Ambuliyar River

The drainage basin of the Ambuliyar is located roughly between the lines of longitude of 79° 0′ E and 79° 10′ E, and the lines of latitude of 10° 10′ N and 10° 20′ N, and covers 702.58 square kilometres (271.27 sq mi) of land in the Pudukkottai and Thanjavur districts of Tamil Nadu. The basin is bordered by the basin of the Vellar River to the south and west, the basin of the Agniyar River to the north, and the Palk Strait to the east.^[4] In 2001, about 307,000 people inhabited the basin,^[7] in which the largest settlement is Alangudi, which had a population of 12,367 people in 2011.^[2]

Alluvium and pebbly sandstone, along with sand near the coast, are the prominent geological deposits in the river basin. Denudational and fluvial landforms such as pediments, floodplain, and channel fill make up the majority of the central region of the basin. There is tertiary upland and cretaceous lowland in the northwestern part of the basin, and paleotidal flats in the southeastern coastal area. Most of the land in the basin is either barren or used for agriculture. The predominant soil types in the basin are alfisols and vertisols.^[7]

Mean annual rainfall in the watershed is 776 millimetres (30.6 in), with a recorded minimum of 156 millimetres (6.1 in) and recorded maximum of 1,108 millimetres (43.6 in). The large majority of this rainfall occurs during the wet season, between June and December. Precipitation levels during the dry season, which typically lasts between January and May, are greatly reduced. Because the reduced level of rainfall usually cannot match heavy irrigative demand, the river does not normally reach the sea during the first half of the year.^[6]

6 anicuts cross the river and divert water to 2534 storage tanks and retention basins, and also to numerous irrigation canals and ditches. Water from the Ambuliyar and its tributaries is used to irrigate 469.3 square kilometres (181.2 sq mi) of land, mostly for agriculture. However, despite this heavy demand and lack of flow during the dry season, a five-year study recorded an average discharge at the mouth of the river of 63,800,000 cubic metres (2.25×10⁹ cu ft) of water per year. This could be partly due to supplemental flow from the Kaveri River via the Grand Anicut canal, which stretches from the eastern end of Srirangam Island to where it intersects the course of the Vellar River. Besides meeting the Ambuliyar at two points, the canal also intersects its Vellunniyar and Maruthankudiyar tributaries, along with the nearby Agniyar and Maharajasamundram Rivers.^[7]