Ground Water Resources Assessment is carried out at periodical intervals jointly by State Ground Water Departments and Central Ground Water Board under the guidance of the respective State Level Committee on Ground Water Assessment at State Levels and under the overall supervision of the Central Level Expert Group. Such joint exercises have been taken up earlier in 1980, 1995, 2004, 2009, 2011, 2013 and 2017. The latest in this series is 2020.
The assessment involves computation of dynamic ground water resources or Annual Extractable Ground Water Resource, Total Current Annual Ground Water Extraction (utilization) and the percentage of utilization with respect to annual extractable resources (stage of Ground Water Extraction). The assessment units (Talukas/blocks/mandals/firkas) are categorized based on Stage of Ground Water Extraction, which are then validated with long-term water level trends. The assessment prior to that of year 2017 were carried out following Ground Water Estimation Committee (GEC) 97 Methodology, whereas 2017 as well as the present assessment are based on norms and guidelines of the GEC 2015 Methodology.
The main source of replenishable ground water resources is recharge from rainfall, which contributes to nearly 64 % of the total annual ground water recharge. India receives about 119 cm. of rain annually on average, with high spatial variation. A major part of the country receives rainfall mainly during SW Monsoon season spread over the months of June to September, except in Tamil Nadu, where the major contribution is from NE monsoon during the period October– December. There are also States such as Jammu and Kashmir, Himachal Pradesh and Uttarakhand which receive significant rainfall in all seasons.
Over 75 % of the annual rainfall is received in the four rainy months for June to September only thereby leading to large variations on temporal scale. The average annual rainfall is 119 cm, but it has great spatial variations. The areas on Western Ghats, Sub-Himalyan areas in North East and Meghalaya Hills receive heavy rainfall over 250 cm annually, whereas the areas of Northern parts of Kashmir and Western Rajasthan receive rainfall less than 40 cm. A major part of the country including Northern, Central and Eastern parts receives annual normal rainfall between 75 and 150 cm. In general, rainfall decreases westwards in the northern part of the country, whereas it decreases eastwards and then increases toward the coast in Peninsular India.
Type of rock formations and their storage and transmission characteristics have a significant influence on ground water recharge. Porous formations such as the alluvial formations in the Indo Ganga-Brahmaputra basin generally have high specific yields and are good repositories of ground water. Ground water occurrence in the fissured formations occupying nearly two-thirds of the geographical area of the country, on the other hand, is mostly limited to the weathered, jointed and fractured portions of the rocks.
In the present assessment, the total annual ground water recharge has been assessed as 436 bcm. Keeping an allocation for natural discharge, the annual extractable ground water resource works out as 398 bcm. The total annual ground water extraction (as in 2020) has been assessed as 245 bcm. The average stage of ground water extraction for the country as a whole works out to be about 62 %.
The extraction of ground water for various uses in different parts of the country is not uniform. Out of the total 6965 assessment units (Blocks/ Districts/ Mandals/ Talukas/Firkas) in the country,
Apart from this, there are 97 assessment units (1 %), which have been categorized as ‘Saline’ as major part of the ground water in phreatic aquifers is brackish or saline.
Similarly out of 24.33 lakh sq km recharge worthy area of the country, 4.09 lakh sq km (17 %) are under ‘OverExploited’, 0.86 lakh sq km (4 %) are under ‘Critical’, 3.4 lakh sq km (14 %) are under ’Semi-Critical’, 15.67 lakh sq km ( 64 %) are under ‘Safe’ and 0.3 lakh sq km (1 %) are under ‘Saline’ category assessment units. Out of 397.62 bcm of Total Annual Extractable Resources of the country, 50.54 bcm (13 %) are under ‘Over-Exploited’, 12.71 bcm (3 %) are under ‘Critical’, 54.11 bcm (14 %) are under ’Semi-Critical’, 280.26 bcm (70 %) are under ‘Safe’ category assessment units.
In comparison to 2017 assessment, the total numbers of assessment units in the country have increased from 6881 to 6965 with major contribution (in increase) from the State of Karnataka, Haryana and Punjab. The total annual ground water recharge has increased from 432 to 436 bcm, where major increase is noticed in the States of Uttar Pradesh, Andhra Pradesh, Karnataka, Telangana, Gujarat & Chhattisgarh. The changes are attributed mainly to changes in recharge from ‘Other Sources’. Accordingly, the annual extractable resource of GW Resource Assessment, 2020 on comparison GW Resource Assessment, 2017 also shows a increase from 393 to 398 bcm.
The ground water extraction has marginally decreased from 249 to 245 bcm. The overall stage of groundwater extraction has marginally decreased from 63 % to 62 %. The over-exploited assessment units are mostly concentrated in :
In some areas of the country, good continuous rainfall and management practices like ground water augmentation and conservation measures through government and private initiatives have resulted in improvement in ground water situation. Ground water resources assessment, like other fields of science, requires continuous refinements.
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Source : Central Ground Water Board, Department of Water Resources, River Development & Ganga Rejuvenation, Ministry of Jal Shakti, Government of India