Analysis of Rainfall Variability and Frequencies for Sustainability of rain fed Agriculture Development in Abbay basin, Ethiopia

Abstract

The study provides the analysis of rainfall variability in the Blue Nile part of Ethiopia considering dry spell, drought and Intensity-Duration-Frequency (IDF) and their relevance to sustainability of surface water storage and rain fed Agriculture development. Standardized Precipitation Index (SPI) was selected to characterize drought condition. Miduss 2.27 software was used for development of Intensity-Duration-Frequency relationship and frequency analysis of dry spells. SPI requires only rainfall as input data for its calculation of different time scales (2, 3, 6 and 12 months) which has also a capability to provide five major drought characteristics including duration, intensity, severity, severity magnitude and frequency. The Miduss 2.27 software needs computed intensity of different duration and for dry spell frequency analysis it requires climate, soil type and crop type data of the study area. The analysis was made using data from 45 rain gauge stations located inside the study area for the years 1975 to 2008. Unreliable distribution and different amount ofrainfall is shown to be the cause of different intensity of drought. The deficient rain water during kiremt were 16.67-73.33% with 7-15 months (3-6years) spell and a probability of 0.1-0.2 occurrence in 1984, 1987, 1995 and 2002 was affected by drought. During the Belg for more than 70% of the study area was under deficient of rain water with probability of occurence 0.1-0.24 and spell of 4-15 out of 22-35 years. The years 1984 and 1987 were found severe drought years for both seasons for more than 73% of the area. The Belg rainfall was highly sensitive and variable than Kiremt and other time scales for the whole years. Map of SPI-JK (Kiremt) and SPI-3B (Belg) shows that a spatial extent of for all the principal stations and based on these distributions quantiles are estimated. With the help of the general mathematical forms of IDF, related Curves and Maps representing IDF were developed The analysis of dry spell was done to show the practical application of dry spell in Abbay basin based on eleven sites with three soil types and five crop types. The result indicates that 50% of the sites which include Assosa, Aykel, Dangla, B/dar, Nekemt, Adet & Bedelle require supplementary irrigation for optimum productivity, and the remaining parts do not require supplementary irrigation for kiremt season 25% of area, i.e. Gonder, Dangla, Bahirdar Nekernt, Adet, Debrebrhan and Bedelle can produce Teff without supplementary irrigation and 75% require supplementary irrigation. For barley crop 57% of stations covering Aykel, Gonder, Dangla, Bahirdar, Alernketema, Debrebrhan and Gimbi can't produce without supplementary irrigation and the remaining stations can produce (43%) without supplementary irrigation. Sorghum and wheat are possible without supplementary irrigation at stations Aykel, Assosa, Gonder and Gonder, Adet, Gimbi respectively. From the above results, one can conclude that ram fed productivity is affected by prolonged dry spells and drought, and increased productivity requires control and management of rainfall for provision as supplementary irrigation. Furthermore, realizing further productive potential of the area require further management of water for second season production. The results provide reliable and detail information for planners and decision makers to address the phenomenon with respect to current and historical perspective, by looking at