Long-term trends and probability of rainfall for crop planning in Bihar

Around 60% of the Indian agriculture is raindependent. Rainfall is the single most important factor in crop production planning in rainfed ecologies. The information on annual, seasonal and weekly rainfall of a region is useful to design water harvesting structure for agricultural operations, field preparation, sowing, irrigation, fertilizer application and overall in field crop planning (Sharma et al, 1979, Singh et al., 2008). Historically in Bihar, zone I and zone II are receiving too much rainfall to cause flood in the region whereas zone III B is characterized as rainfed region with low, erratic and uncertain rainfall pattern with frequent dry spells during the monsoon season. Hence monsoon cropping is tricky operation in the region as well as sudden crop failures during the kharif season is a common phenomenon due to early withdrawal of monsoon. Rainfall probability pattern has been studied by many scientists in India (Hundal and Kaur, 2002; Suchit and Singh, 2009; Ravindrababu et al., 2010) and conclude that rainfall occurrence is certain at greater than or equal to 80 % probability, while 50% probability is the medium limit of certainty and may involve dry spell risk. Taking into account these climatic and probability factor, the study was conducted for four different locations situated in different agro climatic zones of Bihar for interlinking the rainfall probability with the crop planning pattern in the region.

Around 60% of the Indian agriculture is raindependent. Rainfall is the single most important factor in crop production planning in rainfed ecologies. The information on annual, seasonal and weekly rainfall of a region is useful to design water harvesting structure for agricultural operations, field preparation, sowing, irrigation, fertilizer application and overall in field crop planning (Sharma et al, 1979, Singh et al., 2008. Historically in Bihar, zone I and zone II are receiving too much rainfall to cause flood in the region whereas zone III B is characterized as rainfed region with low, erratic and uncertain rainfall pattern with frequent dry spells during the monsoon season. Hence monsoon cropping is tricky operation in the region as well as sudden crop failures during the kharif season is a common phenomenon due to early withdrawal of monsoon. Rainfall probability pattern has been studied by many scientists in India (Hundal and Kaur, 2002;Suchit and Singh, 2009;Ravindrababu et al., 2010) and conclude that rainfall occurrence is certain at greater than or equal to 80 % probability, while 50% probability is the medium limit of certainty and may involve dry spell risk. Taking into account these climatic and probability factor, the study was conducted for four different locations situated in different agro climatic zones of Bihar for interlinking the rainfall probability with the crop planning pattern in the region.

Trend analysis
The average annual rainfall of Pusa (zone I), Purnea (zone II), Sabour (zone III A) and Patna (zone III B) is 1246.9 mm, 1466.7 mm, 1231.4 mm and 1031.0 mm respectively ( Table 1). The long-term annual rainfall of Patna (zone III B) shows significant decreasing trend (Table 1). In Pusa (zone I), it also shows decreasing trend but statistically non significant. But there is increasing trend of annual rainfall in Purnea (zone II) and Sabour (zone III A), though statistically it is not significant.
There is a decreasing trend of winter rainfall in all the zones though it is statistically not significant. There is significant increasing trend of pre monsoon rainfall during long term in Patna (zone III B). In pre monsoon season, there is increasing trend of rainfall for all the zones except zone II, though it is statistically not significant. But there is decreasing trend of monsoon and post monsoon rainfall, in Pusa (zone I) and Patna (zone III B) though it is not statistically significant.

Expected rainfall amount and crop planning
As discussed above, rainfall at 75% and 90% probability is assured rainfall and at 50% probability is the median limit for taking risk. The minimum weekly rainfall amount expected at 50, 75 and 90% probability level is presented for Pusa, Purnea, Sabour and Patna in Fig 1. At Pusa, the probability of more than 30 mm rainfall from SMW 25 th is above 50 %, but there is some risk. From 27 th SMW, there is probability of more than 75% of getting rainfall of more than 15 mm and farmers can initiate their field preparation operations and from 28 th week expected rain becomes more than 20 mm, 27 th and 28 th week are an ideal time for sowing/transplanting of kharif crop and also for the crop fertilization based upon the rainfall pattern and intensity. In Purnea, the probability of more than 20 mm rainfall from SMW 25 th is above 75 % and more than 10 mm rainfall is above 90%. So, from SMW 25 th , sowing of kharif crop can be started in the region. In Department of Agronomy, Bihar Agricultural University, Sabour-813210, Bihar *E-mail: iitsunil@gmail.com SUNIL and SUJEET [Vol. 15, No. 1 SHORT NOTE_4_PAPER 22  Fig 1: Expected weekly rainfall amount (mm) at different probability level (%) in four districts representi ng different zones of Bihar 28 th and 29 th week probability of receiving more than 30 mm rainfall is more than 75% which is sufficient amount of rainfall for transplanting of rice in the region. In Sabour and Patna region, the probability of more than 10 mm rainfall is above 75 % from 25 th and 26 th SMW respectively. The probability of receiving more than 20 mm rainfall in the regions are from 28 th and 29 th week respectively in which transplanting of rice can be completed.

Long-term trends and probability of rainfall for crop planning in Bihar SUNIL KUMAR * and SUJEET KUMAR
Major crop of the region is rice, which is highly dependent on monsoon rainfall for nursery bed preparation, transplanting and maintenance of water in the field. Analysis indicates the need for selection of crops according to the probability of getting wet weeks preceded by wet weeks with the onset of southwest monsoon in the region. Timely monsoon will favour selection of long duration and high water requiring crops like rice. Late monsoon will lead to selection of crops with medium duration and moisture stress tolerant crops like ragi, finger millet and sorghum, in the moderate rainfall districts, where erratic monsoon behavior is observed. Pulses like green gram and black gram, need to be selected often as intercropping based on varied rainfall situation. Direct seeded rice can be adopted in that situation. Analysis reveals that past rainfall record may be handy tool for future rainfall probability projections.