The water occurring below the saturated zone in the subsurface (beneath the ground) is generally defined as groundwater. Groundwater occurs in permeable geologic formations known as 'Aquifers', i.e., formations having open spaces either cracks, fractures or intergranular spaces, which permit appreciable amount of water to move through them under ordinary field conditions.

Groundwater development status is normally determined by the extent of Dynamic Resources. Dynamic Resource is a variable and replenishable part of groundwater resource added yearly to groundwater system, which can be obtained without disturbing the sub surface storage of groundwater. It is the upper limit of safe yield / exploitation. Groundwater exploitation in excess to dynamic resource will induce irreversible damages to groundwater system unless attempts are made to augment recharge / replenishment to compensate the excess draft. The dynamic resource is estimated taking in to consideration of
Recharge through rainfall
Leakage / seepage through surface water bodies and
Percolation of the irrigation water
The Department of Mines & Geology, Government of Karnataka has classified the state into safe, semi-critical, critical and overdeveloped blocks based on the groundwater resource and its status of development (normal annual recharge and the withdrawal for irrigation). Out of the categorized 380 watersheds, 324 are in safe block while 56 are classified as overdeveloped. Considerable portion of the state has already crossed the safe limit of groundwater exploitation.
Classification of ground water blocks
Safe-block: Available groundwater is less than 70% and also between 70-90% where there is no long-term significant decline of groundwater trend in both pre monsoon and post monsoon periods.

Semi-critical: Available groundwater is between 70-90% and where either post monsoon or pre monsoon significant decline in long term groundwater trend is observed.

Critical: The percentage of utilization of available groundwater resource is between 90-100% and either pre-monsoon or post monsoon groundwater trend shows significant decline.

Overdeveloped: The percentage of utilization of available groundwater resource is more than 100% and both pre monsoon and post monsoon groundwater trend shows significant decline.

In Karnataka, ground water in more than 37% of rural habitations and surface water in some rivers are contaminated at the points of effluent discharge and also around urban areas. Habitations in Bagalkot, Bangalore Urban, Bijapur, Chamarajnagar, Chitradurga, Haveri, Mandya, Tumkur, Bellary, Davanagere, Kodagu, Kolar, Raichur and Koppal districts have serious ground water quality problems, ranging from 50-79%. More specifically, excess fluoride in ground water is a major problem in 14 districts, ranging from 10-67% of the total habitations of each district. Similarly, excess brackishness in 13 districts (in the range of 10-27% of the habitations), excess nitrate in 8 districts (10-51% of habitations) and excess iron in 12 districts (10-63% of habitations) is adversely affecting drinking water quality.
Depleting ground water resources
Ground water survey reports of 1999 reveal that there is depletion of ground water resources in 56 watersheds in the State, which are spread over 35 taluks in 10 districts, covering 5692 villages.

Based on the study of historical data, ground water levels are reported to be declining in all parts of the State, except in some command areas. In view of the expected rise in demand for surface water for drinking purposes in urban areas and continuing dependence on ground water for rural drinking water supply, the shortage of ground water resources are likely to be felt increasingly during the coming years particularly in some critical areas of the State. With decline in surface water availability for agriculture due to rising domestic demand, ground water extraction for irrigation may increase in near future. In the absence of any regulatory mechanism, there would be greater pressure on existing ground aquifer. Of the total 1895 wells under observation by the Department of Mines & Geology, water level showed an increase in 418 (22%) wells while, decline was observed in 1109 (56%) wells (Mines & Geology, 2002). Uneven and scanty rainfall and the geo-structure have restricted the availability of underground water resources in the State.

Water tanks/lakes are the oldest of the natural rain water harvesting structures. Tanks have lost water holding capacity to an extent of 30% due to situation. Tanks help in maintaining soil moisture and facilitate recharge of ground water.