Development of Rainfall Intensity-Duration-Frequency (IDF) Curve under Changing Climate: Case study in Omo-Gibe Basin, Ethiopia

Abstract

One means of adaptation to climate change is through assessment of rainfall Intensity-DurationFrequency (IDF) curve developed from historical datasets. However, the representativeness of IDF curves for the future time period remains to be the most challenging issue in view of climate change. IDF curves are commonly developed either at national or basin level in a given watershed. Such curves are not readily available for Omo-Gibe River Basin. Thus, the aim of this research is to evaluate the impact of climate change on rainfall IDF relation in Omo-Gibe River Basin. Long term historic daily rainfall time series data of 34 year (1980-2013) of 18 stations were used as an input. Other geospatial datasets like DEM, land use land cover and soil data were utilized in this study as a proxy inputs. In order to evaluate the climate change impact, the most recent data from Coordinated Regional Climate Downscaling Experiment (CORDEX) data of the fifth Intergovernmental Panel on Climate Change scenarios were used as additional source of data. In order to overcome missing data issues of observation, nearest neighbor (NN) method of interpolation was applied. The completed datasets were further passed through rigorous quality controlling process including data consistency check and outlier tests. The CORDEX data were thoroughly evaluated and appropriate bias correction using power transformation method was undertaken. The bias correction was implemented between historical run of CORDEX data of the same period with observed data. Trend analysis was also carried out for observed data; and future projection was made for a mid-21st century. Extrapolation of values for larger return periods from extracted extreme data were undertaken based on the well fitted empirical distribution function and disaggregation was made to get the required short duration rainfall data. Accordingly, the log Pearson type-III, Gumbel's and Log Normal distributions were applied to all stations. Goodness of fit test using Kolmogorov-Smirnov (K-S), Anderson-Darling (A-D) and Chi-square confirmed the appropriateness of the selected distribution. The spatial distribution of rainfall over the basin is not uniform and varied with topography. According to output from 42 stations, the central western part of the basin received mean annual rainfall as high as up to 2000 mm; whereas, the southern low land region received mean annual rainfall below 675mm. Historical rainfall examination of annual and annual maximum had shown declining trends when averaged over the total basin. However, rainfall projection from 18 stations for (2040-2069) from RCP 2.6, RCP 4.5, RCP 8.5 scenarios has shown insignificant decline with annual mean of (968.9-1675.6); (973.6-1784) and 935.9-vi 1868.5) mm respectively; while for baseline period (1046.1-1891.2)mm. From comparative evaluation, it is found that projected mean monthly rainfall pattern over the basin doesn't show much difference from the observed data. However, there is a probable expectation for a little bit decrements of rainfall in months of December-March over the majority of the basin. Besides, under changing climate conditions; RCP 2.6 scenario projection shows the difference falls in (40-73) %; and (31-41) % for RCP 4.5 scenario projection, which is moderate except Bonga and Wolaita Sodo stations where the changes is non-significant. Furthermore, RCP 8.5projection scenario has shown a substantial change (21-57) % except a tremendous change as high as up to its double value particularly in south western part of the basin. Therefore, it is a best to consider the changing climate condition during design of infrastructures over the basin than the conventional ways.