Temporal and spatial variability of rainfall distribution and evapotranspiration across altitudinal gradient in the Bilate River Watershed, Southern Ethiopia

Abstract

Rainfall and evapotranspiration are the two major climatic factors affecting agricultural production. This study examined the extent and nature of rainfall variability from measured data while estimation of evapotranspiration was made from recorded weather data. Analysis of rainfall variability is made by the rainfall anomaly index, coefficient of variance and precipitation concentration index. The FAO-56 reference ET (ETo) approach was used to determine the amount of evapotranspiration. Estimation of the onset, end of growing season and length of growing period was done using Instat software. The results show that mean annual rainfall of the upper (2307 m.a.s.l), middle (1772 m.a.s.l) and lower (1361 m.a.s.l) altitude zones of the watershed are in the order of 1100, 1070 and 785 mm with CV of 12, 15 and 17% respectively. There was a high temporal anomaly in rainfall between 1980 and 2013. The wettest years recorded Rainfall Anomaly Index of +5, +6 and +8 for stations in upper, middle and lower altitude zones respectively, where the driest year recorded value is -5 in all the stations. The average onset date of rainfall for the upper zone is April 3 ± 8 days, for the middle zone April 10 ± 10 days and for the lower zone is April 11 ± 11 days with CV of 23%, 26 and 29% respectively. The average end dates of the rainy season in the upper and middle zones are October 3 ± 5 days and September 25 ± 7 days with CV 5 and 7%. The main rainy season ends earlier in the lower zone; it is on July 12 ± 10 days with CV of 14%.

Keywords: Variability, days of the year (DOY), onset, end date, length of growing period (LGP)