Analysis of trends in rainfall and dry/wet years over a century in the Eastern Ganga Canal command

Abstract

In the present study, an attempt has been made to analyse the variability and trends in annual and seasonal rainfall over the Eastern Ganga Canal command area, India. To determine the trend in annual/seasonal rainfall and rainy days, the non‐parametric Mann−Kendall test and Sen's slope method were used over daily gridded rainfall for the time period 1901–2012. The results of the trend analysis revealed a decreasing trend in annual and monsoon rainfall in the range 18.5−61.7 and 19.4−56.7 mm/decade, respectively. For annual and monsoon rainy days, the results demonstrated a decreasing trend which varied from 1.8 to 2.9 and 1.4 to 2.2 days/decade, respectively, and an increasing trend in the range 0.2−1.4 and 1.0 days/decade, respectively. A variability analysis was performed to highlight the need for selecting an appropriate confidence level in trend analysis. It was observed that the grids showing a statistically non‐significant trend at the 95% confidence level exhibit a statistically significant trend at lower confidence levels. An analysis of dry and wet years on an annual scale revealed that the chances of occurrence of a wet year are more in comparison with the chances of occurrence of a dry year in the command area by a ratio of 14:13. However, data from the last one and a half decades has a contradictory trend with higher chances of occurrence of a dry year to very minimal chances of occurrence of a wet year. Dependable rainfall at various probability levels was estimated using a gamma distribution function. It was observed that the 75% dependable weighted annual and seasonal rainfall over the command area (985.48 and 727.54 mm) was significantly different with respect to the climatic mean weighted annual and seasonal rainfall against the weighted mean annual and seasonal rainfall (1,165.93 and 901.95 mm).