• L ELANGO

Articles written in Journal of Earth System Science

• A box-model approach for reservoir operation during extreme rainfall events: A case study

Extreme rainfall events in an urban area pose various challenges to the water resource managers in terms of flood mitigation, inundation, water conservation and harvesting for drinking water supply. The objective of this study is to apply the box-model approach to evaluate reservoir operation during extreme rainfall events. A large water supply reservoir in Chennai was chosen to carry out this study. A box model, based on input–output parameters, is proposed to simulate the reservoir operation and hydraulic behaviour. Hydrologic Engineering Centre-Hydrologic Modelling System (HEC-HMS) has been used to simulate the reservoir inflow hydrograph and to understand the run-off characteristics of the basin. Three extreme rainfall events occurred in past have been selected for the analysis. Three different scenarios have been framed to assess the reservoir performance. Reducing the initial storage to 50% and releasing water at the beginning of the event gives a possible solution for flood mitigation in reducing the outflow volume by 9–37% and delaying the time to peak by 1–6 h. Though the reduced outflow volume from this reservoir is less, it can help to mitigate the flood inundation to a significant extent. Thus the box-model approach presented here can be utilised as a simple tool to generate the various combinations of outflow hydrographs for any reservoir.

• Estimation of groundwater abstraction induced land subsidence by SBAS technique

The groundwater over pumping induced land subsidence is one of the major geological hazards in the alluvial aquifers. The objective of this study is to assess the rate of land subsidence in Kolkata metropolitan area, India. Land subsidence can be estimated with high precision by Small Baseline Subset (SBAS) analysis. The advantages of this method are generation of a mean deformation map. The ENVISAT ASAR data acquired at six different periods over the study site were processed by SBAS technique. The decline in the piezometric head from the year 2003 to 2010 was about 6 m. Land subsidence velocity was $\sim$8 mm/year at Salt Lake City and Science City (near the eastern metropolitan bypass). The major cause for land subsidence is over pumping of groundwater from the confined aquifers in these areas. There is a reasonable comparison between the maximum region of land subsidence and low value piezometric head contours confirming that the over extraction of the confined aquifer of this region is responsible for land subsidence. It is necessary to control the groundwater pumping so as to arrest the declining trend of piezometric head of study area for managing the problem of land subsidence. Hence, the possible remedial measures that can be taken are reducing groundwater pumping in the study region.

• Percolation pond with recharge shaft as a method of managed aquifer recharge for improving the groundwater quality in the saline coastal aquifer

The deterioration of groundwater quality has become a serious problem for the safe drinking water supply in many parts of the world. Along coastal aquifers, the saline water moves landward due to several reasons even though significant rainfall is available. The objective of the present study is to investigate the impact of a combined recharge structure including a percolation pond and a recharge shaft in improving the groundwater quality of the surrounding area. The area chosen for this study is Andarmadam, Thiruvallur district of Tamil Nadu. As a part of the study, a suitable site was selected for the construction of a percolation pond based on preliminary field investigations in 2012. Three piezometers were also constructed near the percolation pond to investigate the impact of the structure on groundwater recharge. Further, a recharge shaft was added to this structure in 2013 to overcome the clogging issues at the pond bottom and to enhance the recharge. The impact of the percolation pond on groundwater was assessed by comparing the periodical groundwater level fluctuations with rainfall in the area. The fluctuations in groundwater level near the percolation pond show variations before and after the construction of recharge shaft. The amount of water recharged through the percolation pond during the water year 2012–2013 was estimated as $250–300 \rm{m}^{3}$. The volume of recharge was calculated to be increased more than twice after the construction of recharge shaft inside the percolation pond, on the assumption that recharge through the pond surface remained almost same as before. The dilution of ionic concentration in water was three times higher after the construction of recharge shaft. The long-term groundwater quality in the surrounding area of the pond improves gradually with time. The total dissolved solids (TDS) decrease considerably with time due to the dilution of dissolved solids in water with the fresh water recharging into the aquifer. The Wilcox diagram of most of the water samples after the construction of the recharge structure fall in the excellent to good category, indicating improvement in irrigation water quality.

• Use of GRACE time-series data for estimating groundwater storage at small scale

This study integrates the time series of satellite data, i.e. (terrestrial water storage (TWS) extracted from Gravity Recovery and Climate Experiment (GRACE), soil moisture (SM) from Global Land Data Assimilation System (GLDAS), rainfall from Tropical Rainfall Measuring Mission (TRMM)) with in-situ data, i.e. (groundwater (GW) observatory well data and surface water (SW) as reservoir level from Tamil Nadu Public Work Department (TN PWD)) to estimate the water storage of the study area for the period from 2002 March to 2016 December. The study area encompasses three districts of Tamil Nadu State – Chennai, Kancheepuram and Tiruvallur. It was previously observed that the groundwater level in this study area is decreasing at an alarming rate. Trends and residuals of the times series of all the previously stated components were analyzed. Negative trends were observed in the case of $\Delta$SW, $\Delta$SM, $\Delta$GW, $\Delta$TWS-GRACE, but rainfall shows a slight positive trend. Rainfall, being the input for water storage, a marginal increase in rainfall cannot affect the variability of $\Delta$SM, $\Delta$SW, $\Delta$GW and $\Delta$TWS-GRACE. $\Delta$TWS decreases approximately 0.12 cm yr$^{-1}$ averaged water level equal to a total volume −7.5 km$^{3}$. The result of this study shows a decrease in the total storage of groundwater in the study area during the study period from 2002 March to 2016 December. Seawater intrusion due to an increase in sea surface height also shows an impact on $\Delta$TWS derived from grace, in the form of anomalously elevated $\Delta$TWS. A good agreement between $\Delta$TWS-GRACE and $\Delta$TWS- in-situ is found except where sea-water intrusion occurs.

$\bf{Highlights}$

$\bullet$ Areal extent of the study area is less than the optimum suggested for GRACE related investigation. The amplitude of the GRACE signal is 300 km and it is recommended that the study area to be larger than that. Our study is perhaps the first such attempt where the study area is much less than recommended.

$\bullet$ We were able to get a good correlation between $\Delta$GWGRACE and $\Delta$GWINSITU despite the challenge of spatial resolution.

$\bullet$ A significant finding of the investigation is that distinction in estimates of groundwater changes between GRACE estimates and in-situ perception happens in grids which mostly have sea-area or are intensely impact by urbanization related LULC changes.

$\bullet$ The impact of various precipitation mechanisms in the examination area was contemplated.

• # Journal of Earth System Science

Volume 129, 2020
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019