Basinwise Ground Water Potential Of Country

Cubic Km/Year

GROUND WATER - HOW IT IS ASSESSED

Ground water is an important source of irrigation and caters to more than 45% of the total irrigation in the country. The contribution of ground water irrigation to achieve self-sufficiency in food grains production in the past three decades is phenomenal. In the coming years the ground water utilization is likely to increase manifold for expansion of irrigated agriculture and to achieve National targets of food production. Although the ground water is annually replenishable resource, its availability is non-uniform in space and time. Hence, precise estimation of ground water resource and irrigation potential is a prerequisite for planning its development.

A complexity of factors - hydrogeological, hydrological and climatological, control the ground water occurrence and movement. The precise assessment of recharge and discharge is rather difficult, as no techniques are currently available for their direct measurements. Hence, the methods employed for ground water resource estimation are all indirect. Ground water being a dynamic and replenishable resource, is generally estimated based on the component of annual recharge, which could be subjected to development by means of suitable ground water structures.

For quantification of ground water resources proper understanding of the behaviour and characteristics of the water bearing rock formation known as Aquifer is essential. An aquifer has two main functions - (i) to transit water (conduit function) and (ii) to store it (storage function). The Ground water resources in unconfined aquifers can be classified as Static and Dynamic. The static resources can be defined as the amount of ground water available in the permeable portion of the aquifer below the zone of water level fluctuation. The dynamic resources can be defined as the amount of ground water available in the zone of water level fluctuation. The replenishable ground water resource is essentially a dynamic resource which is replenished annually or periodically by precipitation, irrigation return flow, canal seepage, tank seepage, influent seepage, etc.

The methodologies adopted for computing ground water resources, are generally based on the hydrological budget techniques. The hydrologic equation for ground water regime is a specialized form of water balance equation that requires quantification of the items of inflow to and outflow from a ground water reservoir, as well as of changes in storage there in. A few of these are directly measurable, some may be determined by differences between measured volumes or rates of flow of surface water and some require indirect methods of estimation. These items are elaborated as below

I. Items of supply to ground water reservoir

1. Precipitation infiltration to the water table.

2. Natural recharge from stream, lakes and ponds.

3. Ground water inflow into the area under consideration.

4. Recharge from irrigation, reservoirs, and other schemes especially designed for artificial recharge.

II. Items of disposal from ground'water reservoir

1. Evaporation from capillary fringe in areas of shallow water table, and transpiration by phreatophytes and other plants / vegetation.

2. Natural discharge by seepage and spring flow to streams, lakes and ponds.

3. Ground water outflow.

4. Artificial discharge by pumping or flowing wells or drains.

Over the years the ground water assessment techniques have evolved from progressive understanding of ground water occurrence and movement, recharge and discharge processes.

The first attempt to estimate the ground water resources on scientific basis was made in 1979. A High Level Committee known as Ground Water Over Exploitation Committee was constituted by Agriculture Refinance and Development Corporation (ARDC). The committee was headed by the Chairman, CGWB and included as its members representatives from the State Ground Water Organizations and Financial Institutions. This Committee recommended definite norms, for ground water resources computations. Based on these norms the State Governments and the Central Ground Water Board computed the gross ground water recharge as 46.79 m.ha.m. and the net recharge (70% of the gross) as 32.49 m.ha.m. This committee had however recommended that the methodology be revised with increasing availability of data to make it more scientific.

Relying on the experience, gathered during the implementation of ARDC III and IV projects, and based on the knowledge of the behaviour of ground water regime in the highly developed blocks, and on detailed analysis in the Central Ground Water Board, it was later decided that the guidelines of the Over Exploitation Committee should be modified as under

i) The correction factor for normalization of rainfall in the water table fluctuation approach be modified to make it conceptually more correct.

ii) For all the states, where Central Ground Water Board carried out the ground water resource evaluation by water table fluctuation approach, the concept of 100% utilization of recharge as available in November had been adopted as a realistic approach for ground water resource evaluation. It was felt that the water table fluctuation from pre-monsoon to November accounts for the irrecoverable losses, which were not to be again accounted for. The November water level represents a stabilized post-monsoon ground water situation.

Subsequently the necessity was felt for constitution of a committee to go into various aspects of the problems of the ground water development, faced during the implementation of ARDC III & IV credit projects, and make suitable recommendations. Accordingly_, the Government of India constituted a committee under the Chairmanship of Chairman, Central Ground Water Board. This committee examined in depth a large volume of hydrogeological and related data generated by the Central Ground Water Board through nation-wide surveys, exploration and 12 water balance projects, completed till then, and area oriented studies, carried out by the State Ground Water Organizations. The Ground Water Estimation Committee carne up with a revised methodology for assessment of ground water potential and evolved new norms.

These norms of Ground Water Estimation Committee 1984, are currently utilized by the Central Ground Water Board and State Ground Water Departments to compute the ground water resources.

HYDROGEOLOGICAL SITUATION

India is a vast country having diversified geological climatological and topographic set-up, giving rise to divergent ground water situations in different parts of the country. The prevalent rock formations_, ranging in age from Archaean to Recent, which control occurrence and movement of ground water, are widely varied in composition and structure. Similarly, not too insignificant, are the variations of land forms, from the rugged mountainous terrains of the Himalayas₁ Eastern and Western Ghats to the flat alluvial plains of the river valleys and coastal tracts, and the aeolian deserts of Rajasthan. The rainfall pattern, too, show similar region-wise variations. The topography and rainfall virtually control runoff and ground water recharge.

The high relief areas of the northern and north-eastern regions occupied by the Himalayan ranges, the hilly tracts of Rajasthan and peninsular regions with steep topographic slope_, and characteristic geological set-up offer high run-off and little scope for rain water infiltration. The ground water potential in these terrains are limited to intermoutain valleys.

The large alluvial tract in the Sindhu Ganga - Brahmaputra plains extending over a distance of 2000 kms. from Punjab in the west to Assam in the east, constitutes one of the largest and most potential ground water reservoir in the world. The aquifer systems are extensive, thick, hydraulically interconnected and moderate to high yielding. To the north of this tract all along the Himalayan foot hills, occur the linear belt of Bhabar piedmont deposits, and the Tarai belt down slope with characteristic auto flowing conditions.

Almost the entire Peninsular India, is occupied by a variety of hard and fissured formations, including crystalline, trappean basalt and consolidated sedimentaries (including carbonate rocks), with patches of semi consolidated sediments in narrow intracratonic basins. Rugged topography, compact and fissured nature of the rock formations, combine to give rise to discontinuous aquifers, with limited to moderate yield potentials. The near surface weathered mantle, forms the all important ground water reservoir, and the source for circulation of ground water through the underlying fracture systems. In the hard rock terrain, deep weathered pediments, low-lying valleys and abandoned river channels, generally contain adequate thickness of porous material, to sustain ground water development under favourable hydrometeorological conditions. Generally, the potential water saturated fracture systems occur down to 100 m depth, and in cases yield even upto 30 Litres per Second (Lps). The friable semi consolidated sandstones also form moderate yielding aquifers, and auto flowing zones in these formations are not uncommon.

The coastal and deltaic tracts in the country form a narrow linear strip around the peninsula. The eastern coastal and deltaic tract and the estuarine areas of Gujarat are receptacles of thick alluvial sediments. Though highly productive aquifers occur in these , tracts salinity hazards impose quality constraints for ground water development. In this terrain ground water withdrawal requires to be regulated so as not to exceed annual recharge and not to disturb hydro-chemical balance leading to sea water ingress.

The quality of ground water in both hard rock and alluvial terrains is by and large fresh and suitable for all uses. The specific conductance is generally less than 1000 us/cm at 25 ºC. But in coastal areas, estuarine tracts of Gujarat, Rann of kutch and arid tracts of Rajasthan, the degree of mineralization in ground water is rather high and salinity hazards are not uncommon.

The salinity hazards in ground water are also noticed in the inland areas of Punjab, Haryana, Uttar Pradesh, Rajasthan and Gujarat, generally confined to arid and semi-arid tracts.

The varied modes of ground water occurrence in the country may be broadly summarized as:

a) Porous formations comprising unconsolidated and semi consolidated sediments. Aquifers interconnected, often extensive, both continuous and discontinuous; modrate to very high yield potentials.

b) Consolidated and fissured formations. Aquifers discontinuous; Limited yield potentials.