Rainfall pattern and moisture availability index in relation to rice (Oryza sativa L.) crop planning in eastern plateau region of India P. BANIK * and R.C. SHARMA Agricultural and Ecological Research Unit, Indian Statistical Institute, 203, Barrackpore Trunk Road, Kolkata 700 108. * Corresponding Author (e-mail pbanik@isical.ac.in) ABSTRACT Studies were undertaken to identify the quantum and distribution of rainfall with the frequency of dry spell occurrence during monsoon at Giridih, Jharkhand state, India. Rainfall at different confidence levels was calculated using mixed gamma distribution. The normal onset of monsoon at Giridih was 24 th standard meteorological week (SMW) and the mean monsoon rainfall was 1112 mm. If monsoon onsets two weeks earlier (22 nd SMW) than the normal (24 th SMW) the total monsoonal rainfall was more than the normal with increased number of dry spells. The co-efficient of variation of June and September rainfall was very high. Moisture availability index (MAI) indicated possibility of rice cultivation from 25 th SMW and the flowering stage of rice should be completed within 39 th SMW (normal withdraw of monsoon). Thus to minimize crop failure, conventional cultivation of 135 ± 10 days rice could be replaced by 95 ± 10 days one, particularly in upland (without bund; direct seeded rice) and medium land (low bund land) situation. Key words: Monsoon rainfall, mixed gamma distribution, potential evapotranspiration, moisture availability index, rice. Journal of Agrometeorology 11 (1): 54-58 (June 2009) MATERIALS AND METHODS Daily rainfall data were collected from Damodar Valley Corporation and Indian Statistical Institute, Giridih, Jharkhand (previously in Bihar state) for the years 1969 to 2004 (35 years). Rainfall was measured with IMD specified manual and automatic rain gauges. To study the characteristics of monsoon, the individual years were grouped according to the weeks of commencement of rainy season. Onset of monsoon during different years were identified as the week which received more than 20 mm of rain in 1 or 2 consecutive days, provided that the probability of at least 10 mm of rain in the subsequent week is more than 0.7 (Virmani, 1975). Similarly, the date of withdrawal of monsoon is defined as the date up to which a rainfall of at least 30 mm has been received in a week with no subsequent rainfall for at least 3 consecutive weeks towards the end the monsoon season (Shanker et al., 1992). Total rainfall for standard meteorological weeks (SMW) was computed for all years. The mean, mode and median dates, standard deviation and co-efficient of variation regarding onset, withdrawal and duration of southwest monsoon were calculated as advocated by Chandel (1978). Rainfall at different confidences was done using mixed Gamma distribution (Shanker et al., 1982). The equations used for computation of minimum assured rainfall are mixed Gamma distribution: G(X) = q + pF(X) …………………….(1) Agriculture in the eastern plateau covering total area of Jharkhand and parts of West Bengal and Orissa is characterized by rainfed low productive mono-cropped rice dominated system. The climate here is sub-humid, sub- tropical with hot and dry summer as also with very few winter showers. The amount of total rainfall ranges from 900 - 1500 mm. The normal annual rainfall is 1372 mm, out of which 85 to 90 per cent is received in monsoon months (Sen et al., 1984). Even in monsoon months (June to September), the crops are subjected to moisture stress due to occasional dry spells. Although the rainfall is high, the distribution is erratic making the crop vulnerable even during monsoon. The farmer or agricultural planner must know the risk he is going to take in his endeavor. The study by using average or normal rainfall cannot include this risk factor. In rainfed and low productive eastern plateau region, rainfall being the only water resource, agricultural planning should be based on potentially high rainwater utilization technologies. This in turn calls for better understanding of rainfall probability characteristics to fit agro-climatologically suited cropping systems (Singandhupe et al., 2000). The scientific analysis of availability and water balance for a given location may help farmers in preparing their crop calendar and improving rice productivity and cropping intensity (Tomar, 2002). Again Kharif rice being the dominant one knowledge of rainfall pattern can be effective in crop/variety choice with less risk of failure in monsoon season. With this in view the study was undertaken to identify the quantum and distribution of rainfall with the frequency of dry spell occurrence during monsoon.