Groundwater as a part of the hydrologic cycle

Figure 1. The water cycle depicting how groundwater is replenished. Click on the image to see a larger version.

Groundwater plays a key role in the hydrologic cycle. The hydrologic cycle is a sequence of water transformations that occur in the circulation from the atmosphere onto the surface and into the subsurface regions of the earth, and then back from the surface to the atmosphere once again. As surface water deposits such as snow melt and precipitation recharge the groundwater, it slowly drains gradually towards a discharge point.

When precipitation falls on a land surface, part of the water runs off into the lakes and rivers. Some of the water from melted snow and rainfall seep into the soil and moves down into the saturated zone. This process is called recharge. Places where recharge occurs is referred to as recharge areas. Groundwater does not stay below the surface forever and it does not lie still. This water eventually resurfaces above ground. This is called discharge. Most groundwater will return to the surface of the Earth in areas of groundwater discharge such as wetlands, lakes, springs, and rivers. The length of time water stays in the groundwater region of the hydrologic cycle varies. Groundwater can reside for days, weeks, or up to tens of thousands of years. Compared to groundwater, the average lifetime of river water to completely replace itself is about two weeks. Groundwater discharge can contribute significantly to surface water flow. In drier periods, the flow of some streams may be supplied completely by groundwater. The capacity of water moving from the saturated zone into streams is determined by the rate of discharge and the capacity of water running over the land is determined by the rate of recharge. For example, during rainfall, the volume of water running into streams and rivers depends on how much rainfall the material underground can absorb. When there is a surplus of water on the surface that can’t be absorbed into the groundwater zone any longer, it runs off into streams and lakes.

Groundwater as a source of energy

Groundwater can be used as a source of heat. Heat pumps from a ground source are receiving attention as energy efficient commercial and residential heating and cooling systems. Though the initial costs are higher than air source energy systems, which is due to the additional costs of underground installations, the increased energy efficiency of ground source systems makes them more attractive. Research into the use of geothermal water has been carried out in a number of institutions across Canada. The City of Moose Jaw has developed a geothermal heating system for a public swimming pool and recreational facility. In Ottawa, Carleton University uses groundwater to heat and cool its buildings. Since 1995, Sussex, New Brunswick, The Health Centre complex has been utilizing an aquifer for thermal energy storage.

Estimating the total volume of groundwater on Earth is difficult. There have been estimates that have claimed the quantity of groundwater in the earth could cover the entire surface of the Earth to a depth of 120 metres. By contrast, the volume of surface water in lakes, rivers, reservoirs, and swamps could be contained in a depth of about one fourth of a metre. Recent research has revealed estimations range from 7,000,000 to 330,000,000 cubic kilometres of total groundwater volume. However, all the estimates imply if we do not include water frozen in ice caps, glaciers and permanent snow; groundwater makes up almost the entire volume of the earth's usable fresh water.