Rainfall analysis in relation to rice crop for Jaintia Hills district of Meghalaya

The long term rainfall data for Sali rice season (June-November) of Jaintia Hills district, Meghalaya has been analyzed to estimate expected weekly rainfall at various probability levels. Based on expected rainfall at 50 and 75% probability levels and water requirement, a crop calendar for Sali rice has been prepared for the district. The calendar is assumed to be applicable for the whole district irrespective of terrain differences as rice is mostly grown in comparatively plain lands in bunded condition. 22 nd to 24 th meteorological weeks have been suggested best for sowing/transplanting to avoid any kind of water stress during the critical growth periods. Amount of water required to maintain at least 5 cm of standing water in the field up to the dough stage has also been clacilated.

Jaintia Hills district is a major rice-producing belt in the state of Meghalaya. It forms the eastern part of Meghalaya and covers about 4000 sq. km area and situated between latitude of 25°5' N to 25°4' N and longitude of 91°51' E to 92°45' E. Three types of rice are grown in the district namely Sali, Ahu and Boro rice. Both area and production of Sali rice are much higher as compared to other two types of rice in the district (Anon., 2002). Sali rice is direct sown/ transplanted in the month of June-July and harvested in the month of November-December. Rice cultivation is mostly rainfed in nature and cultivated in the plateaus between the hills in bunded condition and hence, the effect of terrain difference or slope variation on soil moisture status may be neglected.
Although North Eastern region falls in very high rainfall zone with lowest rainfall variability, the amount of rainfall received over the years showed a decreasing trend with erratic distribution pattern probably under the influence of global climate change (Saikia et al., 2007). Information of expected rainfall during the crop season will help the farmer to take up cultural and management practices in time by minimizing associated risk factors, if any. Rainfall analysis in relation to paddy crop in coastal saline soils at Panvel, Maharastra has been done to evolve rainfall based cropping system to utilize the amount of received rainfall efficiently for improving crop production (Mahale and Dhane, 2003). Rana and Thakur (1998) have suggested rice based cropping pattern in Kulu valley, Himachal Pradesh considering the rainfall amount at different probability levels. Crop planning in relation to rainfall pattern in Paotna valley of Himachal Pradesh has been suggested by Rana and Chakor (1999). Gupta et al. (1975) suggested that the rainfall at 80% probability level can safely be taken as sure rainfall, while that of 50% is the medium limit for taking a dry risk. Considering decreasing pattern of rainfall over NE region this analysis is carried out to evolve a crop calendar for Sali rice at 75% probabilistic weekly rainfall for Jaintia Hills district, Meghalaya.

MATERIALS AND METHODS
The rainfall data of Jaintia Hills District, Meghalaya for Sali rice season, for the period of 1986-2004, has been collected from IMD, Pune. Probability analysis of weekly (Meteorological Standard Week) rainfall for the probability levels of 50, 60, 75 and 90%, respectively, have been done following 'Ranking order method' (Doorenbos and Pruitt, 1984) using the formula given below. This method assumes that rainfall is more or less normally distributed.   Where, n = number of records m = rank number In rainy season, pan evaporation can directly be multiplied with crop coefficients (Kc) to determine weekly crop water requirement (ETcrop). Rice crop coefficients were used for dry and wet seasons in 'light to moderate wind' conditions as suggested by Doorenbos and Pruitt (1977).
The water requirement to maintain 5 cm of standing water in the rice paddy fields, from transplanting to the dough stage, has been determined by adding 50 mm of water to the crop water requirement. It is done for both 50 and 75% of probabilities.
Based on water requirement and probabilistic rainfall at 50 and 75%, a Sali rice calendar has been prepared for Jaintia Hills District for optimization of cultural and management practices.

RESULTS AND DISCUSSION
Weekly rainfall at different probability levels (  In case of rice paddy, usually 5 cm of standing water is recommended up to the heading stage when the grains start hardening. This practice helps the crop to avoid any kind of water stress and thereby to meet the total water requirement in its growth and development periods. This task is a easy one in places with irrigation facilities. But in Meghalaya the extent of irrigation coverage is very poor and performance of rice crop is purely dependent on monsoon rainfall. During this season, due to abundance of rainwater, rice often avoids water stress and bunded condition ensures sufficient standing water during the lean periods. From the analysis (Table 2) it was found that at 50% probability level, weekly irrigation water requirement may vary between 0-50 mm between 22 nd (transplanting) and 36 th (dough stage) weeks to manage crop water requirement as well as 5 cm of standing water in the field. At 75% probability level the range is 30-60 mm for the same period.

Rice crop calendar
Weekly water requirement for Sali rice has been estimated and it was found to be between 89-94 mm per week from vegetative to fruit development stages (Table 2). During the active vegetative stage the water requirement is high owing to high temperature during late May and June. Pan evaporation shows a deep from 27 th week (1 st week of July) to 30 th Week (last week of July) due to continuous cloudiness and heavy rainfall during this period. But no change in water demand during this period has been observed. Again, in 31 st and 32 nd weeks we observe high values of pan evaporation with low probable rainfall amount at 75%. These 15 days is much of significance with respect to the panicle initiation (PI) stage of rice. Hence, planning should be made so that critical PI stage can avoid this low rainfall period. Again, 36 th week onward (1 st week of September) the expected rainfall at 75% probability starts decreasing with every succeeding week owing to start of retreating phenomenon of monsoon from Indian land mass.
Considering all of the above factors a crop calendar for Sali rice has been prepared at 75% probability level of rainfall (Table 3). It is suggested that most optimum period to transplant or direct seed the crop is in between 22-24 th weeks so as to avoid falling of PI stage during the lean period of 31-33 rd weeks. In any case, transplanting can be done up to 28 th week (Mid July) but due to reduced rainfall and lowering of air temperature during September and October may hamper proper fruit setting, development and yield. Irrigation facility should be built up to irrigate the crop at least during 31-33 rd weeks period, to meet the challenge of any eventual drought which will prove to be life saving.
Crop Calendar for rice will help in taking up 30.8, 9.2, 0.0, 0.0 0, 0, 0, 0 3.6, 0, 0, 0 0, 0, 0, 0 Rainfall analysis for rice crop planning timely management and cultural practices in relation to weather conditions as agriculture in Meghalaya is rainfed in nature. Cropping intensity can also be increased by incorporation of more crops in a year if crop calendar is followed properly. Appropriate irrigation facilities should be created to avoid any kind of uncertainties in relation to water availability under the changing scenario of global climate change.