Catchment: The area that receives rainfall and which can be used for harvesting is the catchment area. The roof is the primary catchment area in a building while the land is the catchment area in a farm.
Runoff: The falling rain hits a surface and flows off it. This component is the runoff.
Runoff Coefficient: The runoff component varies for different surfaces. For instance, the runoff generated from a hard surface like a roof or a road surface is much more than the runoff generated from a soil or landscaped surface. This is governed by the runoff coefficient. A hard surface will have a higher runoff coefficient compared to a softer/ absorbent surface.
Rainwater Harvesting Potential: The area of the catchment, quantum of rainfall over a period of time and the Runoff coefficient combine to give the Rainwater Harvesting potential.
In the above scenario, a roof catchment area of 100 square meters has the potential to generate an annual runoff of 23760 gallons for an annual rainfall of 1000 mm.
Aquifer: The water reserves below ground are contained in sand, gravel, permeable rocks, rock fissures and this constitutes the aquifer. In the United States, some notable aquifers are the Edwards aquifer in Central Texas, the Ogallala aquifer in central United States which spans across portions of eight states.
Withdrawal: The term withdrawal in the context of groundwater is the process of extraction of ground water from the aquifer.
Recharge: The movement of water from the surface to deeper ground is called Recharge. Recharge can be both natural and artificial through human intervention. Recharge is extremely important towards sustainable ground water management.
Groundwater overdraft: When the volume rate of withdrawal of ground water exceeds the volume rate of recharge, groundwater overdraft occurs and is quantified by the difference between extraction and recharge. For example, in a given year, the total ground water extraction from an aquifer is 1000 m3 while the recharge is only 500 m3. The annual ground water overdraft in this case is 500 m3.