - pressure data.
Click [OK] to close the dialog, and return to the Water Levels window. In the time - water level grid, two new columns will appear beside the drawdown column. The first column contains the correction due to barometric effects; the second column contains the new corrected drawdown value. The following equation is used:
The corrected drawdown measurements can then be used in the analysis, to calculate the aquifer parameters.
An example demonstrating a barometric trend analysis is available in Exercise 6: Adding Barometric Correction.
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