How to Read a Groundwater Hydrograph
- How do you read a groundwater hydrograph?
- Is the vertical axis depth or elevation?
- What kind of record produced the line?
- Does “provisional” change how you use the graph?
- What can explain a rise or fall?
- Does this well represent the local water table?
- How should you compare the present with the past?
- Can a high water level establish drinking safety?
- Sources
How do you read a groundwater hydrograph?
Read the well identifier, dates, vertical-axis label and data notes before interpreting the line. Compare like measurements over time; a change in depth below land surface has a different sign from a change in elevation. A chart cannot establish drinking-water safety. In the United States, use state-certified testing and local health-authority advice for that decision.
A groundwater hydrograph plots water level against time. The useful first question is what was measured, where, and when. Save the source page with the graph so its explanation remains available. This is a desk-reading guide; site-specific interpretation belongs with a hydrogeologist or qualified well professional.
Is the vertical axis depth or elevation?
The USGS parameter definitions distinguish depth below land surface from elevation above a named vertical datum, the reference used for heights. Read the full label and units.
These invented readings demonstrate the arithmetic at one unchanged location:
| Measurement | Earlier | Later | Meaning |
|---|---|---|---|
| Depth below land surface | 20 feet | 25 feet | Water level fell 5 feet |
| Elevation above the same datum | 480 feet | 475 feet | Water level fell 5 feet |
The differences are 25 − 20 = 5 feet deeper and 475 − 480 = −5 feet in elevation. Read the ticks; keep units and datums consistent.
What kind of record produced the line?
USGS distinguishes field-visit measurements from instrumented records. Visits produce discrete measurements; installed instruments collect readings at designated intervals. Methods and frequency can change within a record.
Read the legend: individual observations, a daily statistic, or a combined record? USGS's Best Available Time Series combines discrete and instrumented data into one value per day.
A practical reading rule follows: a connecting line supplies no additional measurements. Keep gaps visible; an unobserved period does not establish a steady level.
Does “provisional” change how you use the graph?
Yes. The USGS provisional-data statement says such information has not received final approval and may be revised. Instrument problems or physical changes at the measurement site can affect accuracy, and later inspections can lead to substantial revisions.
Record the retrieval date and data status beside any value you copy. If a point appears unusual, preserve its date and qualification and ask the data provider about it. Do not silently delete it or announce a new record from an unchecked screenshot. USGS specifically cautions about provisional data in safety and consequential business decisions.
What can explain a rise or fall?
USGS identifies pumping, recharge and seasonal changes as influences on well levels. Recharge means replenishment of groundwater. Pumping can lower levels in the pumped well and affect nearby wells; severe drought can also lower levels.
That list supplies possible explanations, not a diagnosis of a particular dip. As an editorial approach, write the observation before suggesting a cause: “The recorded level fell during this interval” is a different statement from “A neighbour caused the fall.” Ask a hydrogeologist what local records would be needed to investigate the explanation.
Does this well represent the local water table?
Check the aquifer information. In a confined aquifer, pressure can raise water in a well above the aquifer's top. An unconfined aquifer has a water table at atmospheric pressure. A plotted well level therefore needs its geological setting; it is not automatically the upper edge of saturation beneath every nearby property.
Our aquifer introduction explains those terms. For comparing records, ask the data provider whether the wells monitor comparable aquifers and conditions rather than selecting them solely because their map symbols are close together.
How should you compare the present with the past?
Use the accompanying statistical explanation. The USGS National Groundwater Conditions guide describes monthly groundwater-level plots that compare recent monthly means with historical monthly levels shown as percentile groups. Those historical comparisons are different from simply joining the earliest and latest points on a graph.
Keep the reporting period visible when saving a comparison. A useful original note template is:
- Identity: source URL and well identifier.
- Display: chosen dates, axis label, units and legend.
- Observation: exact values and dates copied from the record.
- Qualification: missing periods, data status and relevant notes.
- Question: what remains unresolved and who can answer it.
For example, “Is this point an individual reading or a monthly mean?” gives the provider something specific to resolve. Keep their reply with the saved chart.
Can a high water level establish drinking safety?
No. A level graph is not a contaminant test. CDC advises state-certified laboratory testing and local health-department interpretation. For the separate household tasks, see when to test private-well water and how to read the laboratory report. Do not use this graph to choose treatment or alter well equipment.