Tanka System – A Basic Rainwater Harvesting Technique

Rainfall is the main source of water for augmenting ground water levels, soil moisture and surface water. Water is essential for agricultural activity, for growing fodder to feed livestock and to fulfill domestic requirement of all humans.

Rainwater harvesting has been a natural tendency in all ancient Indian and other civilizations in different parts of the world and has been practiced for more than 4000
years because even in those times the people recognized the fact that without water no form of life is possible on earth.

The Indus Valley Civilization settled on the banks of the Indus River and other parts of western and northern India about 4500 years ago had one of the most sophisticated
water supply and sewage systems in the world

The people of the Thar Desert in Rajasthan designed, developed and constructed many different structures for Rain Water Harvesting and among them; the most Common Rainwater collection technique has been through the ‘Tanka’ system

Tanka is a paved underground tank of differing shapes from square, rectangular to usually cylindrical, having holding capacity ranging from 1000 liters to 1,000,000 liters

The traditional construction material also varies from simple mud plaster to lime mortar or cement mortar. On top of the tank there is a roof cover with mild slope towards the center where there are inlet points to let the rain water falling on this catchment surface flows into the Tank (Tanka). There is also a covered opening from where the water is drawn using a rope and bucket as and when water is required.

Apart from the roof surface of Tanka which acts as catchment for rain water, even rainwater from house rooftop, courtyard or artificially prepared catchments flows are diverted towards the Tanka

The water collected in a Tanka is highly valued commodity for every member of the family and is used carefully so as to ensure that it lasts for many months – sometimes even till the subsequent monsoon…

A Tanka and the water it brings to households in an arid zone provides water security and saves family members (specially the rural womenfolk) from the burden of traveling long distances to get water for every day requirements.

SILVERON, in the year 2003-04 built two Tankas funded by Coca Cola India at Kaladera village and these were inaugurated by Hon’ble Justice B.N. Kirpal, former
Chief Justice of India.

Rainwater Harvesting Tanka constructed by Silveron at Girl’s School (Storage Capacity of 100,000 liters water).
Rainwater Harvesting Tanka constructed by Silveron at Girl’s School
(Storage Capacity of 100,000 liters water).

SILVERON did not build small prototype models but instead constructed big practical structures which are still operational. The rain water falling on the roof of the school/college building also finds its way into the tanks.

Apart from collecting rain water in the Tanka – Team Silveron also made provision of taking the over flow of the water from the Tanka (in the event of heavy rainfall) by a PVC pipe to a Ground Water Recharge Shaft near the school hand pump.

These two Tankas were built in educational institutions so that the students could see and understand all aspects of rain water harvesting from roof top catchment – channelizing the runoff – storage on surface – recharge into the ground.

Such old time water harvesting systems can still be seen along Naneghat in the Western Ghats. Every fort in the area had its own rain water harvesting and storage system that are still in use today. Forts like Raigad in Maharashtra and Jaigarh at Jaipur near Amber have tanks built in their courtyards that collected and provided water.

Seeing the Tanka from the Thar Desert of Rajasthan, India to the Tanka at El Morro Fort, old San Juan, Puerto Rico we must salute the spirit of our ancestors and attribute the Tanka as a rainwater harvesting technique ingenious to the men who desired to survive in the toughest conditions.

The rainwater storage system in the Castillo San Felipe del Morro Fort explained
The rainwater storage system in the Castillo San Felipe del Morro Fort explained

Rainwater can be a reliable source of water if collected from runoff areas such as roofs
and other surfaces and stored appropriately. If the catchment area is big, this system can provide huge quantity of good quality fresh potable water.

Water – Makes or Breaks Societies

The power of water is such that no nation or entity can fight it forever, and this is the main reason why water has always influenced the peace & stability on our planet. In the near future, the richest people and countries will be classified on the basis of the amount of water they own or control and not on the basis of the size of the land they
occupy, the industries they run or other assets that they possess.

Just as the energy from the Sun or nature’s forces like cyclones, hurricanes or earthquakes cannot be divided and are for all people to share and bear likewise the air, snowfall, rivers, rain and the groundwater are indivisible and its abundance or lack thereof are destined to be shared by all, be it for good or bad.

Human Right to Water
The Human Right to Water

The UN has declared access to clean drinking water as a basic human right. Any attempt to control water as a social, economic or political tool goes against the principles of humanity and encroaches upon fundamental human rights.

One individual or entity thus cannot be permitted to appropriate a resource that belongs to all, especially the most important resource for life – water.

Surface and groundwater are public resources which should be managed by public entities. Since, water is as shared resource for all people in a region it needs to be handled and allocated by public institutions and agencies of a government of the people. A government chosen by the people is finally responsible to look after the needs of the people.

When the control of water is designed to benefit individuals or companies, as the recent events in California have showcased, few become rich and in the clatter of their
riches, the voice of so many water-starved people goes unheard.

In order to maintain peace & stability in any country, the administrative authorities from the local communities in the villages right through to the state and national levels must ensure the safety of its water resources and stock. It is essential for the state to make clean water available to all its citizens, without jeopardizing the availability of this resource to future generations.

Just like the Police acts as a government’s primary agency for maintaining law & order and is tasked with apprehending criminals, the water management agencies bear the vital mandate of safeguarding and controlling access to our most important shared resource. And just like the Police does not to merely publish the annual statistics on crimes committed, the water management authorities cannot merely collect data or keep on only classifying locations with depleting water resources as critical.

The water management agencies have to maintain a strict vigil on the water table in a region. With the help of advanced telemetry, these agencies should monitor the monthly depletion of the water table and undertake aggressive and proactive actions to stop it.

Such proactive actions to reduce groundwater depletion include installing new artificial water recharge structures to restore more water than that being extracted. These actions additionally include constant silt removal efforts to prevent sedimentation from reducing water holding capacity of ponds, lakes and reservoirs.

Through such measures, the water management agencies can report to the public about their the availability of water and also help establish a proper balance between water extraction and renewal so that the water table is not allowed to decline.

One thing is for sure – the demand for water will never decline, rather it will continue to rise due to many reasons – the biggest of them being, rise in human population and increased livestock. This factor by itself requires expansion of agricultural activity,  additional infrastructure and other human activity – all of which call for consuming more and more water every year.

Understanding Rain Water Harvesting

Understanding the process of water withdrawal is the best way to understand rain water harvesting since both actions have exactly opposite impact on the ground water table.

A cone of depression is formed as soon as we start the submersible pump to extract ground water. When we stop extraction of water, the cone of depression is filled by ground water from surrounding soil to maintain a uniform water level. The net resultant effect of this action – decrease in ground water level since there was extraction from stored stock.

When the rain water is harvested into the ground, a water mount is formed in the ground. This water mount gradually dissipates supplying its water to the ground water. The net resultant effect of this action is increase in ground water level since it is addition of water to the existing stored stock.

When the rain water falls on the ground and enters the soil surface. This is known as infiltration. When water comes in contact with very dry soil it infiltrates very quickly to begin with. This is due to the affinity of soil particles towards water. The behavior is just like that of a thirsty man starting to drink water at a fast rate and slowing down as the thirst is quenched.

In this case as the soil becomes wet with water continues to move down due to the force of gravity acting on it and in the process wetting the soil further down. The infiltration rate also gradually decreases with time till it reaches a constant value

We at Silveron have experimented and experienced the effect on infiltration rate when the water is provided at lower rate as compared to sudden flooding. It is always our endeavor to ensure that the artificial recharge structures constructed by us for harvesting rain water do not impose over the natural infiltration processes.

We support the soil to continue with the infiltration at its own constant value and on passing forward the excess water for recharge to other Silveron recharge shafts at other locations, channelized through an underground network of pipes.

We support the free movement of water from a region where it has higher total potential to one of lower total potential. It may be the gravitational potential or/and metric potential due to the soil particles. It may be downward flow or horizontal flow or both.

  1. We are preventing wastage of rain water by spill over/runoff, since we are accepting delivery of the entire quantity of rain water available for recharge.
  2. We are not interfering with the natural dynamics of infiltration of water in the soil since we understand that none of the many factors which influence the shape of the infiltration functions can ever be controlled.
Inter-connecting Rainwater Harvesting Shafts
Inter-connecting Rain Water Harvesting Shafts
As seen from top of recharge shafts
Inter-connection as seen from top of recharge shafts

Wake Up and Act

No time for shortcuts

Acute thirst does not see quality of water & acute sleep does not wait for a cozy bed’  

Today the crisis of water scarcity is staring straight into our eyes and threatening the very existence of life on earth.

We all need to realize that water scarcity is no more a fashion topic for seminars and discussions followed by cocktails and dinners and media briefing at 5 star venues.

We must understand the gravity of the situation and act in all seriousness on the issue since water scarcity is a serious problem and calls for immediate intervention through hard & focused action at the ground level.

The web pages are filled with guidelines about roof top rain water harvesting ‘roof top harvesting’ is an incorrect terminology since roof top is only a catchment area and the collected water has still to be harvested. The web sites are filled with photos and pencil sketches of water being diverted from roof tops into collection barrels.

Rain water from roof top being collected in containers/drums.
Rain water from roof top being collected in containers and drums.

Can we or our livestock or our agriculture survive on rain water collected from roof top in PVC drums/barrels?

Falling rain drops are not selective and falls everywhere – on the roof as well as on the open roads and grounds. The total catchment area which is provided by all roofs added together is negligible as compared to the rest of the open areas.

It is in these open areas from where rain water gets an opportunity to runs off, leaving the land high and dry. The focus should be on stopping runoff and harvesting this huge quantity of available water.

As always some quantity of this rain water, percolates into the ground in the natural process and our efforts should be to facilitate the movement of additional quantity of rain water into the ground by artificially created rain water harvesting structures. The debate about legality and illegality of rain water harvesting becomes irrelevant by the very fact that no power on earth can control the natural recharge process of rain water or prevent rain water from harvesting into the ground in the natural process.

Further, ever since the existence of planet earth, the natural water cycle has resulted in rains and the rain water has been percolating into the ground since then and which stands even today as what we call ground water.

All this water has percolated down the soil in the natural process and filtered through the soil. We see no logic in first catching the rain water then filtering it /de-silting it and then harvesting it into the soil, when the soil itself is a very good filter.

We need to immediately start making wide spread rain water harvesting efforts on avery large scale, if we really want to stand up for not just facing but for solving the water scarcity problem for ever.

Waterlogging – Flooding – Water Harvesting

Any soil where the ground water table gets too high to permit any agricultural activity, can be classified as waterlogged soil.

Waterlogging is a result of excessive rainfall at a particular site:

  1. Where the soil is unable to store large quantity of water.
  2. Where the external drainage of rain water by way of runoff is poor.
  3. Where the internal drainage by way of movement of water within the soil is poor.

Waterlogging creates an ecological imbalance by adversely affecting the soil & plant relationship, since all plants require air, especially oxygen to a greater or lesser depth in the soil for growth.

The Waterlogged soil is nearly saturated with water most of the time such that the aeration is restricted and anaerobic conditions prevail. With this depletion of oxygen in root zone, the micro organisms which support plant growth are affected adversely and in turn the plant growth is restricted. Waterlogging reduces the temperature of the soil and increases dampness which disturbs the biological activity in the soil.

Waterlogging restricts all operations related to soil enrichment and soil development. In irrigated agricultural land, waterlogging is often accompanied by soil salinity as waterlogged soils prevent leaching of the salts imported by the irrigation water and the adverse effects are accelerated by the salts brought from lower parts of soil by the capillary water.

This increase in salinity not only interferes with the absorption of nutrients by the plant roots thereby damaging the plantation but also spoils the physical state of the soil by making it less permeable for water and more suited for runoff which in turn hurts the adjoining land and vegetation. Even fodder grown in such soil may cause diseases in live stock.

It will be interesting to note that in California farmers are working to restore groundwater by purposely flooding crops. This may be supported by the availability of soil which has good absorption capacity as well as proper external and internal drainage.

California farmers are pumping groundwater faster than it can be replenished. One farmer is spending millions of dollars trying to restore it by deliberately flooding his crops when there is water to spare. It’s caught the attention of other farmers, especially since new state laws could soon restrict groundwater use

Amy Quinton, NPR

We at Silveron would recommend development of small water harvesting structures as per Silveron design rather than flooding the entire field. It is our observation and experience which makes us to believe that flooding has its own negative impact on the soil. This may not be apparently visible in the initial years but in the long run flooding has a tendency to spoil the soil quality by consequently reducing the water absorption capacity of the soil.

Our best advice would always be to avoid flooding and instead create safe passage for water to get into the soil at various locations. There after give the water the freedom to move from particle to particle in direction of its choice with no fear of evaporation losses. Every drop which has been sent into the soil has to become a part of the ground water and has to benefit some one or the other.

We agree with every talk about climate change and reduction in rain/snowfall. One more aspect which is equally valid is that a decade ago the ground water table was at a much higher level and has now fallen down drastically due to over exploitation or any other reason. This clearly indicates that the soil had the capability and capacity to hold water so why not use this empty space and refill it with water. Even the soil will be happy to welcome and accommodate its long lost asset, water.

Standing water in a field
Natural performance of Rain Water Harvesting Shaft

The Silveron design of water harvesting provides absolute freedom to every available water molecule to recharges the soil from the root zone to the aquifer – where ever the soil welcomes it.

Water harvesting is the only solution to combat water scarcity be it anywhere in the world.