Water storage variations in major river basins of India

January 18th, 2016

A. Soni & Dr. T. H. Syed, Department of Applied Geology, Indian School of Mines, India

Freshwater resources in various parts of the world are threatened by an increasing global population and agricultural demand. In India, increased demand for freshwater is essentially driven by rapid growth in urbanization, agriculture, population and economic development. With a primarily agrarian economy, agriculture in India is largely supported by groundwater-based irrigation. Overall, groundwater caters to more than 80% of the rural water requirement, 50% of urban and around 50% of total irrigation.1

In spite of being an essential component of terrestrial water storage, which encompasses all forms of water stored above and below the land surface, the level of groundwater monitoring and management in many developing countries is rudimentary. With an unaccounted number of bore and dugwells catering to domestic and commercial needs, it is nearly impossible to quantify and monitor the extraction of groundwater with ground-based monitoring networks.

Since its inception in 2002, Gravity Recovery and Climate Experiment (GRACE) satellites have been instrumental in assessing terrestrial water storage (TWS) at varied spatial scales over monthly time periods. These satellite-based estimates in conjunction with ancillary in-situ observations and model simulations have also enabled, for the first time, large scale assessment of groundwater storage variations.

As an integrated measure of water resources, and groundwater storage at its core, TWS is a key part of the terrestrial and global hydrologic cycle that has an important control over water, energy and biogeochemical fluxes. Therefore, analysis of spatio-temporal variations in TWS and its relationship with major hydrologic states and fluxes is crucial for an enhanced understanding of the terrestrial hydrologic cycle and proper management of the available freshwater resources.

This article presents the results of a comprehensive analysis of TWS over the major Indian river basins (Ganga, Godavari, Krishna and Mahanadi) for the period of 2003-2012.2 The article builds upon the results obtained by the synergistic utilization of GRACE data and various observations and numerical simulations of land surface hydrological states and fluxes. Also examined were the contributions of groundwater and soil moisture in terrestrial water storage variations. Additionally, basin-wise trends in TWS and groundwater storage were analysed in terms of their magnitude and relation with coupled ocean-atmosphere patterns like ENSO.

Analysis of TWS and major hydrologic fluxes

Before analyzing the GRACE-based TWS estimates they were compared with those obtained by water balance computations. While the comparison extended confidence on the satellite-based estimates, it also highlighted some of the important differences and limitations of the methods involved. Results revealed significant agreement between GRACE-derived and flux-derived TWS changes over the Ganga (R = 0.87, p-value < 0.05), Godavari (R = 0.89, p-value < 0.05), Krishna (R = 0.73, p-value < 0.05) and Mahanadi basins (R = 0.88, p-value < 0.05).

In all the basins an average RMSE of ~48 mm was obtained. In order to evaluate the dominance of individual hydrologic fluxes towards TWS variations, correlations between the time series of hydrologic fluxes and GRACE derived TWS changes for each basin were investigated. Results showed that TWS in all basins is highly correlated with precipitation and with evapotranspiration to some extent but barely with runoff. The strongest correlations suggest that precipitation is the most significant driver of monthly variations of TWS in all of the river basins, while contributions from runoff are insignificant. Furthermore, analysis of lag correlation between TWS anomaly and hydrologic fluxes showed time lags of 1-2 months in all the basins.

Variations in terrestrial water storage and groundwater storage

Trends in groundwater storage, estimated by removing the contributions of soil moisture from TWS observed by GRACE, were observed to be similar to those of TWS (indicating groundwater to be a major component of TWS) and also vary significantly from one basin to the other. During the study period, a significant decline was observed in groundwater storage (-1.28 ± 0.20 mm/month) over the Ganga basin. In contrast, groundwater storage over the Krishna and Godavari basins showed a significant increasing trend, at the rate of 0.97 ± 0.21 mm/month and 0.74 ± 0.21 mm/month respectively.

In India, a major portion of the freshwater requirement, particularly for irrigation, is met by groundwater extraction. Since the Ganga basin is one of the most densely populated and heavily irrigated basins in the world, it may be concluded that groundwater withdrawal in this region is the highest when compared to the other basins. By contrast, the increasing trend in the Krishna and Godavari basins is a possible indication of the dependence on surface water resources relative to groundwater storage. It may also be suggested that in these basins groundwater storage recovery, due to the recharge from nearby aquifers, is more effective than in those lying within the Ganga basin.

Impact of ENSO on groundwater storage

In order to establish the climatic controls of groundwater storage, the study compared time variations of groundwater storage with that of El Niño Southern Oscillation (ENSO) indices (i.e. Niño 3.4 and BEST). ENSO is one of the most important phenomena influencing the Indian Monsoon. Over continental India, during the El Niño (La Niña) year, the warm (cold) extremes induce the majority of the episodes of below (above) normal precipitation.

Results of this analysis revealed negative correlation between groundwater storage and ENSO in all the basins, with the exception of the Ganga (see Table 1). In the Ganga river basin groundwater was observed to be continuously decreasing and the highest decline rate was observed during the strongest El Niño event (2009-2010). In fact, the impact of ENSO on groundwater storage is most prominent during the strongest El Niño (2009-2010) and La Niña (2010-2011) episodes. Hence, in general, El Niño (La Niña) events are associated with low (high) groundwater storage anomaly in all these Indian river basins.

This inference supports the claim that the effect of ENSO on Indian monsoon directly or indirectly influences groundwater storage variations. Beyond these conclusions, this comprehensive approach helps to understand the role of hydrologic fluxes in controlling TWS variations and how groundwater storage would respond to climate change signals.

References:

Mall RK, Gupta A, Singh R, Singh RS, Rathore LS. 2006, Water resources and climate change: An Indian perspective, Current Science 90: 1610-1626. A. Soni and T. H. Syed (2015): Diagnosing Land Water Storage Variations in Major Indian River Basins using GRACE observations, Global and Planetary Change, 133, doi: 10.1016/j.gloplacha.2015.09.007, p263-271.

Soni is a research scholar in the Department of Applied Geology, Indian School of Mines, Dhanbad, India. Her current research is focuseed towards basin-scale water storage variations and spatio-temporal variability of evapotranspiration. T. H. Syed is an Associate Professor in the Department of Applied Geology, Indian School of Mines, Dhanbad, India. His research interests span a variety of land surface hydrology and climate-related topics with particular emphasis on large-scale water balance studies, surface and groundwater variations. This article is based on the original research published in the Elsevier journal “Global and Planetary Change”, entitled “Diagnosing Land Water Storage Variations in Major Indian River Basins using GRACE observations”.

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.