Ground Water Observation Wells
The Ganga basin has a vast reservoir of groundwater, replenished every year at a very high rate. The mean annual replenishable groundwater in India as a whole has been assessed at 433 BCM per annum, of which about 202.5 billion cumec per annum (46.8%) lies in the states of the Ganga basin. The conjunctive use of groundwater for irrigation within the canal command areas not only ensures steady supply to the cultivated fields on time but also helps reduce water logging and salinization due to consequent downward movement of subsurface moisture. The most extensively used water sources for irrigation in the basin are the groundwater wells.
On an average, each square kilometer of the Ganga Basin receives a million cubic meters of water as rainfall. Thirty percent of this is lost as evaporation, while the remainder eventually exits the land surface as run off and/or seeps down into the subsoil as groundwater recharge, a portion of which often oozes out at lower levels into streams͘ In the course of the water‘s movement either overland or below the surface, various chemical compounds become dissolved in it. Some of these extraneous chemical constituents are derived from the residues of pesticides and chemical fertilizers, which are added to the soil every year for better yield of crops. The sub-basin wise distribution of number of ground water observation wells are given below:
Sub-basin wise number of ground water observation wells
The groundwater usage for irrigation in the states falling under Ganga basin exceeded 104.7 billion cum per annum as of 2008 and accounted for nearly 50 percent of the groundwater irrigated area of the entire country. The net annual groundwater availability for irrigation, domestic and industrial usage in the states of the Ganga basin has been assessed at 187.4 BCM per annum. Some 60 percent of this potential has already been utilized. The extent of groundwater utilization for irrigation is highest in Uttar Pradesh (45.36 BCM per annum), followed by Madhya Pradesh (16.08 BCM per annum), West Bengal (10.84 BCM per annum) and Rajasthan (11.6 BCM per annum). The net irrigated area percentage in the basin by ground water wells are Uttar Pradesh and Uttarakhand (73%), West Bengal (59%), Madhya Pradesh (64%), Delhi (88%) Rajasthan (71%). Apart from irrigation, groundwater resources are also being heavily tapped for industrial and domestic uses majoring both urban and rural areas. Throughout the alluvial area of the Ganga basin, the major urban water supply schemes are dependent upon groundwater resources. Similarly, a large number of industries also withdraw significant amounts of groundwater, especially from the easily accessible aquifers in the alluvial zone. As per India-WRIS there are 5745 Ground Water Observation sites out of which 303 sites are Monitoring sites of National Hydrograph Network Stations, 4005 sites are CGWB Ground Water Monitoring sites. The spatial distribution of ground water observation wells in the Basin is shown here.