Trends in the rainfall pattern over India
P. Guhathakurta and M. Rajeevan
National Climate Centre India Meteorological Department PUNE. INDIA 411005 email@example.com
Monthly, seasonal and annual rainfall time series of 36 meteorological sub-divisions of India were constructed using a fixed but a large network of about 1476 rain-gauge stations. These rainfall series are thus temporally as well as spatially homogenous. Trend analysis was carried out to examine the long-term trends in rainfall over different sub divisions. Also monthly contributions of each of the monsoon months to annual rainfall in each year were computed and the trend analysis was performed. It has been found that the contribution of June, July and September rainfall to annual rainfall is decreasing for few sub-divisions while contribution of August rainfall is increasing in few other subdivisions.
In the context of climate change, it is pertinent to ascertain whether the
characteristics of Indian summer monsoon also is changing. The Indian summer monsoon (June to September) rainfall is very crucial for the economic development, disaster management, hydrological planning for the country. Earlier, Mooley and Parthasarathy (1984), Parthasarathy et al. (1993), Parthasarathy et al. (1994), constructed all India rainfall series based on 306 uniformly distributed stations. They have also used area weighted method to calculate all India rainfall using rainfall data of the 306 districts outside the hilly regions like Jammu and Kashmir, Himachal Pradesh, Hills of west Uttar Pradesh, Sikkim and Arunachal Pradesh, Bay Islands and Arabian Sea Island. Presently this time series is updated by the Indian Institute of Tropical Meteorology, Pune (www.tropmet.res.in) and this rainfall time series was extensively used by many researchers. At present there are more than 500 districts in the country. Using only 306 raingauge stations, it may not be possible to represent all the districts and prepare district-wise rainfall climatology. All the districts are having geographical area more than 100 square km (except Andaman and Nicobar Islands). Only one station in the district may not produce reliable district rainfall climatology as rainfall is highly variable (WMO, 1983). Spatial homogeneity cannot be achieved with one or two stations in a district. Long term trends of Indian monsoon rainfall for the country as a whole as well as for smaller regions have been studied by several researchers. Most of the studies are based on the rainfall series constructed by Parthasarathy et al. (1994). They have found that the monsoon rainfall is without any trend and mainly random in nature over a long period of time, particularly on the all India time scale (Mooley and Parthasarathy, 1984). But on the spatial scale, existence of trends was noticed by Parthasarathy (1984) and Rupa Kumar et al. (1992). Parthasarathy (1984) found that the monsoon rainfall for the two subdivisions viz. sub-Himalayan West Bengal & Sikkim and the Bihar Plains are having decreasing trends while for the four sub-
divisions viz. Punjab, Konkan & Goa, West Madhya Pradesh and Telangana are having increasing trends. Using the network of 306 stations and for the period 18711984, Rupa Kumar et al. (1992) identified the areas having decreasing and increasing trends of monsoon rainfall. The past performances of the monsoon rainfall may give an indication of the future scenario. But in order to do so we should also understand the climatology in a better way. The construction of a homogeneous rainfall data series (spatially as well as temporally) was the first step in this study. India Meteorological Department (IMD) has a good network of rain gauge stations. From the vast data set archived at the National Data Centre, IMD, Pune, a network of 1476 rain-gauge stations was selected which have only 10% or less...