The Need for Rainwater Harvesting
India, and particularly urban centers like Bangalore, are facing an escalating crisis of groundwater depletion. Groundwater, which serves as a primary source for drinking, domestic, and agricultural needs, is being extracted at unsustainable rates. The natural replenishment of these underground aquifers has been severely disrupted due to rapid urbanization, reduced open soil areas, and inadequate recharge practices.
One of the most pressing indicators of this crisis is the increasing dependence on borewells. As rivers run dry and surface water sources become unreliable or polluted, borewells have become the go-to solution for both residential and commercial water needs. However, this dependence has led to indiscriminate drilling and over-extraction, resulting in falling water tables and, in many cases, the complete failure of borewells.
A core issue lies in the imbalance between groundwater extraction and natural recharge. In many urban developments, the volume of water being drawn out far exceeds what is naturally percolated back into the earth. Hardscaping, reduced green cover, and inadequate drainage systems further exacerbate this imbalance.
In the local context, Bangalore is among the fastest-depleting metropolitan aquifers in India. Once known for its lakes and tank systems, much of this natural infrastructure has been lost to urban sprawl. This makes rainwater harvesting not just a sustainable practice, but a critical intervention to mitigate water scarcity. By capturing and recharging rainwater, we can reduce our reliance on borewells, improve groundwater levels, and create a more resilient urban water system.
Bangalore receives an average annual rainfall of approximately 970 mm, which is more than sufficient to meet the city’s domestic water needs if harvested and managed effectively. However, the city continues to face severe and recurring water shortages, especially during the summer months. This paradox highlights a critical issue—not the lack of rain, but the lack of infrastructure and systems to capture, store, and recharge rainwater.
It is estimated that over 90% of the rainwater that falls on Bangalore’s surface is lost as runoff. This is primarily due to impervious surfaces such as roads, pavements, rooftops, and built-up areas that dominate the urban landscape. Instead of allowing rainwater to percolate into the ground and replenish aquifers, the city’s current design channels it into drains, leading to flash flooding during monsoons and dry borewells in summer.
This inefficiency underlines the urgent need to shift from drainage-centric planning to a rainwater harvesting-centric approach—where rainfall is seen as a valuable asset, not a waste product. Implementing decentralized rainwater recharge systems across buildings, campuses, and public spaces is the most practical way forward to close this loop
Moreover, Bangalore’s dependence on groundwater is alarmingly high. Out of the total daily water demand of approximately 2632 million litres per day (MLD), nearly 1372 MLD—more than 52%—is fulfilled by groundwater sources. This overreliance is unsustainable and directly linked to falling water tables, poor recharge, and deteriorating water quality.
According to the 2023 data from the Central Ground Water Authority (CGWA), urban Bengaluru has officially been classified as an “overexploited” zone, meaning that the annual extraction of groundwater significantly exceeds its natural annual recharge. This status is a red flag and mandates urgent and coordinated interventions to reduce groundwater withdrawal and improve recharge efforts at every possible scale—from individual plots to large campuses.
Adding to the crisis, climate data indicates that Bengaluru has experienced an average temperature increase of nearly 1°C over the past 42 years. This gradual yet persistent rise in ambient temperature is a clear marker of urban heat island effects
Higher temperatures have a direct impact on the urban water cycle. As a result, even when rainfall occurs, the effective retention and recharge of water are compromised. This contributes to the drying up of shallow aquifers and open wells, worsening water scarcity even in the presence of rainfall.
The rising heat and falling groundwater are thus not isolated problems—they are interlinked symptoms of the same systemic failure in urban water and land management.
Rainwater Harvesting System at GIIS
Catchment and Collection
In this project, the rainwater harvesting system has been designed to maximize groundwater recharge through a combination of recharge pits and direct borewell injection, ensuring efficient use of available rainfall.
Rainwater from the building’s terrace areas is collected via strategically placed downpipes. One section of the terrace directs water straight into percolation pits located onsite, allowing natural percolation into the groundwater.
The terrace area facing the plaza has a dedicated collection system where rainwater is channeled through downpipes into a sedimentation tank. From this pit, water is further conveyed into a direct borewell recharge system, which injects filtered rainwater into deeper groundwater aquifers.
To manage excess rainwater during heavy rainfall events, any overflow from these two systems is carefully routed to the site’s main well. This secondary storage ensures that surplus water is not wasted but retained for groundwater recharge or other utility.
This multi-tiered system effectively balances natural infiltration via recharge pits with enhanced recharge through borewell injection, optimizing groundwater replenishment for the project site.
Recharge Mechanisms: Direct Borewell Recharge and Percolation Pits
Direct Borewell Recharge (DBR)
Direct borewell recharge involves collecting, thoroughly filtering, and injecting rainwater directly into existing borewells to replenish sub-soil and groundwater aquifers. This method accelerates groundwater recharge by bypassing the slower natural percolation process and delivering clean rainwater straight into deeper aquifers.
Immediate impacts of DBR include:
- Significant improvement in the quality of water extracted from existing borewells.
- Overall rise in sub-soil and groundwater table levels.
- Successful recharging of previously dry or non-productive borewells.
The filtration stage is critical to ensure that only clean water enters the aquifers, preventing contamination and maintaining groundwater health. This technique provides rapid and efficient groundwater recharge, making it highly effective in urban settings where space for infiltration is limited.
Percolation Pits
Percolation pits are constructed in a network across the site to collect and manage rainwater runoff. These pits are typically filled with layers of gravel — often 40 mm and 20 mm “jelly” (rounded gravel stones) — which act as filters to clean the water as it percolates through.
Rainwater flows into the pits, filters through these gravel layers, and slowly infiltrates into the ground, naturally recharging the groundwater table.
Advantages of percolation pits include:
- Simple construction techniques and low maintenance requirements, making them cost-effective and affordable.
- Effective conservation of unconfined aquifers by promoting natural infiltration and groundwater recharge.
- Management of excess stormwater, reducing surface runoff and the risk of urban flooding.
Together, direct borewell recharge and percolation pits form a complementary system that balances rapid aquifer replenishment with sustainable, natural infiltration.
Design
Long-term average rainfall data for Bengaluru indicates an annual precipitation of approximately 970 mm. However, observations over the past five years show rainfall often exceeding 1000 mm. To maintain a realistic estimate for this project, an average rainfall of 1100 mm over the monsoon period of 4 months (approximately 120 days) is assumed.
This translates to an average rainfall intensity of about:
=1100(mm)/120 (days) = 9.166mm ≈ 10mm/day
Therefore, Now;
Harvested Volume=Catchment Area×Rainfall Depth×Runoff Coefficient×Unit Conversion
= 1950(sqmt) * 0.01 * 0.95 * 1000
= 18,525 litres/day
On an annual basis, this translates to:
18,525×120 days=22,23,000 litres/year
Calculation Methodology – Site Groundwater
Harvested Volume=Catchment Area×Rainfall Depth×Runoff Coefficient×Unit Conversion
= 6500(sqmt) * 0.01 * 0.85 * 1000
= 55,250 litres/day
On an annual basis, this translates to:
18,525×120 days=66,30,000 litres/year
Interpretation
This volume of over 8.8 million litres per year represents a significant amount of rainwater that can be effectively harvested and recharged into the groundwater system via the implemented mechanisms. This contributes directly to reducing borewell dependence and improving local water security for the project site.