The Iranian plateau, with mountains running north-west to south-east, a cold desert formed from a dried up sea, dense forests of oak, walnut, pistachio and valleys with figs, pomegranates, oranges, lemons and mulberries, is a region of contrasts. As forests gave way to pastures and later cultivation, it was seen that the ground of what is to a large extent, a cold desert, would yield abundantly wherever water was available. Wheat, barley, the opium poppy, cotton and tobacco grew and the country was artificially irrigated from pre historic times by a wide network of canals called Ghanat/ Qanat or kariz. A quanāt is a gently sloping underground channel with a series of vertical access shafts, similar to that of a well, used to transport water from an aquifer under a hill. Quanāts create a reliable supply of water for human settlements and irrigation in hot, arid and semi-arid climates.

Zoroastrians, since these ancient quanats, were well known for their ability in protecting water and their ecological consciousness, particularly in connection with water systems, as is seen in the Gabarbands of Central Asia. Continuity in this tradition is observed in the Tanka system of Gujarat. While Gujarat is famous for its step wells and baolis, these are systems of gathering water. The unique water harvesting system of the Tanka provides pure drinking water at Parsi homes in Bharuch, South Gujarat. On analysis, this water meets WHO Water Quality Standards of purity.

The Tankas, as part of the cultural heritage of the Zoroastrians living in an area which has scanty rainfall, are an ecologically valuable system of water harvesting and perhaps, the need of the hour. Essentially a form of rooftop rainwater harvesting, the Tanka system was practiced by the Parsis as almost a religious ritual, and stories are told of an obstinate Parsi from Jambusar village who died of thirst while travelling, rather than drink anything but Tanka water!

The Tankas of Bharuch are found all over town, across differing social strata. In the Parsi Vad, or in clusters of smaller homes, these were constructed within the kitchen area, while in larger homes there was a separate space for the Tanka. The largest Tanka found during Parzor's field recordings, is located at Mr. Rohinton Jambusarwala’s home ‘Gulshan’. It is 70 ft. x 20 ft. and 30 ft. deep. He explained how he adheres strictly to traditions handed down orally through several generations to ensure the purity of the water he collects.

When the rains begin at the start of the monsoon, the first rains clean the rooftops. During the start of the monsoon season, seven glasses of water kept outdoors are examined regularly for their purity. The day of the collection of rainwater can be recognized through the smell and taste of the water in these glasses. At an exact point of time, recognized through smell and taste, the water is deemed suitable for collection. The sluices are then opened; the water passes through seven types of filters, including a cork filtering system, before it is allowed into the Tanka below the house. Pipes lead from all parts of the roof to the final boiya a copper colander. This boiya is covered when the water is being collected, with layers of the same white muslin cloth used for making the sudreh.

Each Tanka has a small fish engraved on its upper side, as an indicator of the limit of its capacity; no water must be filled over this symbol. Architecturally, the Tanka withstands this huge volume of water because it is built using arches, recalling the fact that thousands of years ago the Persians invented the arch and squinch as well as water harvesting through the qanats and kariz systems.

Tankas in Vadis (Mansions) can be very big and those in the Parsi Vad (Mohallas) are usually smaller. Apart from providing drinking water, since the roof of the Tanka creates the floor of the house, they keep the building cool throughout the year. The Tankas, built without cement or concrete, have withstood devastating earthquakes in Gujarat for the past 300 years. Even when houses fell during the 1917 earthquake, Tankas, full of enormous volumes of water, remained unaffected. If sunlight falls directly on the stored water, it is believed that the water can become contaminated. Tankas are therefore carefully planned to prevent any type of contamination. No direct sunlight is allowed into the Tanka, and the well from which the water is drawn is vertical while the storage space is horizontal to it. The container or ghara, must be lowered very gently into the well; if it hits against the Tanka, there is the danger of a crack. The last Tanka to be built in Bharuch was over eighty years ago. Women of the community sang songs, danced and performed ceremonies, while a priest prayed over it in blessing. A Tanka is emptied during the monsoons, only about once in four to five years; the Gulshan Tanka, because of its size, is emptied once in ten years. Yet, the water remains pure and free of any harmful bacteria. When emptied, some water is stored in huge ceramic containers, because without the pressure of the water, there is a danger of the structure collapsing, if the monsoons are delayed for more than 15 days. When totally emptied, a crystalline disinfectant and sealing agent, khari, is used to seal up any cracks in the structure.