5.4. Rainfall-runoff relationship in Akaki sub-catchments (Bulbula and Kebena) Fig. 7(a) shows the monthly rainfall distributions at Intoto station in the Kebena (KE) catchment for the 20202022 periods. In 2020, rainfall started in March and continued until September. The rainfall began to intensify in April, and peaked in July and August. Significant rainfall was recorded in September, followed by insignificant rainfall in the Further data quality assessments were performed to confirm whether these data observed in these plots are due to observation errors or climatic conditions. Suspicious data were identified using time series comparisons of areal rainfall and river flow data at each site (Fig. 6). The JournalofHydrology629(2024)1306398

G. Kebede Mengistie et al. Fig. 6. Comparisons between areal rainfall and river flow data collected by citizen scientists at BK station. Fig. 7. Monthly rainfall and river flow over KE catchment (a, c) and BUL catchment (b, d) for the observation periods (20202022). Note that plots (a) and (c) refer to monthly rainfall at Intoto station and monthly river flow over KE catchment respectively. Plots (b) and (d) refer to monthly rainfall at Sendafa station and monthly river flow over BUL catchment respectively.

7(c). In 2020, the river flow was significantly high in July, August and September. The peak flow occurred in August. This agrees with the observed rainfall. The peak flow was observed in September for 2021, which shows lag time of one month compared to the timing of the rainfall peak. In contrast, in 2022, the river flow peaked in July abruptly dropping afterwards. The slight difference between monthly rainfall and river flow patterns suggests that other rainfall characteristics also affect runoff generation. Overall, there was a significant difference in the hydrograph patterns of the three years with higher river flow in 2020 than the other two years.

Whereas in 2021, it began in February but declined in March. For the remaining months, the rainfall of 2021 followed a similar pattern as that of 2020 but with relatively small magnitude. In 2022, the rainfall onset was in June with the July and August rainfall amounts higher than same months in 2021. In 2020 and 2021, nearly equal peak rainfall amounts occurred in July and August but in 2022 the peak rainfall occurred in July.