New Delhi, Jan 31 : During 2004 to 2020, the percentage of unit of groundwater resources categorised as "safe" have declined from 2009 (73 per cent) to 2020 (64 per cent), Economic Survey 2021-22 data released Monday, showed. The report shows the per cent of number of assessment units of India recorded under different categories -- safe, semi-critical, critical, over-exploited and saline) during 2004-2020. "During this period, units categorised as "safe", have declined from 2009 (73 per cent) to 2020 (64 per cent). "Semi-critical" units have increased from 9 per cent in 2009 to 15 per cent in 2020. The share of "Critical" units has remained in the range of 3-5 per cent during 2004-2020. The share of "over-exploited" units, accounted for 14-17 per cent of total assessment units during 2004-20. In addition, approximately one per cent of assessment units have been categorised as "saline"," the report stated. The categorisation of ground water resource assessment units across India in 2020 shows over-exploitation of ground water resources, i e extraction exceeding the annually replenishable ground water recharge is concentrated in north-west and parts of southern India. Ground Water Resources Assessment of states and union territories is carried out jointly by state, groundwater and nodal departments and Central Ground Water Board at periodic intervals, and the Dynamic Ground Water Resources of India is published by compiling the state and union territory wise ground water resources assessed. Such ground water assessments have been undertaken in 2004,2009, 2011, 2013, 2017 and 2020. Further, citing that reservoirs are an important source of water resources for the country, the report added that they are particularly prone to seasonality, and are greatly impacted by rainfall and temperature patterns. The report showed the capacity at full reservoir levels in 138 monitored reservoirs of India along with the live storage during June-December 2021, June 2020-May 2021, and the 10 year average during the months of June - May. Reservoir live storage is at its peak during monsoon months and lowest in summer months, requiring careful planning and coordination of storage, release and utilisation of reservoirs. /IANS