How to Assess Ground Water Resources

Ground water is an important source of irrigation and caters to more than 50% 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 climatologically, 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 behavior 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 Recharge 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 Draft (withdrawal) 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. Artificial discharge by pumping or flowing wells or drains.
4. Ground water outflow.

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:-

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

2. 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 came up with a revised methodology for assessment of ground water potential and evolved new norms 1984.

In the light of voluminous data generated due to a number of studies, which led to revision of parameters for ground water resource estimation and the difficulties experienced while computing ground water resources based on the 1984 methodology, the Government of India constituted a committee in 1995 for reviewing the existing methodology of ground water estimation and to look into all related issues. This committee constituted under the Chairmanship of Chairman CGWB submitted its report on the revised ground water estimation methodology in 1997.

These norms of Ground Water Estimation Committee 1997, are currently utilized by the Central Ground Water Board and State Ground Water Departments to compute the ground water resources.
 

 

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