Rainfall Variability and Extreme Rainfall Events Over Jharkhand State

Abstract

Rainfall is the primary source of surface and groundwater recharge. The state receives 91% of its annual rainfall due to the southwest monsoon, which is its principal rainy season. The contribution of winter, Pre-Monsoon, and Post-Monsoon season’s rainfall amount to about 2%, 3%, and 4%, respectively, of the annual total rainfall. Therefore, the temporal and spatial distribution of rainfall plays a vital role not only in the agricultural community but also in water resources management. This study investigates the rainfall variability over Jharkhand by using past 116 years data from 1901 to 2016. The extreme annual one-day rainfall depths investigated by using normal frequency distribution. The period examined for extreme rainfall events is 1986–2016. The data showed that the annual daily maximum rainfall received at any time ranged between 34.2 mm (minimum) to 341.0 mm (maximum) over Jamshedpur, indicating a very large range of fluctuation during the period of study. The magnitude of one-day annual maximum rainfall corresponds to return period were investigated. The depth of maximum daily rainfall is found highest over Jamshedpur. The highest rainfall observed in a day is: 224.3 mm at Ranchi, 341.0 mm at Jamshedpur and 240.4 mm at Dalton-ganj during the study period. The return period of extremely heavy rainfall (greater than or equal to 204.5 mm) in a day is 6 years, and the probability of exceedance is found 18% at Jamshedpur. Heavy rainfall events with more 402than 64.5 mm rain in a day was examined by using past 31 years daily rainfall data of Ranchi, Jamshedpur, and Daltonganj stations. To study the presence of a trend in rainfall, a widely used nonparametric Mann-Kendall test is applied. The linear trend line suggests that there is a decreasing trend in seasonal rainfall. The value of Kendall Score is found –2.82, which reveals that the decreasing trend in seasonal rainfall over Jharkhand is significant. An increasing trend is found in annual heavy rainy days over Jamshedpur, and the value of Kendall test statistics Zs for Jamshedpur is +2.43, which reveals that the increasing trend in an annual number of heavy rainy days is significant at 5% significance level. However, no significant trend is found over Ranchi and Daltonganj.