Study of season onset based on water requirement assessment

Abstract

The criteria of season onset and withdrawal in Indonesia determined by Agency for Meteorology Climatology and Geophysics (BMKG) which need a month to confirm the period of the wet season and dry season. The determination of season onset based on the amount of 10-day rainfall equal to or more than 50 millimeters (mm) followed by the next two 10-day rainfall (the amount of monthly rainfall equal or more than 150 mm) for the wet season and vice versa. The determination of 150 mm/month based on water requirement using the reference of evapotranspiration (ETo) in the territory of Indonesia at 5 mm per day. This paper evaluates the determination of season onset by comparing it based on the ETo value of 5 mm/day and ETo from observation data of pan evaporation in the islands of Java and Bali in the period 2003 - 2017. The result of the study shows that the difference of the dry season onset up to 3 months which 25.9 % of stations have no difference, 40.7% have 1-month difference, 22.2% have 2 months difference and 11.1% have 3 months difference. The difference of wet season onset show 40.7% of stations have 1-month difference and 55.6% have no difference. The ETo value of 5 mm/day for criteria of season onset is appropriate and the difference of up to 3 months is still sufficient for early warning purpose.