Analysis of Rainfall Onset, Cessation, Length of Growing Season and Dry Spell in Ada'a District, Central Rift Valley of Ethiopia

This study attempts to analyze seasonal rainfall variability in terms of its onset, end date, length of growing period. Two different methods were used. The first was done with INSTAT and the second with Hargreaves method. Hence, this study presents a detail account of the rainfall variability statistics with a view to achieving one specific objective for possible application to cropping system management in the study area. The results show that seasonal rainfall amount was found to decrease with significance for 42 years period (1970-2012) (P<0.05) while monthly rainfall statistics showed a high variation (CV oscillating between 80.6 and 34.4 % across months). Minimum and maximum temperatures showed an increasing trend at inter annual scale (0.21ºC and 0.14ºC per decade respectively). The onset window varies between June 9 (DOY 159) and July 5 (DOY 186) across methods. The probability of dry spells longer than 15 days remain 0 from the end of June till third dekad of August. While, the dry spell length probability of greater than 7 and 10 days curves converge to their minimum only during the peak rain period (June and August or DOY 180-222) and turn upward again around September (240-280 DOY), signaling the end of the growing season. Therefore, more attention is needed for planning of agricultural water for crop production. The introduction of different agronomic practices that prolongs soil moisture holding capacity and reduce evaporation loss could be used as management options in order to offset different magnitude of dry spell.