Rainwater harvesting/storage in an underground cistern locally known as Tanka is an age old common practice of western Rajasthan. Various types of tanka from rectangular to circular in shape and capacity ranging from as small as 1000 liters to 500,000 liters are prevalent in this region. The availability of water in these tankas was usually round the year for drinking purposes for the whole family. The traditional construction of tanka varies from simple mud plaster to lime mortar, however wide variations have been observed.
Central Arid Zone Research Institute, Jodhpur over a 4 decades research has perfected the technology of tanka construction for various types of users. CAZRI has developed improved design of tanka for capacity ranging from 5000 liters for individual family to 600,000 liters for community use. The most common construction material for improved tanka is stone masonry with cement plaster and cement concrete. However Ferro-cement and prefabricated PVC tanks are also used in some modern buildings in urban areas. In improved design of tanka provision has been made for silt trap at inlet to control inflow of silt in flowing runoff. Construction procedure has been improved for cost efficiency and longer life span. A tanka of 21 m capacity is sufficient to meet the drinking water requirement of a family of 6 persons for whole the year.
Steps for design and construction of Tanka
Estimate domestic water requirement of a family based on 7-10 liters of water per day per person.
Estimate water requirement for livestock (40 liters per day per animal).
Estimate other water requirements like nursery raising or life saving irrigation to nearby plants based on the number of plants and average daily water requirement of individual plant /tree species.
Determine total daily water requirement by adding water requirement as estimated above for domestic, livestock and other needs.
Determine annual water requirement by multiplying t he total daily water requirement as estimated above by 365 or no. of days for which water is required.
Estimate Tanka Capacity by multiplying annual water requirement as estimated above by a factor of 1.1 to take into account the possible seepage and evaporation losses.
If tanka capacity as estimated above is in liters, determine tanka capacity in cubic meter by dividing tanka capacity by 1000.
Determine annual rainfall at 60% probability for district particular.
Select suitable area preferably at lower side of field if it is to be constructed in field or near to the intended use.
Determine catchment area needed for 1 m3 runoff based on catchment conditions.
Determine total catchment area by multiplying the capacity of the tanka to the catchment area as determined above for 1 m3 runoff.
Ascertain whether required catchment area as estima ted above is available or not. If not, reduce the tanka capacity or improve t he catchment conditions.
For circular tanka, depth (or diameter) can by determined by D = (1.27 x V) 0.33 where D is diameter as well as depth in meters and V is capacity in cubic meters.
For rectangular tanka, decide any two dimensions fr om length, width and depth and determine the remaining third dimension by using the equation V = L x B x H where L, B, H and V are length (m), Width (m), depth (m) and Volume (m3 ) respectively.
Above calculated dimensions are inner dimensions of tanka. Add appropriate quantities keeping in view the construction type i. e. masonry or cement concrete. Add extra allowance of 5-10 cm for filling sand envelope around tanka.
First step for construction of tanka is excavation as per dimensions determined above. Excavated sand should not be dumped far away as same can be used for preparation of catchment.
Construction of tanka with local minor variation within limit is acceptable as per site specific needs.
Good quality construction material should be used in exact proportion and construction should be thoroughly supervised by a responsible person.
After construction, curing is very important for the strength and lifespan of the structure.
For proper maintenance of tanka, some quantity of water should always remain in the tanka.
Tanka catchment should be properly cleaned atleast once in year before monsoon.
Tanka should be desilted atleast once in a year before monsoon.
Tanka should be covered from top for the safety and economy of water.
Whenever there is any doubt on quality of water, it should be checked by nearest water quality testing centre and acted on as per advice.
A good tanka is a wealth for generations; its water should be used economically.