Groundwater is the water stored in the pores and other openings in rocks below ground. It is a precious resource which must be safeguarded for the benefit of people and the environment.
1. Groundwater comprises 96% of all liquid fresh water on earth
Think of this groundwater stored beneath our feet as the savings in your bank, or the financial reserves of a nation.
Most (97%) of the world’s water is sea water. Of the freshwater most (nearly 69%) is ice. Groundwater comprises 96% of the freshwater which we can utilise.
2. However, from a water resource point of view, what matters is how much natural replenishment, or recharge, takes place
Recharge rates vary from a few to hundreds of millimetres per year. In dry regions recharge ranges from zero to a few mm per year. In humid regions recharge rates represent a higher proportion of rainfall.
3. If no abstraction (pumping) takes place, then recharged water discharges naturally to swamps, streams and rivers
This natural outflow is important because it sustains springs and river flows in dry periods and supports wetland ecosystems (lakes and swamps) which are important to people and livestock
4. Water in streams, rivers and lakes and groundwater are closely connected
- Globally about 25% of the flow in rivers comes from groundwater
- Globally about 90% of groundwater recharge joins rivers
Rivers may be fed by groundwater, or they may feed groundwater. Whether a stream is gaining or losing depends on location and time of year
5. Some of the groundwater recharge can be safely abstracted for human use
Even the smallest abstraction reduces the natural outflow by an equivalent amount, but a certain level of water abstraction may be judged to be acceptable.
6. Deciding how much groundwater is exploitable depends on many factors. Three main options exist
- Sustainable yield: Limit abstractions to no more than the long-term average recharge. “Living off the interest or earnings”
- Mixed strategy: Planned depletion for a limited period followed by abstraction at a sustainable rate.“Spending some of the savings followed by living off the interest or earnings”
- Mining: Long-term progressive depletion, reducing the groundwater reserves over time. “Spending the savings”
8. More people use ground-water for drinking than use rain water or surface water
About half the population of Africa and Asia use groundwater directly from springs, wells and boreholes, while an unknown additional number served by piped systems enjoy groundwater use. [Source of data: Joint Monitoring Programme of WHO and UNICEF]
9. In rural areas groundwater is often the cheapest source of safe drinking water
- The capital cost of a borehole and handpump is about USD40 per person (say USD1.50 per person per year).
- The recurrent costs are about USD4.50 per person per year.
- The total is about USD6.00 per person per year.
- Piped schemes cost about twice as much.
[Source: WASHCost Working Paper 8, 2013, IRCwash.org]
10. In urban areas many people use shallow ground-water despite the fact that it is very vulnerable to pollution
The high cost of connection to piped water services makes it more attractive to use a private well despite the poor water quality.