Abstract

Rainfall of Pusa, Purnia, Sabour and Gaya representing different Agro-ecological zones (Zone I, II, IIIA, IIIB respectively) of Bihar were analyzed on monthly, seasonal and annual time scales using long period data. Mann-Kendall non-parametric test was employed for observational trend detection of monthly, seasonal and annual precipitation for different period viz. 1971-1980 (DP1), 1981-1990 (DP2), 1991-2000 (DP3), 2001-2010 (DP4) and for long period 1971-2010 (LP). Zone IIIA shows significant decreasing trend of 42.85 mm/year annual rainfall in recent decadal period (DP4). Zone III B shows significant decreasing trend of annual rainfall during long period (LP) but there is increasing trend of 4.57 mm/year during recent decadal period (DP4) and 0.43 mm/year during long period (LP) in pre monsoon/summer season. August and September months show significant decreasing trend of 2.12 and 2.62 mm/year rainfall respectively in LP in Zone III B, which may affect the transplanting/vegetative phase of the rice crop. Decreasing trend of rainfall in post monsoon season in all the zones during recent decadal period (DP4) as well as LP is delaying/affecting the sowing of wheat crop.

Key words: Rainfall, trend, Mann-Kendall test, least-squares, periods, agro ecological zones.