Surface-water resources. The most reliable method to assess the surface-water resources is through an accurate & continual study of water flows through rivers & streams over a period of years. The run-off records of the various river systems in India are incomplete & therefore, the assessment of the water resources from actual gauged discharge records is not possible for the whole country. However, the rainfall records are available for a long period & attempts have, therefore, been made to roughly assess the water resources on the concept that the natural run-off (assuming that there is neither any upstream development from the surface or ground-water resources nor any increment or decrement in the ground-water storage) is equal to the total volume of precipitation minus the volume of water lost into the atmosphere through evapo-transpiration.

Dr A. N. Khosla, an eminent engineer, evolved the following relationships in the run-off as the function of rainfall & temperature, assuming that the temperature is the main determinant of evapo-transpiration :

On the basis of this relationship, he estimated the mean annual natural run-off (normally referred to as the surface-water resources) of the six water-resources regions of the country as 167.23 mham. The region-wise distribution is given in Table 1.

Later, based on the specific analysis of the stream-flow data from 80 sources, carried out by the Central Water & Power Commision, the Irrigation Commission (1972) has worked out the water resources of the country at 178 mham. This figure includes, besides surface run-off, ground-water run-off (effluent discharge) which is contributed by the ground-water storage as the base flow of the rivers(as shown later the present figure of ground water run-off is roughly assessed at about 45 mham) as well as the sub-surface run-off contributed by rainfall & seepage from surface-water resources, but excludes whatever influent recharge(tank storage) that may have taken place from the flood flows.

The above estimate represents the average annual river flow. All this flow cannot be utilised owing to the highly variable character of the flow & other limitations imposed by the physiographic factors. The concentration of flow in a four month period necessitates the construction of storage works, if the flood flows passing to the sea are to be utilised. However, suitable sites for the construction of storage dams are limited. The utilisable quantum also depends on the availability of suitable sites for the diversion of water, the quality of water being suitable, the proximity of water to the land fit for cultivation & the dependability of flow. In India, the irrigation projects are usually designed for 75 percent dependability, so that the designed quantity of water is available for at least 75 percent of the year. In view of the limitations imposed by these considerations, the total utilisable flow has been estimated at 66.6 mham, by the Irrigation Commission(1972).

Region-wise details of the total available & the utilisable surface-water resources are given in Table 2.

With further growth of irrigation, it is likely that sub-surface flow contributing to the total annual run-off may appreciably increase, thus augmenting the present availability of surface-water resources.This increase, however, maybe compensated for by the anticipated reduction in the ground-water run-off owing to the continued development of the ground-water resources. It is therefore, visualised that the total surface-water resources may remain, more or less, steady at the same level as at present.