Stage of Groundwater Extraction

The stage of groundwater extraction measures how intensively groundwater is being withdrawn within an assessment unit during a year. It is the annual groundwater extraction for all uses expressed as a percentage of the unit's annual extractable groundwater resource.

Calculation

For an assessment unit, the calculation is: Stage of groundwater extraction (%) = Annual groundwater extraction for all uses ÷ Annual extractable groundwater resource × 100. Both quantities must cover the same area and annual assessment period.

• Annual extraction includes groundwater withdrawn for irrigation, domestic and industrial uses.
• Annual extractable groundwater resource equals annual groundwater recharge minus natural groundwater discharge provided for during the year.

Interpretation and classification

A higher percentage indicates greater pressure on the annually replenishable groundwater resource. It measures extraction intensity, not the total groundwater stored in all aquifers.

• An assessment unit is safe when the stage is up to 70 per cent.
• It is semi-critical when the stage is above 70 per cent and up to 90 per cent.
• It is critical when the stage is above 90 per cent and up to 100 per cent.
• It is over-exploited when annual extraction exceeds the annual extractable resource, meaning the stage is above 100 per cent.

Illustration

If annual extraction is 80 million cubic metres and the annual extractable resource is 100 million cubic metres, the stage is 80 per cent. The assessment unit is therefore semi-critical.