Can urban runoff be harvested to meet Indian cities’ growing water demands?

Every city has a water story. Rain falls on its roads, roofs, fields and open spaces. Some of it enters the soil, some returns to the atmosphere and some flows away as runoff. What happens to this water can shape whether a city faces shortages, flooding or both.

Rapid urbanisation and climate change are making water management increasingly difficult for Indian cities. Water scarcity, flooding, pollution and ecosystem degradation are now closely connected to the way cities manage rainfall and other water sources. National policies and legislative frameworks address these challenges, but their effectiveness depends on how they are implemented at the city and sub-regional levels.

One tool that can help cities understand their water situation is water budgeting. It compares the water entering a system with the water being used, lost or stored over a given period. The basic components are simple. Rain and other water entering an area form the inputs. Evaporation, plant use, runoff and water pumped out of the system are outputs. Lakes, soil and underground aquifers provide storage.

Runoff is generated when rainwater cannot enter the ground. This can happen when the soil is already saturated or when roads, buildings and other built surfaces prevent infiltration and groundwater recharge. The water then often enters stormwater drains or flows into streams, rivers, lakes and eventually the sea. Some of this runoff could instead be captured and used.

A recent  study by Indra Mani Tripathi and others, published in Sustainable Cities and Society: Advances, examines this possibility in three cities with very different climates: Bhopal, Bhuj and Kozhikode. Using the Soil and Water Assessment Tool, the researchers carried out a comparative water budget to assess how much runoff could potentially be harvested to strengthen urban water security. The three cities tell very different stories.

Bhopal: Plenty of rain, but much of it runs away

Bhopal’s city catchment covers 244 square kilometres. Cropland occupies 46 per cent of the area, and built-up land accounts for 38 per cent. The city receives around 1,100 to 1,200 mm of rainfall during the southwest monsoon. Yet much of this water does not remain available within the system. Of the annual rainfall of 1,191 mm, around 46 per cent becomes surface runoff. Impervious surfaces, storm sewers and channelised paths quickly carry water away, reducing natural infiltration. Only 9 per cent of rainfall percolates into the soil, while deep recharge is negligible.

Evapotranspiration accounts for another 44 per cent of the water loss. Baseflow contributions are almost absent. Most runoff and water yield occur during the monsoon between June and September, while flows are negligible during the dry season. Bhopal is therefore largely dependent on surface water, with substantial atmospheric losses and limited groundwater recharge. This becomes more significant when the city’s water demand is considered.

The gross water requirement is estimated at 377.84 ± 35.74 MLD for 2024 and 464.12 ± 43.90 MLD for 2031. The city currently draws water from Kolar Dam, the Narmada River, Kerwa Dam and Upper Lake. Their combined present capacity is 514 MLD. Current withdrawals total 330 ± 11 MLD after accounting for a 25 per cent leakage estimate, with 160 MLD from Kolar, 145 MLD from the Narmada, 20 MLD from Kerwa and 115 MLD from Upper Lake. Proposed capacity expansions could increase total supply to 575 MLD.

Against this demand, the estimated annual runoff of 134,275.01 ML ± 12,061.49 ML presents a substantial opportunity. For Bhopal, capturing runoff before it leaves the catchment could become an important part of managing its water needs.

Bhuj: Water arrives quickly and disappears just as fast

The story changes sharply in Bhuj. Located within the Hamirsar Lake catchment, Bhuj has a watershed area of 44.47 square kilometres. The landscape consists mainly of cropland, shrubland and urban areas, with little forest or water cover. More than 90 per cent of the rainfall arrives during the monsoon. About 34 per cent becomes surface runoff, while 58 per cent is lost through evapotranspiration. Only 6 per cent contributes to groundwater recharge and around 5 per cent to baseflow.

Rainfall produces quick runoff peaks, particularly in July and August, followed by a long dry period. This creates a city that faces both flood risk during the monsoon and water scarcity afterwards. Bhuj’s gross water requirement is estimated at 29.74 ± 2.81 MLD for 2024 and 37.44 ± 3.54 MLD for 2031.

The municipality receives around 55 per cent of its water from the Narmada, outside the Hamirsar catchment. After accounting for a 30 per cent leakage loss, the municipal supply is 22.75 ± 0.975 MLD. The remaining supply comes from tubewells and borewells. The estimated annual runoff of 6,586.23 ± 2,542.76 ML therefore offers an opportunity to capture and store monsoon water, particularly for non-domestic uses. For a city with a long dry season, the challenge is not simply to find more water. It is to hold on to the water that arrives during the monsoon.

Kozhikode: Abundant rainfall, but flooding remains a concern

Kozhikode presents another contrast. The Kallayi River catchment covers 52.8 square kilometres, with built-up land accounting for 61 per cent and forest 36 per cent. Small water bodies cover around 2 per cent. Its humid tropical climate produces a very different water budget. Around 65 per cent of rainfall contributes to streamflow, supported by 44 per cent surface runoff and 31 per cent baseflow. Groundwater recharge is around 30 per cent, although deep aquifer replenishment is only 2 per cent.

Evapotranspiration accounts for 24 per cent. Rainfall peaks in July, producing the highest runoff and water yield, while flows become negligible during the dry months. Unlike Bhopal, Kozhikode’s humid climate, lateritic soils and shallow aquifers allow measurable groundwater contributions to streams despite urbanisation. Yet abundant water does not mean that the city has no water challenge.

Kozhikode’s gross water requirement is estimated at 90.42 ± 8.55 MLD for 2024 and 110.55 ± 10.45 MLD for 2031. Its main water sources include Peruvannamuzhi Dam, which supplies 40 MLD, and Koolimadu, which supplies 72 MLD. However, leakage and production losses account for 39 per cent of the supplied water. The net municipal supply is 75 ± 12.32 MLD.

A large share of residents also depends on wells. Around 64.2 per cent of the population within the planning area relies on well water, while 30 per cent has access to municipal water. Around 58 per cent of municipal water is sourced from the Chaliya River, outside the Kallayi catchment. The estimated annual runoff of 69,646.21 ± 15,693.15 ML could therefore serve two purposes: meeting part of the city’s non-domestic water demand and helping reduce flooding.

Three cities, three water budgets

Putting the three cities together shows why urban water management cannot follow a single model. Bhopal has substantial runoff potential, estimated at 134,275.01 ML per year, but limited groundwater recharge and significant evapotranspiration. Bhuj has much less runoff, at 6,586.23 ML per year, but faces severe water scarcity after its short monsoon. Kozhikode receives much more rainfall and has a runoff potential of 69,646.21 ML per year but must also manage flooding.

The monsoon drives the water cycle in all three cities, but the months outside the monsoon reveal their differences. Bhuj experiences severe water scarcity because of limited rainfall and high evapotranspiration. Bhopal faces the possibility of pre-monsoon water stress alongside increasing evapotranspiration trends that are likely linked to climatic shifts. Kozhikode maintains a more balanced water budget after the monsoon.

Their dependence on water from outside their immediate catchments also creates another common concern. Bhopal depends on external sources for 74 per cent of its supply, while Bhuj and Kozhikode depend on external and groundwater sources for 55 per cent and 58 per cent respectively. This dependence can increase economic costs and place pressure on water sources and ecosystems beyond the city.

Making every drop count

The comparison points to the importance of understanding water at the city level. For Bhopal, capturing a portion of the large volume of runoff could help address the gap between supply and demand. For Bhuj, storing monsoon runoff could help carry water into the long dry season. For Kozhikode, runoff harvesting could contribute to non-domestic water needs while also helping manage flooding.

The study therefore advocates tailored runoff harvesting systems as a way to improve urban water self-sufficiency. The lesson from these three cities is not that runoff can replace existing water sources. Rather, understanding where rainwater goes, how much is lost and how much could potentially be captured gives cities another way to plan for their water needs.

A water budget makes this movement visible. And once a city can see where its water is coming from, where it is going and what is being lost, it can begin to ask a more practical question: how much of that water can be retained and used before it leaves the city?