The science behind India’s heatwaves

20 Apr 2016 | 15:18 BST | Posted by Subhra Priyadarshini

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COMMENTS

Perception change on groundwater can protect it from depletion

Partha Sarathi Datta

While I appreciate the innovative approach of the NASA’s GRACE studies, it is desirable to remove the uncertainties in the GRACE data processing, which earlier seems to have overestimated very rapid groundwater depletion in the Northwest India aquifer during 2002-2008. However, it is not clear to me why the article clubbed this study with another study on India’s heat wave stressing on the need for understanding the science.

Based on terrestrial water storage-change data from the Gravity Recovery and Climate Experiment Satellites of NASA, US, soil-water variations data and hydrological modeling, Rodell et al. (2009), estimated that groundwater got depleted at a mean rate of 4.0+1.0 cm yr-1 equivalent height of water (17.7+ 4.5 km3 yr-1) from the aquifers of Punjab, Haryana and Rajasthan including Delhi during August 2002 to October 2008, and depletion was equivalent to a net loss of 109 km3 of water. Although, the observation record is brief, in the absence of any unusual trend in rainfall during 2002-08, over-consumption of groundwater may be the cause. Such modeling can be useful to study impacts and trends on a regional scale; but, rarely address the stakeholders priorities, and may not be useful for local scale water management. The water table fluctuation data of the CGWB, India also suggest groundwater depletion in northern India at 13.2 km3 yr-1 rate (very close to the GRACE data; doi:10.1038/nindia.2009.270;Aug 12). However, non-availability of a large amount of hydrological data over a long period and uncertain nature of long-term data on runoff, etc. in many areas make these approaches grossly inadequate and provide incorrect estimation of recharge.

In this context, for information of the readers, the assessments form both GRACE and CGWB agree well with the earlier estimates of groundwater recharge by Datta et al. (1973), Datta and Goel (1977) and Goel et al. (1977) using the tritium tracing technique extensively. In India, these studies, along with radioactive isotopes (3H, 14C) and stable isotopes (2H, 18O) fingerprinting of water, used extensively for over four decades, have provided detailed insight into the hidden groundwater situation in Punjab, Haryana, Western UP, Rajasthan and Delhi, and have been internationally acclaimed. The average recharge varies widely (1-50%) from region to region and within the parts of a region, both in space and time, depending on the frequency, intensity and distribution of rainfall, evaporation and soil clay content. The average annual recharge was estimated to be 20% of the rainfall in western UP; 16% in Haryana; and 18% in Punjab. It was observed that irrigation water infiltrate considerably to groundwater, and insignificant recharge takes place when the annual water input (rainfall + irrigation) is <40 cm. Datta (2001) further estimated 1-8% recharge in Delhi area, and reported that contemporary groundwater recharge is very limited as compared to abstraction.

By comparing the results of different basins, Datta et al. (1979) indicated a relatively higher efficiency of winter rains in inducing groundwater recharge. Datta et al. (1979, 1980) showed that higher potential evaporation during monsoon months is likely to reduce the net groundwater recharge for a certain amount of water input. Groundwater recharge is significantly high in the North alluvial plains where rainfall is plenty and thick piles of unconsolidated formations are conducive for recharge. Both the groundwater draft and the stage of groundwater development are high in the states of Haryana, Punjab, Rajasthan and Delhi, while the recharge is quite low in the Rajasthan and Delhi. Datta et al. (2001); and Datta (2005) observed that in Delhi, annual recharge being very small as compared to withdrawal, the groundwater has become vulnerable to water table decline by 2-8 m to 30-40 m in different parts during 1960-2000. In the past decades, water table in 77% area of the Punjab State has fallen by 25-30 cm a year.

Groundwater resource characteristics in a basin differ from location to location due to the heterogeneity in hydrogeological set-up and complexity of groundwater hydraulics; and accurate estimation of groundwater availability and depletion is complex and challenging task. Hence, each area/region should be treated separately. Volumetric assessment of groundwater recharge depends on the underground lateral and vertical extent of each of the aquifer. To protect groundwater from depletion, it is very desirable to have a comprehensive status on dynamic water resources and its renewability by thorough micro-level investigations. The status of such knowledge by satellite based modeling; hydraulic data as well as isotopic methods are quite limited to arrive at reliable volumetric assessment of available potential at which a management response is required. Additional work is desirable regarding the number, type and characteristics of aquifers. Disastrous situation of groundwater is essentially man-made in the Northwestern states due to indiscriminate groundwater withdrawal. Due to variations in the social, economic and political factors no single template for management can be developed. The future strategies on groundwater abstraction monitoring should be based on obtaining hydrogeologic characteristic of the groundwater flow field under natural and stressed conditions. Groundwater vulnerability to depletion can be partly minimized or controlled by management goals restricting unplanned groundwater abstraction, in the potential groundwater recharge zones, through strict regulatory measures. Fusion of different techniques, and choices based on the best obtainable detailed scientific information, can help in assessing groundwater potential. Management guided by ethical considerations, offer the best hope to protect water from depletion.

References

Datta, P.S. (2005) Groundwater ethics for its sustainability, Current Science, 89, No. 5, 10 September, 1-6.

Datta P.S., Desai B.I. and Gupta S.K. (1980). Hydrological investigations in Sabarmati basin – I. groundwater recharge estimation using tritium tagging method. Proc. Ind. Natn. Sci. Acad., Phys. Sci., 46, No.1, 84-98.

Datta P.S., Desai B.I. and Gupta S.K. (1979). Comparative study of groundwater recharge rates in parts of Indo-Gangetic and Sabarmati alluvium plains. Mausam, 30, 1, 129-133.

Datta P.S. and Goel P.S. (1977). Groundwater recharge in Punjab state (India) using tritium tracer. Nordic Hydrology, 8, 225-236.

Datta P.S., Goel P.S., Rama and Sangal S.P. (1973). Groundwater recharge in western Utter Pradesh. Proc. Ind. Acd. Sci., 78, Sec. A, 1-12.

Datta P.S., Gupta S.K.and Sharma S.C. (1980a). A conceptual model of water transport through the unsaturated soil zone. Mausam, 31, 1, 9-18.

Datta, P.S., Rohilla, S.K. and Tyagi, S.K. (2001) Integrated approach for water resources management in Delhi region: Problems and perspectives. In: Regional Management of Water Resources, Proc. Maastricht Assembly Symposium, July, 2001, IAHS Publ. No. 268, 1-8.

Goel P.S., Datta P.S., and Tanwar B.S. (1977). Measurement of vertical recharge to groundwater in Haryana state (India) using tritium tracer. Nordic Hydrology, 8, 211-224.

Rodell Matthew, Velicogna Isabella and Famiglietti James S. (2009). Nature, Vol. 460, 999-1002 (20 August 2009).