Breaks in monsoon and related precursors

A comprehensive analysis of eleven break monsoon situations that occurred during the period 1987 to 1997 have been attempted in the study. The various features like daily rainfall departures, wind anomalies and the satellite derived Outgoing Long wave Radiation (OLR) associated with the commencement/cessation of the break monsoon condition are studied with a view to identifying the precursors associate the break situation. The results reveal that there is progressive decrease of below normal rainfall departures 5 days prior to the actual break day in the latitude belts south of 20° N. During the period of the revival of the monsoon, the time section of the daily rainfall departures shows that the daily rainfall departure first starts becoming above normal in the southern most latitudinal belt 5° N to 10°N from the second day onwards after the cessation of the break. Similarly, the easterly anomalies in the zonal wind are first noticed in the southern latitude even 5 days prior to the starting of the break in the lower and middle troposphere. The maximum easterly anomalies in the lower and the middle troposphere move northwards upto 20° N. The composite latitudinal time section of OLR anomaly show a large area of negative OLR anomaly extending from 20°S to 10°N. The area is defined as the Southern. Hemispheric Convective Zone ( SHCZ). The negative OLR anomaly (10 Wm-2 is noticed around 5° S to 0° N. It increases to 20 Wm-2 on the second day of the break on the same latitudinal belt. The daily OLR anomaly pattern shows that the area of the negative OLR anomaly around the equatorial region increases with the approach of a break epoch. The forecasting aspects of the commencement / cessation of the break have been also discussed.

Study of rainfall departure over catchments of Bihar plains

Station wise daily rainfall data of sixty years is used to study rainfall departure and variability in Kosi, Kamala/Bagmati/Adhwara and Gandak/Burhi Gandak catchments during monsoon season. Station and catchment wise rainfall time series have been made to compute rainfall departure and Coefficient of Variation (CV). Southern Oscillation Index (SOI), Multivariate ENSO Index (MEI) and ENSO strength based on percentile analysis are used to ascertain their impact on rainfall distribution in the category as excess, normal, deficient and scanty. Results indicate that the variability is greater over Kosi as compared to the other catchments. Probability of normal rainfall is found 0.75 and there is no possibility of scanty rain over the catchments during El Nino and La Nina year. Similarly probabilities of normal, deficient, excess rainfall are found as 0.67, 0.18 and 0.15 respectively during mixed year. SOI has emerged as principal parameter which modifies the departure during El Nino and La Nina year. MEI along with ENSO strength are more prominent during mixed year particularly to ascertain deficient and excess rain in weak and strong- moderate La Nina respectively .

Drought severity assessment in south Bihar Agro-Climatic zone

The present study has been focused to recognize and quantify the drought condition in Bihar Agro-climatic Zone-III, which falls in the drought-prone region of Bihar. The zone covers the whole South Bihar region, comprising of seventeen districts. The drought severity assessment in the study area has been carried out by two methods i.e., the rainfall departure analysis and the probability analysis. The research work has been carried out at a grid level consisting of fifty-five grids. The analysis of work has been further evaluated district-wise and in addition the result has been interpreted for the two agro-climatic zones {zone-iii (a) and zone-iii (b)}. For decades, the area is under the influence of frequent droughts due to uneven distribution of rainfall. The grid forty-seven of Bhagalpur district having maximum drought frequency of once in 1.87 years and also faces a maximum number of sixty-two drought years in the period of 116 years, as resulted from rainfall departure method. The maximum annual rainfall departure of -82.99% was observed for the grid-32 (G-32) of Banka district, during the year 1978-79, with a return period of once in 2.7 years. The probability analysis concludes that the areas influenced by grids 37,42,45, and 47 are drought-prone (probability of 75% mean rainfall being less than 80%). The departure as well as the probability analysis indicates that the grid-47 as drought-prone. This shows the compatibility of the two methods in meteorological drought assessment in general and particularly the most drought severe condition of the grid-47.

Indicator-Based Assessment of Drought in Bhilwara District, Rajasthan

Management of water resources helps to sustain even in drastic conditions resulted due to unprecedented disruption in rainfall patterns. Change and irregularity in the pattern of the Indian monsoon are the outcomes of human-induced activities. Inadequate water availability affecting various sectors namely domestic, industrial, and agricultural sectors that are dependent upon it. To cope under such drastic conditions adaptability and planning prior to its occurrence plays a significant role. It is under this context, the present study investigated drought characteristics which include drought frequency and severity caused by prolonged dry spells in Bhilwara district, Rajasthan. The district falls in the water-scarce regions with arid to semi-arid conditions and with an average annual rainfall of 658.03 mm. Due to high rainfall variability, the region is frequently subsumed under drought-like conditions. Comprehensive analysis using daily rainfall data from 1973-2018 for 12 rain gauge stations in Bhilwara district has been carried out. Most of the stations were drought-prone assessed by probability analysis using Weibull’s plotting position formula. The departure analysis showed that Kotri station has a maximum drought frequency of 1 in 2 years while other stations were having a drought frequency of 1 in 3 to 4 years. Prioritization of drought-prone stations based on rainfall departure analysis helps to initiate an immediate mitigation process based on the ranking of its proneness. Asind and Bhilwara station with RDI 0.91 has maximum proneness and its calls for effective planning for drought management. Based on seasonal departure analysis it is found that 1980, 1981, 1985, 1987, 2000, 2002, 2008, 2015, and 2017 are drought years. The result has been supplemented using NDVI so that the vegetation condition can also be assessed during water stress conditions. The study highlighted that the frequency of drought has increased owing due to climate change and thus, poses serious challenges if not tackle adequately.

Identification of Meteorological Drought Characteristics and Drought Year Based on Rainfall Departure Analysis

Dholpur district is located in the semi-arid regions of Rajasthan. The region is affected due to unreliable rainfall pattern, which resulted in the development of a drought-like situation. The mean annual rainfall in the district is 596.76 mm. The present study has been conducted to identify drought-prone station, drought year and drought characteristics. Departure analysis of seasonal and annual rainfall helps to determine drought year and drought characteristics. The study shows that the region in under widespread drought condition in 1986, 1987, 1989, 1991, 2002, 2006, and 2007. The drought severity in the region was classified into four categories, namely extreme, severe, moderate and mild. Maximum extreme drought events occur in Baseri and Dholpur station. Urmilsagar station faced severe drought events 4 times followed by Rajakhera station with three times. Occurrences of moderate and mild drought events are frequent in the region. All the stations are drought-prone based on probability analysis of annual rainfall. Relative departure index (RDI) has been used to decide the relative drought proneness of the various station and based on which Dholpur station (RDI= 1.04) followed by Baseri station (RDI=1.0) are relatively higher drought-prone. The region faces natural water scarcity and droughts. Therefore effort is needed for drought preparedness, mitigation and management measures.