Ground-water resources. The assessment of ground-water resources is much more difficult, as it involves the evaluation of the various hydrologic components within the framework of a complex geological environment. There are as yet numerous gaps in the available information & the stage has not reached when any precise basin-wise assessment of the resource can be made except in highly generalised terms.

An attempt was made by the Central Working Group (constituted for working out the Fourth Five-Year Plan proposals for Minor Irrigation in 1968) to work out the rainfall contribution of ground-water recharge by classifying the geological formations in India into 7 categories & assuming, on the basis of the best-informed judgement, different percentage factors for the ground-water recharge for these various categories. The categories & the percentage factors assumed are indicated in Table 3.

On the above basis, the average contribution to ground-water recharge in the country as a whole works out at 10 percent of the total rainfall. Since then, some more reliable regional estimates (though still based on short-cut generalised methods) have been made by the state ground-water organisations, & it is felt that the figure of 10 percent contribution, on the average, maybe somewhat conservative. A figure of 12.5 percent is considered to be more realistic.

Reckoning that the total annual volume of rainfall is about 400 mham, on the basis of a mean rainfall of 120 cm over the geographical area of 328 mham, the mean rainfall contribution to the ground-water recharge comes to 50 mham.

As mentioned before, the under-ground aquifers are supplemented from sources other than rainfall, seepage from canals, water-courses, field channels ,ponds, tanks, return flow from irrigation in the fields & the influent recharge from rivers. As assessed later, the total volume of surface water presently utilised for irrigation is about 23.5 mham. It is visualised that about 35 percent of this total volume diverted for irrigation would pass as deep percolation to ground-water storage. Hence, the addition to the ground-water recharge on this account maybe taken as about 8.2 mham. The total volume of ground-water presently used for irrigation is estimated at about 10.5 mham. Since in the case of ground-water used for irrigation, the loss in the long conveyance systems is not involved & the efficiency of irrigation is somewhat better, it may be more realistic to assume a figure of 25 percent as the deep percolation contributing to ground-water recharge. The addition to ground-water recharge on this account will thus be 2.75 mham. The total contribution to ground-water recharge from these sources may thus be taken as 10.95(say 11) mham.

Assuming that about 4 mham, maybe the influent recharge from the flood flows, the total mean availability of ground-water resources at present maybe reckoned as about 65 mham. It is roughly visualised to be accounted for as below:

1. Withdrawal use:
   1. Irrigation         10.5 mham
   2. Other uses        1.5 mham
   3. Total                12.00 mham
2. Loss due to evapo-transpiration from the water table                  8.00 mham
3. Ground-water rainfall (effluent recharge to rivers & streams)        45.00 mham

According to the assessment of the National Commission on Agriculture, the contribution from surface-water resources to the ground-water recharge is likely to increase to about 35 mham on the full development of irrigation-25 mham from seepage from canals, tanks etc. & 10 mham as influent recharge (to be increased as a result of special measures for induced recharge). Adding the contribution of rainfall, the total ultimate ground-water resources may be of the order of 85 mham. Reckoning that about 50 mham will be lost as ground-water run-off & evapo-transpiration, the ultimate usable ground water resources may be assessed at 35 mham. The above assessment does not include the additional supplies that may temporarily be obtained by the mining of water in certain regions.