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Watershed , Patna District, Bihar
Background Information
The watershed enjoys a typical subtropical climate. The winter season starts from the month of October and continues up to February. Summer season starts from April and continues up to mid–June. The rainy season lasts from mid–June to the end of September, which receives the south-west monsoon, accounting for about 90% of the total rainfall. The area receives an average normal monsoon rainfall of about 1100 mm/year.
The area represents a monotonously flat topography. The topographical variation within the area indicates a general slope from south-west to north-east with minor variations. The elevation of the area varies from about 42 to 55m amsl.
The area is drained by the mighty River Ganga forming the northern boundary of the project area. Other rivers around the area are Sone, Punpun and their tributaries. The River Ganga forms the levee or upland all along its southern bank.
The area forms a part of the Sone megafan and the active channel of Sone river is situated at its western side. At least 5 palaeochannels of Sone traverse the area. Unlike the active channel, the palaeochannels of Sone typically display moderate meanderings and migration at few stretches. The active East Patna Fault (EPF) lies immediately east of the area and it is suspected that another active fault lineament lies along Bishunpur - Khagaul within the area. The uplift along EPF and subsidence at the north/north western part have induced avulsions in the Sone channel in discrete steps westwards.
Within the entire area of concern, not a single stream flows northward or westward to join either Ganga or Sone. The remnants of Sone palaeochannels in patches presently serve as ‘pynes’, which are channels led off from the rivers to transmit water to the fields. Depressions in the palaeochannels still form temporary water bodies.
Figure 1.5: Cut-off water body and the levee along the paleochannel.
Along the major paleo-courses of the Sone river and the Ganges river, detached levees and well-spread back-swamp areas are observed with the help of satellite data. The levees are generally well-drained more gravelly and coarse grained flood deposits whereas the back-swamp areas are underlain by mostly thick clayey and silty deposits.
Figure 1.6: Back swamp area and levee along the Ganga river.
The Ganga River, which has entrenched on to the Sone megafan, flows along the northern boundary of the area. From Bahura, where Sone joins Ganga, up to Danapur, channels stretches are alternatively occupied by Sone and Ganga, e.g. - the Sone River indicated in the Survey of India topographical map (1976) has presently been occupied by Ganga River. Slope disruption in the Ganga River channel and its multiple channeling due to uplift along the Bishunpur - Khagaul Fault has created a large broad island on Ganga.
Soils of the area are predominantly sandy loam with clay loam at places, having low to medium nutrients status. It is generally alkaline with pH values ranging from 6.3 to 8.2. Traditionally, soils in the area are classified on the basis of mode of deposition and have accordingly been divided into two groups viz. (i) Recent alluvium and (ii) Older alluvium.
Land Use
The western part of city of Patna, the capital of Bihar, extends over this watershed. The landscape and land use of this watershed is fast changing as the expansion of the city is bringing newer areas under urban land use.
Agriculture and groundwater use
Agriculture in the area is mainly rain-fed. In some areas, winter crops are also grown using groundwater, drawn mainly through shallow tube wells of 20 to 60m depth.
The area forms a part of the Gangetic plains underlain by immensely thick alluvial deposits. It holds about 700 m thick pile of alluvial sediments of Quaternary Age comprising various grades of clay, silt and sand. A thick clay layer commences from the surface which gets mixed with sand towards southern and western parts.
The stratigraphic sequence of the geological formation in Patna district is as follows:
Age Formation
Quaternary (Pleistocene to Recent) Newer Alluvium
  Older Alluvium
Achaean Crystallines-Granites and Gneisses Quartzites etc.
Newer alluvium
The Newer alluvium is light coloured and poor in calcareous matter. It contains lenticular beds of sand and gravel and peat beds. It is morpho-stratigraphically classified in two categories:
Diara formation/ Channel alluvium of Recent Age.
Vaishali/ Fathua/ Terrace alluvium of Middle to Upper Holocene Age.
Older alluvium
The Older alluvium (called Bhangar in the Ganges valley) forms slightly elevated terraces, generally above the flood level. This is dark coloured and in general it is rich in concretion and nodules of impure calcium carbonate known as kankar. These kankars are of different shape and sizes.
Archaean Formation
The granites, gneisses and quartzites of Archaean age occurring in the southern districts of Bihar state namely Munger, Nalanda and Gaya, seem to extend northward up to the Ganga in Patna district and beyond and are overlain unconformably by the Quaternary deposits as revealed by seismic refraction study by Geological Survey of India. The entire alluvial thickness overlying the Archaean basement in the area is believed to be more than 700 m.
Figure 1.7: Morphostratigraphic map of the watershed.