Modeling the potential for floodwater recharge to offset groundwater depletion: a case study from the Ramganga basin, India

The Ganges basin faces considerable spatial and temporal imbalance between water demand and availability. Lack of water storage infrastructure has led to this mismatch, wherein there are limited options to store flood water during the wet season and limited groundwater and surface water resources during the dry season. In this current study, a semi-coupled hydrological modeling framework is used to test scenarios that can help bridge this imbalance. A hydrological model (SWAT), groundwater model (MODFLOW) and flood inundation model (HEC-RAS) were applied to the Ramganga basin in India (*19,000 km2) to understand the baseline hydrologic regime and to test scenarios with distributed managed aquifer recharge (MAR) interventions, which when applied to at the basin scale to co-address flooding and groundwater depletion has come to be known as Underground Taming of Floods for Irrigation. The scenarios with MAR, which used available basin runoff to recharge groundwater, yielded favorable results in flood reduction and groundwater level improvement throughout the sub-basin. Groundwater levels improved within 5 years of introducing MAR, resulting in a groundwater elevation increase of up to 7 mwhen compared to baseline conditions. The HEC-RAS model indicated that a 20% reduction in basin outflow converted a 15-year flood peak to an 8-year flood peak, a 5-year peak to 3 years and a 2-year peak to 1 year. In addition, this resulted in a 10% reduction in the inundated area in all return periods tested. Therefore, distributed MAR practices can be effective in reducing the negative impacts from larger return period floods and increasing the groundwater levels

Reviving the “Ganges Water Machine”: where and how much?

Runoff generated in the monsoon months in the upstream parts of the Ganges River basin (GRB) contributes substantially to downstream floods, while water shortages in the dry months affect agricultural production in the basin. This paper examines the potential for subsurface storage (SSS) in the Ganges basin to mitigate floods in the downstream areas and increase the availability of water during drier months. The Soil and Water Assessment Tool (SWAT) is used to estimate “sub-basin” water availability. The water availability estimated is then compared with the sub-basinwise unmet water demand for agriculture. Hydrological analysis reveals that some of the unmet water demand in the subbasin can be met provided it is possible to capture the runoff in sub-surface storage during the monsoon season (June to September). Some of the groundwater recharge is returned to the stream as baseflow and has the potential to increase dry season river flows. To examine the impacts of groundwater recharge on flood inundation and flows in the dry season (October to May), two groundwater recharge scenarios are tested in the Ramganga sub-basin. Increasing groundwater recharge by 35 and 65 % of the current level would increase the baseflow during the dry season by 1.46 billion m3 (34.5 % of the baseline) and 3.01 billion m3 (71.3 % of the baseline), respectively. Analysis of pumping scenarios indicates that 80 000 to 112 000 ha of additional wheat area can be irrigated in the Ramganga sub-basin by additional SSS without reducing the current baseflow volumes. Augmenting SSS reduces the peak flow and flood inundated areas in Ramganga (by up to 13.0 % for the 65 % scenario compared to the baseline), indicating the effectiveness of SSS in reducing areas inundated under floods in the sub-basin. However, this may not be sufficient to effectively control the flood in the downstream areas of the GRB, such as in the state of Bihar (prone to floods), which receives a total flow of 277 billion m3 from upstream sub-basins

Use of geophysical and hydrochemical tools to investigate seawater intrusion in coastal alluvial aquifer, Andhra Pradesh, India

India has a very long coastline and 25 % of the country’s population live in the coastal zone. Urban centers are located along the coast and three out of four metro cities are located on the coast. The high population density along the banks of major rives and coast Increasing population and demand for water putting the coastal aquifers under stress and causing sea water inrush and salinity upcoming in the coastal aquifers. Apart from sea water contamination, urban waste releases and agriculture inputs threatening the coastal groundwater aquifer systems. Generally coastal areas receive more pollutant loads from different sources including geogenic and anthropogenic sources. Central Godavari delta is located adjacent to the Bay of Bengal Coast, Andhra Pradesh, India and is drained by Pikaleru, Kunavaram and Vasalatippa drains. The area is occupied by recent Quaternary alluvium and gone through a series of marine transgression and regression. The entire study area comes under Godavari central canal command area, water is available throughout year except first week of June and last week of April in the canals. Water requirements for irrigation met from surface water in the delta. There is no groundwater pumping for agriculture as wells as for domestic purpose due to brackish nature of the groundwater at shallow depths. The groundwater depths varying from 0.8 to 3.4 m dug wells and in bore wells located near the coast 4.5–13.3 m. The established groundwater flow direction is to be towards Bay of Bengal from Amalapuram. Geophysical and hydrochemical tools were applied to identify the source of the salinity and to assess the saline water intrusion in the Godavari delta. Electrical Resistivity Tomography (ERT) surveys were carried out at several locations in the deltaic region to delineate the aquifer geometry and to identify saline water aquifer zones. The results inferred from ERT indicate 12–15 m thick loamy sands were existed from surface to subsurface and it is followed by 18–25 m thick clay layers. The thickness of clay is being increased toward Sea from inland. The low resistivity values in the delta are attributed to existence of the thick marine clays in the subsurface and relative high resistivities are attributed to existence of fresh water. The resistivity values similar to saline water 0.86) and SO4 -2/Cl- (0.05) indicated marine palaeo salinity, dilution of marine clays and dissolution of evaporites. The high SO4 -2/Cl in the post monsoon is attributed to dilution groundwater salinity due to rainfall infiltration and irrigation return flows in the delta. The low Na+2/Cl- ratios in upstream of the delta are due to sand exposures and isolated fresh water lances in the perched aquifers

Reviving the Ganges water machine: potential and challenges to meet increasing water demand in the Ganges River Basin

Although the Ganges River Basin (GRB) has abundant water resources, the seasonal monsoon causes a mismatch in water supply and demand, which creates severe water-related challenges for the people living in the basin, the rapidly growing economy and the environment. Addressing these increasing challenges will depend on how people manage the basin’s groundwater resources, on which the reliance will increase further due to limited prospects for additional surface storage development. This report assesses the potential of the Ganges Water Machine (GWM), a concept proposed 40 years ago, to meet the increasing water demand through groundwater, and mitigate the impacts of floods and droughts. The GWM provides additional subsurface storage (SSS) through the accelerated use of groundwater prior to the onset of the monsoon season, and subsequent recharging of this SSS through monsoon surface runoff. It was identified that there is potential to enhance SSS through managed aquifer recharge during the monsoon season, and to use solar energy for groundwater pumping, which is financially more viable than using diesel as practiced in many areas at present. The report further explores the limitations associated with water quality issues for pumping and recharge in the GRB, and discusses other related challenges, including availability of land for recharge structures and people’s willingness to increase the cropping intensity beyond the present level