Piezometer Trend Calculator
Calculates the rate of change of a piezometric (groundwater) level between two readings, in m/day and m/week, with the total change. Used by geotechnical and mine-water teams to trend pore-pressure changes that affect slope and dam stability.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter water level 1 and water level 2 (m) from the same piezometer.
- Enter the time between readings Δt in days.
- Read the trend (positive = rising, negative = falling), the weekly trend and the total change.
How it works
The trend is the change in level divided by the elapsed time: trend = (level 2 − level 1) ÷ Δt, in m/day, and × 7 for m/week. A positive trend is a rising level, a negative trend a falling one.
A rising phreatic surface increases pore-water pressure, which reduces effective stress and lowers slope stability, so a persistent upward trend is a warning. Trending the rate over successive readings, not a single level, is the basis of a piezometer-driven TARP.
Worked example
Water table rising 0.2 m/day. A piezometer reads 45.0 m, then 46.4 m seven days later. Total change = +1.4 m. Trend = 1.4 ÷ 7 = +0.2 m/day (rising), or 1.4 m/week.
Common mistakes
- Confusing depth-to-water with water-level elevation — a falling water table can look like a rising number if you log depth-to-water.
- Entering Δt in the wrong unit; it must be in days for the m/day and m/week outputs.
- Swapping level 1 and level 2, which reverses the sign of the trend.
Frequently asked questions
Does a positive trend mean the water level is rising?
Yes. Trend = (level 2 − level 1) ÷ Δt, so positive means rising and negative means falling.
Why does a rising water level matter for slope stability?
A rising phreatic surface raises pore-water pressure, which reduces effective stress on potential failure surfaces and lowers the factor of safety.
What units are used?
Levels and total change are in metres; the trend is in m/day and m/week. Enter the time between readings in days.
Should I log level elevation or depth-to-water?
Log a consistent quantity. This tool treats level 2 > level 1 as a rise, so use water-level elevation (or invert your sign convention if you record depth-to-water).
Can I use it for two readings weeks apart?
Yes. Any two readings from the same piezometer work — enter the elapsed time in days and the trend is normalised per day and per week.
Related tools
- Piezometric Level Calculator
- Pore Pressure Ratio Calculator
- Crack Displacement Calculator
- Prism Movement TARP Checker
- Prism Movement Rate Calculator
- Rolling Average Movement Calculator
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Tip: Enter any known values to calculate the remaining results.
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