Dry and wet spell probabilities and rainy season characteristics for crop planning in Gwalior district of Madhya Pradesh

Daily rainfall data of 33 years (1980–2012) of Gwalior were used to find the probabilities of occurrence of dry and wet spells, onset and withdrawal of rainy season. If the dry spell coincides with critical moisture requiring stages of the crop, it is damaging whereas during ripening stage, it may be beneficial. On the other hand, if wet spell coincides with the critical moisture requiring stages of the crop, it will be most beneficial for increasing productivity. In order to analyse the probability of occurrence of certain amount of rainfall, the standard 7 day period (weekly) has been taken to establish the dry and wet spell frequency based on Markov chain model considering a week receiving rainfall less than 20 mm as a dry period and 20 mm or more as a wet period. The consecutive dry weeks indicates for the need of supplemental irrigations and moisture conservation practices whereas, successive wet weeks give an idea of excessive runoff water availability for rainwater harvesting and to take up suitable measures to control soil erosion. Using this information in crop planning could be of great help in judicious and efficient use of rainwater, conservation of ground water and in increasing the productivity of crops. Higher values of coefficient of variation (CV) of weekly rainfall indicate the erratic distribution of rainfall. The CV was less than 150 per cent from 25^th to 37^th standard week, indicated that the rainfall is consistent during this period. The data on onset and withdrawal of rainy season indicated that the monsoon starts effectively from 27^th SMW (2–8 07) and remain active upto 40^th SMW (1–7 Oct). Therefore, the mean length of the rainy season was found to be 13 weeks (91 days). The probability of occurrence of wet week could be expected from 27^th -37^th SMW with 50 per cent probability. During rainy season, the probability of wet week preceded by another wet week was more than 75 per cent from 29^th -30^th SMW and 32^th SMW. There are more than 50 per cent chances that 2 consecutive dry weeks may occur during 22^nd-24^th SMW and 38^th-40^th SMW. On the other hand, probability of 2 consecutive wet weeks was found more than 50 per cent during 29^th SMW and 33 SMW.