Dry and Wet Spell Analysis and Moisture Adequacy Index (MAI) Estimation for Assessing the Agro Climatic Potentiality for Crop Planning in the Central Brahmaputra Valley Zone (CBVZ) of Assam, India

Analysis of rainfall and other weather parameters helps to develop and modify the management practices for stabilizing crop production and crop planning in the rainfed ecosystem at certain level. Lack of knowledge on wet and dry spell hampers scientific crop planning in the region. The present study has been undertaken with the objectives to investigate probable occurrence of dry and wet spell and to find out the Moisture Adequacy Index (MAI) for the districts of Nagaon and Morigaon under Central Brahmaputra Valley Zone (CBVZ) of Assam. In this study, Markov Chain model was used to estimate the initial, conditional probabilities of dry and wet weeks along with the probability of two and three consecutive wet and dry weeks considering 20 mm rainfall as threshold limit. MAI was calculated using Thornthwaite’s (1955) soil water balance method. The average rainfall of Nagaon and Morigaon were found to be 1775.27 mm and 1734.37 mm respectively. The study indicated that in CBVZ, probability of occurrence of dry week is higher from 1st week to 14th week. The range of probability varies from 56.7 per cent to 100 per cent during this period. The period from 25th week to 30th week is best suited for transplanting of sali rice with 100 per cent probability of wet spell and also continuously high MAI value of 1.0 throughout the period. The MAI value increases from 15th week with onset of pre-monsoon and ranges between 0.9-1.0 throughout the kharif period and then decreases with withdrawal of rainfall. Based on MAI values, the area is suitable for growing of a third crop using residual soil moisture. There is an ample scope for water harvesting from July to September, which can be utilized as crop saving irrigation as well as pre-sowing irrigation for succeeding rabi crops, which are generally sown on residual soil moisture. The results through analysis have been used for agricultural planning at CBVZ region.