It takes 4 to 6 measurements per year over a period of at least 5 years to obtain useful data on water level trends.
You don't need to measure your own well to obtain information about groundwater levels in your area. The Oregon Water Resources Department manages a number of observation wells throughout Oregon. These wells are monitored frequently and provide valuable information for people living in the surrounding area on seasonal fluctuations and long-term trends. The location of these wells, along with long-term trends in water levels are available here.
Taking your own measurements
If you are interested in taking your own water depth measurements:
Make sure to use approved sanitary procedures to prevent bacteria or other surface contaminants from entering the system.
Measure the static water level
Measure the static water level, rather than the pumping or recovering water level by letting your well rest for several hours before measuring. Pumping and recovering water levels do not reflect the water level of the surrounding aquifer, and should not be used as indicators of whether a well is going dry.
Static water level
The water level in the aquifer from which you are pumping, measured after the well has rested for several hours (so as not to measure the pumping or recovering water level).
Pumping water level
The water level in the well during drawdown. Pumping causes the formation of a cone of depression. This drawdown always exceeds the drawdown in the surrounding aquifer.
Recovering water level
The water level after pumping has stopped, but before the well has fully returned to the static water level.
Electric depth gauge
Use an electric depth gauge. This is the easiest, and most common method to measure your well's static water level. This equipment typically consists of two wires with an electronic sensor that indicates when, and at what depth, the tape has hit water. The tapes can be purchased or constructed.
For tips see the OSU online publication Measuring Well Water Depth.