Development of Rainfall Intensity Duration Frequency Curves for the City of Addis Ababa under the Changing Climate

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Addis Ababa University


Hydrologic design of water management infrastructures is on the basis of specific design storms derived from historical rainfall events available in the form of intensity-duration-frequency (IDF) curves. However, it is expected that the frequency and magnitude of future extreme rainfalls will change due to the increase in greenhouse gas concentrations in Earth’s atmosphere. Developing intensity, duration, and frequency (IDF) of rainfall with the reflection of future climate change is required for designing and managing water infrastructures. Thus, this study presents a rainfall Intensity-Duration-Frequency (IDF) curves for Addis Ababa city under base and climate change scenarios. For this purpose, historical daily rainfall data was collected from Ethiopian National Meteorological Service Agency and bias-corrected CORDEX rainfall data for RCP 8.5 climate scenario was used. Appropriate methods for filling of rainfall data gaps and calculating areal rainfall were employed. Rainfall data tests for independence, stationarity, homogeneity, trend and outliers were also conducted using relevant methods. The data were found to be independent, stationary, homogenous with no trend and outliers. By using EasyFit computer program, the probability distribution that best fits the historical and future rainfall data was selected The Generalized Extreme Value (GEV) distribution was found to be the best fit distribution. Daily historically rainfall and climate scenario data were generated and disaggregated in to hourly basis using the method indicated in ERA Drainage Manual. Expected rainfall quantiles (XT) for 10min up to 180min durations were computed for return periods of 2, 5, 10, 25, 50, and 100 years using general extreme value (GEV) frequency analysis. Future Rainfall intensities under climate change scenario RCP8.5 were found to be greater than the historical rainfall intensities by 27%–79% in the period2010-2039; 37%-88% in 2040-2069; and 55%-104% in 2069-2099.; Hence, intense rainfall events are expected in the future and design of stormwater management systems should take this into account.



Intensity Duration Frequency Curves, RCP8.5, Climate change, extreme rainfall events, Addis Ababa