ANALYSIS OF RAINFALL INTENSITY DURATION FREQUENCY RELATIONSHIP FOR RWANDA

Abstract

IDF (Rainfall Intensity Duration Frequency) is one of the most commonly used tools in water resources engineering particularly for planning, designing and operating water resource project or running various project against flood. Rainfall IDF curves permit the estimation of return period of an observed rainfall event for specified duration. The present study is aimed at developing detailed rainfall IDF relationship for Rwanda based on selected twenty six (26) meteorological rainfall stations. The rainfall stations have been selected based on country representatively and reliable rainfall records varying from 14 years to 83 years of record. The temporal variability of annual maximum rainfall data for duration of 24 hours was further investigated using Mann-Kendall trend analysis. Results revealed that data for twenty two (22) rainfall stations have no significant trend at 5% significance level, i.e. the series is independent and identically distributed. Three stations namely; Kabaya, Ruhengeri and Rwaza have showed increasing trends while Nemba station showed decreasing trend. Using Hydrognomon model collected 24 hours annual maximum rainfall data were disaggregated into higher resolution data such as 0.5hr, 1hr, 3hr, 6hr and 12hr and were fitted to the probability distributions .Based on smallest standard error of estimate best fitted distribution were selected for each duration . EVI was the dominant distribution which fitted the rainfall data for different periods. Quantiles estimation for different return periods was prepared based on the best fitted distribution. Parameters of IDF curves were computed and curves were established. Rainfall stations cluster analysis was made based on annual maximum rainfall for 24 hours duration. The primary identification of homogeneous regions was done by using LMRD. Arc GIS software was used for delineation of five (5) identified regions. Finally, regional quantile, IDF parameters and IDF maps were developed for each region. The results of the study are believed to avail basic information on rainfall intensity, duration and frequency relationship for Rwanda.