Runoff Water Storage Structures: Ponds and Tanks

Every drop of rain that falls on a hillside or across a field has the potential to be captured, stored, and put to productive use – if the right infrastructure is in place. In rainfed farming regions, where water availability is seasonal and often unpredictable, runoff water storage structures like ponds and tanks are among the most practical and time-tested solutions. These structures collect surface runoff during rainfall and store it for use in irrigation, livestock watering, and even domestic needs – bridging the gap between the wet season and the dry one. According to the Food and Agriculture Organization (FAO), water harvesting and small-storage technologies are key interventions with the potential to rapidly improve yields in rainfed agriculture.

What are runoff water storage structures? 🔗

Runoff water storage structures are engineered systems built to intercept, collect, and retain water flowing over land surfaces after rainfall. Rather than letting that water drain away – often causing erosion in the process – these structures capture it for controlled future use. The primary goals are threefold: storing water for direct use in irrigation and livestock; enhancing soil moisture in surrounding fields; and facilitating groundwater recharge by allowing stored water to slowly seep into the soil. The most common forms are ponds and tanks, each suited to different topographies and intended uses. When well-designed, they form the backbone of on-farm water security, reducing dependence on groundwater extraction and providing a buffer during dry spells.

Ponds: types and where they are used 🔗

Ponds are open, shallow water bodies that collect surface runoff from surrounding catchment areas. Their design, depth, and construction method vary based on the terrain where they are built. There are three main types commonly used in agricultural water harvesting.

Dugout-cum-embankment ponds 🔗

These are the most common type found in hilly and mildly sloping terrain. As the name suggests, they combine two construction techniques: soil is excavated from the centre to form a basin, and that same excavated earth is used to build an embankment around the perimeter. The embankment raises the effective storage capacity beyond what excavation alone could achieve. These structures are positioned in the lower or middle reaches of hill slopes, where runoff naturally converges. According to research from the Central Soil and Water Conservation Research and Training Institute (CSWCRTI), dugout-cum-embankment ponds are particularly suited to mild sloping topography, and in semi-arid black soil regions they have demonstrated significant potential to stabilise crop production.

Dugout ponds 🔗

In flatter regions, where there is no slope to assist embankment construction, simple dugout ponds are used. Here, a pit is excavated directly into the ground and it fills with runoff and, in some cases, groundwater seepage. These ponds are simpler and cheaper to build than embankment types, but their capacity depends entirely on the depth of excavation. According to guidance from Asia Farming, the ideal dugout pond for Indian conditions is typically 10 metres by 10 metres and at least 3 metres deep, sized to hold roughly half the maximum expected runoff to remain functional even during drought years.

Embankment ponds 🔗

In hilly or undulating areas where a natural depression or valley already exists, an embankment-type pond can be constructed by building a dam or earthen bund across that depression. The embankment acts as a barrier that holds back incoming runoff, effectively creating a reservoir without requiring extensive excavation. Soil used for the dam should have enough clay content to compact well and retain water. Any dam that collects runoff must also incorporate a properly designed spillway – a channel to safely discharge excess water and prevent the embankment from being overtopped and damaged, as noted by Backwoods Home Magazine.

Sizing a pond: the role of rainfall and catchment area 🔗

The size of a pond or tank is not arbitrary. It must be calculated based on the annual rainfall of the area and the catchment area – the total land surface from which runoff will flow into the structure. A larger catchment with higher rainfall will generate more runoff and therefore requires greater storage capacity. Conversely, in areas with lower or erratic rainfall, the pond must be large enough to capture every usable rainfall event during the wet season. Losses from evaporation and seepage must also be factored into capacity planning. Lining the pond with clay, plastic sheeting, or concrete significantly reduces seepage losses and extends the usable life of the structure. According to Mississippi State University Extension, on-farm storage ponds must be designed to hold enough water to cover the entire growing season – especially in areas where groundwater is deep or unreliable as a backup.

Tanks: storage and percolation 🔗

Tanks are larger and generally deeper than ponds. They are designed to store greater volumes of water, often for distribution to fields through pipelines or channels. Within agricultural water management, two types of tanks serve distinct purposes.

Storage tanks 🔗

Storage tanks are intended for direct use. They can be constructed above or below ground using materials such as concrete, ferrocement, plastic, or stone masonry, depending on cost and availability. Their enclosed nature protects stored water from evaporation and contamination – a key advantage over open ponds. However, as noted by BTL Liners, tanks are more expensive to build than open ponds and require more maintenance, particularly for plumbing and pump systems. They are best suited for situations where water quality is a concern or where precise distribution to specific field areas is needed.

Percolation tanks 🔗

Percolation tanks serve a fundamentally different function from storage tanks. Rather than holding water for direct use, they are designed to spread it over a large area of permeable soil so that it seeps gradually into the ground, recharging groundwater aquifers. These tanks are typically constructed across ephemeral (seasonal) streams in areas with fractured rock formations or permeable soils. As water percolates downward through soil layers, it replenishes underground aquifers – raising water tables in surrounding wells and bore wells. According to a comprehensive review published in the journal Water (MDPI), percolation tanks are part of a broader class of run-off conservation structures – alongside contour bunds, gully plugs, and bench terracing – that together form cost-effective solutions for groundwater management in areas where natural recharge is insufficient.

The strategic placement of percolation tanks matters greatly. A catchment area at least 10 times larger than the tank’s water spread area is generally recommended to ensure adequate inflow. In India, states like Maharashtra, Gujarat, and Rajasthan rely heavily on percolation tanks to combat seasonal water scarcity. According to NIPSTec, in drought-prone areas of Maharashtra, Rajasthan, and Uttar Pradesh, percolation tanks have demonstrated substantial gains in groundwater availability, directly benefiting rainfed farmers who depend on well irrigation during the dry season.

Diversion bunds and channels: directing runoff to where it’s needed 🔗

Constructing a storage structure is only half the solution. To feed these structures effectively, diversion bunds (earthen embankments) and diversion channels are constructed across slopes to intercept surface runoff and redirect it toward storage ponds, tanks, or fields. A diversion bund is built along the contour of a slope; runoff flowing downhill is intercepted by the bund and channelled laterally to the intended storage structure or irrigated field. This approach serves two purposes simultaneously: it fills the storage structure during rainfall events, and it slows the flow of water across agricultural fields, increasing the time water spends in contact with the soil and improving soil moisture levels. In Bihar, for example, rectangular catchment basins called Ahars – built with earthen embankments to impound rainfall – have historically served this dual function of storage and field moisture enhancement, as documented by the Meghalaya Public Health Engineering Department’s water harvesting report.

Water spreading techniques and soil moisture enhancement 🔗

Beyond discrete storage structures, water spreading techniques aim to distribute runoff over as wide a surface area as possible, maximising infiltration and soil moisture replenishment. This is particularly useful in areas where soils have moderate to high permeability. Shallow, flat-bottomed ditches and contour bunds allow runoff to slow down and spread laterally across fields rather than concentrating in channels and flowing away. When water spreads evenly across a field surface, a larger soil volume absorbs moisture – directly benefiting crops with improved root-zone water availability. Percolation tanks, in this context, function as a large-scale water spreading tool: they hold runoff long enough for it to percolate into the soil rather than evaporate or drain away. According to Wikipedia’s entry on groundwater recharge, when water pools in low-lying areas or surface depressions rather than flowing uniformly across the land, the concentration of infiltration under those depressions can significantly increase the local groundwater table – a process called depression-focused recharge.

Construction essentials: from site selection to lining 🔗

The construction process for any runoff storage structure follows a broadly similar sequence. First, site selection determines where runoff naturally concentrates, where soil conditions are suitable, and where the structure can serve the most fields or communities. For embankment-type structures, a narrow valley with steep side slopes is ideal – it maximises storage volume with the least amount of earthwork. For dugout ponds, low-lying farm areas with naturally poor drainage are preferred. Second, excavation shapes the basin and, for embankment types, provides the soil material for dam construction. Third, lining – with clay, plastic, or concrete – reduces seepage where required. Fourth, inlet and outlet structures are built to control water entering and leaving the pond. The inlet is often designed as a chute spillway that prevents erosion as water enters the basin at speed. The outlet manages the water level and enables controlled release for irrigation. Finally, an emergency spillway must be included in any embankment-type structure to handle unexpectedly heavy inflows without overtopping and breaching the embankment.

Regular maintenance – including desilting, repairing embankments, and clearing inlet channels – is essential for long-term effectiveness. Silt accumulation is a persistent problem: muddy runoff gradually reduces the storage capacity of both ponds and percolation tanks over time, as highlighted by Chaitanya Products’ water harvesting resource.

Benefits beyond irrigation 🔗

The value of runoff storage structures extends well beyond crop irrigation. These structures provide a reliable source of water for livestock throughout the year, particularly in semi-arid regions where surface water dries up after the monsoon. They support fish farming and aquaculture, creating an additional income stream for farm households. Where percolation tanks recharge local aquifers, surrounding wells remain productive for longer into the dry season – reducing the energy costs of pumping from deeper groundwater. There is also a role in flood mitigation: during intense rainfall, storage structures absorb excess runoff that would otherwise cause downstream flooding and soil erosion. Research cited by Wikipedia on rainwater harvesting found that in Caribbean smallholder farming systems, capturing and storing runoff significantly reduced the risks of crop loss from both water scarcity and soil erosion during high rainfall seasons.

What do you think? Given the dual role of runoff storage structures in both supplying water directly and recharging groundwater, how should farmers and planners prioritise between storage tanks and percolation tanks in regions experiencing groundwater depletion? And with climate patterns growing increasingly unpredictable, are current approaches to sizing ponds and tanks – based on historical rainfall data – still adequate for future water security?

References

1. https://www.fao.org/land-water/water/water-management/water-storage/en/
2. http://www.cswcrtiweb.org/Technology/English/3.pdf
3. https://www.asiafarming.com/building-a-farm-pond-in-india-a-comprehensive-guide-to-uses-advantages-and-subsidies
4. https://www.backwoodshome.com/water-development-for-the-homestead/
5. https://extension.msstate.edu/publications/farm-water-storage-systems-and-surface-water-for-irrigation
6. https://www.btlliners.com/types-of-rainwater-harvesting-systems
7. https://www.mdpi.com/2073-4441/17/7/976
8. https://www.therainwaterharvesting.com/percolation-tanks-for-effective-groundwater-recharge/
9. https://megphed.gov.in/rainwater/Chap5.pdf
10. https://en.wikipedia.org/wiki/Groundwater_recharge
11. https://www.chaitanyaproducts.com/blog/percolation-tanks-a-component-for-ground-water-recharging/
12. https://en.wikipedia.org/wiki/Rainwater_harvesting