On Variability of Hydrological Wet season, seasonal rainfall and rainwater potential of the river basins of India (1813-2006)

Abstract

In tropical monsoonal climate rainfall is subject to strong seasonality. Further, attributes of rainy season (starting date, ending date and duration) exhibit large spatial and temporal variations. Numerous notions of rainy season exist in the real world and the literature, e.g. green season, growing season, wet season, monsoonal rainy season and wet period. For Indian region, a hydrological wet season (HWS) is defined as ‘continuous period with each of the monthly rainfall greater than 50mm’. Climatological and fluctuation features of important parameters of the HWS (starting and ending dates and duration, seasonal rainfall/rainwater and surplus rainfall/rainwater potential) are described for the 11 major and 36 minor river basins as well as the West Coast Drainage System (WCDS) and the whole country using highly quality-controlled monthly rainfall from well spread network of 316 raingauge stations from earliest available year (sometimes going back to 1813) up to 2006. The average period of the HWS is in close agreement with the (southwest) monsoonal rainy season (MRS) over the basins in the core monsoon rainfall regime (CMRR- basins between parallels of 16°N and 32°N and west of meridian 88°E). In the mixed monsoon rainfall regime (MMRR- south peninsula, northeast and extreme north), the HWS is of longer duration compared to the MRS due to extra-monsoonal rainfall over there. For the whole country, the mean starting date of the HWS is 30 May, ending 11 October and duration 135 days. Fluctuations of the HWS parameters do not show significant long term trend anywhere. However, nature of recent tendency according to a subjective deduction from a set of time series displays (actual and 9-point Gaussian low-pass filtered values and the Cramer’s tk-statistic for 31-term running means) is reported. Sixteen basins across the country experienced multiple wet seasons in some years, important characteristics of these wet seasons are discussed. The average HWS rainwater (and hydrological rainwater potential, HRWP) of the major basins (in billion cubic meters or bcm) is as: Indus 147.392 (34.884), Ganga 740.507 (316.383), Brahmaputra 430.168 (285.992), Godavari 282.584 (122.865), Krishna 180.95 (18.922), Sabarmati 19.06 (6.912), Mahi 28.537 (12.777), Narmada 87.532 (49.110), Tapi 45.646 (16.366), Mahanadi 173.181 (98.141), Cauvery 97.478 (27.046) and the WCDS 270.825 (179.399). For the whole country, the mean wet season rainwater is 2911.064 bcm, of which 1856.947 bcm evaporates and the remaining as surplus rainwater (or HRWP) 1054.117 bcm goes through different surface hydrological processes (soil moisture recharge, deep percolation, surface storage and streamflow/riverflow). Declining tendency is seen in the HWS rainfall/rainwater and surplus rainfall/HRWP over most of the minor basins in the CMRR in recent years/decades. Over the country, the HWS rainfall during 1965-2006 was less by 4.15% compared to the period 1912-1964, and the surplus rainfall/HRWP during 1978-2006 less by 11.93% compared to 1915-1977.