Can traditional tanks help Bengaluru revive its declining groundwater levels?

Can traditional tanks help Bengaluru in reviving its groundwater levels and in preventing the growing water crisis that the city is currently facing?

Traditional tanks in South India

The use of tanks is a long-standing tradition especially in southern India and traditional tanks have been known to play an important role in recharging local groundwater levels besides providing other advantages such as flood protection, water storage, ecosystem services, as well as recreational and aesthetic value.  

There are around 200,000 to 500,000 traditional tanks in south India. Efforts at tank restoration and promotion to enhance groundwater availability by means of MAR, have been recently undertaken at the national level as well as by local governments such as Tamil Nadu and Karnataka, with support from local NGOs and international institutions such as the European Union (EU), the National Bank for Agricultural and Rural Development (NABARD), the World Bank, and the International Water Management Institute (IWMI). 

However, very few studies exist that have looked at the effectiveness of tank recharge in the complex fractured hydrogeology of Peninsular India informs this paper titled “Assessing the role of groundwater recharge from tanks in crystalline bedrock aquifers in Karnataka, India, using hydrochemical tracers” published in the Journal of Hydrology X.

This study examined the usefulness of tanks in helping groundwater recharge in comparison to other structures in the city of Bengaluru in two subcatchments (Aralamallige and Hadonahalli), and two rural areas of Thippagondanahalli (TG Halli) and Berambadi catchment in the southern part of Karnataka. All study areas are located in the Cauvery River Basin, with exception of the north-eastern part of Bengaluru, which falls into the South Pennar basin.

Water tanks of urban Bengaluru

A number of  water tanks are found in urban Bengaluru and TG Halli and Berambadi catchments and mainly consist of river-fed tanks constructed by using anicuts to check water along a hydraulic gradient, and rain-fed tanks constructed by digging a square cavity of shallow (few metres) depth into the ground. Tank embankments can be site-specific, but typically, river-fed tanks have little embankment aside from the check dam, while smaller, rain-fed tanks are usually without raised embankments. 

Bengaluru city has cascading, large (> 1.2 ha), medium (0.4–1.2 ha), and small size (< 0.4 ha) tanks with seasonal to perennial water storage. The city is divided into three main river valleys, and most of the city’s tanks have been constructed by using anicuts to intercept rainwater and dam existing streams, and drains connect a series of cascading tanks to each other. Nowadays, the main tank inflows are from rainfall, and urban runoff, including waste water. The depth of water bodies in Bengaluru ranges from 0.5 m to a maximum of 9 m. 

Traditional tanks in rural areas of TG Halli and Berambadi

The TG Halli catchment includes two large reservoirs, namely TG Halli reservoir and Hesaraghatta Tank, which is located about 30 km upstream from TG Halli along the Arkavathy River and about 600 small cascading in-stream tanks, which have been constructed by checking monsoon runoff. The Berambadi catchment area has 53 tanks in addition to the Berambadi tank, which fill up for short periods after strong monsoon rains. 

Water situation in urban Bengaluru 

Water demand in Bengaluru is rising over the years and has been unable to meet the needs of the growing population in the city.  Water drawn from the Cauvery River, and brought to the city via a large pipeline now serves as the city’s main drinking water source, particularly in the centre of Bengaluru, where the density of piped network supplying water is the greatest. Groundwater through borewells also continues to be extracted at frightening rates in the city as well as in the outskirts of the city, where it is used for drinking water as well as for small-scale agricultural activities. 

State of water resources in rural TG Halli and Berambadi

In rural TG Halli, groundwater abstraction has led to a decline in surface water flows. In the Berambadi catchment, groundwater is the primary source of water and increasing groundwater abstraction for agricultural production has led to water table declines of up to 50 m. However, the recent good 2018 monsoon has also helped groundwater to recover across the catchment.

There are 2 to 3 cropping seasons a year, namely in summer (January to May, always irrigated), kharif (May to September, frequently irrigated), and rabi (September to January, only irrigated if rainfalls are insufficient) and thus the highest groundwater is abstracted from deeper levels during the summer season. 

Findings

Tanks are a source of groundwater contamination in urban areas

In Bengaluru city, tanks are a possible source of contamination due to vertical infiltration to groundwater in the dry season. In the rural catchments, especially Berambadi, contamination is due to agricultural pollutants and tanks pose a negligible risk to groundwater quality.

Tanks do not play a role in groundwater recharge in rural areas

Tanks do not play much of a role in groundwater recharge and affect quality in rural areas and groundwater recharge mainly occurs due to rainfall and groundwater recycling from irrigation.

Tanks play a limited role in aiding groundwater recharge in Bengaluru city

In the urban setting in Bengaluru, the presence of more  impermeable surfaces increases the relative effect of recharge from point sources such as tanks and rivers. However, where present, pipe leakage from public-water-supply accounts for the majority of the recharge. 

Location-specific differences are seen for example, in the city centre where shallow groundwater levels are predominant and groundwater (as well as mains water leakage) is likely to interact and possibly recharge tanks, and in the outer areas of Bengaluru. 

The study thus finds that groundwater recharge from tank systems cannot be generalised and depends on the locations and area concerned, and especially on water level fluctuations and alternative recharge sources available. The paper recommends further studies to quantify tank recharge and revisions to the current guidelines for national groundwater recharge estimations, using a less generalised approach, to avoid over-estimating the role tanks play in groundwater recharge.