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There are numerous measurement systems available for measuring groundwater levels (head / stage). What period of time should measurements be collected in order to draw solid conclusions about the average groundwater level, min/ max, trends, etc?

I would reckon at least one year to take account for seasonal effects, although longer would be preferable.

Long measurement periods (several years) are often unwanted, due to the need for fast results. So the optimum between research duration and solid conclusions lies somewhere in between I presume.

EDIT: There is a factory which is investigating the possible use of groundwater for their industrial process. Enviromental law dictates that the effects of the substraction on the long term groundwaterlevels have to be investigated. The first question then becomes: what is the current long term variability in groundwaterlevel? Only then we can continue with answering what the effects of the substraction will be.

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    $\begingroup$ I think this question it too broad. The correct amount of time to measure something depends on the question you want to answer with the data. For instance, if you want to know the average ground water level over the course of a year, then one year is the proper amount of time to measure. If you are interested in how the ground water level varies through climate cycles such as ice ages, then one year is not long enough. $\endgroup$ – Chris Mueller Apr 30 '14 at 16:22
  • $\begingroup$ @Timror - I also think you should clarify the thing you are interested in, so the question becomes answerable. $\endgroup$ – tobias47n9e Apr 30 '14 at 18:36
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Terminology

The term "average" isn't a specific term. Are you talking daily average or annual average? The question "What is the average?" begs the question of "Across what timeframe?"

To explain that point, let's look at a chart:

Groundwater depletion

This is the groundwater level of a well in Georgia (US). The "Average" for this well in December 2003 would be 176.9 feet below ground level. However, for the year 2003 altogether, it would be around 177 feet below ground level. For the previous decade, the average would be around 175 below ground level.

Timeframe

To answer the question, the best timeframe is the largest timeframe you can afford to gather.

If we look at the Raw data for the above chart, what we see is that for the year 2003, the groundwater levels went up! If you only gather one year of data, this well would show a dramatic rise in water levels, which would clearly not be accurate.

If you extended the search back five years, you would see a large downward trend. March 1998 shows 173 feet depth and March 2003 shows 178 feet depth. This trend indicates a dramatic loss of water volume. However, without a larger timeframe, it's impossible to tell if this is just a negative trend or a cyclical one.

Even in the chart above, the mid-1960s may just have been a high point in the water table. (Indeed, Georgia saw 15% more percipitation in 1967 than is historically average and 1999-2001 were historically low.)

Summary

The best timeframe to gather data is the longest timeframe that you can get.

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As your question involves environmental law, the correct answer to this question will be based on whichever country or agency would be in charge of providing a permit; as you already noted, commercial considerations will probably end up with doing the minimum requirement. Delving into local law or contacting the appropriate agency will probably provide you with the necessary information.

Afterwards, I'd suggest you look at available historical data first: you may not necessarily be required to do a lengthy survey in order to get the answers on your reference situation, first. The data may already be available, and require you to run scenario's for possible effects of extraction.

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